1.  Introduction

At times, it is necessary to use native (non-Java) codes (e.g., C/C++) to overcome the memory management and performance constraints in Java. Java supports native codes via the Java Native Interface (JNI).

JNI is difficult, as it involves two languages and runtimes.

I shall assume that you are familiar with:

1. Java.
2. C/C++ and the GCC Compiler (Read "GCC and Make").
3. (For Windows) Cygwin or MinGW (Read "How to Setup Cygwin and MinGW").

2.  Getting Started

2.1  JNI with C

Step 1: Write a Java Class HelloJNI.java that uses C Codes

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public class HelloJNI {  // Save as HelloJNI.java

   static {

      System.loadLibrary("hello"); // Load native library hello.dll (Windows) or libhello.so (Unixes)

                                   //  at runtime

                                   // This library contains a native method called sayHello()

   }

   // Declare an instance native method sayHello() which receives no parameter and returns void

   private native void sayHello();

   // Test Driver

   public static void main(String[] args) {

      new HelloJNI().sayHello();  // Create an instance and invoke the native method

   }

}

The static initializer invokes System.loadLibrary() to load the native library "hello" (which contains a native method called sayHello()) during the class loading. It will be mapped to "hello.dll" in Windows; or "libhello.so" in Unixes/Mac OS X. This library shall be included in Java's library path (kept in Java system variable java.library.path). You could include the library into Java's library path via VM argument -Djava.library.path=/path/to/lib. The program will throw a UnsatisfiedLinkError if the library cannot be found in runtime.

Next, we declare the method sayHello() as a native instance method, via keyword native which denotes that this method is implemented in another language. A native method does not contain a body. The sayHello() shall be found in the native library loaded.

The main() method allocates an instance of HelloJNI and invoke the native method sayHello().

Step 2: Compile the Java Program HelloJNI.java & Generate the C/C++ Header File HelloJNI.h

Starting from JDK 8, you should use "javac -h" to compile the Java program AND generate C/C++ header file called HelloJNI.h as follows:

> javac -h . HelloJNI.java

The "-h dir" option generates C/C++ header and places it in the directory specified (in the above example, '.' for the current directory).

Before JDK 8, you need to compile the Java program using javac and generate C/C++ header using a dedicated javah utility, as follows. The javah utility is no longer available in JDK 10.

> javac HelloJNI.java

> javah HelloJNI

Inspect the header file HelloJNI.h:

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/* DO NOT EDIT THIS FILE - it is machine generated */

#include <jni.h>

/* Header for class HelloJNI */

#ifndef _Included_HelloJNI

#define _Included_HelloJNI

#ifdef __cplusplus

extern "C" {

#endif

/*

 * Class:     HelloJNI

 * Method:    sayHello

 * Signature: ()V

 */

JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *, jobject);

#ifdef __cplusplus

}

#endif

#endif

The header declares a C function Java_HelloJNI_sayHello as follows:

JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *, jobject);

The naming convention for the C function is Java_{package_and_classname}_{function_name}(JNI_arguments). The dot in package name is replaced by underscore.

The arguments are:

• JNIEnv*: reference to JNI environment, which lets you access all the JNI functions.
• jobject: reference to "this" Java object.

We are not using these arguments in this hello-world example, but will be using them later. Ignore the macros JNIEXPORT and JNICALL for the time being.

The extern "C" is recognized by C++ compiler only. It notifies the C++ compiler that these functions are to be compiled using C's function naming protocol instead of C++ naming protocol. C and C++ have different function naming protocols as C++ support function overloading and uses a name mangling scheme to differentiate the overloaded functions. Read "Name Mangling".

Step 3: Implementing the C Program HelloJNI.c

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// Save as "HelloJNI.c"

#include <jni.h>        // JNI header provided by JDK

#include <stdio.h>      // C Standard IO Header

#include "HelloJNI.h"   // Generated

// Implementation of the native method sayHello()

JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {

   printf("Hello World!\n");

   return;

}

Save the C program as "HelloJNI.c".

The JNI header "jni.h" provided by JDK is available under the "<JAVA_HOME>\include" and "<JAVA_HOME>\include\win32" (for Windows) or "<JAVA_HOME>\include\linux" (for Ubuntu) [Check Mac OS X] directories, where <JAVA_HOME> is your JDK installed directory (e.g., "c:\program files\java\jdk10.0.x" for Windows).

The C function simply prints the message "Hello world!" to the console.

Step 4: Compile the C program HelloJNI.c

Finding the right compiler for your operating platform (Windows, Mac OS X, Ubuntu), for your JDK (32-bit, 64-bit), and figuring out the correct compiler options is the hardest part to get the JNI working!!!

(Windows) 64-bit JDK

We are going to use Cygwin. You need to take note of the followings for Windows:

• Windows/Intel uses these instruction sets: x86 is a 32-bit instruction set; i868 is an enhanced version of x86 (also 32-bit); x86_64 (or amd64) is a 64-bit instruction set.
• A 32-bit compiler can run on 32-bit or 64-bit (backward compatible) Windows, but 64-bit compiler can only run on 64-bit Windows.
• A 64-bit compiler could produce target of 32-bit or 64-bit.
• If you use Cygwin's GCC, the target could be native Windows or Cygwin. If the target is native Windows, the code can be distributed and run under Windows. However, if the target is Cygwin, to distribute, you need to distribute Cygwin runtime environment (cygwin1.dll). This is because Cygwin is a Unix emulator under Windows.
• The above explains for the many versions of GCC under Cygwin.

For 64-bit JDK, you need to find a compiler that produces target of 64-bit native Windows. This is provided by MinGW-W64. You can install MinGW-W64 under Cygwin, by selecting packages "mingw64-x86_64-gcc-core" (C compiler) and "mingw64-x86_64-gcc-g++" (C++ compiler). The executables are "x86_64-w64-mingw32-gcc" (C Compiler) and "x86_64-w64-mingw32-g++" (C++ Compiler), respectively.

First, set the environment variable JAVA_HOME to point the JDK installed directory (e.g., "c:\program files\java\jdk10.0.x"). Follow the steps HERE.

Next, use the following commands to compile HelloJNI.c into hello.dll. In Windows, we reference the environment variable JAVA_HOMEas %JAVA_HOME% in the command.

> x86_64-w64-mingw32-gcc -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o hello.dll HelloJNI.c

The compiler options used are:

• -IheaderDir: for specifying the header directory. In this case "jni.h" (in "%JAVA_HOME%\include") and "jni_md.h" (in "%JAVA_HOME%\include\win32"), where JAVA_HOME is an environment variable set to the JDK installed directory.
• -shared: to generate share library.
• -o outputFilename: for setting the output filename "hello.dll".

You can also compile and link in two steps:

// Compile-only "HelloJNI.c" with -c flag. Output is "HElloJNI.o"

> x86_64-w64-mingw32-gcc -c -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" HelloJNI.c

// Link "HelloJNO.o" into shared library "hello.dll"

> x86_64-w64-mingw32-gcc -shared -o hello.dll HelloJNI.o

You need check the resultant file type via the "file" utility, which indicates "Hello.dll" is a 64-bit (x86_64) native Windows DLL.

> file hello.dll

hello.dll: PE32+ executable (DLL) (console) x86-64, for MS Windows

Try nm, which lists all the symbols in the shared library and look for the sayHello() function. Check for the function name Java_HelloJNI_sayHello with type "T" (defined).

> nm hello.dll | grep say

00000000624014a0 T Java_HelloJNI_sayHello

(Windows) 32-bit JDK [Obsolete?]

For 32-bit JDK, you need to find a 32/64-bit compiler that produces target of 32-bit native Windows. This is provided by MinGW-W64 (and the older MinGW). You can install MinGW-W64 under Cygwin, by selecting packages "mingw64-i686-gcc-core" (C compiler) and "mingw64-i686-gcc-g++" (C++ compiler). The executables are "i886-w64-mingw32-gcc" (C Compiler) and "i686-w64-mingw32-g++" (C++ Compiler), respectively.

First, set the environment variable JAVA_HOME to point the JDK installed directory (e.g., "c:\program files\java\jdk9.0.x"). Follow the steps HERE.

Next, use the following command to compile HelloJNI.c into hello.dll:

> i886-w64-mingw32-gcc -Wl,--add-stdcall-alias -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o hello.dll HelloJNI.c

The compiler options used are:

• -Wl: The -Wl to pass linker option --add-stdcall-alias to prevent UnsatisfiedLinkError (symbols with a stdcall suffix (@nn) will be exported as-is and also with the suffix stripped). (Some people suggested to use -Wl,--kill-at.)
• -I: for specifying the header files directories. In this case "jni.h" (in "%JAVA_HOME%\include") and "jni_md.h" (in "%JAVA_HOME%\include\win32"), where %JAVA_HOME% is an environment variable set to the JDK installed directory.
• -shared: to generate share library.
• -o: for setting the output filename "hello.dll".
• -D __int64="long long": define the type (add this option in front if error "unknown type name '__int64'")

(Ubuntu) 64-bit JDK

1. Set environment variable JAVA_HOME to point to the JDK installed directory (which shall contains the include subdirectory to be used in the next step):

   $ export JAVA_HOME=/your/java/installed/dir$ echo $JAVA_HOME

2. Compile the C program HelloJNI.c into share module libhello.so using gcc, which is included in all Unixes:

   $ gcc -fPIC -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -shared -o libhello.so HelloJNI.c

3. Run the Java Program:

   $ java -Djava.library.path=. HelloJNI

(Mac OS X) 64-Bit JDK

1. Set environment variable JAVA_HOME to point to the JDK installed directory (which shall contains the include subdirectory to be used in the next step):

   $ export JAVA_HOME=/your/java/installed/dir
   
      // for my machine @ /Library/Java/JavaVirtualMachines/jdk1.8.0_xx.jdk/Contents/Home
   
   $ echo $JAVA_HOME

2. Compile the C program HelloJNI.c into dynamic share module libhello.dylib using gcc, which is included in all Unixes/Mac OS:

   $ gcc -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/darwin" -dynamiclib -o libhello.dylib HelloJNI.c

3. Run the Java Program:

   $ java -Djava.library.path=. HelloJNI

Step 4: Run the Java Program

> java HelloJNI

You may need to explicitly specify the Java library path of the "hello.dll" (Windows), "libHello.so" (Unixes), "libhello.dylib" (Mac OS X) via VM option -Djava.library.path=/path/to/lib, as below. In this example, the native library is kept in the current directory '.'.

> java -Djava.library.path=. HelloJNI

2.2  JNI with C++

Instead of a C program, we can use a C++ program (called HelloJNI.cpp) for the above example.

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// Save as "HelloJNI.cpp"

#include <jni.h>       // JNI header provided by JDK

#include <iostream>    // C++ standard IO header

#include "HelloJNI.h"  // Generated

using namespace std;

// Implementation of the native method sayHello()

JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {

	cout << "Hello World from C++!" << endl;

   return;

}

Compile the C++ programs into shared library as follows. See "JNI with C" section for explanation.

(Windows) 64-bit JDK

On Cygwin, you need to install mingw64-x86-gcc-g++ package.

> x86_64-w64-mingw32-g++ -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o hello.dll HelloJNI.cpp

(Ubuntu) 64-bit JDK

$ g++ -fPIC -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -shared -o libhello.so HelloJNI.cpp

(Mac OS X) 64-bit JDK

[TODO]

Run the Java Program

> java HelloJNI

or

> java -Djava.library.path=. HelloJNI

Notes: If you encounter "java.lang.UnsatisfiedLinkError: hello.dll: Can't find dependent libraries", you need to find a "DLL dependency walker" to track down the dependent libraries. Search for the libraries (under Cygwin) and include the libraries in the environment variable PATH. In my case, the dependent library is "libstdc++-6.dll" located at "cygwin64\usr\x86_64-w64-mingw32\sys-root\mingw\bin".

2.3  JNI with C/C++ Mixture

Step 1: Write a Java Class that uses Native Codes - HelloJNICpp.java

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public class HelloJNICpp {

   static {

      System.loadLibrary("hello"); // hello.dll (Windows) or libhello.so (Unixes)

   }

   // Native method declaration

   private native void sayHello();

   // Test Driver

   public static void main(String[] args) {

      new HelloJNICpp().sayHello();  // Invoke native method

   }

}

Step 2: Compile the Java Program & Generate the C/C++ Header file HelloJNICpp.h

javac -h . HelloJNICpp

The resultant header file "HelloJNICpp.h" declares the native function as:

JNIEXPORT void JNICALL Java_HelloJNICpp_sayHello(JNIEnv *, jobject);

Step 3: C/C++ Implementation - HelloJNICppImpl.h, HelloJNICppImpl.cpp, and HelloJNICpp.c

We shall implement the program in C++ (in "HelloJNICppImpl.h" and "HelloJNICppImpl.cpp"), but use a C program ("HelloJNICpp.c") to interface with Java.

C++ Header - "HelloJNICppImpl.h"

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#ifndef _HELLO_JNI_CPP_IMPL_H

#define _HELLO_JNI_CPP_IMPL_H

#ifdef __cplusplus

        extern "C" {

#endif

        void sayHello ();

#ifdef __cplusplus

        }

#endif

#endif

C++ Implementation - "HelloJNICppImpl.cpp"

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#include "HelloJNICppImpl.h"

#include  <iostream>

using namespace std;

void sayHello () {

    cout << "Hello World from C++!" << endl;

    return;

}

C Program interfacing with Java - "HelloJNICpp.c"

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#include <jni.h>

#include "HelloJNICpp.h"

#include "HelloJNICppImpl.h"

JNIEXPORT void JNICALL Java_HelloJNICpp_sayHello (JNIEnv *env, jobject thisObj) {

    sayHello();  // invoke C++ function

    return;

}

Compile the C/C++ programs into shared library ("hello.dll" for Windows).

(Windows) 64-bit JDK

On Cygwin, you need to install mingw64-x86-gcc-g++ package.

> x86_64-w64-mingw32-g++ -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o hello.dll HelloJNICpp.c HelloJNICppImpl.cpp

(Ubuntu) 64-bit JDK

$ g++ -fPIC -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -shared -o libhello.so HelloJNICpp.c HelloJNICppImpl.cpp

Step 4: Run the Java Program

> java HelloJNICpp

or

> java -Djava.library.path=. HelloJNICpp

Notes: If you encounter "java.lang.UnsatisfiedLinkError: hello.dll: Can't find dependent libraries", you need to find a DLL dependency walker to track down the dependent libraries. Search for the libraries (under Cygwin) and include the libraries in the PATH. In my case, the dependent library is "libstdc++-6.dll" located at "cygwin64\usr\x86_64-w64-mingw32\sys-root\mingw\bin".

2.4  JNI in Package

For production, all Java classes shall be kept in proper packages, instead of the default no-name package.

Step 1: JNI Program - myjni\HelloJNI.java

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package myjni;

public class HelloJNI {

   static {

      System.loadLibrary("hello"); // hello.dll (Windows) or libhello.so (Unixes)

   }

   // A native method that receives nothing and returns void

   private native void sayHello();

   public static void main(String[] args) {

      new myjni.HelloJNI().sayHello();  // invoke the native method

   }

}

This JNI class is kept in package "myjni" - to be saved as "myjni\HelloJNI.java".

Step 2: Compile the JNI program & Generate C/C++ Header

// change directory to package base directory

> javac -h include myjni\HelloJNI

The output of compilation is "myjni\HelloJNI.class".

In this example, we decided to place the header file under a "include" sub-directory. The generated output is "include\myjni_HelloJNI.h".

The header file declares a native function:

JNIEXPORT void JNICALL Java_myjni_HelloJNI_sayHello(JNIEnv *, jobject);

Take note of the native function naming convention: Java_<fully-qualified-name>_methodName, with dots replaced by underscores.

Step 3: C Implementation - HelloJNI.c

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#include <jni.h>

#include <stdio.h>

#include "include\myjni_HelloJNI.h"

JNIEXPORT void JNICALL Java_myjni_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {

   printf("Hello World!\n");

   return;

}

Compile the C program:

// for Windows 64-bit JDK

> x86_64-w64-mingw32-gcc -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o hello.dll HelloJNI.c

You can now run the JNI program:

> java -Djava.library.path=. myjni.HelloJNI

2.5  JNI in Module (JDK 9)

[TODO]

2.6  JNI in Eclipse [To Check]

Writing JNI under Eclipse is handy for development Android apps with NDK.

You need to install Eclipse and Eclipse CDT (C/C++ Development Tool) Plugin. Read "Eclipse for C/C++" on how to install CDT.

Step 1: Create a Java Project

Create a new Java project (says HelloJNI), and the following Java class "HelloJNI.java":

public class HelloJNI {

   static {

      System.loadLibrary("hello"); // hello.dll (Windows) or libhello.so (Unixes)

   }

   // Declare native method

   private native void sayHello();

   // Test Driver

   public static void main(String[] args) {

      new HelloJNI().sayHello();  // Allocate an instance and invoke the native method

   }

}

Step 2: Convert the Java Project to C/C++ Makefile Project

Right-click on the "HelloJNI" Java project ⇒ New ⇒ Other... ⇒ Convert to a C/C++ Project (Adds C/C++ Nature) ⇒ Next.

The "Convert to a C/C++ Project" dialog appears. In "Project type", select "Makefile Project" ⇒ In "Toolchains", select "MinGW GCC" ⇒ Finish.

Now, you can run this project as a Java as well as C/C++ project.

Step 3: Generate C/C++ Header File (Pre JDK-10)

Create a directory called "jni" under the project to keep all the C/C++ codes, by right-click on the project ⇒ New ⇒ Folder ⇒ In "Folder name", enter "jni".

Create a "makefile" under the "jni" directory, by right-click on the "jni" folder ⇒ new ⇒ File ⇒ In "File name", enter "makefile" ⇒ Enter the following codes. Take note that you need to use tab (instead of spaces) for the indent.

# Define a variable for classpath

CLASS_PATH = ../bin

# Define a virtual path for .class in the bin directory

vpath %.class $(CLASS_PATH)

# $* matches the target filename without the extension

# Pre JDK-10. JDK 10 removes the javah utility, need to use javac -h instead [TO CHECK]

HelloJNI.h : HelloJNI.class

	javah -classpath $(CLASS_PATH) $*

This makefile create a target "HelloJNI.h", which has a dependency "HelloJNI.class", and invokes the javah utility on HelloJNI.class(under -classpath) to build the target header file.

Right-click on the makefile ⇒ Make Targets ⇒ Create ⇒ In "Target Name", enter "HelloJNI.h".

Run the makefile for the target "HelloJNI.h", by right-click on the makefile ⇒ Make Targets ⇒ Build ⇒ Select the target "HelloJNI.h" ⇒ Build. The header file "HelloJNI.h" shall be generated in the "jni" directory. Refresh (F5) if necessary. The outputs are:

make HelloJNI.h

javah -classpath ../bin HelloJNI

Read "GCC and Make" for details about makefile.

Alternatively, you could also use the CMD shell to run the make file:

// change directory to the directory containing makefile

> make HelloJNI.h

You can even use the CMD shell to run the javah (Pre JDK-10):

> javah -classpath ../bin HelloJNI

Step 4: C Implementation - HelloJNI.c

Create a C program called "HelloJNI.c", by right-click on the "jni" folder ⇒ New ⇒ Source file ⇒ In "Source file", enter "HelloJNI.c". Enter the following codes:

#include <jni.h>

#include <stdio.h>

#include "HelloJNI.h"

JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {

   printf("Hello World!\n");

   return;

}

Modify the "makefile" as follows to generate the shared library "hello.dll". (Again, use tab to indent the lines.)

# Define a variable for classpath

CLASS_PATH = ../bin

# Define a virtual path for .class in the bin directory

vpath %.class $(CLASS_PATH)

all : hello.dll

# $@ matches the target, $< matches the first dependency

hello.dll : HelloJNI.o

	gcc -Wl,--add-stdcall-alias -shared -o $@ $<

# $@ matches the target, $< matches the first dependency

HelloJNI.o : HelloJNI.c HelloJNI.h

	gcc -I"D:\bin\jdk1.7\include" -I"D:\bin\jdk1.7\include\win32" -c $< -o $@

# $* matches the target filename without the extension

HelloJNI.h : HelloJNI.class

	javah -classpath $(CLASS_PATH) $*

clean :

	rm HelloJNI.h HelloJNI.o hello.dll

Right-click on the "makefile" ⇒ Make Targets ⇒ Create ⇒ In "Target Name", enter "all". Repeat to create a target "clean".

Run the makefile for the target "all", by right-click on the makefile ⇒ Make Targets ⇒ Build ⇒ Select the target "all" ⇒ Build. The outputs are:

make all

javah -classpath ../bin HelloJNI

gcc -I"D:\bin\jdk1.7\include" -I"D:\bin\jdk1.7\include\win32" -c HelloJNI.c -o HelloJNI.o

gcc -Wl,--add-stdcall-alias -shared -o hello.dll HelloJNI.o

The shared library "hello.dll" shall have been created in "jni" directory.

Step 5: Run the Java JNI Program

You can run the Java JNI program HelloJNI. However, you need to provide the library path to the "hello.dll". This can be done via VM argument -Djava.library.path. Right-click on the project ⇒ Run As ⇒ Run Configurations ⇒ Select "Java Application" ⇒ In "Main" tab, enter the main class "HelloJNI" ⇒ In "Arguments", "VM Arguments", enter "-Djava.library.path=jni" ⇒ Run.

You shall see the output "Hello World!" displayed on the console.

2.7  JNI in NetBeans

[TODO]

3.  JNI Basics

JNI defines the following JNI types in the native system that correspond to Java types:

1. Java Primitives: jint, jbyte, jshort, jlong, jfloat, jdouble, jchar, jboolean for Java Primitive of int, byte, short, long, float, double, char and boolean, respectively.
2. Java Reference Types: jobject for java.lang.Object. It also defines the following sub-types:
   1. jclass for java.lang.Class.
   2. jstring for java.lang.String.
   3. jthrowable for java.lang.Throwable.
   4. jarray for Java array. Java array is a reference type with eight primitive array and one Object array. Hence, there are eight array of primitives jintArray, jbyteArray, jshortArray, jlongArray, jfloatArray, jdoubleArray, jcharArray and jbooleanArray; and one object array jobjectArray.

The native functions receives argument in the above JNI types and returns a value in the JNI type (such as jstring, jintArray). However, native functions operate on their own native types (such as C-string, C's int[]). Hence, there is a need to convert (or transform) between JNI types and the native types.

The native programs:

1. Receive the arguments in JNI type (passed over by the Java program).
2. For reference JNI type, convert or copy the arguments to local native types, e.g., jstring to a C-string, jintArray to C's int[], and so on. Primitive JNI types such as jint and jdouble do not need conversion and can be operated directly.
3. Perform its operations, in local native type.
4. Create the returned object in JNI type, and copy the result into the returned object.
5. Return.

The most confusing and challenging task in JNI programming is the conversion (or transformation) between JNI reference types (such as jstring, jobject, jintArray, jobjectArray) and native types (C-string, int[]). The JNI Environment interface provides many functions to do the conversion.

JNI is a C interface, which is not object-oriented. It does not really pass the objects.

[C++ object-oriented interface?!]

4.  Passing Arguments and Result between Java & Native Programs

4.1  Passing Primitives

Passing Java primitives is straight forward. A jxxx type is defined in the native system, i.e,. jint, jbyte, jshort, jlong, jfloat, jdouble, jchar and jboolean for each of the Java's primitives int, byte, short, long, float, double, char and boolean, respectively.

Java JNI Program: TestJNIPrimitive.java

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public class TestJNIPrimitive {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Declare a native method average() that receives two ints and return a double containing the average

   private native double average(int n1, int n2);

   // Test Driver

   public static void main(String args[]) {

      System.out.println("In Java, the average is " + new TestJNIPrimitive().average(3, 2));

   }

}

This JNI program loads a shared library myjni.dll (Windows) or libmyjni.so (Unixes). It declares a native method average() that receives two int's and returns a double containing the average value of the two int's. The main() method invoke the average().

Compile the Java program into "TestJNIPrimitive.class" and generate the C/C++ header file "TestJNIPrimitive.h":

javac -h . TestJNIPrimitive.java

C Implementation - TestJNIPrimitive.c

The header file TestJNIPrimitive.h contains a function declaration Java_TestJNIPrimitive_average() which takes a JNIEnv* (for accessing JNI environment interface), a jobject (for referencing this object), two jint's (Java native method's two arguments) and returns a jdouble (Java native method's return-type).

JNIEXPORT jdouble JNICALL Java_TestJNIPrimitive_average(JNIEnv *, jobject, jint, jint);

The JNI types jint and jdouble correspond to Java's type int and double, respectively.

The "jni.h" and "win32\jni_mh.h" (which is platform dependent) contains these typedef statements for the eight JNI primitives and an additional jsize.

It is interesting to note that jint is mapped to C's long (which is at least 32 bits), instead of of C's int (which could be 16 bits). Hence, it is important to use jint in the C program, instead of simply using int. Cygwin does not support __int64.

// In "win\jni_mh.h" - machine header which is machine dependent

typedef long            jint;

typedef __int64         jlong;

typedef signed char     jbyte;

// In "jni.h"

typedef unsigned char   jboolean;

typedef unsigned short  jchar;

typedef short           jshort;

typedef float           jfloat;

typedef double          jdouble;

typedef jint            jsize;

The implementation TestJNIPrimitive.c is as follows:

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#include <jni.h>

#include <stdio.h>

#include "TestJNIPrimitive.h"

JNIEXPORT jdouble JNICALL Java_TestJNIPrimitive_average

          (JNIEnv *env, jobject thisObj, jint n1, jint n2) {

   jdouble result;

   printf("In C, the numbers are %d and %d\n", n1, n2);

   result = ((jdouble)n1 + n2) / 2.0;

   // jint is mapped to int, jdouble is mapped to double

   return result;

}

Compile the C program into shared library (jni.dll).

gcc -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o myjni.dll TestJNIPrimitive.c

Now, run the Java Program:

java -Djava.library.path=. TestJNIPrimitive

C++ Implementation - TestJNIPrimitive.cpp

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#include <jni.h>

#include <iostream>

#include "TestJNIPrimitive.h"

using namespace std;

JNIEXPORT jdouble JNICALL Java_TestJNIPrimitive_average

          (JNIEnv *env, jobject obj, jint n1, jint n2) {

   jdouble result;

   cout << "In C++, the numbers are " << n1 << " and " << n2 << endl;

   result = ((jdouble)n1 + n2) / 2.0;

   // jint is mapped to int, jdouble is mapped to double

   return result;

}

Compile the C++ program:

g++ -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" -shared -o myjni.dll TestJNIPrimitive.cpp

4.2  Passing Strings

Java JNI Program: TestJNIString.java

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public class TestJNIString {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Native method that receives a Java String and return a Java String

   private native String sayHello(String msg);

   public static void main(String args[]) {

      String result = new TestJNIString().sayHello("Hello from Java");

      System.out.println("In Java, the returned string is: " + result);

   }

}

This JNI program declares a native method sayHello() that receives a Java String and returns a Java String. The main() method invokes the sayHello().

Compile the Java program and generate the C/C++ header file "TestJNIString.h":

javac -h . TestJNIString.java

C Implementation - TestJNIString.c

The header file TestJNIString.h contains this function declaration:

JNIEXPORT jstring JNICALL Java_TestJNIString_sayHello(JNIEnv *, jobject, jstring);

JNI defined a jstring type to represent the Java String. The last argument (of JNI type jstring) is the Java String passed into the C program. The return-type is also jstring.

Passing strings is more complicated than passing primitives, as Java's String is an object (reference type), while C-string is a NULL-terminated char array. You need to convert between Java String (represented as JNI jstring) and C-string (char*).

The JNI Environment (accessed via the argument JNIEnv*) provides functions for the conversion:

1. To get a C-string (char*) from JNI string (jstring), invoke method const char* GetStringUTFChars(JNIEnv*, jstring, jboolean*).
2. To get a JNI string (jstring) from a C-string (char*), invoke method jstring NewStringUTF(JNIEnv*, char*).

The C implementation TestJNIString.c is as follows.

1. It receives the JNI string (jstring), convert into a C-string (char*), via GetStringUTFChars().
2. It then performs its intended operations - displays the string received and prompts user for another string to be returned.
3. It converts the returned C-string (char*) to JNI string (jstring), via NewStringUTF(), and return the jstring.

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#include <jni.h>

#include <stdio.h>

#include "TestJNIString.h"

JNIEXPORT jstring JNICALL Java_TestJNIString_sayHello(JNIEnv *env, jobject thisObj, jstring inJNIStr) {

   // Step 1: Convert the JNI String (jstring) into C-String (char*)

   const char *inCStr = (*env)->GetStringUTFChars(env, inJNIStr, NULL);

   if (NULL == inCStr) return NULL;

   // Step 2: Perform its intended operations

   printf("In C, the received string is: %s\n", inCStr);

   (*env)->ReleaseStringUTFChars(env, inJNIStr, inCStr);  // release resources

   // Prompt user for a C-string

   char outCStr[128];

   printf("Enter a String: ");

   scanf("%s", outCStr);    // not more than 127 characters

   // Step 3: Convert the C-string (char*) into JNI String (jstring) and return

   return (*env)->NewStringUTF(env, outCStr);

}

Compile the C program into shared library.

gcc -I"<JAVA_HOME>\include" -I"<JAVA_HOME>\include\win32" -shared -o myjni.dll TestJNIString.c

Now, run the Java Program:

java -Djava.library.path=. TestJNIString

In C, the received string is: Hello from Java

Enter a String: test

In Java, the returned string is: test

JNI Native String Functions

JNI supports conversion for Unicode (16-bit characters) and UTF-8 (encoded in 1-3 bytes) strings. UTF-8 strings act like null-terminated C-strings (char array), which should be used in C/C++ programs.

The JNI string (jstring) functions are:

// UTF-8 String (encoded to 1-3 byte, backward compatible with 7-bit ASCII)

// Can be mapped to null-terminated char-array C-string

const char * GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy);

   // Returns a pointer to an array of bytes representing the string in modified UTF-8 encoding.

void ReleaseStringUTFChars(JNIEnv *env, jstring string, const char *utf);

   // Informs the VM that the native code no longer needs access to utf.

jstring NewStringUTF(JNIEnv *env, const char *bytes);

   // Constructs a new java.lang.String object from an array of characters in modified UTF-8 encoding.

jsize GetStringUTFLength(JNIEnv *env, jstring string);

   // Returns the length in bytes of the modified UTF-8 representation of a string.

void GetStringUTFRegion(JNIEnv *env, jstring str, jsize start, jsize length, char *buf);

   // Translates len number of Unicode characters beginning at offset start into modified UTF-8 encoding

   // and place the result in the given buffer buf.

// Unicode Strings (16-bit character)

const jchar * GetStringChars(JNIEnv *env, jstring string, jboolean *isCopy);

   // Returns a pointer to the array of Unicode characters

void ReleaseStringChars(JNIEnv *env, jstring string, const jchar *chars);

   // Informs the VM that the native code no longer needs access to chars.

jstring NewString(JNIEnv *env, const jchar *unicodeChars, jsize length);

   // Constructs a new java.lang.String object from an array of Unicode characters.

jsize GetStringLength(JNIEnv *env, jstring string);

   // Returns the length (the count of Unicode characters) of a Java string.

void GetStringRegion(JNIEnv *env, jstring str, jsize start, jsize length, jchar *buf);

   // Copies len number of Unicode characters beginning at offset start to the given buffer buf

UTF-8 strings or C-strings

The GetStringUTFChars() function can be used to create a new C-string (char*) from the given Java's jstring. The function returns NULLif the memory cannot be allocated. It is always a good practice to check against NULL.

The 3rd parameter isCopy (of jboolean*), which is an "in-out" parameter, will be set to JNI_TRUE if the returned string is a copy of the original java.lang.String instance. It will be set to JNI_FALSE if the returned string is a direct pointer to the original String instance - in this case, the native code shall not modify the contents of the returned string. The JNI runtime will try to return a direct pointer, if possible; otherwise, it returns a copy. Nonetheless, we seldom interested in modifying the underlying string, and often pass a NULL pointer.

Always invoke ReleaseStringUTFChars() whenever you do not need the returned string of GetStringUTFChars() to release the memory and the reference so that it can be garbage-collected.

The NewStringUTF() function create a new JNI string (jstring), with the given C-string.

JDK 1.2 introduces the GetStringUTFRegion(), which copies the jstring (or a portion from start of length) into the "pre-allocated" C's char array. They can be used in place of GetStringUTFChars(). The isCopy is not needed as the C's array is pre-allocated.

JDK 1.2 also introduces the Get/ReleaseStringCritical() functions. Similar to GetStringUTFChars(), it returns a direct pointer if possible; otherwise, it returns a copy. The native method shall not block (for IO or others) between a pair a GetStringCritical() and ReleaseStringCritical() call.

For detailed description, always refer to "Java Native Interface Specification" @ http://docs.oracle.com/javase/7/docs/technotes/guides/jni/index.html.

Unicode String

Instead of char*, it uses a jchar* to store the Unicode characters.

C++ Implementation - TestJNIString.cpp

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#include <jni.h>

#include <iostream>

#include <string>

#include "TestJNIString.h"

using namespace std;

JNIEXPORT jstring JNICALL Java_TestJNIString_sayHello(JNIEnv *env, jobject thisObj, jstring inJNIStr) {

   // Step 1: Convert the JNI String (jstring) into C-String (char*)

   const char *inCStr = env->GetStringUTFChars(inJNIStr, NULL);

   if (NULL == inCStr) return NULL;

   // Step 2: Perform its intended operations

   cout << "In C++, the received string is: " << inCStr << endl;

   env->ReleaseStringUTFChars(inJNIStr, inCStr);  // release resources

   // Prompt user for a C++ string

   string outCppStr;

   cout << "Enter a String: ";

   cin >> outCppStr;

   // Step 3: Convert the C++ string to C-string, then to JNI String (jstring) and return

   return env->NewStringUTF(outCppStr.c_str());

}

Use g++ to compile the C++ program:

g++ -I"<JAVA_HOME>\include" -I"<JAVA_HOME>\include\win32" -shared -o myjni.dll TestJNIString.cpp

Take note that C++ native string functions have different syntax from C. In C++, we could us "env->", instead of "(env*)->". Furthermore, there is no need for the JNIEnv* argument in the C++ functions.

Also take note that C++ support a string class (under the header <string> which is more user-friendly, as well as the legacy C-string (char array).

[TODO] Is C++ string class supported directly?

4.3  Passing Array of Primitives

JNI Program - TestJNIPrimitiveArray.java

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public class TestJNIPrimitiveArray {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Declare a native method sumAndAverage() that receives an int[] and

   //  return a double[2] array with [0] as sum and [1] as average

   private native double[] sumAndAverage(int[] numbers);

   // Test Driver

   public static void main(String args[]) {

      int[] numbers = {22, 33, 33};

      double[] results = new TestJNIPrimitiveArray().sumAndAverage(numbers);

      System.out.println("In Java, the sum is " + results[0]);

      System.out.println("In Java, the average is " + results[1]);

   }

}

C Implementation - TestJNIPrimitiveArray.c

The header "TestJNIPrimitiveArray.h" contains the following function declaration:

JNIEXPORT jdoubleArray JNICALL Java_TestJNIPrimitiveArray_average (JNIEnv *, jobject, jintArray);

In Java, array is a reference type, similar to a class. There are 9 types of Java arrays, one each of the eight primitives and an array of java.lang.Object. JNI defines a type for each of the eight Java primitive arrays, i.e, jintArray, jbyteArray, jshortArray, jlongArray, jfloatArray, jdoubleArray, jcharArray, jbooleanArray for Java's primitive array of int, byte, short, long, float, double, char and boolean, respectively. It also define a jobjectArray for Java's array of Object (to be discussed later).

Again, you need to convert between JNI array and native array, e.g., between jintArray and C's jint[], or jdoubleArray and C's jdouble[]. The JNI Environment interface provides a set of functions for the conversion:

1. To get a C native jint[] from a JNI jintArray, invoke jint* GetIntArrayElements(JNIEnv *env, jintArray a, jboolean *iscopy).
2. To get a JNI jintArray from C native jint[], first, invoke jintArray NewIntArray(JNIEnv *env, jsize len) to allocate, then use void SetIntArrayRegion(JNIEnv *env, jintArray a, jsize start, jsize len, const jint *buf) to copy from the jint[] to jintArray.

There are 8 sets of the above functions, one for each of the eight Java primitives.

The native program is required to:

1. Receive the incoming JNI array (e.g., jintArray), convert to C's native array (e.g., jint[]).
2. Perform its intended operations.
3. Convert the return C's native array (e.g., jdouble[]) to JNI array (e.g., jdoubleArray), and return the JNI array.

The C implementation "TestJNIPrimitiveArray.c" is:

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#include <jni.h>

#include <stdio.h>

#include "TestJNIPrimitiveArray.h"

JNIEXPORT jdoubleArray JNICALL Java_TestJNIPrimitiveArray_sumAndAverage

          (JNIEnv *env, jobject thisObj, jintArray inJNIArray) {

   // Step 1: Convert the incoming JNI jintarray to C's jint[]

   jint *inCArray = (*env)->GetIntArrayElements(env, inJNIArray, NULL);

   if (NULL == inCArray) return NULL;

   jsize length = (*env)->GetArrayLength(env, inJNIArray);

   // Step 2: Perform its intended operations

   jint sum = 0;

   int i;

   for (i = 0; i < length; i++) {

      sum += inCArray[i];

   }

   jdouble average = (jdouble)sum / length;

   (*env)->ReleaseIntArrayElements(env, inJNIArray, inCArray, 0); // release resources

   jdouble outCArray[] = {sum, average};

   // Step 3: Convert the C's Native jdouble[] to JNI jdoublearray, and return

   jdoubleArray outJNIArray = (*env)->NewDoubleArray(env, 2);  // allocate

   if (NULL == outJNIArray) return NULL;

   (*env)->SetDoubleArrayRegion(env, outJNIArray, 0 , 2, outCArray);  // copy

   return outJNIArray;

}

JNI Primitive Array Functions

The JNI primitive array (jintArray, jbyteArray, jshortArray, jlongArray, jfloatArray, jdoubleArray, jcharArray andjbooleanArray) functions are:

// ArrayType: jintArray, jbyteArray, jshortArray, jlongArray, jfloatArray, jdoubleArray, jcharArray, jbooleanArray

// PrimitiveType: int, byte, short, long, float, double, char, boolean

// NativeType: jint, jbyte, jshort, jlong, jfloat, jdouble, jchar, jboolean
NativeType * Get<PrimitiveType>ArrayElements(JNIEnv *env, ArrayType array, jboolean *isCopy);

void Release<PrimitiveType>ArrayElements(JNIEnv *env, ArrayType array, NativeType *elems, jint mode);

void Get<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array, jsize start, jsize length, NativeType *buffer);

void Set<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array, jsize start, jsize length, const NativeType *buffer);
ArrayType New<PrimitiveType>Array(JNIEnv *env, jsize length);

void * GetPrimitiveArrayCritical(JNIEnv *env, jarray array, jboolean *isCopy);

void ReleasePrimitiveArrayCritical(JNIEnv *env, jarray array, void *carray, jint mode);

The GET|Release<PrimitiveType>ArrayElements() can be used to create a new C's native array jxxx[] from the given Java jxxxArray. GET|Set<PrimitiveType>ArrayRegion() can be used to copy a jxxxArray (or a portion from start of length) to and from a pre-allocated C native array jxxx[].

The New<PrimitiveType>Array() can be used to allocate a new jxxxArray of a given size. You can then use the Set<PrimitiveType>ArrayRegion() function to fill its contents from a native array jxxx[].

The Get|ReleasePrimitiveArrayCritical() functions does not allow blocking calls in between the get and release.

5.  Accessing Object's Variables and Calling Back Methods

5.1  Accessing Object's Instance Variables

JNI Program - TestJNIInstanceVariable.java

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public class TestJNIInstanceVariable {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Instance variables

   private int number = 88;

   private String message = "Hello from Java";

   // Declare a native method that modifies the instance variables

   private native void modifyInstanceVariable();

   // Test Driver

   public static void main(String args[]) {

      TestJNIInstanceVariable test = new TestJNIInstanceVariable();

      test.modifyInstanceVariable();

      System.out.println("In Java, int is " + test.number);

      System.out.println("In Java, String is " + test.message);

   }

}

The class contains two private instance variables: a primitive int called number and a String called message. It also declares a native method, which could modify the contents of the instance variables.

C Implementation - TestJNIInstanceVariable.c

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#include <jni.h>

#include <stdio.h>

#include "TestJNIInstanceVariable.h"

JNIEXPORT void JNICALL Java_TestJNIInstanceVariable_modifyInstanceVariable

          (JNIEnv *env, jobject thisObj) {

   // Get a reference to this object's class

   jclass thisClass = (*env)->GetObjectClass(env, thisObj);

   // int

   // Get the Field ID of the instance variables "number"

   jfieldID fidNumber = (*env)->GetFieldID(env, thisClass, "number", "I");

   if (NULL == fidNumber) return;

   // Get the int given the Field ID

   jint number = (*env)->GetIntField(env, thisObj, fidNumber);

   printf("In C, the int is %d\n", number);

   // Change the variable

   number = 99;

   (*env)->SetIntField(env, thisObj, fidNumber, number);

   // Get the Field ID of the instance variables "message"

   jfieldID fidMessage = (*env)->GetFieldID(env, thisClass, "message", "Ljava/lang/String;");

   if (NULL == fidMessage) return;

   // String

   // Get the object given the Field ID

   jstring message = (*env)->GetObjectField(env, thisObj, fidMessage);

   // Create a C-string with the JNI String

   const char *cStr = (*env)->GetStringUTFChars(env, message, NULL);

   if (NULL == cStr) return;

   printf("In C, the string is %s\n", cStr);

   (*env)->ReleaseStringUTFChars(env, message, cStr);

   // Create a new C-string and assign to the JNI string

   message = (*env)->NewStringUTF(env, "Hello from C");

   if (NULL == message) return;

   // modify the instance variables

   (*env)->SetObjectField(env, thisObj, fidMessage, message);

}

To access the instance variable of an object:

1. Get a reference to this object's class via GetObjectClass().
2. Get the Field ID of the instance variable to be accessed via GetFieldID() from the class reference. You need to provide the variable name and its field descriptor (or signature). For a Java class, the field descriptor is in the form of "L<fully-qualified-name>;", with dot replaced by forward slash (/), e.g.,, the class descriptor for String is "Ljava/lang/String;". For primitives, use "I" for int, "B"for byte, "S" for short, "J" for long, "F" for float, "D" for double, "C" for char, and "Z" for boolean. For arrays, include a prefix "[", e.g., "[Ljava/lang/Object;" for an array of Object; "[I" for an array of int.
3. Based on the Field ID, retrieve the instance variable via GetObjectField() or Get<primitive-type>Field() function.
4. To update the instance variable, use the SetObjectField() or Set<primitive-type>Field() function, providing the Field ID.

The JNI functions for accessing instance variable are:

jclass GetObjectClass(JNIEnv *env, jobject obj);

   // Returns the class of an object.

jfieldID GetFieldID(JNIEnv *env, jclass cls, const char *name, const char *sig);

  // Returns the field ID for an instance variable of a class.

NativeType Get<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID);

void Set<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID, NativeType value);

  // Get/Set the value of an instance variable of an object

  // <type> includes each of the eight primitive types plus Object.

5.2  Accessing Class' Static Variables

Accessing static variables is similar to accessing instance variable, except that you use functions such as GetStaticFieldID(), Get|SetStaticObjectField(), Get|SetStatic<Primitive-type>Field().

JNI Program - TestJNIStaticVariable.java

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public class TestJNIStaticVariable {

   static {

      System.loadLibrary("myjni"); // nyjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Static variables

   private static double number = 55.66;

   // Declare a native method that modifies the static variable

   private native void modifyStaticVariable();

   // Test Driver

   public static void main(String args[]) {

      TestJNIStaticVariable test = new TestJNIStaticVariable();

      test.modifyStaticVariable();

      System.out.println("In Java, the double is " + number);

   }

}

C Implementation - TestJNIStaticVariable.c

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#include <jni.h>

#include <stdio.h>

#include "TestJNIStaticVariable.h"

JNIEXPORT void JNICALL Java_TestJNIStaticVariable_modifyStaticVariable

          (JNIEnv *env, jobject thisObj) {

   // Get a reference to this object's class

   jclass cls = (*env)->GetObjectClass(env, thisObj);

   // Read the int static variable and modify its value

   jfieldID fidNumber = (*env)->GetStaticFieldID(env, cls, "number", "D");

   if (NULL == fidNumber) return;

   jdouble number = (*env)->GetStaticDoubleField(env, cls, fidNumber);

   printf("In C, the double is %f\n", number);

   number = 77.88;

   (*env)->SetStaticDoubleField(env, cls, fidNumber, number);

}

The JNI functions for accessing static variable are:

jfieldID GetStaticFieldID(JNIEnv *env, jclass cls, const char *name, const char *sig);

  // Returns the field ID for a static variable of a class.

NativeType GetStatic<type>Field(JNIEnv *env, jclass clazz, jfieldID fieldID);

void SetStatic<type>Field(JNIEnv *env, jclass clazz, jfieldID fieldID, NativeType value);

  // Get/Set the value of a static variable of a class.

  // <type> includes each of the eight primitive types plus Object.

5.3  Callback Instance Methods and Static Methods

You can callback an instance and static methods from the native code.

JNI Program - TestJNICallBackMethod.java

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public class TestJNICallBackMethod {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Declare a native method that calls back the Java methods below

   private native void nativeMethod();

   // To be called back by the native code

   private void callback() {

      System.out.println("In Java");

   }

   private void callback(String message) {

      System.out.println("In Java with " + message);

   }

   private double callbackAverage(int n1, int n2) {

      return ((double)n1 + n2) / 2.0;

   }

   // Static method to be called back

   private static String callbackStatic() {

      return "From static Java method";

   }

   // Test Driver

   public static void main(String args[]) {

      new TestJNICallBackMethod().nativeMethod();

   }

}

This class declares a native method called nativeMethod(), and invoke this nativeMethod(). The nativeMethod(), in turn, calls back the various instance and static methods defined in this class.

C Implementation - TestJNICallBackMethod.c

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#include <jni.h>

#include <stdio.h>

#include "TestJNICallBackMethod.h"

JNIEXPORT void JNICALL Java_TestJNICallBackMethod_nativeMethod

          (JNIEnv *env, jobject thisObj) {

   // Get a class reference for this object

   jclass thisClass = (*env)->GetObjectClass(env, thisObj);

   // Get the Method ID for method "callback", which takes no arg and return void

   jmethodID midCallBack = (*env)->GetMethodID(env, thisClass, "callback", "()V");

   if (NULL == midCallBack) return;

   printf("In C, call back Java's callback()\n");

   // Call back the method (which returns void), baed on the Method ID

   (*env)->CallVoidMethod(env, thisObj, midCallBack);

   jmethodID midCallBackStr = (*env)->GetMethodID(env, thisClass,

                               "callback", "(Ljava/lang/String;)V");

   if (NULL == midCallBackStr) return;

   printf("In C, call back Java's called(String)\n");

   jstring message = (*env)->NewStringUTF(env, "Hello from C");

   (*env)->CallVoidMethod(env, thisObj, midCallBackStr, message);

   jmethodID midCallBackAverage = (*env)->GetMethodID(env, thisClass,

                                  "callbackAverage", "(II)D");

   if (NULL == midCallBackAverage) return;

   jdouble average = (*env)->CallDoubleMethod(env, thisObj, midCallBackAverage, 2, 3);

   printf("In C, the average is %f\n", average);

   jmethodID midCallBackStatic = (*env)->GetStaticMethodID(env, thisClass,

                                 "callbackStatic", "()Ljava/lang/String;");

   if (NULL == midCallBackStatic) return;

   jstring resultJNIStr = (*env)->CallStaticObjectMethod(env, thisClass, midCallBackStatic);

   const char *resultCStr = (*env)->GetStringUTFChars(env, resultJNIStr, NULL);

   if (NULL == resultCStr) return;

   printf("In C, the returned string is %s\n", resultCStr);

   (*env)->ReleaseStringUTFChars(env, resultJNIStr, resultCStr);

}

To call back an instance method from the native code:

1. Get a reference to this object's class via GetObjectClass().
2. From the class reference, get the Method ID via GetMethodID(). You need to provide the method name and the signature. The signature is in the form "(parameters)return-type". You can list the method signature for a Java program via javap utility (Class File Disassembler) with -s (print signature) and -p (show private members):

   > javap --help
   
   > javap -s -p TestJNICallBackMethod
   
     .......
   
     private void callback();
   
       Signature: ()V
   
     private void callback(java.lang.String);
   
       Signature: (Ljava/lang/String;)V
   
     private double callbackAverage(int, int);
   
       Signature: (II)D
   
     private static java.lang.String callbackStatic();
   
       Signature: ()Ljava/lang/String;
   
     .......

3. Based on the Method ID, you could invoke Call<Primitive-type>Method() or CallVoidMethod() or CallObjectMethod(), where the return-type is <Primitive-type>, void and Object, respectively. Append the argument, if any, before the argument list. For non-void return-type, the method returns a value.

To callback a static method, use GetMethodID(), CallStatic<Primitive-type>Method(), CallStaticVoidMethod() or CallStaticObjectMethod().

The JNI functions for calling back instance method and static method are:

jmethodID GetMethodID(JNIEnv *env, jclass cls, const char *name, const char *sig);

   // Returns the method ID for an instance method of a class or interface.

NativeType Call<type>Method(JNIEnv *env, jobject obj, jmethodID methodID, ...);

NativeType Call<type>MethodA(JNIEnv *env, jobject obj, jmethodID methodID, const jvalue *args);

NativeType Call<type>MethodV(JNIEnv *env, jobject obj, jmethodID methodID, va_list args);

   // Invoke an instance method of the object.

   // The <type> includes each of the eight primitive and Object.

jmethodID GetStaticMethodID(JNIEnv *env, jclass cls, const char *name, const char *sig);

   // Returns the method ID for an instance method of a class or interface.

NativeType CallStatic<type>Method(JNIEnv *env, jclass clazz, jmethodID methodID, ...);

NativeType CallStatic<type>MethodA(JNIEnv *env, jclass clazz, jmethodID methodID, const jvalue *args);

NativeType CallStatic<type>MethodV(JNIEnv *env, jclass clazz, jmethodID methodID, va_list args);

   // Invoke an instance method of the object.

   // The <type> includes each of the eight primitive and Object.

5.4  Callback Overridden Superclass' Instance Method

JNI provides a set of CallNonvirtual<Type>Method() functions to invoke superclass' instance methods which has been overridden in this class (similar to a super.methodName() call inside a Java subclass):

1. Get the Method ID, via GetMethodID().
2. Based on the Method ID, invoke one of the CallNonvirtual<Type>Method(), with the object, superclass, and arguments.

The JNI function for calling the overridden superclass' instance method are:

NativeType CallNonvirtual<type>Method(JNIEnv *env, jobject obj, jclass cls, jmethodID methodID, ...);

NativeType CallNonvirtual<type>MethodA(JNIEnv *env, jobject obj, jclass cls, jmethodID methodID, const jvalue *args);

NativeType CallNonvirtual<type>MethodV(JNIEnv *env, jobject obj, jclass cls, jmethodID methodID, va_list args);

6.  Creating Objects and Object Arrays

You can construct jobject and jobjectArray inside the native code, via NewObject() and newObjectArray() functions, and pass them back to the Java program.

6.1  Callback the Constructor to Create a New Java Object in the Native Code

Callback the constructor is similar to calling back method. First, get the Method ID of the constructor by passing "<init>" as the method name and "V" as the return-type. You can then use methods like NewObject() to call the constructor to create a new java object.

JNI Program - TestJavaConstructor.java

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public class TestJNIConstructor {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Native method that calls back the constructor and return the constructed object.

   // Return an Integer object with the given int.

   private native Integer getIntegerObject(int number);

   public static void main(String args[]) {

      TestJNIConstructor obj = new TestJNIConstructor();

      System.out.println("In Java, the number is :" + obj.getIntegerObject(9999));

   }

}

This class declares a native method getIntegerObject(). The native code shall create and return an Integer object, based on the argument given.

C Implementation - TestJavaConstructor.c

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#include <jni.h>

#include <stdio.h>

#include "TestJNIConstructor.h"

JNIEXPORT jobject JNICALL Java_TestJNIConstructor_getIntegerObject

          (JNIEnv *env, jobject thisObj, jint number) {

   // Get a class reference for java.lang.Integer

   jclass cls = (*env)->FindClass(env, "java/lang/Integer");

   // Get the Method ID of the constructor which takes an int

   jmethodID midInit = (*env)->GetMethodID(env, cls, "<init>", "(I)V");

   if (NULL == midInit) return NULL;

   // Call back constructor to allocate a new instance, with an int argument

   jobject newObj = (*env)->NewObject(env, cls, midInit, number);

   // Try running the toString() on this newly create object

   jmethodID midToString = (*env)->GetMethodID(env, cls, "toString", "()Ljava/lang/String;");

   if (NULL == midToString) return NULL;

   jstring resultStr = (*env)->CallObjectMethod(env, newObj, midToString);

   const char *resultCStr = (*env)->GetStringUTFChars(env, resultStr, NULL);

   printf("In C: the number is %s\n", resultCStr);

   return newObj;

}

The JNI functions for creating object (jobject) are:

jclass FindClass(JNIEnv *env, const char *name);

jobject NewObject(JNIEnv *env, jclass cls, jmethodID methodID, ...);

jobject NewObjectA(JNIEnv *env, jclass cls, jmethodID methodID, const jvalue *args);

jobject NewObjectV(JNIEnv *env, jclass cls, jmethodID methodID, va_list args);

   // Constructs a new Java object. The method ID indicates which constructor method to invoke

jobject AllocObject(JNIEnv *env, jclass cls);

  // Allocates a new Java object without invoking any of the constructors for the object.

6.2  Array of Objects

JNI Program - TestJNIObjectArray.java

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import java.util.ArrayList;

public class TestJNIObjectArray {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Native method that receives an Integer[] and

   //  returns a Double[2] with [0] as sum and [1] as average

   private native Double[] sumAndAverage(Integer[] numbers);

   public static void main(String args[]) {

      Integer[] numbers = {11, 22, 32};  // auto-box

      Double[] results = new TestJNIObjectArray().sumAndAverage(numbers);

      System.out.println("In Java, the sum is " + results[0]);  // auto-unbox

      System.out.println("In Java, the average is " + results[1]);

   }

}

For illustration, this class declares a native method that takes an array of Integer, compute their sum and average, and returns as an array of Double. Take note the arrays of objects are pass into and out of the native method.

C Implementation - TestJNIObjectArray.c

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#include <jni.h>

#include <stdio.h>

#include "TestJNIObjectArray.h"

JNIEXPORT jobjectArray JNICALL Java_TestJNIObjectArray_sumAndAverage

          (JNIEnv *env, jobject thisObj, jobjectArray inJNIArray) {

   // Get a class reference for java.lang.Integer

   jclass classInteger = (*env)->FindClass(env, "java/lang/Integer");

   // Use Integer.intValue() to retrieve the int

   jmethodID midIntValue = (*env)->GetMethodID(env, classInteger, "intValue", "()I");

   if (NULL == midIntValue) return NULL;

   // Get the value of each Integer object in the array

   jsize length = (*env)->GetArrayLength(env, inJNIArray);

   jint sum = 0;

   int i;

   for (i = 0; i < length; i++) {

      jobject objInteger = (*env)->GetObjectArrayElement(env, inJNIArray, i);

      if (NULL == objInteger) return NULL;

      jint value = (*env)->CallIntMethod(env, objInteger, midIntValue);

      sum += value;

   }

   double average = (double)sum / length;

   printf("In C, the sum is %d\n", sum);

   printf("In C, the average is %f\n", average);

   // Get a class reference for java.lang.Double

   jclass classDouble = (*env)->FindClass(env, "java/lang/Double");

   // Allocate a jobjectArray of 2 java.lang.Double

   jobjectArray outJNIArray = (*env)->NewObjectArray(env, 2, classDouble, NULL);

   // Construct 2 Double objects by calling the constructor

   jmethodID midDoubleInit = (*env)->GetMethodID(env, classDouble, "<init>", "(D)V");

   if (NULL == midDoubleInit) return NULL;

   jobject objSum = (*env)->NewObject(env, classDouble, midDoubleInit, (double)sum);

   jobject objAve = (*env)->NewObject(env, classDouble, midDoubleInit, average);

   // Set to the jobjectArray

   (*env)->SetObjectArrayElement(env, outJNIArray, 0, objSum);

   (*env)->SetObjectArrayElement(env, outJNIArray, 1, objAve);

   return outJNIArray;

}

Unlike primitive array which can be processed in bulk, for object array, you need to use the Get|SetObjectArrayElement() to process each of the elements.

The JNI functions for creating and manipulating object array (jobjectArray) are:

jobjectArray NewObjectArray(JNIEnv *env, jsize length, jclass elementClass, jobject initialElement);

   // Constructs a new array holding objects in class elementClass.

   // All elements are initially set to initialElement.

jobject GetObjectArrayElement(JNIEnv *env, jobjectArray array, jsize index);

   // Returns an element of an Object array.

void SetObjectArrayElement(JNIEnv *env, jobjectArray array, jsize index, jobject value);

   // Sets an element of an Object array.

7.  Local and Global References

Managing references is critical in writing efficient programs. For example, we often use FindClass(), GetMethodID(), GetFieldID() to retrieve a jclass, jmethodID and jfieldID inside native functions. Instead of performing repeated calls, the values should be obtained once and cached for subsequent usage, to eliminate the overheads.

The JNI divides object references (for jobject) used by the native code into two categories: local and global references:

1. A local reference is created within the native method, and freed once the method exits. It is valid for the duration of a native method. You can also use JNI function DeleteLocalRef() to invalidate a local reference explicitly, so that it is available for garbage collection intermediately. Objects are passed to native methods as local references. All Java objects (jobject) returned by JNI functions are local references.
2. A global reference remains until it is explicitly freed by the programmer, via the DeleteGlobalRef() JNI function. You can create a new global reference from a local reference via JNI function NewGlobalRef().

Example

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public class TestJNIReference {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // A native method that returns a java.lang.Integer with the given int.

   private native Integer getIntegerObject(int number);

   // Another native method that also returns a java.lang.Integer with the given int.

   private native Integer anotherGetIntegerObject(int number);

   public static void main(String args[]) {

      TestJNIReference test = new TestJNIReference();

      System.out.println(test.getIntegerObject(1));

      System.out.println(test.getIntegerObject(2));

      System.out.println(test.anotherGetIntegerObject(11));

      System.out.println(test.anotherGetIntegerObject(12));

      System.out.println(test.getIntegerObject(3));

      System.out.println(test.anotherGetIntegerObject(13));

   }

}

The above JNI program declares two native methods. Both of them create and return a java.lang.Integer object.

In the C implementation, we need to get a class reference for java.lang.Integer, via FindClass(). We then find the method ID for the constructor of Integer, and invoke the constructor. However, we wish to cache both the class reference and method ID, to be used for repeated invocation.

The following C implementation does not work!

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#include <jni.h>

#include <stdio.h>

#include "TestJNIReference.h"

// Global Reference to the Java class "java.lang.Integer"

static jclass classInteger;

static jmethodID midIntegerInit;

jobject getInteger(JNIEnv *env, jobject thisObj, jint number) {

   // Get a class reference for java.lang.Integer if missing

   if (NULL == classInteger) {

      printf("Find java.lang.Integer\n");

      classInteger = (*env)->FindClass(env, "java/lang/Integer");

   }

   if (NULL == classInteger) return NULL;

   // Get the Method ID of the Integer's constructor if missing

   if (NULL == midIntegerInit) {

      printf("Get Method ID for java.lang.Integer's constructor\n");

      midIntegerInit = (*env)->GetMethodID(env, classInteger, "<init>", "(I)V");

   }

   if (NULL == midIntegerInit) return NULL;

   // Call back constructor to allocate a new instance, with an int argument

   jobject newObj = (*env)->NewObject(env, classInteger, midIntegerInit, number);

   printf("In C, constructed java.lang.Integer with number %d\n", number);

   return newObj;

}

JNIEXPORT jobject JNICALL Java_TestJNIReference_getIntegerObject

          (JNIEnv *env, jobject thisObj, jint number) {

   return getInteger(env, thisObj, number);

}

JNIEXPORT jobject JNICALL Java_TestJNIReference_anotherGetIntegerObject

          (JNIEnv *env, jobject thisObj, jint number) {

   return getInteger(env, thisObj, number);

}

In the above program, we invoke FindClass() to find the class reference for java.lang.Integer, and saved it in a global static variable. Nonetheless, in the next invocation, this reference is no longer valid (and not NULL). This is because FindClass() returns a local reference, which is invalidated once the method exits.

To overcome the problem, we need to create a global reference from the local reference returned by FindClass(). We can then free the local reference. The revised code is as follows:

   // Get a class reference for java.lang.Integer if missing

   if (NULL == classInteger) {

      printf("Find java.lang.Integer\n");

      // FindClass returns a local reference

      jclass classIntegerLocal = (*env)->FindClass(env, "java/lang/Integer");

      // Create a global reference from the local reference

      classInteger = (*env)->NewGlobalRef(env, classIntegerLocal);

      // No longer need the local reference, free it!

      (*env)->DeleteLocalRef(env, classIntegerLocal);

   }

Take note that jmethodID and jfieldID are not jobject, and cannot create global reference.