JNI调用的helloworld(JNI_OnLoad映射方式)

  本示例展示JNI的基本示例,helloworld级别的,不过是用JNI_OnLoad映射的方式。

  直接看代码,先看包含native method的Person.java的代码:

 package helloworld;

 /**
* author : Zhou Shenshen
* date : 2016.7.16
* desc : It is a class to show how to use JNI.
* 演示调用JNI本地方法的写法和加载动态库
* email : zhouss@iPanel.cn
*/
public class Person{ /* Load the shared library libperson.so 加载C语言编写的动态库*/
static{
String classDir = Person.class.getResource("/").getPath();
System.load(classDir + "helloworld/libperson.so");
} /* Two native implemented by person.c 两个native method通过person.c来实现 */
public native void show();
public native void say(String content);
}

这里native method的定义不用多说,注意就是调用System的load时,传入的参数是库的全名,linux下包括lib和.so,而loadLibrary方法则是传入库的名字,没有前后缀。这里采用绝对路径定位和加载库,不用在设定库的参数。

  接下来看person.c的代码:

 #include <jni.h>
#include <stdio.h> /* Declare two function map to java native method 声明映射过来的本地函数 */
void native_show(JNIEnv * env,jobject obj);
void native_say(JNIEnv * env,jobject obj,jstring string); /* The array to map java method and c function 映射用的JNINativeMethod数组 */
static JNINativeMethod methods[] = {
{"show","()V",(void *)native_show},
{"say","(Ljava/lang/String;)V",(void *)native_say}
}; /* This function will be exec when libperson.so been loading. */
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved){
JNIEnv * env = NULL;
jclass cls = NULL;
jint result = -;
if((*vm)->GetEnv(vm,(void **)&env,JNI_VERSION_1_6) != JNI_OK){
printf("err!!");
return JNI_ERR;
}
cls = (*env)->FindClass(env,"helloworld/Person");  //通过类路径字符串找到对应类
(*env)->RegisterNatives(env,cls,methods,sizeof(methods)/sizeof(JNINativeMethod));//调用RegisterNatives来注册本地方法,完成映射
return JNI_VERSION_1_6;
} /* Define the c native function. 本地方法映射过来的实现*/
void native_show(JNIEnv * env,jobject obj){
printf("hello!!\n");
}
void native_say(JNIEnv * env,jobject obj,jstring string){
const char * str = (*env)->GetStringUTFChars(env,string,); //将jstring转成c语言中的字符串。
printf("content : %s\n",str);
}

  然后写个主函数测试JNI调用的情况:

 package helloworld;
/**
* author : Zhou Shenshen
* date : 2016.7.16
* desc : As the entrance for the example.
* email : zhouss@iPanel.cn
*/
public class Main{
public static void main(String[] args){
Person p = new Person();
p.show();
p.say("I am XiaoMing!");
}
}

  编译运行的脚本如下:

#!/bin/sh

#test the libperson.so is exist.
if [ -e libperson.so ];then
#change to .. directory to run this example
cd ..
java helloworld.Main
exit ;
fi #Compiler *.java to *.class
javac Person.java Main.java
#Test the JAVA_HOME
if [ $JAVA_HOME != "" ]; then
#Compiler *.c to *.o,then link to shared library *.so.
gcc -fPIC -c -I$(echo $JAVA_HOME)/include/ -I$(echo $JAVA_HOME)/include/linux person.c
gcc -shared -o libperson.so person.o
#change to .. directory to run this example
cd ..
java helloworld.Main
exit
else
echo "JAVA_HOME is empty variable"
exit
fi

  linux下bash下运行的结果如下:

hello!!
content : I am XiaoMing!

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