转载自： https://docs.oracle.com/javase/7/docs/technotes/tools/findingclasses.html

How Classes are Found

SDK Tools

• How the Java Launcher Finds ClassesHow Javac and Javadoc Find Classes
  • How the Java Launcher Finds Bootstrap Classes
  • How the Java Launcher Finds Extension Classes
  • How the Java Launcher Finds User Classes
  • How the Java Launcher Finds JAR-class-path Classes
• Class Loading and Security Policies

How the Java Launcher Finds Classes

The Java launcher, java, initiates the Java virtual machine. The virtual machine searches for and loads classes in this order:

• Bootstrap classes - Classes that comprise the Java platform, including the classes in rt.jar and several other important jar files.
• Extension classes - Classes that use the Java Extension mechanism. These are bundled as .jar files located in the extensions directory.
• User classes - Classes defined by developers and third parties that do not take advantage of the extension mechanism. You identify the location of these classes using the -classpath option on the command line (the preferred method) or by using the CLASSPATH environment variable. (See Setting the Classpath for Windows or Solaris.)

In effect, these three search paths are joined to form a simple class path. This is similar to the "flat" class path previously used, but the current model has some important differences:

• It is relatively difficult to accidentally "hide" or omit the bootstrap classes.
• In general, you only have to specify the location of user classes. Bootstrap classes and extension classes are found "automatically".
• The tools classes are now in a separate archive (tools.jar) and can only be used if included in the user class path (to be explained shortly).

How the Java Launcher Finds Bootstrap Classes

Bootstrap classes are the classes that implement the Java 2 Platform. Bootstrap classes are in the rt.jar and several other jar files in the jre/lib directory. These archives are specified by the value of the bootstrap class path which is stored in the sun.boot.class.path system property. This system property is for reference only, and should not be directly modified.

It is very unlikely that you will need to redefine the bootstrap class path. The nonstandard option, -Xbootclasspath, allows you to do so in those rare cicrcumstances in which it is necessary to use a different set of core classes.

Note that the classes which implement the Java 2 SDK tools are in a separate archive from the bootstrap classes. The tools archive is the SDK's/lib/tools.jar file. The development tools add this archive to the user class path when invoking the launcher. However, this augmented user class path is only used to execute the tool. The tools that process source code, javac and javadoc, use the original class path, not the augmented version. (For more information, see How Javac and Javadoc Find Classes, below.)

How the Java Launcher Finds Extension Classes

Extension classes are classes which extend the Java platform. Every .jar file in the extension directory, jre/lib/ext, is assumed to be an extension and is loaded using the Java Extension Framework. Loose class files in the extension directory will not be found. They must be contained in a .jar file (or .zip file). There is no option provided for changing the location of the extension directory.

If the jre/lib/ext directory contains multiple .jar files, and those files contain classes with the same name, such as:

smart-extension1_0.jar contains class smart.extension.Smart
smart-extension1_1.jar contains class smart.extension.Smart


the class that actually gets loaded is undefined.

How the Java Launcher Finds User Classes

User classes are classes which build on the Java platform. To find user classes, the launcher refers to the user class path -- a list of directories, JAR archives, and ZIP archives which contain class files.

A class file has a subpath name that reflects the class's fully-qualified name. For example, if the class com.mypackage.MyClass is stored under /myclasses, then /myclasses must be in the user class path and the full path to the class file must be /myclasses/com/mypackage/MyClass.class. If the class is stored in an archive named myclasses.jar, then myclasses.jar must be in the user class path, and the class file must be stored in the archive as com/mypackage/MyClass.class.

The user class path is specified as a string, with a colon (:) separating the class path entries on Solaris, and a semi-colon (;) separating entries on Microsoft Windows systems. The java launcher puts the user class path string in the java.class.path system property. The possible sources of this value are:

• The default value, ".", meaning that user class files are all the class files in the current directory (or under it, if in a package).
• The value of the CLASSPATH environment variable, which overrides the default value.
• The value of the -cp or -classpath command line option, which overrides both the default value and the CLASSPATH value.
• The JAR archive specified by the -jar option, which overrides all other values. If this option is used, all user classes must come from the specified archive.

How the Java Launcher Finds JAR-class-path Classes

A JAR file usually contains a "manifest" -- a file which lists the contents of the JAR. The manifest can define a JAR-class-path, which further extends the class path (but only while loading classes from that JAR). Classes accessed by a JAR-class-path are found in the following order:

• In general, classes referenced by a JAR-class-path entry are found as though they were part of the JAR file. The JAR files that appear in the JAR-class-path are searched after any earlier class path entries, and before any entries that appear later in the class path.
• However, if the JAR-class-path points to a JAR file that was already searched (for example, an extension, or a JAR file that was listed earlier in the class path) then that JAR file will not be searched again. (This optimization improves efficiency and prevents circular searches.) Such a JAR file is searched at the point that it appears, earlier in the class path.
• If a JAR file is installed as an extension, then any JAR-class-path it defines is ignored. All the classes required by an extension are presumed to be part of the SDK or to have themselves been installed as extensions.

How Javac and JavaDoc Find Classes

The javac and javadoc tools use class files in two distinct ways:

• Like any Java application, javac and javadoc must load various class files in order to run.
• To process the source code they operate on, javac and javadoc must obtain information on object types used in the source code.

The class files used to resolve source code references are mostly the same class files used to run javac and javadoc. But there are some important exceptions:

• Both javac and javadoc often resolve references to classes and interfaces that having nothing to do with the implementation of javac or javadoc. Information on referenced user classes and interfaces may be present in the form of class files, source code files, or both.
• The tools classes in tools.jar are only used to run javac and javadoc. The tools classes are not used to resolve source code references unless tools.jar is in the user class path.
• A programmer may want to resolve boot class or extension class references using an alternative Java platform implementation. Both javac and javadoc support this with their -bootclasspath and -extdirs options. Use of these options does not modify the set of class files used to run the javac or javadoc tools themselves.

If a referenced class is defined in both a class file and source file, javadoc always uses the source file (javadoc never compiles source files). In the same situation javac uses class files, but automatically recompiles any class files it determines to be out of date. The rules for automatic recompilation are documented in the javac document for Windows or Solaris.

By default, javac and javadoc search the user class path for both class files and source code files. If the -sourcepath option is specified, javac and javadoc search for source files only on the specified source file path, while still searching the user class path for class files.

Class Loading and Security Policies

To be used, a class or interface must be loaded by a class loader. Use of a particular class loader determines a security policy associated with the class loader.

A program can load a class or interface by calling the loadClass method of a class loader object. But usually a program loads a class or interface simply by referring to it. This invokes an internal class loader, which can apply a security policy to extension and user classes. If the security policy has not been enabled, all classes are "trusted". Even if the security policy is enabled, it does not apply to bootstrap classes, which are always "trusted."

When enabled, security policy is configured by system and user policy files. The Java 2 SDK includes a system policy file that grants "trusted" status to extension classes and places basic restrictions on user classes.

To enable or configure the security policy, refer to Security Features.

Note: Some security programming techniques that worked with the Java 1.1 platform are incompatible with the class loading model of the Java 2 Platform.