JDK and JRE File Structure

This document gives an overview of the JDK directories and the files they contain. This page may also be used as a reference for the file structure of the JRE. To see the file structure of the Runtime Environment, just look at the description of the contents of the JDK’s jre directory on this page. The file structure of the JRE is identical to that of the JDK’s jre directory.

Development Files and Directories

This section describes the files and directories that are required to develop apps for the Java platform. (The directories that are not required include demos, Java source code, and C header files. They are discussed in the following section, Additional Files and Directories.) The following chart shows the most important directories:

Assuming the JDK software is installed at /jdk1.5.0, here are some of the most important directories:

The root directory of the JDK software installation. Contains copyright, license, and README files. Also contains src.jar, the archive of source code for the Java 2 platform.
The executables for all the development tools contained in the Java 2 JDK. The PATH environment variable should contain an entry for this directory. For more information on the tools, see the JDK Tools.
Files used by the development tools. Includes tools.jar, which contains non-core classes for support of the tools and utilities in the JDK. Also includes dt.jar, the DesignTime archive of BeanInfo files that tell interactive development environments (IDE’s) how to display the Java components and how to let the developer customize them for the application.
The root directory of the Java runtime environment used by the JDK development tools. The runtime environment is an implementation of the Java 2 platform. This is the directory referred to by the java.home system property.
Executable files for tools and libraries used by the Java platform. The executable files are identical to files in /jdk1.5.0/bin. The java launcher tool serves as an application launcher, in place of the old jre tool that shipped with 1.1 versions of the JDK software. This directory does not need to be in the PATH environment variable.
Code libraries, property settings, and resource files used by the Java runtime environment. Includes:
  • rt.jar — the bootstrap classes (the RunTime classes that comprise the Java platform’s core API).
  • charsets.jar — character-conversion classes.
Aside from the ext subdirectory (described below) there are several additional resource subdirectories not described here.
Default installation directory for Extensions to the Java platform. This is where the JavaHelp jar file goes when it is installed, for example.
  • localedata.jar — locale data for java.text and java.util.
Contains files used for security management. These include the security policy (java.policy) and security properties (java.security) files.
Contains the .so file used by the Java HotSpot Client Virtual Machine, which is implemented with Java HotSpotTM technology. This is the default VM.
Contains the .so file used by the Java HotSpot Server Virtual Machine.
Jar files containing support classes for applets can be placed in the lib/applet/ directory. This reduces startup time for large applets by allowing applet classes to be pre-loaded from teh local file system by the applet class loader, providing the same protections as if they had been downloaded over the net.
Font files for use by platform.

Additional Files and Directories

This section describes the directory structure for the demos, Java source code, and C header files. Here is the directory structure:

The additional files and directories shown above are:

Archive containing source code for the Java 2 platform.
Examples, with source code, that show you how to program for the Java platform.
Applets that can be used on a Web page.
Examples that use Java 2DTM and JFC/Swing components.
Examples of using the Java Platform Debugging Architecture. Includes source code for the javadt and jdb utilities.
Demos for use with Java Plug-in product.
Example classes and C code that demonstrate access to poll(2) functionality from the Java 2 Platform.
C-language header files that support native-code programming using the Java Native Interface and the Java Virtual Machine Debugger Interface.
Contains troff-formatted man pages for the Java 2 JDK tools.

Source: Oracle.com https://docs.oracle.com/javase/8/docs/technotes/tools/windows/jdkfiles.htm

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