What are jar files in java

Lesson: Packaging Programs in JAR Files

The Java™ Archive (JAR) file format enables you to bundle multiple files into a single archive file. Typically a JAR file contains the class files and auxiliary resources associated with applets and applications.

The JAR file format provides many benefits:

• Security: You can digitally sign the contents of a JAR file. Users who recognize your signature can then optionally grant your software security privileges it wouldn’t otherwise have.
• Decreased download time: If your applet is bundled in a JAR file, the applet’s class files and associated resources can be downloaded to a browser in a single HTTP transaction without the need for opening a new connection for each file.
• Compression: The JAR format allows you to compress your files for efficient storage.
• Packaging for extensions: The extensions framework provides a means by which you can add functionality to the Java core platform, and the JAR file format defines the packaging for extensions. By using the JAR file format, you can turn your software into extensions as well.
• Package Sealing: Packages stored in JAR files can be optionally sealed so that the package can enforce version consistency. Sealing a package within a JAR file means that all classes defined in that package must be found in the same JAR file.
• Package Versioning: A JAR file can hold data about the files it contains, such as vendor and version information.
• Portability: The mechanism for handling JAR files is a standard part of the Java platform’s core API.

This lesson has four sections:

Using JAR Files: The Basics

This section shows you how to perform basic JAR-file operations, and how to run software that is bundled in JAR files.

Working with Manifest Files: The Basics

This section explains manifest files and how to customize them so you can do such things as seal packages and set an application’s entry point.

Signing and Verifying JAR Files

This section shows you how to digitally sign JAR files and verify the signatures of signed JAR files.

Using JAR-related APIs

This section introduces you to some of the JAR-handling features of the Java platform. The JAR file format is an important part of the Java platform’s extension mechanism. You can learn more about that aspect of JAR files in the The Extension Mechanism trail of this tutorial.

Questions and Exercises: JAR

Test what you’ve learned about JAR.

Additional References

The documentation for the Java Development Kit (JDK) includes information about the Jar tool:

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Using JAR Files: The Basics

JAR files are packaged with the ZIP file format, so you can use them for tasks such as lossless data compression, archiving, decompression, and archive unpacking. These tasks are among the most common uses of JAR files, and you can realize many JAR file benefits using only these basic features.

Even if you want to take advantage of advanced functionality provided by the JAR file format such as electronic signing, you’ll first need to become familiar with the fundamental operations.

To perform basic tasks with JAR files, you use the Java Archive Tool provided as part of the Java Development Kit (JDK). Because the Java Archive tool is invoked by using the jar command, this tutorial refers to it as ‘the Jar tool’.

As a synopsis and preview of some of the topics to be covered in this section, the following table summarizes common JAR file operations:

AppletClassName.class archive width=width height=height> 

This section shows you how to perform the most common JAR-file operations, with examples for each of the basic features:

Creating a JAR File

This section shows you how to use the Jar tool to package files and directories into a JAR file.

Viewing the Contents of a JAR File

You can display a JAR file’s table of contents to see what it contains without actually unpacking the JAR file.

Extracting the Contents of a JAR File

You can use the Jar tool to unpack a JAR file. When extracting files, the Jar tool makes copies of the desired files and writes them to the current directory, reproducing the directory structure that the files have in the archive.

Updating a JAR File

This section shows you how to update the contents of an existing JAR file by modifying its manifest or by adding files.

Running JAR-Packaged Software

This section shows you how to invoke and run applets and applications that are packaged in JAR files.

Additional References

The documentation for the JDK includes reference pages for the Jar tool:

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