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What is a JAR File and How To Open One

A JAR (Java ARchive) file is a package file format typically used to aggregate many Java class files and associated metadata and resources into one file for distribution. JAR files are built on the ZIP file format and have the .jar file extension.

What’s Inside a JAR File

A JAR file can contain the following:

  • Java class files
  • Associated metadata and resources like images and configuration files
  • A manifest file – META-INF/MANIFEST.MF
  • Optional signature files
  • Package namespaces

Java class files contain the actual Java bytecode that will be executed by the JVM (Java Virtual Machine). These files have the .class extension.

The manifest file includes metadata about the JAR file’s contents and is located at META-INF/MANIFEST.MF inside the archive. It has details like the package name, version, main class, dependencies, etc.

Signature files are used for integrity checking and authentication. They ensure the code has not been tampered with.

Package namespaces help avoid naming conflicts between class files from different sources.

Advantages of Using JAR files

JAR files provide the following advantages:

  • Convenience – Multiple class files and associated resources can be bundled into a single file. This is easier to distribute.
  • Compression – JAR files allow efficient compression of contents, reducing file size.
  • Package Sealing – Contents can be signed and sealed to verify package integrity.
  • Package Versioning – Manifest contains metadata like version and vendor.
  • Portability – Runs on any platform with Java support.

In summary, JAR files simplify distribution and installation of Java software.

Opening and Viewing JAR Files

There are several ways to open, view, and work with JAR files:

1. Extract the Contents

Since JAR files are based on the ZIP format, you can rename a JAR file to .zip and extract all the contents using any zip extraction utility.

For example, on Windows you can right-click on the JAR > Select Extract All. On Mac, double-click the JAR file to unzip.

This will let you view the individual files inside the archive.

2. View Manifest Contents

To quickly view the manifest file (META-INF/MANIFEST.MF) without fully extracting, use:

jar tf <name>.jar

This will list all contents, including the manifest which you can inspect.

3. View Class Files

Use the jar utility to view extracted .class files:

jar xf <name>.jar
jar tvf <name>.jar

The first command extracts contents and second lists the class files.

4. Run the Java Program

To directly execute the Java program contained in a JAR:

java -jar <name>.jar

This will launch the main class specified in the manifest. Any command line arguments can follow the JAR name.

5. View JAR Contents Via IDE

Modern Java IDEs like Eclipse and IntelliJ IDEA provide a user-friendly GUI view of the archive contents through their UI, along with options to extract files.

This is convenient for inspecting files and metadata.

Modifying JAR Files

While JAR files are designed for final distribution, their contents can be modified if needed:

  • Change the manifest by editing the META-INF/MANIFEST.MF text file before creating the JAR
  • Delete unwanted class files
  • Add new classes and resources
  • Repackage the contents into a new JAR archive

Use care when doing this to avoid breaking signatures or functionality.

Creating JAR Files

While pre-built JAR files for libraries and frameworks can be directly used, you may need to create your own:

1. jar Command

The jar command line tool can assemble JAR files:

jar cvf myProgram.jar *.class

This will package all .class files from current dir into myProgram.jar

2. IDE Support

IDEs like Eclipse, IntelliJ and NetBeans have built-in support for building JARs from your project contents.

3. Build Tools

Build tools like Maven and Gradle can also assemble and manage JAR files.

Best Practices

Follow these best practices when working with JAR files:

  • Validate integrity with signed JARs
  • Document contents in the manifest
  • Use package namespaces to avoid conflicts
  • Set main class and dependencies in manifest
  • Compress contents for efficiency
  • Version for release management
  • Use IDEs or build tools to create JARs

Conclusion

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