JAR File (.jar file)

What is a JAR File?

A JAR file (.jar) is a logical container that has all necessary parts of a Java software application or software library bundled together and compressed into one file. JAR files use lossless compression algorithms to save storage space and make the consolidated content easier to share.

Techopedia Explains the JAR File Meaning

JAR files are like ZIP files for software applications and software libraries that require a Java Runtime Environment to execute. Developers use JAR files to streamline the distribution and deployment of Java applications and libraries.

File Formats Using Extension .jar

When a file has a .jar extension, it indicates a JAR file format. This means that the file is either an executable Java application or it contains a library that provides code and resources for other Java applications to use.

If the JAR file contains a manifest file with a specified Main-Class attribute, it can be executed by the Java Runtime Environment as a standalone Java application. (The manifest file will point to a class that specifies what method should be invoked to execute the JAR.)

If the JAR file doesn’t contain a manifest file, that means the JAR contains a library that provides reusable Java classes and resources for other Java applications. This type of JAR file can be added to another Java application’s classpath to give the app access to the library’s functionalities.

JAR File Components

JAR files have three main components: class files, resource files, and metadata.

Java class files

Java class files are compiled .class files that contain the bytecode that a Java Virtual Machine executes. Each class file corresponds to a single Java class.

Resource files

Resource files are non-code components like images and text files that a Java application uses at runtime. This type of file supports an application’s functionality.

Metadata

Metadata is data that describes what is in the JAR file. The most important piece of metadata in a JAR file is the manifest file (MANIFEST.MF), which resides in a directory called META-INF. The manifest file can contain configuration data, such as which class to run when executing the JAR file (Main-Class attribute), as well as version information, package sealing policies, and specification and implementation references.

JAR file metadata can also include security and signing information to verify the integrity and source of the JAR’s contents.

How JAR Files Work

The Java Development Kit (JDK), Java Virtual Machine (JVM), and Java Runtime Environment (JRE)? are interconnected components that facilitate the development and execution of Java applications

The Java Development Kit has tools for creating, viewing, and extracting JAR files, and the Java Virtual Machine makes it possible to execute applications or use libraries in the JAR file across various computing environments.

The Java Runtime Environment uses the JAR file format to bundle Java libraries and components needed to run Java applications.

To run a Java application packaged in a JAR file, you can use the command java -jar filename.jar. The Java Virtual Machine will read the JAR file, unzip the necessary files into memory, read the manifest file to find the main class, and start the application from its main method.

How to Open a JAR File

As long as JRE is installed and set as the default program for JAR files, you should be able to double-click a JAR file to run it.? It’s important to note, however, that double-clicking might not work on systems where multiple Java versions are installed or in environments where Java is not set to handle JAR files by default. In such cases, users may need to manually associate JAR files with the Java runtime or use a command line interface (CLI) to run them.

How to Open a JAR File?

Use Cases of JAR Files

In web development, JAR files are used to bundle Java classes, libraries, resources, and configuration files that can be deployed to Java EE (Enterprise Edition) application servers.

Although the standard packaging for web applications is .war (Web ARchive), JAR files are often included within WAR files as libraries. JAR files can also be used to deliver code and resources as plug-ins or extensions for Java applications.

In more complex Java applications built with microservices, JAR files can be used to package individual service components. This approach facilitates the independent deployment, scaling, and management of each microservice.

Advantages and Disadvantages of JAR Files

JAR files can simplify application delivery and enhance portability across different platforms. When multiple JAR files contain the same class files, however, it can lead to dependency conflicts and runtime errors.

Another potential problem is that threat actors can use JAR files in phishing campaigns to trick their victims into executing malicious code.

Proactive measures like code review, vulnerability scanning, and adhering to secure coding best practices can help mitigate the risk of malware infection through JAR files.

The Bottom Line

JAR files provide developers with an effective way to distribute and deploy Java applications and libraries. Bundling multiple Java class files and associated resources into a single, compressed file simplifies application delivery, enhances portability across different platforms, and supports modular programming practices.