Demystifying Jar, Fat Jar, War, and Ear Files in Java

Naveen Metta
4 min readDec 1, 2023

Introduction

In the realm of Java development, packaging and deploying applications come in various flavors, each tailored to specific use cases. In this comprehensive exploration, we’ll unravel the differences between Jar, Fat Jar, War, and Ear files, exploring their characteristics, use cases, and key considerations. Along the way, we’ll provide code snippets to illustrate their creation and highlight additional insights.

Jar Files
Java Archive (Jar) files are a fundamental packaging format in Java. They are used to aggregate and compress multiple files into a single archive, making it convenient to distribute and deploy Java applications.

Creating a Jar File:
To create a basic Jar file, you can use the following command:

jar cf MyJar.jar -C /path/to/classes .

Here, -C specifies the directory where the compiled classes are located.

Fat Jar Files
Fat Jar files, also known as Uber Jars or executable Jars, take the concept of Jar files a step further by including all dependencies within the archive. This self-contained approach simplifies deployment, as the application can be run without relying on external libraries.

Creating a Fat Jar File:
Using tools like Maven or Gradle, you can generate a Fat Jar easily. Here’s an example using Maven:

<!-- Add the following plugin configuration to your pom.xml file -->
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-assembly-plugin</artifactId>
<version>3.3.0</version>
<configuration>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<manifest>
<mainClass>com.example.MainClass</mainClass>
</manifest>
</archive>
</configuration>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>

With this configuration, running mvn clean package will generate a Fat Jar in the target directory.

War Files
Web Application Archive (War) files are specialized for Java web applications. They contain everything needed to deploy a web application, including servlets, JSP pages, HTML files, and static content. War files are deployable on web servers such as Apache Tomcat.

Creating a War File:
To create a War file using Maven, add the following configuration to your pom.xml:

<!-- Add this configuration to your pom.xml file -->
<packaging>war</packaging>

When you run mvn clean package, Maven will generate a War file in the target directory.

Ear Files
Enterprise Archive (Ear) files are used for packaging and deploying enterprise applications. They can contain multiple modules, including EJB (Enterprise JavaBeans) modules and Web modules. Ear files provide a way to encapsulate and distribute entire Java EE applications.

Creating an Ear File:
Creating an Ear file typically involves configuring your build tool. Here’s an example using Gradle:

plugins {
id 'ear'
}

ear {
archiveName = 'my-application.ear'
from 'web-module/build/libs/web-module.war'
from 'ejb-module/build/libs/ejb-module.jar'
}

In this example, the ear block configures the Ear plugin, specifying the name of the Ear file and including the Web and EJB modules.

Comparing the Formats
Now that we’ve covered the basics of Jar, Fat Jar, War, and Ear files, let’s compare them based on key criteria:

Use Cases:
Jar: Suitable for standalone applications with a simple structure.

Fat Jar: Ideal for standalone applications with external dependencies bundled within.

War: Tailored for Java web applications to be deployed on servlet containers like Tomcat.

Ear: Designed for enterprise-level applications with multiple modules, including EJBs.

Size:
Jar: Compact size as it only includes application-specific files.

Fat Jar: Larger due to the inclusion of all dependencies.

War: Size depends on the web application’s content and dependencies.

Ear: Size can vary based on the complexity of the enterprise application.

Deployment:
Jar: Deployed as a standalone application.

Fat Jar: Also deployed as a standalone application, simplifying deployment.

War: Deployed on servlet containers like Tomcat.

Ear: Deployed on Java EE application servers such as WildFly or JBoss.

Dependencies:
Jar: Requires external dependencies to be available in the classpath.

Fat Jar: Self-contained with all dependencies included.

War: Relies on servlet containers to manage dependencies.

Ear: Manages dependencies internally, often requiring Java EE application servers.

Isolation and Modularity:
Jar: Limited isolation; dependencies are resolved at runtime.

Fat Jar: Encapsulates dependencies, but limited modularity.

War: Encapsulates web application components for better isolation.

Ear: Supports modularity with distinct modules for different functionalities.

Additional Considerations

Versioning:
Jar: Versioning is typically handled at the file level.

Fat Jar: Versioning can be more complex due to bundled dependencies.

War: Versioning is essential for web applications, often specified in the deployment descriptor.

Ear: Versioning is crucial for managing the overall enterprise application.

Scalability:
Jar: Well-suited for small to medium-sized applications.

Fat Jar: May become unwieldy for large projects; careful consideration of dependencies is needed.

War: Scalable for web applications, especially when deployed on cloud platforms.

Ear: Provides scalability for enterprise-level applications with modular components.

Development Environment:
Jar: Simple and easy for local development and testing.

Fat Jar: Ensures consistent dependencies across different environments.

War: Mimics the production environment closely for accurate testing.

Ear: Requires a Java EE application server for comprehensive testing.

Conclusion
In this extended exploration, we delved deeper into Jar, Fat Jar, War, and Ear files, considering additional aspects such as versioning, scalability, and development environments. Each packaging format serves distinct purposes, and choosing the right one depends on the nature and requirements of your Java application.

When making decisions about packaging formats, consider factors like isolation, modularity, deployment environment, and the overall architecture of your application. Whether you opt for the simplicity of a Jar file, the self-contained nature of a Fat Jar, the web-centric approach of a War file, or the enterprise-level capabilities of an Ear file, understanding the strengths and use cases of each format empowers you to make informed decisions in your Java development journey.

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Naveen Metta

Java Backend Engineer who loves to share his experience in Enterprise Application development.