How to run a jar with lib?

2 min read 05-10-2024

Running a JAR with External Libraries: A Comprehensive Guide

Have you ever encountered a Java program packaged as a JAR file that needs additional libraries to run? This situation is common, especially when dealing with external dependencies like database connectors or specialized frameworks. This article guides you through the process of running a JAR with its required libraries, ensuring a smooth execution.

The Problem: JARs Needing External Help

Imagine you have a JAR file, myProgram.jar, containing your awesome Java application. However, it depends on external libraries like mysql-connector-java.jar to interact with a MySQL database. If you simply try to run java -jar myProgram.jar, you'll get an error because the JVM can't find the necessary libraries.

Solution: Bringing the Libraries Together

The solution lies in providing the JVM with a path to the external libraries. We achieve this using the -cp (classpath) flag when executing the java command. There are two main approaches:

1. Combining JARs into a Single Classpath:

This involves concatenating the paths to the main JAR and all the external libraries, separated by colons (:) on Unix-like systems or semicolons (;) on Windows. For example:

java -cp myProgram.jar:mysql-connector-java.jar com.example.MyProgram

Here, com.example.MyProgram is the fully qualified name of the class containing the main method in your JAR file.

2. Using a Manifest File:

A more structured approach involves using a manifest file within your JAR. This file, named META-INF/MANIFEST.MF, defines the Class-Path attribute, specifying the relative or absolute paths to the external libraries.

To use this method:

Create a META-INF/MANIFEST.MF file within your JAR.
Add the Class-Path attribute:
```
Class-Path: mysql-connector-java.jar 
```
Package your JAR file. Now, you can run the JAR using:
```
java -jar myProgram.jar 
```

The JVM will automatically read the manifest file and include the specified libraries in the classpath.

Optimizing the Solution

Relative Paths: Use relative paths in the Class-Path attribute if the external libraries are located within the same directory as the JAR file or in a subdirectory.
JAR File Order: The order of JARs in the classpath is important. The main JAR should usually be listed first, followed by the external libraries.
Dependency Management Tools: For complex projects with many dependencies, tools like Maven or Gradle are highly recommended. They handle dependency management, packaging, and automatic classpath generation.

Additional Considerations:

JAR File Execution: If you're running a JAR with a class named Main.class, you can execute it directly using:
```
java -cp myProgram.jar:mysql-connector-java.jar Main
```
Environment Variables: You can set environment variables like CLASSPATH to include common libraries that are used across multiple projects. However, this is generally discouraged as it can lead to conflicts and ambiguity.

Conclusion

Running a JAR with external libraries is a common task in Java development. By understanding the concepts of classpath and manifest files, you can effectively execute your JARs and manage their dependencies. If you are dealing with complex projects or extensive dependencies, consider using dependency management tools like Maven or Gradle to streamline your workflow.