How to Run a JAR File with External Libraries and Avoid \'java.lang.ClassNotFoundException\'?

Calling "java -jar MyFile.jar" with Extended Classpath Options

When attempting to run a compiled JAR file incorporating external libraries and facing a "java.lang.ClassNotFoundException", it's crucial to understand the fundamentals behind the Java execution process.

Java provides two options for specifying classpaths:

• -jar: Includes the JAR file and its manifest-defined classpath.
• -cp: Manually specifies the complete classpath, including all dependencies.

Incorrect Usage

Attempting to combine both -jar and -cp concurrently is incorrect and will result in the aforementioned error.

Solution

There are two valid approaches to resolve this issue:

1. Manifest-Defined Classpath:

• Create a manifest file within the JAR specifying the required classpath.
• Use java -jar to execute the JAR, which will automatically load the specified dependencies.

2. Explicit Classpath:

• Use java -cp to define the complete classpath, including the main JAR and all external JARs.
• Specify the main class explicitly on the command line.

For Example:

java -cp 'MyProgram.jar:libs/*' main.Main

Note: The dir/* syntax adds all JAR files from the specified directory to the classpath.

Additional Tips:

• For the manifest-defined approach, use Ant's task to dynamically generate the manifest file with the desired classpath before building the JAR.
• If the -jar option is more convenient, ensure that all dependencies are included in the JAR's manifest.

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