The magic behind the eclipse generated executable jar file

Question

I can make a executable jar file with Eclipse. Let's say it is called ast.jar, and the Main class is ASTExplorer. I can verify that this works with java -jar ast.jar.

Then, I unzip the jar file in a separate directory and verify that the ASTExplorer is in astexplorer directory. But when I executed this command java -cp . astexplorer.ASTExplorer I get this error.

java -cp . astexplorer/ASTExplorer
Exception in thread "main" java.lang.NoClassDefFoundError: org/eclipse/swt/widgets/Composite
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)

The thing is that there is no org/eclipse/swt directory in the jar file.

What's the magic behind the executable jar file? Why doesn't it run when unzipped?

Answer 1

When Eclipse packages a runnable jar, it includes the dependencies as nested jar files, and also includes a special classloader that understands how to find classes in nested jars. It works only with nested jars and when you extract everything you prevent it from working.

I've looked in an Eclipse-produced runnable jar, and here's what I think you'd need to do:

1. Extract everything to a directory. The dependency jars are extracted at the top level directory
2. Delete the org/eclipse/jdt/internal directory

3. Run your main class from the top level directory with

   java -cp .;dep-jar;dep-jar;... your.main.class

I tried this with a runnable jar I happen to have lying around and it works. Step 2 isn't strictly necessary.