从jar文件中，从类中调用main方法，该类从jar外部实现一个类

Question

Suppose I have a jar file called 'MyJar.jar' with a class defined like this:

import SomeInterface;

public class MyClass implements SomeInterface {
   static public void main ( String args[] ) {
     ...
   }
}

Note, however, that the interface 'SomeInterface' is NOT in the jar file. Instead its referenced in another jar file called 'SomeJar.jar'

My manifest-mf file does include something like this:

Main-Class: MyClass

Now, assume that I try to run the 'main' method on this 'MyClass' class from my jar file using what seems like workable syntax, ie:

java -cp SomeJar.jar -jar MyJar.jar

In this case, I get something like:

Error: Could not find or load main class MyClass

However, if I rework the syntax like this, everything works fine:

java -cp SomeJar.jar;MyJar.jar MyClass

For a sanity check, I did do the former call with a 'HelloWorld' class with an appropriate manifest file entry and that worked. It seems that this problem only occurs when the class in question implements an interface that isn't immediately available to the jar file, but instead exists outside of it. (This might happen for exending a class as well, but I haven't checked) As you can see, I can make things work... just not with the 'java -jar' syntax that I thought would work. Additionally, with the former way, I DO NOT get a "NoClassDefFoundError" error (which I thought I would), regardless of whether I include the "-cp" classpath in the "java -jar" call or not.

Any thoughts why the former "java -jar" call does not work (and does not seem to give any good errors on WHY its not working) and yet the latter "java" call works fine?

Answer 1

The -cp and -jar options both set (rather than append to) the underlying classpath. Whichever option is on the command-line last will override all former -cp and -jar options.