How to determine which jar a class is loaded from?

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Did you ever encounter an runtime exception saying a method not found? How can the code compile if the method is not defined? Or after you made some changes to the code, you don't see it takes effect, even after you clean built, and redeploy for a couple of times? When these situations occurs, besides trying to pull your hair out, you can in fact find out which jar the class in trouble is loaded from. More than often, there is another jar file sitting the the classpath which contains the exact same class.
 

If you are handy in shell scripts, you may come up with a simple script using "find" to track down all the jar files in the class path that contains the class you are looking for.


    If you are not a script guru, you can try the following:
    Put this code snippet inside any one of the method of any one of the class:     Class klass = ClassInTrouble.class;
    CodeSource codeSource = klass.getProtectionDomain().getCodeSource();
    if ( codeSource != null) {
        System.out.println(codeSource.getLocation());
    }



When the method that contains this sniplet is called, it will print out something like:
file://c:/dev/source/lib/myjar.jar
 
Will the codeSource be null? The answer is yes. When a jar file is loaded by the system class loader, it's codeSource will be null. What jars will be loaded by the system class loader? the rule of thumb is that all the jars in the class path (not the ones you package in your application) will be loaded by the system class loader. Obviously the above code can't used to find out which jar a class is loaded from, if the jar is loaded by the system class loader. You can use the "verbose" java command line argument when you start the application:
java -verbose app

it will print out every class in each jar the system class loader loads, ie:

[Opened C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.Object from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.io.Serializable from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.Comparable from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.CharSequence from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.String from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.reflect.GenericDeclaration from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
......