Where on the file system was my Java class loaded from?

Question

I think this is a situation every Java programmer runs into if they do it long enough. You're doing some debugging and make a change to class. When you go to re-run the program, these changes don't seem to be picked up but rather the old class still seems to be running. You clean and rebuild everything, same issue. Sometimes, this can come down to a classpath issue, of the same class being on the classpath more than once, but there doesn't seem to be an easy way to figure out where the class being loaded is coming from...

Is there any way to find the file path for the class that was loaded? Preferable something that would work either if the class was loaded from a .class file or a .jar file. Any Ideas?

Answer 1

Simply run java using the standard command line switch"-verbose:class" (see the java documentation). This will print out each time a class is loaded and tell you where it's loaded from.

Answer 2

If you wanted to do it programmatically from inside the application, try:

URL loc = MyClass.class.getProtectionDomain().getCodeSource().getLocation();

(Note, getCodeSource() may return null, so don't actually do this all in one line :) )

Answer 3

public static URL getClassURL(Class klass) {
    String name = klass.getName();
    name = "/" + convertClassToPath(name);
    URL url = klass.getResource(name);
    return url;
}

public static String convertClassToPath(String className) {
    String path = className.replaceAll("\\.", "/") + ".class";
    return path;
}

Just stick this somewhere, and pass it the Class object for the class you want to find the definition of. It should work regardless of where it called from, since it calls getResource() on the class being searched for.

public static void main(String[] args) {
    System.out.println(getClassURL(String.class));       
}

Sample output: jar:file:/System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Classes/classes.jar!/java/lang/String.class

Answer 4

As the class needs to come from somewhere in the class path, I would recommend to simply print the class path and check if there's an older version of you class somewhere.