Find where java class is loaded from

Question

Here's an example:

package foo;

public class Test
{
    public static void main(String[] args)
    {
        ClassLoader loader = Test.class.getClassLoader();
        System.out.println(loader.getResource("foo/Test.class"));
    }
}

This printed out:

file:/C:/Users/Jon/Test/foo/Test.class

Another way to find out where a class is loaded from (without manipulating the source) is to start the Java VM with the option: -verbose:class

getClass().getProtectionDomain().getCodeSource().getLocation();

This is what we use:

public static String getClassResource(Class<?> klass) {
  return klass.getClassLoader().getResource(
     klass.getName().replace('.', '/') + ".class").toString();
}

This will work depending on the ClassLoader implementation: getClass().getProtectionDomain().getCodeSource().getLocation()

Jon's version fails when the object's ClassLoader is registered as null which seems to imply that it was loaded by the Boot ClassLoader.

This method deals with that issue:

public static String whereFrom(Object o) {
  if ( o == null ) {
    return null;
  }
  Class<?> c = o.getClass();
  ClassLoader loader = c.getClassLoader();
  if ( loader == null ) {
    // Try the bootstrap classloader - obtained from the ultimate parent of the System Class Loader.
    loader = ClassLoader.getSystemClassLoader();
    while ( loader != null && loader.getParent() != null ) {
      loader = loader.getParent();
    }
  }
  if (loader != null) {
    String name = c.getCanonicalName();
    URL resource = loader.getResource(name.replace(".", "/") + ".class");
    if ( resource != null ) {
      return resource.toString();
    }
  }
  return "Unknown";
}

Edit just 1st line: Main.class

Class<?> c = Main.class;
String path = c.getResource(c.getSimpleName() + ".class").getPath().replace(c.getSimpleName() + ".class", "");

System.out.println(path);

Output:

/C:/Users/Test/bin/

Maybe bad style but works fine!

Typically, we don't what to use hardcoding. We can get className first, and then use ClassLoader to get the class URL.

        String className = MyClass.class.getName().replace(".", "/")+".class";
        URL classUrl  = MyClass.class.getClassLoader().getResource(className);
        String fullPath = classUrl==null ? null : classUrl.getPath();