Java exception not caught

Question

Why are some exceptions in Java not caught by catch (Exception ex)? This is code is completely failing out with an unhandled exception. (Java Version 1.4).

public static void main(String[] args) {
    try {
        //Code ...
    } catch (Exception ex) {
        System.err.println("Caught Exception");
        ex.printStackTrace();
        exitCode = app.FAILURE_EXIT_CODE;
    }
    finally {
        app.shutdown();
    }
    System.exit(exitCode);
}

I get a Exception in thread "main" java.lang.NoSuchMethodError

But this works

public static void main(String[] args) {
    int exitCode = app.SUCCESS_EXIT_CODE;
    try {
        //Code ...
    } catch (java.lang.NoSuchMethodError mex){
        System.err.println("Caught NoSuchMethodError");
        mex.printStackTrace();
        exitCode = app.FAILURE_EXIT_CODE;
    } catch (Exception ex) {
        System.err.println("Caught Exception");
        ex.printStackTrace();
        exitCode = app.FAILURE_EXIT_CODE;
    }
    finally {
        app.shutdown();
    }
    System.exit(exitCode);
}

I get Caught NoSuchMethodError java.lang.NoSuchMethodError:

I thought catching exceptions would catch all exceptions? How can I catch all exceptions in java?

Answer 1

Because some exceptions don't derive from Exception - e.g. Throwable and Error.

Basically the type hierarchy is:

       Object
         |
      Throwable
     /         \
Exception      Error

Only Throwables and derived classes can be thrown, so if you catch Throwable, that really will catch everything.

Throwable, Exception and any exception deriving from Exception other than those derived from RuntimeException count as checked exceptions - they're the ones that you have to declare you'll throw, or catch if you call something that throws them.

All told, the Java exception hierarchy is a bit of a mess...