Why's a simple change to rt.jar causing the Java Runtime Environment to crash silently?

Question

This is what I'm doing:

1. extract contents of my JRE's rt.jar
2. extract src.zip of my JDK (same version)

Now, if I copy Runtime.java from the extracted src folder and compile it using javac.exe without any modifications and then put it in the extracted rt folder to finally put everything back in a jar file using jar.exe, everything works as expected. The JRE runs fine.

However, if I make the slightest change to Runtime.java and compile it and put it in rt.jar, the JRE crashes whenever I attempt to start it. This is an example of a slight change that causes the silent crash:

/** Don't let anyone else instantiate this class */
    private Runtime() {
        System.out.println("This is a test.");
    }

Instead of:

/** Don't let anyone else instantiate this class */
    private Runtime() {}

Could anyone tell me why this is causing my JRE to crash?

Thanks in advance.

Answer 1

That's pretty strange, as I did the same trick with many classes in rt.jar in past.

Can you provide us with the crashed process output?

Answer 2

It's possible that System.out has not been initialised at the time that the Runtime() constructor runs. Usually console output is not considered a "slight" change, but at the wrong time it can invoke way too much stuff that may not be set up at all yet.

Answer 3

You're doing this all wrong. You can't distribute that modified JRE for a start, so it is only useful inside your organization . Install a SecurityManager and don't grant your codebase any of the RuntimePermissions you're trying to protect against.

Answer 4

@Tom - I advise you NOT to try to do this:

• You cannot distribute the modified rt.jar file without violating the Sun binary license.

• Even if you did, you would not be allowed to call it Java.

• As you are finding, there are lots of complications that arise when you make changes, particularly when those changes might interfere with the JVM's behind the scenes initialization. And when things blow up during initialization, the JVM often cannot report the problem in an intelligible way.

• If you do succeed in making the modified rt.jar work for one JRE, there is no guarantee that the same hacks will work for a different version.

• Nobody in their right mind would knowingly use a modified JVM (especially one modified by a third-party) in a production app.

EDIT : judging from your other questions, I guess you are trying to reverse engineer or modify some third party Java application with a custom launcher. If you provided more information on what you were really trying to do, we might be able to suggest the right way to do it ... rather than using "desperate measures" such as modifying the JRE.