An alternative of software like VisualVM to programmatically find running java applications' values etc. by searching heap dumps?

Question

I'm not experienced with java applications but I found out that finding static pointers etc. to these applications' memory addresses is often (nearly) impossible, apparently because of the java engine that handles the code (correct me if this way of naming it is wrong please).

Now, I've used VisualVM (https://visualvm.dev.java.net/) and it's great. I can select my java process and create a heap dump. It then shows me all classes and their values.

Can I use this method to continousely poll the heap dump and receive object values, for example the X Y and Z of a game? How would I programmatically interact with such application, and if this should not be done with VisualVM, what would be an alternative?

Edit: this is what I need to do: I need to be able to find all classes with properties that have a certain value. For example: I'd search for the X coordinate (a float) and it should return the class "PlayerCoordsHandler" (just an example) and the corresponding float with it's value... or alternatively just a way to find this same float again (after restarting for example). This process does not have to be programmatic, aslong as requesting the value of the now known property (x float) can be retrieved programmatically (for example with a command line utility or reading from a file).

Edit2: The target application is a windows executable (but made with java) and launches it's own java VM. It's not possible to add java parameters for debugging. This does not seem to be required though, as VirtualVM is able to debug the process just fine. Anyone knows how?

Thanks in advance.

Answer 1

Triggering a heapdump is usefull for post-mortem analysis of say memory leaks, but as the Java garbage collector moves objects around, you cannot use the memory values of a heapdump to reliably access those objects.

If you need a way to query internal values from outside of the application you could look into setting up an RMI service API via which you can retrieve the values you need.

Another method (if you just need to test something) could be to connect to the process via de Java debugging API.

If you know the JRE location that is used, you could rename java.exe and write a (C/C++) wrapper that adds the debug options listed by Carl and calls the renamed_java.exe in turn.

Another posibility might be to add or update classes in the .jar file of the application. You do not need the source to do this.

Tom, are you trying to reverse engineer an application that specifically tries to obfuscate its working? If so you might get further if you contact the manufacturer and ask them what possibilities they see for what you try to achieve?

Answer 2

Have a look at YourKit. You can monitor CPU, memory and threads live, and generate dumps whenever you want. It can even compare different memory dumps to show you which objects were added/removed.

It's not free though, it has a 15 day (or 30 day?) fully functional eval period. If free is not a real concern it's definitely a great tool.

Answer 3

If you want to poll the values of specific objects while your Java application is running you would probably find that using JMX is a better and more efficient approach rather than using a heap dump. With JMX you can define what values should be exposed and use tools such as VisualVM or JConsole to view them at runtime.

Answer 4

I good starting point is the jps and jstat tools added in Java 6 (i think). jps gives you the pid and main class for each application. jstat give you more details about process

Answer 5

You can easily generate a heap dump by creating your own JMX connection to the JVM, just like VisualVM does it. Analyzing the heapdump is very possible (the data is there and totally disconnected from the JVM so there is no interference from the gc).

However, unless it is a very specific scenario you are looking for you are probably much better off giving the heapdump to MAT and find a good workflow in there to use.

Edit: In this particular case it is probably better to create some kind of specific API to access the values from the outside (and maybe publish the values as MBeans using JMX). Taking a heap dump is way to much work if all you want to do is monitoring a few values.

Edit2: Based on your edits, it seems to me like you could really benefit from publishing your own MBean over JMX. I have to run for a meeting but, unless someone else does it while I am away, I will try to remember to give you some pointers later. Either in an edit of this one or in a new post.

Answer 6

It looks like you want to debug running Java applications.

The "official" Java debugger is JDB. I believe it's part of the JDK. It has the ability to set breakpoints, examine heaps, list and display and even change variables, show running threads and so on. The usual debugger stuff. But it's command line, which makes it a pain in the neck to work with.

Instead, it makes a lot of sense to use an IDE with integrated debugger. I use Eclipse. You can do all the usual debuggery things, including displaying windows with variables. You can set conditional breakpoints and there's much more. Specifically in answer to your question, you can set up watch expressions, which will be evaluated during the program's execution and their displays refreshed with new values when they change.

You may not want to run your Java app inside the IDE; or it may be running in a Web application server. That's no problem for JDB or Eclipse (or other IDEs, like NetBeans or IntelliJ Idea): They can connect to a running JVM and debug remotely with the same level of convenience.

A program being debugged like this, remotely or otherwise, run somewhat more slowly than if it were not. Your game, while being debugged, will run at rather bad-looking FPS; but it should still respond more or less normally to gameplay interaction.

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Remote debugging:

To be able to attach your EclipseNetBeans debugger to a running Java process you need to start that process with the following Java options…

-Xdebug -Xrunjdwp:transport=dt_socket,address=3704,server=y,suspend=n