Thursday, November 14, 2013

Using mvnDebug to remote attach debuggers to Java processes

I thought I would expand on these answers for OSX and Linux folks (not that they need it):
I prefer to use mvnDebug too. But after OSX maverick destroyed my Java dev environment (or it could have been that SSD install), I am starting from scratch and stubbled upon this post, and thought I would add to it. 
$ mvnDebug vertx:runMod
-bash: mvnDebug: command not found
DOH! I have not set it up on this box after the new SSD drive and/or the reset of everything Java when I installed Maverick.
I use a package manager for OSX and Linux so I have no idea where mvn really lives. (I know for brief periods of time.. thanks brew.. I like that I don't know this.)
Let's see:
$ which mvn
There you are... you little b@stard.
Now where did you get installed to:
$ ls -l /usr/local/bin/mvn

lrwxr-xr-x  1 root  wheel  39 Oct 31 13:00 /
                  /usr/local/bin/mvn -> /usr/local/Cellar/maven30/3.0.5/bin/mvn
Aha! So you got installed in /usr/local/Cellar/maven30/3.0.5/bin/mvn. You cheeky little build tool. No doubt by homebrew...
Do you have your little buddy mvnDebug with you?
$ ls /usr/local/Cellar/maven30/3.0.5/bin/mvnDebug 
Good. Good. Very good. All going as planned.
Now move that little b@stard where I can remember him more easily.
$ ln -s /usr/local/Cellar/maven30/3.0.5/bin/mvnDebug /usr/local/bin/mvnDebug
  ln: /usr/local/bin/mvnDebug: Permission denied
Darn you computer... You will submit to my will. Do you know who I am? I am SUDO! BOW!
$ sudo ln -s /usr/local/Cellar/maven30/3.0.5/bin/mvnDebug /usr/local/bin/mvnDebug
Now I can use it from Eclipse (but why would I do that when I have IntelliJ!!!!)
$ mvnDebug vertx:runMod
Preparing to Execute Maven in Debug Mode
Listening for transport dt_socket at address: 8000
Internally mvnDebug uses this:
MAVEN_DEBUG_OPTS="-Xdebug -Xnoagent -Djava.compiler=NONE  \
So you could modify it (I usually debug on port 9090).
This blog explains how to setup Eclipse remote debugging (shudder)
Ditto Netbeans
Here is some good docs on the -Xdebug command in general.
"-Xdebug enables debugging capabilities in the JVM which are used by the Java Virtual Machine Tools Interface (JVMTI). JVMTI is a low-level debugging interface used by debuggers and profiling tools. With it, you can inspect the state and control the execution of applications running in the JVM."
"The subset of JVMTI that is most typically used by profilers is always available. However, the functionality used by debuggers to be able to step through the code and set breakpoints has some overhead associated with it and is not always available. To enable this functionality you must use the -Xdebug option."
-Xrunjdwp:transport=dt_socket,server=y,suspend=n myApp
Check out the docs on -Xrunjdwp too. You can enable it only when a certain exception is thrown for example. You can start it up suspended or running. Anyway.. I digress.

BOON! There I said it. Happy?

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