Debugging in Maven?

Question

Is it possible to launch a debugger such as jdb from Maven? I have a pom.xml file that compiles the project successfully. However, the program hangs somewhere and I would really like to launch jdb or an equivalent debugger to see what's happening.

I compile using mvn compile and launch using:

mvn exec:java -Dexec.mainClass="com.mycompany.app.App"

I was expecting something like:

mvn exec:jdb -Dexec.mainClass="com.mycompany.app.App"

to launch the debugger but, as usual, my expectations are incongruent with maven's philosophy.

Also, I couldn't find any documentation (on Maven's website or google) to describe how debugging works. I suspect that I have to use some plugin.

Clone

Answer 1

Just as Brian said, you can use remote debugging:

mvn exec:exec -Dexec.executable="java" -Dexec.args="-classpath %classpath -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=1044 com.mycompany.app.App"

Then in your eclipse, you can use remote debugging and attach the debugger to localhost:1044.

Stockbreeder

Answer 2

If you are using Maven 2.0.8+, run the mvnDebug command in place of mvn and attach a debugger on port 8000.

For Maven <2.0.8, uncomment the following line in your %M2_HOME%/bin/mvn.bat (and maybe save the modified version as mvnDebug.bat):

@REM set MAVEN_OPTS=-Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000

More details in MNG-2105 and Dealing with Eclipse-based IDE.

Corvine

under Mac OS X you have to create a link to mvnDebug first: ln -s /usr/share/maven/bin/mvnDebug /usr/bin/mvnDebug – Unify

mvnDebug is really nice! I did this before with the mavenOpts but this is cumbersome... – Bedraggled

This saved me! It's a valid workaround for this Eclipse / M2E issue – Concordant

I was not able to make it work in my windows machine, but it is a script that takes care of all environment problems, like cygwin. Finally I was able to debug a Jenkins plugin. Very good tip. – Aikens

What does and attach a debugger on port 8000 means? – Mikael

mvnDebug gives me the following message: ERROR: JDWP unable to get necessary JVMTI capabilities. – Richey

@NisargPatil I thought Run -> Attach to Process, but that didn't work for me. What did was creating a new configuration: Run -> Edit Configurations -> Add New Configuration (plus sign) -> Remote JVM Debug. there I just changed the port to 8000 and left the rest as it was – Wag

Answer 3

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, 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
/usr/local/bin/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 
/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  \
-Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000"

So you could modify it (I usually debug on port 9090).

This blog explains how to setup Eclipse remote debugging (shudder)

http://javarevisited.blogspot.com/2011/02/how-to-setup-remote-debugging-in.html

Ditto Netbeans

https://blogs.oracle.com/atishay/entry/use_netbeans_to_debug_a

Ditto IntelliJ http://www.jetbrains.com/idea/webhelp/run-debug-configuration-remote.html

Here is some good docs on the -Xdebug command in general.

http://docs.oracle.com/cd/E13150_01/jrockit_jvm/jrockit/jrdocs/refman/optionX.html

"-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.

Seedcase

Answer 4

I found an easy way to do this -

Just enter a command like this -

>mvn -Dtest=TestClassName#methodname -Dmaven.surefire.debug test

It will start listening to 5005 port. Now just create a remote debugging in Eclipse through Debug Configurations for localhost(any host) and port 5005.

Source - https://doc.nuxeo.com/display/CORG/How+to+Debug+a+Test+Run+with+Maven

Tillett

Answer 5

If you are using Netbeans, there is a nice shortcut to this. Just define a goal exec:java and add the property jpda.listen=maven

Tested on Netbeans 7.3

Defamation

Yup, that would have been my answer as well. If you do not fork the exec plugin and run as part of the maven build with the "java" goal, then like that you will debug the maven build, including the exec plugin. Netbeans is simplicity itself. – Span

This should be marked as answer. Helped me in solving the issue in Netbeans. – Dorotheadorothee

Answer 6

If you don't want to be IDE dependent and want to work directly with the command line, you can use 'jdb' (Java Debugger)

As mentioned by Samuel with small modification (set suspend=y instead of suspend=n, y means yes which suspends the program and not run it so you that can set breakpoints to debug it, if suspend=n means it may run the program to completion before you can even debug it)

On the directory which contains your pom.xml, execute:

mvn exec:exec -Dexec.executable="java" -Dexec.args="-classpath %classpath -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=1044 com.mycompany.app.App"

Then, open up a new terminal and execute:

jdb -attach 1044

You can then use jdb to debug your program!=)

Sources: Java jdb remote debugging command line tool

Theravada

Answer 7

Why not use the JPDA and attach to the launched process from a separate debugger process ? You should be able to specify the appropriate options in Maven to launch your process with the debugging hooks enabled. This article has more information.

Legend

Answer 8

I use the MAVEN_OPTS option, and find it useful to set suspend to "suspend=y" as my exec:java programs tend to be small generators which are finished before I have manage to attach a debugger.... :) With suspend on it will wait for a debugger to attach before proceding.

Beauvais

Answer 9

I couldn't make any of the other solutions work and then I tried starting maven with the following.

mvn exec:exec -Dexec.executable="java" -Dexec.args="-classpath %classpath -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=8000 com.example.Main"

This worked for me and then I searched some more and found this other answer. IntelliJ can run Maven project in debug mode but ignores breakpoints

The fact is that my application is a spring-boot application. Now, the link referred to above may be right and the problem is that the spring-boot plugin creates a new fork that somehow invalidates the interaction between debugger and target app. I'm saying this because I got some info on how this may work from here

In any case, I don't know how the two solutions are related to each other. Is the first solution also preventing spring-boot from creating a fork??

Sadly I have no clue at the moment.

Laure

Answer 10

Here is how I do it.

For example:

mvn clean azure-functions:run -DenableDebug

Then, after it starts up, you'll see a link in the Terminal that says "Attach Debugger".

To me, that is the easiest way to do it.

Asperity

Answer 11

i have tried the following Command and it works: mvndebug compile exec:java -Dexec.mainClass="hello.HelloWorld"

Excrement