Grails Goodness: Debugging App in Forked Mode

Since Grails 2.2 by default the run-app command will launch the Grails application in a separate Java Virtual Machine. This is called forked Tomcat execution in Grails. This way the class path of the Grails build system and the application will not intervene and also both processes will have their own memory settings. We can see the settings in grails-app/conf/BuildConfig.groovy where we find the configuration property When we want to debug our application in an IDE like IntelliJ IDEA we cannot use the Debug command, because this will only allow us to debug the Grails build system. We will not reach breakpoints in our source code. But Grails 2.3 introduces an extra argument for the run-app command: --debug-fork. If we use this extra argument the JVM running the Grails application will stop and listen for a debug session to be attached and then continue. We can configure a Debug configuration in IntelliJ IDEA (or another IDE) to attach to the waiting Grails application and use breakpoints and other debugging tools like we are used to.

Suppose we have a Grails application named forked-debug and we have created a project in IDEA for this application. We click on the Select Run/Debug Configuration button and select Edit Configurations…:

IDEA opens a dialog where we can change the Grails command and set JVM options. We add the option --debug-fork to the Command Line field in this dialog:

We click the OK button to save our change and close the dialog window. Next we can run our Grails application using our changed run configuration:

IDEA starts our application in the console window we can see Listening for transport dt_socket at address: 5005:

Now it is time to create a new debug configuration. We click on the Select Run/Debug Configuration button again and select Edit Configurations…. We add a new type of configuration, so we click on the + sign and type Remote:

We select the Remote type and the dialog window shows now a lot of input fields which we can leave to the default values. It is good to given this configuration a new name, for example Grails:forked-debug (debug):

We click the OK button to close the dialog window. Our Grails application is still waiting for a debug session to be attached, so we use our new configuration with the Debug button:

In the console window of our Grails application we can see the application is not continuing to start and finally we can reach the appellation via web browser. We can now place breakpoints in our source code and when we hit them we can use all debugging tools from IDEA:

We could also have used the argument --debug-fork from a command-line and then use the IDEA debug configuration to attach to that instance of the Grails application.

Code written with Grails 2.3.4 and IntelliJ IDEA 13 is used.

Original article

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About mrhaki

My name is Hubert A. Klein Ikkink also known as mrhaki. I work at the great IT company JDriven. Here I work on projects with Groovy & Grails, Gradle and Spring. At JDriven we focus on SpringSource technologies. All colleagues want to learn new technologies, support craftmanship and are very eager to learn. This is truly a great environment to work in. You can contact me via Google+ or @mrhaki.

6 thoughts on “Grails Goodness: Debugging App in Forked Mode

  1. Hubert,

    Thanks for this info – i have just upgraded to Grails 2.3.4 and intellij 13 and hit this “no more simple debug click” wall right away. Your solution works! Thanks a lot :-)


  2. Hi Hubert,

    Thanks for this, but it didn’t help me right away. It’s worth mentioning that this only works with the Tomcat plugin applied. I had no success with the default Jetty plugin. I tried Grails 2.3.5 and 2.3.7.

    I guess the clue was the phrase ‘forked Tomcat execution’!

    People wanting this way of debugging to work should first head to, if they’re currently using Jetty.



  3. Hi Ian,

    Since the latests releases of Grails the default servlet container is already Tomcat. I’ve not tried this with Jetty (yet), so good observation. Something for me to look into and see if with Jetty we could also run in forked mode.

    Kind regards, Hubert Klein Ikkink

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