Debugging Java Application Running on Docker
In this post I will explain how you can debugging Java Application that is running on Docker like a boss. To be precise I will explain how we can debug the application that uses Maven. I will show you how we can attach our debugger on IntelliJ Idea.
Let’s assume we have a SpringBoot Application. To run our application we use mvn spring-boot:run. If we were to use IDE directly to run our application, we can directly use debugging feature provided by IDE. But, when we run our application on Docker we execute our application with help of command [mvn spring-boot:run].
When we use IDE to debug our application it adds some meta-data information on run command. The meta-data information to be precise is a JVM Arguments. This is what JVM Arguments looks like:
-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005
To send this meta-data information [i.e JVM Arguments] when we run our application, we need to do something like below :
mvn spring-boot:run -Drun.jvmArguments="-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005"
You can see that we are sending those meta information with help of -Drun.jvmArguments.
Options on agentlib:jdwp #
transport=dt_socketmeans the way used to connect to JVM.address=5005: TCP/IP port exposed, to connect from the debuggersuspend=y: ify, tell the JVM to wait until debugger is attached to begin execution, otherwise (ifn), starts execution right away.
Debugging with IntelliJ Idea #
Open IntelliJ Idea and Go to Edit Coniguration ( Run > Edit Configuration)
Click + Icon. [Alt + Insert] then Type remote and select Remote 
You will see dialog like below. 
From the information you can see that I copied the Remote Debugging information from this dialog. If you didn’t change any of the option from previous section you are good to go. Just a quick note though, on Host if you are using Linux 'localhost' will work but, if you are on Windows / Mac replace localhost with IP Docker Machine is running on. Before you click Debug make sure you already executed your application with meta information. If you change the address option when you added meta information just replace the port number with same.
You should be able to debug your application. If you were stuck on some step please comment below.