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In a typical debugging scenario, you would set breakpoints to tell the debugger when to suspend your program. A breakpoint usually corresponds to the moment that marks the starting point for further investigation.

Deciding where to set a breakpoint can be challenging. There may be situations where the exact line is unclear, or you may prefer to pause the program based on time rather than specific code.

In this article, we'll take a look at IntelliJ IDEA's Pause Program functionality – a lesser-known debugging technique that can be extremely powerful in some scenarios, including those described above. We will discuss its use cases and limitations, as well as discover the secret step by step.

What is Pause?

Pause Program is a feature of the IntelliJ IDEA debugger that allows you to arbitrarily pause your application at any point in time. Furthermore, you don't even need to know the application code. In fact, you can just ignore it!

To pause a program, click Pause Program on the debugger toolbar. Then, the program will stop right in the middle of whatever you are doing.

Limitations

At first glance, a paused program may look exactly like one that has been suspended at a breakpoint. However, this is only true to a certain extent.

It would be correct to consider Pause Program a kind of thread dump plus. You can still inspect both variables and threads the way you normally would. However, some of the more advanced features, such as Evaluate Expression, will not work.

Use cases

There are countless ways to use Pause Program. It can often be used interchangeably with traditional breakpoints. However, there are scenarios where using Pause Program is a more appropriate approach. Let's consider some of them.

Irresponsive apps

If you encounter a user interface (UI) freeze, it is usually due to the UI thread being blocked.

Pause Program could be useful in this case, as it allows you to pause the application while it is unresponsive and examine the UI thread's call stack. This is usually enough to diagnose the problem.

Missing fonts

As mentioned earlier, Pause Program allows you to simply ignore the source code, which may be missing for you anyway. Although this scenario is not very common, when you come across it, breakpoints would not be of any help.

This is where Pause Program comes into play!

Locks

If you suspect a synchronization problem, such as a deadlock or livelock, Pause Program can help you find the exact threads and locks that are causing the problem.

In this case, pause the program and inspect the thread list. It will show which threads are blocked. When you navigate to the execution point, you will also see the critical sections where they are locked. This information can help guide you toward a solution.

Secret step-by-step tip

As I mentioned earlier, Pause Program limits your access to some of the debugger's advanced functionality. If you have tried to use certain features while an application is paused, you may have seen an error message that says Cannot evaluate methods after Pause action.

However, there is a shortcut to this restriction.

After you have paused an application, continue performing any action step by step. Step Into or Step Over will do. Once this is done, you will be in a regular debugging session, similar to when you suspend an application using a breakpoint. All advanced features are now unlocked!

Conclusion

That's it for today! I hope you find these tips and tricks helpful.

If you are interested in more articles related to debugging and profiling, check out some of my other articles:

• Debugger.godMode() – Hack a JVM Application with the Debugger
• Debugger Slow Troubleshooting
• Debugging Inactive Applications
• What's Wrong with createDirectories()? - CPU Profile Guide

If there's anything specific about debugging in Java that you want me to cover, don't hesitate to get in touch! Your opinion will help prioritize and publish the content that is most interesting to you.