How to Debug Java Applications in IntelliJ IDEA

Edited 16 minutes ago by ExtremeHow Editorial Team

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Debugging Java applications in IntelliJ IDEA can be a vital skill for developers. This guide provides a comprehensive understanding of how to debug Java applications using IntelliJ IDEA, with each step explained in detail.

Introduction to debugging

Debugging is the process of finding and resolving defects or problems within a program. In Java development, debugging helps when an application does not work as expected. It is essential to ensure that the software is reliable and performs well.

Setting up IntelliJ IDEA for debugging

Before we begin, you need to install IntelliJ IDEA on your machine as well as set up your Java project. Make sure your project builds correctly without any errors. After opening your project in IntelliJ IDEA, you are ready to start debugging.

Using breakpoints

Breakpoints allow you to stop the execution of your program at a specific point. This will enable you to examine the state of the application at a particular moment. To set a breakpoint, follow these steps:

• Open the Java file where you want to set the breakpoint.
• Find the line where you want to stop the execution.
• Click on the left margin next to the line number. A red dot will appear, indicating that the breakpoint is set.

You can add multiple breakpoints to your code to help you locate errors.

Running in debug mode

After setting the breakpoint, you need to run your application in debug mode:

1. Open the Run/Debug configuration located in the top right corner of IntelliJ IDEA.
2. Select the "Debug" option associated with your main class or configuration.
3. The application will start, and execution will stop at the first breakpoint.

Navigating while debugging

Once execution is stopped at a breakpoint, you have several options for navigating through your code:

• Step Over (F8): This command moves to the next line of code without going into the method call.
• Step Into (F7): Use this to step into methods on the current line, so you can see how a method executes.
• Step Out (Shift+F8): If you have entered a method and want to go back to the calling method, use this command.
• Resume Program (F9): Continues execution until the next breakpoint.

Example:

public class Main { public static void main(String[] args) { int result = sum(5, 10); System.out.println("Result: " + result); } public static int sum(int a, int b) { return a + b; } }
public class Main { public static void main(String[] args) { int result = sum(5, 10); System.out.println("Result: " + result); } public static int sum(int a, int b) { return a + b; } }

Here, we are interested in debugging sum method, so we set a breakpoint at return a + b;. When running the debugger, you can step into sum method to see how the execution flows.

Inspecting variables and expressions

When debugging, you can inspect the values of variables and the results of expressions:

• Hover over a variable to see its current value.
• Use the debugger panel at the bottom to view all current variables and their state.
• Add watch expressions if you want to monitor specific expressions or variables during execution.

Modifying code on the fly

IntelliJ IDEA allows you to modify certain parts of the code without restarting the application during debugging:

• This feature is called "hotswap", which can save time during debugging sessions.
• Note that HotSwap has its limitations. It can handle changes to method bodies, but not method signatures or field changes.

Debugging multi-threaded applications

Debugging multi-threaded applications is more complex due to concurrent execution. IntelliJ IDEA provides tools to handle this:

1. Use the Threads view in the Debug panel to see all threads that are currently running.
2. Simply suspend the thread of interest and observe its condition.
3. Set specific breakpoints for certain threads.

Evaluating the expression

IntelliJ IDEA lets you evaluate expressions to test the results without changing your code:

• Pause execution where necessary.
• Open the Expression Evaluation dialog (Alt+F8) to enter the expressions you want to test.
• The system returns the results immediately.

Bringing everyone together

With the steps explained, you have a solid foundation for debugging Java applications using IntelliJ IDEA. Let's summarize the key points:

1. Set breakpoints to control where code execution stops.
2. Run your application in debug mode to start a debugging session.
3. Use navigation commands to understand the flow of execution.
4. Inspect variables, use watch expressions, and evaluate expressions to understand the state of your program.
5. Handle multi-threaded execution by focusing on individual threads.

These techniques help you become proficient at debugging, leading to more efficient and error-free Java applications.

Taking advantage of advanced features

IntelliJ IDEA provides advanced features for more sophisticated debugging. Understanding these tools can significantly enhance your debugging skills:

Conditional Breakpoint

Conditional breakpoints are triggered only when the specified condition is true. This is useful when you need to break execution only for specific scenarios:

• Set breakpoints as usual.
• Right-click the breakpoint and select More, then select Set Condition.
• Enter the condition using a Java expression.

Exception Breakpoint

These breakpoints are triggered when exceptions occur. They enable you to catch errors that might not trigger normal breakpoints. To set an exception breakpoint:

• Go to the Breakpoints dialog (Ctrl+Shift+F8).
• Add an exception breakpoint and specify the exception you're interested in.
• You can choose to catch "caught" and/or "uncaught" exceptions.

Improving debugging sessions

IntelliJ IDEA's debugging settings allow you to customize your experience to suit your development needs:

Log Point

Log points are non-intrusive breakpoints. Instead of stopping execution, they log messages to the console:

• Set a breakpoint where you want the log point.
• Modify it to log the message instead of stopping execution.

Using Run/Debug Configurations

Setting specific debug configurations allows you to adjust the way your application starts:

• Open the Run/Debug Configurations dialog.
• Set specific VM options, environment variables, or program arguments as needed.

Conclusion

Debugging Java applications in IntelliJ IDEA becomes more manageable when you use the platform's extensive features. By setting breakpoints, using threads, evaluating expressions, and familiarizing yourself with advanced tools like conditional and exception breakpoints, you can efficiently resolve errors and maintain smooth code execution. Always remember, the goal is not just to fix bugs, but to understand the underlying issues affecting your application.

Whether you are dealing with simple bugs or complex multi-threaded problems, IntelliJ IDEA provides you with the right tools to diagnose them and solve them effectively. Enjoy debugging!

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