how to run standalone tomcat in debug mode

Charlotte

3 min read

·

02-02-2025

20

0

Share

Running Tomcat in debug mode is essential for developers. It allows you to step through your code, inspect variables, and identify issues during runtime. This guide will walk you through the process of running a standalone Tomcat instance in debug mode, covering various scenarios and troubleshooting tips. We'll focus on using a Java debugger, which is the most common method.

Setting Up Your IDE for Debugging

Before you start, ensure your Integrated Development Environment (IDE) is configured correctly. This guide assumes you're using IntelliJ IDEA or Eclipse, but the principles are similar for other IDEs.

IntelliJ IDEA:

1. Create a Run/Debug Configuration: In IntelliJ, go to "Run" -> "Edit Configurations...". Add a new "Remote" configuration. Give it a name (e.g., "Tomcat Remote Debug").
2. Specify the Debug Port: This is crucial. It needs to match the port Tomcat will listen on for debugging. The default is often 8000, but you can change it (more on this later). Enter the port number in the "Port" field.
3. Save the Configuration: Click "OK" to save your debug configuration.

Eclipse:

1. Create a Debug Configuration: In Eclipse, go to "Run" -> "Debug Configurations...". Create a new "Remote Java Application" configuration. Give it a name.
2. Specify Connection Properties: Set the "Project" to your web application project. Enter the debug port (usually 8000) in the "Connect" tab.
3. Save the Configuration: Click "Apply" and then "Debug".

Configuring Tomcat for Debugging

Now, let's configure Tomcat itself to listen for debugger connections. This involves modifying the catalina.sh (or catalina.bat on Windows) script.

Modifying the catalina.sh (or catalina.bat) Script

Locate your Tomcat installation directory. Open the bin/catalina.sh (Linux/macOS) or bin/catalina.bat (Windows) file. You need to add a Java option to enable debugging.

The crucial line to add before the exec command is:

JAVA_OPTS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000 $JAVA_OPTS"

• -agentlib:jdwp: This tells the JVM to load the Java Debug Wire Protocol agent.
• transport=dt_socket: Specifies the transport method (socket).
• server=y: Indicates that Tomcat will act as a debug server.
• suspend=n: This is very important. n means Tomcat will not suspend execution until a debugger attaches. If you use y, Tomcat will wait for you to connect your debugger before starting.
• address=8000: Sets the port number for the debug connection. Make sure this matches the port in your IDE's debug configuration. You can change this to any available port. Avoid ports already in use by other applications.

Important Considerations:

• Port Number: Choose a port that's not already used by another application. If port 8000 is occupied, select another available port (e.g., 8080, 8001, etc.). Remember to update both your IDE configuration and the catalina.sh/catalina.bat file accordingly.
• suspend=n vs. suspend=y: Using suspend=n (recommended) allows Tomcat to start normally. You then attach the debugger later. Using suspend=y will pause Tomcat until your debugger connects.
• Security: Be mindful of security when exposing the debug port. Consider restricting network access to the debug port if necessary, especially in production environments. Don't run Tomcat in debug mode on a production server.

Starting Tomcat and Attaching the Debugger

After saving the changes to catalina.sh or catalina.bat, start Tomcat:

./catalina.sh start  # Linux/macOS
catalina.bat start   # Windows

Once Tomcat is running, go to your IDE and start your debugging session using the configuration you created earlier. Your IDE should connect to the Tomcat instance, allowing you to set breakpoints and step through your code.

Troubleshooting

• Connection Refused: This usually means Tomcat isn't listening on the specified port, or there's a firewall blocking the connection. Double-check the port number in both your IDE and catalina.sh/catalina.bat.
• Tomcat Not Starting: Verify that the changes you made to catalina.sh/catalina.bat are correct and that there are no syntax errors.
• Debugger Not Attaching: Ensure Tomcat is actually running and that the firewall isn't interfering.

By carefully following these steps, you can effectively run your standalone Tomcat server in debug mode, greatly simplifying the process of identifying and resolving issues in your Java web applications. Remember to switch back to a standard startup configuration once debugging is complete. Running with the debug options enabled in production is strongly discouraged due to performance implications and security risks.