Java Debugger (JDB)

 

参考

 https://www.geeksforgeeks.org/java-debugger-jdb/

https://docs.oracle.com/en/java/javase/17/docs/specs/man/jdb.html

 

Java Debugger (JDB) is a command-line tool used by Java developers to debug Java code efficiently.

 

JDB Architecture

The architecture of Jdb consists of the following components:

1. Java Debug Interface (JDI): JDI is a set of APIs that provides a standard interface for interacting with the JVM. Jdb uses JDI to communicate with the JVM and perform debugging tasks.
2. Debuggee: Debuggee is the Java program being debugged by Jdb. Jdb attaches itself to the debuggee process and interacts with it through JDI.
3. Jdb client: Jdb client is the user interface that allows developers to interact with Jdb. It provides a command-line interface for entering debugging commands.
4. Java Virtual Machine (JVM): JVM is the runtime environment in which Java code is executed. Jdb uses the JVM to run the debuggee process and perform debugging tasks.

Options for the jdb command

-sourcepath dir1:dir2:...

Uses the specified path to search for source files in the specified path. If this option is not specified, then use the default path of dot (.).

-attach address

Attaches the debugger to a running JVM with the default connection mechanism.

Jdb-Basic Commands

Here are some of the most commonly used JDB commands

• run – Starts the execution of the program.
• stop at <class>:<line> – Sets a breakpoint at the specified line in the specified class.
• stop in <class>.<method> – Sets a breakpoint at the beginning of the specified method in the specified class.
• step – Executes the current line of code and stops at the next line. If the current line contains a method call, JDB steps into the method and stops at the first line of the method.
• next – Executes the current line of code and stops at the next line. If the current line contains a method call, JDB executes the method call and stops at the next line after the method call.
• cont – Continues the execution of the program until the next breakpoint or until the program completes.
• list – Lists the source code around the current execution point.
• print <expression> – Prints the value of the specified expression.
• locals – Lists the local variables in the current frame.
• classes – Lists all loaded classes.
• methods <class> – Lists all methods in the specified class.
• thread – Lists all threads and their current status.
• suspend – Suspends all threads.
• resume – Resumes all threads.
• help – Displays a list of available JDB commands.
• exit – Exits JDB.

For example, first start the jar

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005 -jar myapp.jar

Then at another terminal

jdb -attach 5005

如果要添加代码目录可以使用-sourcepath

jdb -attach 5005 -sourcepath /mnt/e/app/src/main

之后可以运行如下命令

 

	命令示例
set breakpoint	stop at com.example.app.Scheduler:139
continue	Cont
打印变量	pint this
	dump this
	next
	step