jdb

jdb is a command-line debugger for the Java programming language that ships with the Java Development Kit (JDK). It provides a lightweight debugging interface without a graphical user interface and is designed to work with the Java Platform Debugger Architecture (JPDA), communicating with a target Java Virtual Machine (JVM) through the Java Debug Wire Protocol (JDWP).

jdb can debug a program started under the debugger or attach to a running JVM that is listening for a debugger connection. This makes it suitable for headless environments and automated debugging tasks. The tool supports both local debugging and remote debugging over a network connection, depending on how the target VM is started for debugging.

Core capabilities of jdb include setting breakpoints at specific lines or methods, stepping through code, inspecting variables and the call stack, and managing threads. Users can evaluate expressions, print variable values, and control the flow of execution with commands to continue, pause, or terminate the target VM. The command set is designed to be concise and scriptable, making it useful for quick inspections or lightweight automation.

Typical usage involves starting jdb with a main class or attaching to an already running VM. For example, a session might begin with jdb com.example.Main, followed by stop at com.example.Main:42 to set a breakpoint, then run, print x to inspect a variable, where to view the call stack, and cont or step to control execution. While powerful for simple tasks, jdb is primarily a low-overhead alternative to full-featured IDE debuggers.

See also: Java Platform Debugger Architecture, Java Debug Wire Protocol, JDWP, JVMTI, OpenJDK.