Remote Debugging Java Applications With JDWP | Learning Quest

Remote Debugging Java Applications With JDWP | Learning Quest

Most of Java developers have had the need to debug their applications, usually to find and fix an issue there. In many cases, the application to debug (known as the "debuggee") is launched from within the IDE used by the developer, while the debugger is also integrated in the IDE, allowing easy inspection of the program state in a step-by-step manner. Sometimes, however, the debuggee JVM is launched from a separate command line, or by executing it on a separate host. In such scenarios, debugging necessitates launching the JVM with some options suitable for debugging, while your IDE debugger would have to connect to it. This is where JDWP (Java Debug Wire Protocol) comes into play.

What is JDWP?

In order to debug remotely executed JVMs (where the debuggee is separately launched locally or on another machine), the Java platform defines a protocol for communication between the JVM and the debugger. JDWP dictates the format of the commands sent by the debugger (e.g. to evaluate a local variable), and replies by the JVM. The exact way of transporting the packets is not specified and is up to the implementation to define transport mechanisms. What JDWP specifies is the format and layout of packets containing commands and those containing replies. Therefore it is conceptually very simple.

JDWP is only one part of the debugging infrastructure in the Java platform. The endpoints (debugger and debuggee) communicating over JDWP implement other specifications to provide the actual debugging functionality. The JVM implements the JVM Tool Interface (JVMTI) to provide debugging functionality for it, for example, to control executions using breakpoints or inspecting the current object. JVMTI is the low-level layer implemented natively in the JVM. The debugger implements another interface called the Java Debug Inteface (JDI) that provides a high-level way to carry debugging requests from the debugger process. JDI is a pure Java interface. Together, JVMTI, JDWP and JDI form the main layers of the Java Platform Debugger Architecture. Links to official references about all these specifications are provided at the end.

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