JDB - Introduction

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Debugging is a technical procedure to find and remove bugs or defects in a program and get expected results. Debugging includes testing and monitoring. It is very complex when the subunits of a program are tightly coupled. We can debug a program using the debugger tools that follow the prescribed APIs. A debugger allows you to step through every aspect of a code, inspect all the elements, and remove errors, if any.

Debugging Techniques

There are different kinds of techniques to debug a Java program. The old method of debugging is by using print statements at the end of every segment which will print the trace statements on the console. Take a look at the following code.

pubic class Add
{
   public static void main(String ar[])
   {
      int a = ar[0];
      system.out.println("A : " + a);
      
      int b = ar[1];
      system.out.println("B : " + b);
      
      int c = a + b;
      system.out.println("C = a + b : " + c);
   }
}

Here, we have a program that adds two numbers and prints the output. Notice that at each step, we have introduced a print statement that prints the state of the program on the console. This is the traditional approach to debug a program.

In addition, we have advanced concepts that can be used to debug a program such as:

  • stepping
  • breakpoints, and
  • exceptions or watchpoints.

Types of Debugging

We can debug a program using various methods:

  • Using Java bytecode (compiled version of Java code)
  • Using comments inside the programs
  • Attaching class to a running program
  • Remote debugging
  • Debugging on demand
  • Optimized code debugging

Java Debuggers

Here are some examples of Java debuggers that are available in the market:

  • IDEs such as Eclipse, Netbeans, etc. contain their own debuggers (Visual cafe, Borland, JBuilder)
  • Standalone debugger GUIs (such as Jikes, Java platform debugger, and JProbe)
  • Command-line debugger (Sun’s JDB)
  • Notepad or VI driven (stack trace)

This tutorial covers how to use the command-line debugger, jdb.

JDB

The Java debugger (JDB) is a tool for Java classes to debug a program in command line. It implements the Java Platform Debugger Architecture. It helps in detecting and fixing bugs in a Java program using Java Debug Interface (JDI).

JDB in JDK

The following architecture defines the role of JDB in JDK. It contains mainly three units:

  • Java Virtual Machine Tool Interface (JVM TI)
  • Java Debug Wiring Pool (JDWP)
  • Java Debugger Interface (JDI)
JDB Architecture

JVM TI

It is a native programming interface implemented by VM. It provides ways to inspect and debug the state of the application running on the VM. It allows an implementer (VM Implementer) that can be enclosed easily into the debugging architecture. It also uses a third-party channel called JDWP for communication.

JDWP

It defines the format of information and the requests that pass in between the debuggee process and the debugger front end. The primary purpose of having a JDWP is to allow the debuggee and the debugger to communicate when they run under separate VMs or in separate platforms.

JDI

It is a high-level Java interface implemented as front end. It defines the variable information at user code level. It is recommended to use a JDI layer for all debugger development. It uses JDWP for communication with the debuggee JVM.

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