Developing code needs debugging. Java has very good facilities to debug code during development. JVM supports debuggers via the Java Platform Debugger Architecture. This lets you execute code in debug mode and JVM will accept external debugger tools to attach to it via a network, or it will try to attach to a debugger depending on command-line options. JDK contains a client, the jdb tool, which contains a debugger; however, it is so cumbersome to use when compared to the graphical client built into the IDEs that I have never heard of anyone using it for real work.

To start a Java program in debug mode so that JVM will accept a debugger client to attach the options to it, execute the following command:

    -Xagentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=7896

The Xagentlib option instructs the Java runtime to load the jdwp agent. The part of the option that follows -Xagentlib:jdwp= is interpreted by the debugger agent. These options are as follows:

• transport: This should specify which transport to use. It can be a shared memory (dt_shmem) socket or a TCP/IP socket transport but, in practice, you will always use the latter. This is specified in the preceding dt_socket sample.
• server: This specifies if the debugged JVM starts in server mode or client mode. When you start the JVM in server mode, it starts to listen on a socket and accepts the debugger to connect to it. If it is started in client mode, then it tries to connect a debugger that is supposed to be started in server mode, listening on a port. The value of the option is y meaning server mode or n meaning nonserver, a.k.a. client mode.
• suspend: This can also be y or n. If JVM is started in suspend mode, it will not start the Java code until a debugger is attached to it. If it is started with suspend=n, then the JVM starts and when a debugger attaches, it stops as soon as a breakpoint is reached. If you start a standalone Java application, you will usually start the debugging with suspend=y, which is the default. If you want to debug an application in an application server or servlet-container environment, then it is better to start with suspend=n; otherwise, the server does not start until the debugger attaches to it. Starting the Java process in suspend=y mode in case servlet application is only useful when you want to debug the servlet static initializer code, which is executed when the server is starting up. Without suspend mode, you will be required to attach the debugger very fast. It is better that JVM just waits for you in that situation.
• address: This should specify the address that JVM communicates with. If the JVM started in client mode, then it will start to connect to this address. If the JVM runs in server mode, then it will accept connections from the debugger on that address. The address may specify only the port. In this case, the IP address is that of the local machine.

The other options the debugger agent may handle are for special cases. For the topics covered in this book, the preceding options are enough.

The following screenshot shows a typical debugging session where we debug the simplest program in IntelliJ IDE:

When you start a program from the IDE in debug mode, all these options are automatically set for you. You can set breakpoint just by clicking on the source code in the editor. You can have a separate form to add, remove, and edit breakpoints. Breakpoints can be attached to specific lines or specific events, like when an exception is thrown. Breakpoints attached to a specific line can also have conditions that tell the debugger to stop the execution of the code only when the condition is true; for example, if a variable has some predefined value.