Java Just-In-Time (JIT) Compiler: What is it, how does it work, where does it fit into JVM architecture (JIT vs Interpreter)?

Just-In-Time Compilers

The simplest tool used to increase the performance of your application is the Just-In-Time (JIT) compiler. A JIT is a code generator that converts Java bytecode into native machine code. Java programs invoked with a JIT generally run much faster than when the bytecode is executed by the interpreter. The Java Hotspot VM removes the need for a JIT compiler in most cases however you may still find the JIT compiler being used in earlier releases.

The JIT analyzes the behaviour of a program while it runs and looks for opportunities to optimize the bytecode and re-compile it to machine code using the most appropriate native instruction. All mainstream java VMs have a JIT compiler and you can bet that all your java programs are using it right now.

The JIT compiler was first made available as a performance update in the Java Development Kit (JDK) 1.1.6 software release and is now a standard tool invoked whenever you use the java interpreter command in the Java 2 platform release. You can disable the JIT compiler using the -Djava.compiler=NONE option to the Java VM.

JVM Architecture – Role of JIT Compiler

Overview of Java Virtual Machine (JVM) architecture. Source code is compiled down to Java bytecode. Any platform running a JVM can execute Java bytecode. Bytecode is verified, then interpreted or JIT-compiled for the native architecture. The Java APIs and JVM together make up the Java Runtime Environment (JRE).

How do JIT Compilers work?

JIT compilers are supplied as standalone platform-dependent native libraries. If the JIT Compiler library exists, the Java VM initializes Java Native Interface (JNI) native code hooks to call JIT functions available in that library instead of the equivalent function in the interpreter.

The java.lang.Compiler class is used to load the native library and start the initialization inside the JIT compiler.

When the Java VM invokes a Java method, it uses an invoker method as specified in the method block of the loaded class object. The Java VM has several invoker methods, for example, a different invoker is used if the method is synchronized or if it is a native method.

The JIT compiler uses its own invoker. Sun production releases check the method access bit for value ACC_MACHINE_COMPILED to notify the interpreter that the code for this method has already been compiled and stored in the loaded class.

When does the code become JIT compiled code?

When a method is called the first time the JIT compiler compiles the method block into native code for this method and stored that in the code block for that method.

Once the code has been compiled the ACC_MACHINE_COMPILED bit, which is used on the Sun platform, is set.

Related articles

• Java HotSpot VM Options: Client Vs Server And Impact On Performance (singztechmusings.wordpress.com)
• Visualization of JITted code (morepypy.blogspot.com)

Tags: Java bytecode, Java Virtual Machine, Just-in-time compilation, Machine code