Java Magazine - May/June 2012

//java architect /

Part 1

Introduction to JIT Compilation in Java HotSpot VM

COMMUNITY

Use the PrintCompilation switch to observe the effects of Java HotSpot VM compiling methods during runs.

JAVA IN ACTION

BEN EVANS AND PETER LAWREY

The last two functions are huge topics in their own right, so in this article we will focus purely on the execution of code.

ABOUT US

Java HotSpot VM works best when it can accumulate enough statistics to make intelligent decisions about what to compile.

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BEN EVANS’ PHOTOGRAPH BY JOHN BLYTHE

This article is the first article in a two-part series about Java HotSpot VM and just-in-time (JIT) compilation.

Java HotSpot VM is the VM that Oracle acquired with the Sun acquisition, and it is the VM that forms the basis of both the Java Virtual Machine (JVM) and the open source OpenJDK. Like all VMs, Java HotSpot VM’s role is to provide an operating environment for bytecode. In practice, there are three major functions that need to

; ; Executing the instructions and computations that are requested by methods

; ; Locating, loading, and verifying new types (that is, class loading)

; ; Managing memory on behalf of application code

JIT Compilation

Java HotSpot VM is a mixed-mode VM, which means that it starts off interpreting the bytecode, but it can (on a method-by-method basis) compile code into native machine instructions for faster execution.

By passing the switch -XX:+PrintCompilation, you can see entries in the log file that show each method as it is compiled.

This compilation takes place at runtime—after the method has already been run a number of times. By waiting until the method is actually being used, Java HotSpot VM can make sophisticated decisions about how to optimize the code as it compiles the code.