IBM User Guide for Java V7 on Windows > IBM SDK for Java

The AOT compiler

Ahead-Of-Time (AOT) compilation allows the compilation of Java™ classes into native code for subsequent executions of the same program. The AOT compiler works with the class data sharing framework.

The AOT compiler generates native code dynamically while an application runs and caches any generated AOT code in the shared data cache. Subsequent JVMs that execute the method can load and use the AOT code from the shared data cache without incurring the performance decrease experienced with JIT-compiled native code.

The AOT compiler is enabled by default, but is only active when shared classes are enabled. By default, shared classes are disabled so that no AOT activity occurs. When the AOT compiler is active, the compiler selects the methods to be AOT compiled with the primary goal of improving startup time.

Note: Because AOT code must persist over different program executions, AOT-generated code does not perform as well as JIT-generated code. AOT code usually performs better than interpreted code.

In a JVM without an AOT compiler or with the AOT compiler disabled, the JIT compiler selectively compiles frequently used methods into optimized native code. There is a time cost associated with compiling methods because the JIT compiler operates while the application is running. Because methods begin by being interpreted and most JIT compilations occur during startup, startup times can be increased.

Startup performance can be improved by using the shared AOT code to provide native code without compiling. There is a small time cost to load the AOT code for a method from the shared data cache and bind it into a running program. The time cost is low compared to the time it takes the JIT compiler to compile that method.

The -Xshareclasses option can be used to enable shared classes, which might also activate the AOT compiler if AOT is enabled.

Parent: IBM SDK for Java

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