IBM User Guide for Java V7 on Windows > IBM SDK for Java > The JIT compiler

Frequently asked questions about the JIT compliler

Examples of subjects that have answers in this section include disabling the JIT compiler, use of alternative JIT compilers, control of JIT compilation and dynamic control of the JIT compiler.

Can I disable the JIT compiler?

Yes. The JIT compiler is turned on by default, but you can set the appropriate command-line parameter to disable it. (See Disabling the JIT or AOT compiler.)

Can I use another vendor's JIT compiler?

No.

Can I use any version of the JIT compiler with the JVM?

No. The two are tightly coupled. You must use the version of the JIT compiler that comes with the JVM package that you use.

Can the JIT compiler "decompile" methods?

No. After a method has been compiled by the JIT compiler, the native code is used instead for the remainder of the execution of the program. An exception to this rule is a method in a class that was loaded with a custom (user-written) class loader, which has since been unloaded (garbage-collected). In fact, when a class loader is garbage-collected, the compiled methods of all classes loaded by that class loader are discarded.

Can I control the JIT compilation?

Yes. See JIT and AOT problem determination. In addition, advanced diagnostic settings are available to IBM engineers.

Can I dynamically control the JIT compiler?

No. You can pass options to the JIT compiler to modify its behavior, but only at JVM startup time, because the JIT compiler is started up at the same time as the JVM. However, a Java™ program can use the java.lang.Compiler API to enable and disable the JIT compiler at run time.

How much memory does the code cache consume?

The JIT compiler uses memory intelligently. When the code cache is initialized, it consumes relatively little memory. As more methods are compiled into native code, the code cache is grown dynamically to accommodate the needs of the program. Space previously occupied by discarded or recompiled methods is reclaimed and reused. When the size of the code cache reaches a predefined upper limit, it stops growing. The JIT compiler will stop all future attempts to compile methods, to avoid exhausting the system memory and affecting the stability of the application or the operating system.

Parent: The JIT compiler

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