[转载]PrintCompilation 参数解释

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本文详细介绍了JVM中的编译与优化机制，包括如何处理同步方法、原生方法及异常处理等，并深入探讨了当类加载导致已编译代码失效时，JVM是如何标记这些代码为不可用（notentrant）和僵尸状态（zombie），最终通过垃圾回收机制移除无效代码。

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英文不好，所以就不翻译了：

b    Blocking compiler (always set for client)
*    Generating a native wrapper
%    On stack replacement (where the compiled code is running)
!    Method has exception handlers
s    Method declared as synchronized
n    Method declared as native
made non entrant    compilation was wrong/incomplete, no future callers will use this version
made zombie         code is not in use and ready for GC

"made not entrant" and "made zombie"

Zombie methods are methods whose code has been made invalid by class loading. Generally the server compiler makes aggressive inlining decisions of non-final methods. As long as the inlined method is never overridden the code is correct. When a subclass is loaded and the method overridden, the compiled code is broken for all future calls to it. The code gets declared "not entrant" (no future callers to the broken code), but sometimes existing callers can keep using the code. In the case of inlining, that's not good enough; existing callers' stack frames are "deoptimized" when they return to the code from nested calls (or just if they are running in the code). When no more stack frames hold PC's into the broken code it's declared a "zombie" - ready for removal once the GC gets around to it.

原文地址：http://blog.joda.org/2011/08/printcompilation-jvm-flag.html