I need to programmatically extract method definition/implementation from Java source file, could you recommend any handy library that I can use? Thanks.
ANTLR is a parser/generator that has grammars for Java (and other languages). It can produce an abstract syntax tree (AST) that you can navigate and manipulate as you wish.
if you look for a simple parser, i'd avise JavaParser (a much heavier implementation is also included in Eclipse)
Before going nuts with libraries, have you considered if Doclets could fit for your purposes? While meant for custom Javadoc parsing, Doclets actually get access to a lot of details about the classes themselves which can be used for whatever purpose.
Another similar not-exactly-intended possibility is the rather badly documented Annotation Processor functionality which, despite its name, can be used for plain code generation and general messing around and about within whatever you're loading through the class loaders.
来源：https://stackoverflow.com/questions/4273284/java-source-code-parser