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Core Java® Volume II—Advanced Features, Ninth Edition by Gary Cornell, Cay S. Horstmann

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10.7. Bytecode Engineering

You have seen how annotations can be processed at runtime or at the source code level. There is a third possibility: processing at the bytecode level. Unless annotations are removed at the source level, they are present in the class files. The class file format is documented (see http://docs.oracle.com/javase/specs/jvms/se7/html). The format is rather complex, and it would be challenging to process class files without special libraries. One such library is the Bytecode Engineering Library (BCEL), available at http://jakarta.apache.org/bcel.

In this section, we use BCEL to add logging messages to annotated methods. If a method is annotated with

@LogEntry(logger=loggerName)

then we add the bytecodes for the following ...

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