What is "javassist"

Javassist (Java programming assistant) is a Java library providing a means to manipulate the Java bytecode of an application. In this sense Javassist provides the support for structural reflection, i.e. the ability to change the implementation of a class at run time.

Bytecode manipulation is performed at load-time through a provided class loader.

Javassist enables Java programs to define a new class at runtime and to modify a class file when the JVM loads it. Unlike other similar bytecode editors, Javassist provides two levels of API: source level and bytecode level. Using the source-level API, programmers can edit a class file without knowledge of the specifications of the Java bytecode; the whole API is designed with only the vocabulary of the Java language. Programmers can even specify inserted bytecode in the form of Java source text; Javassist compiles it on the fly. On the other hand, the bytecode-level API allows the users to directly edit a class file as other editors.