Javassist (Java programming assistant) is a Java library providing a means to manipulate the Java bytecode of an application.[1] In this sense Javassist provides the support for structural reflection, i.e. the ability to change the implementation of a class at run time.

Developer(s)Shigeru Chiba (programmer)
Stable release
3.25.0-GA / April 16, 2019 (2019-04-16)
Written inJava
Operating systemCross-platform
TypeBytecode Engineering Library
Licensedual license of the MPL and LGPL.

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.


Javassist can be used for the following:

  • For specifying the bytecode using source code – can compile a fragment of source text online (e.g., just a single statement)
  • For aspect-oriented programming (AOP) – can introduce new methods into a class and insert before/after/around advice at both the caller and callee sides
  • For reflection at runtime – can use a metaobject that controls method calls on base-level objects
  • For remote method invocation – can call a method on a remote object running on a web server, an alternative to Java RMI that does not need a stub compiler such as rmic

See also


  1. "Javassist". JBoss Community.

Official website

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