Home – Apache Commons BCEL
Apache Commons BCEL ™
Last Published: 18 Jan 2026
Version: 6.12.0
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Commons BCEL
The Byte Code Engineering Library (Apache Commons BCEL™) is intended to give users a
convenient way to analyze, create, and manipulate (binary)
Java class files (those ending with .class). Classes are
represented by objects which contain all the symbolic information
of the given class: methods, fields and byte code instructions, in
particular.
Such objects can be read from an existing file, be transformed
by a program (e.g. a class loader at run-time) and written to a file again.
An even more interesting application is the creation of classes from scratch
at run-time. The Byte Code Engineering Library (BCEL) may be also useful
if you want to learn about the Java Virtual Machine (JVM) and the format of
Java .class files.
BCEL contains a byte code verifier named JustIce, which usually
gives you much better information about what's wrong with your
code than the standard JVM message.
BCEL is already being used successfully in several projects such
as compilers, optimizers, obsfuscators, code generators
and analysis tools. Unfortunately there hasn't been much development
going on over the past few years. Feel free to help out or you
might want to have a look into the ASM project at objectweb.
Documentation
The package descriptions in the
Javadoc
give an overview of the available features
and various
project reports
are provided.
The
source repository
can be
browsed
, or you can browse/contribute via
GitHub
Release Information
The latest stable release of BCEL is here, you may:
Read the
release notes
Inspect the
extended Clirr report
comparing 5.2 with 6.x
Alternatively, you can pull it from the central Maven repositories through a
dependency
Getting Involved
The
commons developer mailing list
is the main channel of communication for contributors. Please remember that the lists are shared between all commons components, so prefix your email by [bcel].
You can also peruse
JIRA
Alternatively you can go through the
Needs Work
tags in the
TagList report
If you'd like to offer up pull requests via GitHub rather than applying patches to JIRA, we have a
GitHub mirror
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