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JBL - Java Bytecode Library

JBL, as its expanded name might suggest, is a library for manipulating JVM bytecode on the fly. It is written, as one might expect, in Java. Bytecode support for other languages is planned, but currently not supported. JBL attempts to merge the things which make other bytecode libraries, namely ASM, BCEL, and SERP useful, maintaining an equally simply (and sometimes simpler!) api, while not sacrificing usability by enforcing visitor pattern usage. JBL abstracts the internal bytecode in class files to trivial notions, yet also allows the delving into direct class pool manipulation et al. It also provides structures for doing common things (like class reference remapping), and has a simple OO structure making it easy for you to define your own. Additionally, each JBL object has methods you'd logically expect it to have, making it relatively simple to do work even without constantly glancing at the JBL JavaDocs.



Speed and loading time is always essential when deciding on a library, and we are proud to inform you that JBL is //very// fast. JBL has been tested against other popular bytecode manipulation frameworks, mainly ASM, BCEL, and SERP. The benchmark consisted of an iteration of 500 class loads, and the average time of loading. The results are displayed below: take them with a grain of salt, as ASM performs in 0ms simply due to its structure only loading the constant pool of a class. JBL, BCEL, and SERP all parse the entire class in object construction.

  • JBL Time: 1ms
  • ASM Time: 0ms
  • BCEL Time: 3ms
  • SERP Time: 10ms

As is apparent, JBL performs much faster than SERP and BCEL, and almost on par with ASM's performance. The benchmarking code for this experiment can be found in src/test/java/BenchmarkTest.

Defining a ClassFile object

For you to do anything, you must first define a ClassFile object. This is the core of JBL, and can be constructed simply, in many ways. You can pass a byte[], stream, or file. Or use its public no-args constructor. Whatever floats your boat.

ClassFile streamed = new ClassFile(inputStream); //An input stream, can be anything..
ClassFile flat = new ClassFile(new File("D:\myclass.class"); //A file...
ClassFile bytes = new ClassFile(new byte[0]); //! byte array...
ClassFile noArgs = new ClassFile(); //Or even empty, if you wish to start from scratch.

Take note that JBL does minimal verification when loading files, so you may get funky behavior if you pass a truncated stream or such. Another thing to keep in mind is that JBL does a load of logic underneath each class parse, and so wrapping ClassFile instantiation in a try/catch block can be extremely negative to performance. For optimal performance, use the noargs or byte[] constructor. The other two throw IOExceptions, which by Java specification you will be forced to wrap. Use something like:

RandomAccessFile f = new RandomAccessFile(clazz, "r");
byte[] bytes = new byte[(int) f.length()];;

And feed the byte[] to a ClassFile. This can cause performance increases upwards of 700%.

Visitor pattern

Though not enforced, if you are used with ASM or BCEL, you may find this more suitable to your taste. In some cases, ASM visitors will run out of the box when their extending classes are changed to point JBL classes. JBL is packaged with two visitor interfaces: ClassVisitor and MemberVisitor. The latter is a further abstraction over both method and field structures, since they share the same format. In most cases, however, simply manipulating a ClassFile object suffices and may be cleaner than using a visitor. Consider the following example:

public void publicWithVisitor(ClassFile clazz) {
   new ClassAdapter(new ClassVisitor() {
	  public int visitAccessFlags(int flag) {
	     return flag & Modifier.PUBLIC;

public void publicWOVisitor(ClassFile clazz) {
   clazz.setAccessFlags(clazz.getAccessFlags() & Modifier.PUBLIC);

JBL also comes with an abstract Remapper class, which is simply a ClassVisitor implementation for remapping class references. Extending classes need only override remap(String), yet are free to override more specific remapSignature(String) or remapType(String), while still having the capabilities of their ClassVisitor superclass.

Please note that JBL visitors are still very much under heavy development, so the above information may come and go out of date as development continues.


JBL puts a strong emphasis on metadata, its storage, and its ease-of-access. A new, and exciting feature is the introduction of the Metadatable interface. This interface further abstracts attribute pool modification, and, when used in conjunction with XMLAttribute, allows the storage of arbitrary information, like HashMaps, Collections, or even more complex structures like TreeMaps to be stored and later recovered from class files. Both the ClassFile and Member objects implement Metadatable, and though this is still a work in progress, many parts of the interface's implementation have been finished. With JBL, storing and accessing attribute metadata becomes as simple as:

ClassFile clazz = new ClassFile();
HashMap myMap = new HashMap();
clazz.setMetadata("my_map", myMap);
//Later on in your code, or after the dumping/reinstantiation of your class
HashMap myRecoveredMap = (HashMap)clazz.getMetadata("my_map");

Noteworthy notes

This is a bulleted collection of tips and tricks on the usage of JBL, which knowing will speed up developing with JBL.

  • ClassFile and Member objects extends AccessibleMember, giving simple access to access flags via methods akin to setSynchronized(boolean) and setPublic(boolean). All this means is you do not have to handle getFlag() masks directly.
  • All JVM opcodes (currently with the exception of INVOKEDYNAMIC) reside in JBL's Opcode class. Also included are constant pool tag bytes, primitive array designations, and stack growth patterns.
  • You can define your own Pool parser, via the Pool.Parser interface. Pool also contains preset parsers for classes, including ATTRIBUTE_PARSER, CONSTANT_PARSER, and MEMBER_PARSER, among others.
  • A very rough work-in-progress code generator can be found in package tk.jblib.generation.
  • Simple test programs that will help you get the hang of JBL can be found in root package core.


JBL is licensed under the LGPL. More info can be found in LICENSE.txt.


A library for JVM bytecode modification, implemented in Java.







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