The ASM framework is organized around the ClassVisitor , FieldVisitor and MethodVisitor interfaces, which allow one to visit the fields and methods of a class, including the bytecode instructions of each method.

In addition to these main interfaces, ASM provides a ClassReader class, that can parse an existing class and make a given visitor visit it. ASM also provides a ClassWriter class, which is a visitor that generates Java class files.

In order to generate a class from scratch, only the ClassWriter class is necessary. Indeed, in order to generate a class, one must just call its visitXXX methods with the appropriate arguments to generate the desired fields and methods. See the "helloworld" example in the ASM distribution for more details about class generation.

In order to modify existing classes, one must use a ClassReader class to analyze the original class, a class modifier, and a ClassWriter to construct the modified class. The class modifier is just a ClassVisitor that delegates most of the work to another ClassVisitor , but that sometimes changes some parameter values, or call additional methods, in order to implement the desired modification process. In order to make it easier to implement such class modifiers, ASM provides the ClassAdapter and MethodAdapter classes, which implement the ClassVisitor and MethodVisitor interfaces by delegating all work to other visitors. See the "adapt" example in the ASM distribution for more details about class modification.

The size of the core ASM library, asm.jar, is only 31KB, which is much more smaller than the size of the BCEL library (350KB without the class verifier), and than the size of the SERP library (150KB). ASM is also much more faster than these tools. Indeed the overhead of a load time class transformation process is of the order of 60% with ASM, 700% or more with BCEL, and 1100% or more with SERP (see the test/perf directory in the ASM distribution)!