The ASM framework is organized around the {@link org.objectweb.asm.ClassVisitor ClassVisitor} and {@link org.objectweb.asm.CodeVisitor CodeVisitor} interfaces, which allows one to visit the fields and methods of a class, including the bytecode instructions of each method.

In addition to these two main interfaces, ASM provides a {@link org.objectweb.asm.ClassReader ClassReader} class, that can parse an existing class and make a given visitor visit it. ASM also provides a {@link org.objectweb.asm.ClassWriter ClassWriter} class, which is a visitor that generates Java class files.

In order to generate a class from scratch, only the {@link org.objectweb.asm.ClassWriter 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 {@link org.objectweb.asm.ClassReader ClassReader} class to analyse the original class, a class modifier, and a {@link org.objectweb.asm.ClassWriter ClassWriter} to construct the modified class. The class modifier is just a {@link org.objectweb.asm.ClassVisitor ClassVisitor} that delegates most of the work to another {@link org.objectweb.asm.ClassVisitor 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 {@link org.objectweb.asm.ClassAdapter ClassAdapter} and {@link org.objectweb.asm.CodeAdapter CodeAdapter} classes, which implement the {@link org.objectweb.asm.ClassVisitor ClassVisitor} and {@link org.objectweb.asm.CodeVisitor CodeVisitor} 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 ASM library is only 22KB, 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)!