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

In addition to these main abstract classes, 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.

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, the ClassVisitor and MethodVisitor classes delegate by default all the method calls they receive to an optional visitor.