org.objectweb.asm (ASM 5.0.4 Documentation)
Date: 2015-09-18

# Package org.objectweb.asm

Provides a small and fast bytecode manipulation framework.

See: Description

- Interface Summary
  Interface | Description
  --- | ---
  Opcodes | Defines the JVM opcodes, access flags and array type codes.

- Class Summary
  Class | Description
  --- | ---
  AnnotationVisitor | A visitor to visit a Java annotation.
  Attribute | A non standard class, field, method or code attribute.
  ByteVector | A dynamically extensible vector of bytes.
  ClassReader | A Java class parser to make a `ClassVisitor` visit an existing class.
  ClassVisitor | A visitor to visit a Java class.
  ClassWriter | A `ClassVisitor` that generates classes in bytecode form.
  FieldVisitor | A visitor to visit a Java field.
  Handle | A reference to a field or a method.
  Label | A label represents a position in the bytecode of a method.
  MethodVisitor | A visitor to visit a Java method.
  Type | A Java field or method type.
  TypePath | The path to a type argument, wildcard bound, array element type, or static inner type within an enclosing type.
  TypeReference | A reference to a type appearing in a class, field or method declaration, or on an instruction.

## Package org.objectweb.asm Description

Provides a small and fast bytecode manipulation framework.

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. 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, the `ClassVisitor` and `MethodVisitor` classes delegate by default all the method calls they receive to an optional visitor. 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 45KB, which is much smaller than the size of the BCEL library (504KB), and than the size of the SERP library (150KB). ASM is also much 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)!

Since:
  ASM 1.3