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Provides a small and fast bytecode manipulation framework.
The ASM framework is organized
around the {@link org.objectweb.asm.ClassVisitor ClassVisitor},
{@link org.objectweb.asm.FieldVisitor FieldVisitor},
{@link org.objectweb.asm.MethodVisitor MethodVisitor} and
{@link org.objectweb.asm.AnnotationVisitor 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 {@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 analyze
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, the {@link org.objectweb.asm.ClassVisitor
ClassVisitor} and {@link org.objectweb.asm.MethodVisitor 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