Provides a small and fast bytecode manipulation framework.
Provides a small and fast bytecode manipulation framework.
The
ASM framework is organized
around the
ClassVisitor and
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
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 visit
XXX
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 analyse
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
CodeAdapter
classes, which implement the
ClassVisitor and
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 core ASM library,
asm.jar, is only 25KB, 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)! kASM is a subset of ASM, for class generation only (it includes
the
ClassWriter, but not the
ClassReader). The size of this library,
kasm.jar, is only 16KB.