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See:
Description
Interface Summary | |
ClassVisitor | A visitor to visit a Java class. |
CodeVisitor | A visitor to visit the bytecode instructions of a Java method. |
Constants | Defines the JVM opcodes, access flags and array type codes. |
Class Summary | |
ClassAdapter | An empty ClassVisitor that delegates to another ClassVisitor . |
ClassReader | A Java class parser to make a ClassVisitor visit an
existing class. |
ClassWriter | A ClassVisitor that generates Java class files. |
CodeAdapter | An empty CodeVisitor that delegates to another CodeVisitor . |
CodeWriter | A CodeVisitor that generates Java bytecode instructions.
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Label | A label represents a position in the bytecode of a method. |
Type | A Java type. |
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 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 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 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)!
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