The gnu.bytecode
package contains classes to generate, read,
write, and print Java bytecode (.class
) files.
It is used by Kawa to compile Scheme into bytecodes; it should be useful for other languages that need to be compiled into Java bytecodes. (An interesting exercise would be an interactive Java expression evaluator.) The classes here are relatively low-level. If you want to use them to generate bytecode from a high-level language, it is better to use the gnu.expr package, which works at the expression level, and handles all the code-generation for you.
Javadoc generated documentation of the class is available online.
The most important class is ClassType
.
This contains information
about a single class. Note that the difference between ClassType
and java.lang.Class
is that the latter only represents existing
classes that have been loaded into the Java VM; in contrast,
ClassType
can be used to build new classes
incrementally and on the fly.
A ClassType
has a list of Field
objects;
new ones can be added using
the various addField
methods. A ClassType
manages a ConstantPool
.
A ClassType
also has a list of Method
objects;
new ones can be created by the various addMethod
objects.
A Method
contains many methods you can use to generate bytecode.
See Kawa for examples.
Once you have finished generating a ClassType
, you
can write it to a .class
file with
the writeToFile
method. You can also make a
byte array suitable for ClassLoader.defineClass
using the
writeToArray
method. This is used by Kawa to compile and immediately load a class.
You can also print out the contains of a ClassType
in
human-readable
form using the class ClassTypeWriter
. This prints a fair bit of
information of the generated class, including
dis-assembling the code of the methods.
You can also build a ClassType
by reading it from an
existing .class
file by using a ClassFileInput
class. This reads the constant
pool, the fields, methods, superclass, and interfaces.
The gnu.bytecode.dump
class has a main
method
that prints out the information in a named class file.
ZipArchive
as an application:
java gnu.bytecode.ZipArchive [txpq] archive [file ...]See the comments for
ZipArchive.main
.
ZipArchive
does not do compression or uncompressions, and
it is reported that that some programs do not like the archives it creates.
It is probably best suited for listing and extracting from classes.zip-like
archives. (This class has been partially re-written to use java.util.zip.
It may get dropped in the future.)
dump
as an application:
java gnu.bytecode.dump foo.classThis will print out the constant pool, fields, methods, superclass, and implemented interfaces of class
foo
.
It is useful for printing out detailed information about the constant
pool, which javap
does not do.
gnu.bytecode
is currently distributed as part of
Kawa, though it can be used independent
of the rest of Kawa.