neuron303 / krakatau Goto Github PK

Krakatau Bytecode Tools
Copyright (C) 2012-13 Robert Grosse

=== Introduction ===

Krakatau currently contains three tools - a decompiler and disassembler for
Java classfiles and an assembler to create classfiles.

=== Requirements ===

Krakatau requires Python 2.7. The assembler additionally requires a free
Python library called PLY. Note that if you want to do decompilation, you'll
probably want an installation of the JDK as well. For assembly and disassembly,
a Java installation is not strictly necessary, but it is still useful for
testing the resulting classes.

=== Decompilation ===

Usage:
python Krakatau\decompile.py [-nauto] [-path PATH] [-out OUT] [-r] target

PATH : An optional list of directories, jars, or zipfiles to search for
    classes in. Krakatau will attempt to automatically detect and add the
    jar containing core language classes, but you can disable this with
    the -nauto option.

OUT : Directory name where source files are to be written. Defaults to the
    current directory

-r : Decompiles all .class files found in the directory target (recursively)

target : Class name or jar name to decompile. If a jar is specified, all
    classes in the jar will be decompiled. If -r is specified, this should
    be a directory.

The Krakatau decompiler takes a different approach to most Java decompilers.
It can be thought of more as a compiler whose input language is Java bytecode
and whose target language happens to be Java source code. Krakatau takes in
arbitrary bytecode, and attempts to transform it to equivalent Java code. This
makes it robust to minor obfuscation, though it has the drawback of not
reconstructing the "original" source, leading to less readable output than a
pattern matching decompiler would produce for unobfuscated Java classes.

However, it will not always produce valid Java since there are some things
that are difficult or impossible to decompile at all thanks to the limitations
of the Java language. In most cases, Krakatau will try to at least produce
readable pseudocode, but sometimes it may just throw an exception.

Note: decompilation produces a file called "cache.txt" in the current
    directory. This stores information about the standard library so it
    doesn't have to be loaded every time. However, if you change the library,
    you should delete the cache.

Warning: Output on Windows uses UNC-style paths, which means that depending on
    the input class name, it may create files which are difficult or impossible
    to access through Windows Explorer or other non-UNC aware tools.

=== Assembly ===

Usage:
python Krakatau\assemble.py [-out OUT] [-g] [-jas] [-r] [-q] target

OUT : Directory name where class files are to be written. Defaults to the
    current directory

-g : If specified, SourceFile and LineNumberTable attributes giving the
    offset of every instruction will be added. This is useful for debugging
    exceptions, as it will let the JVM print the origin of the exception
    in the traceback. If these attributes are already specified manually,
    they will not be overridden.

-jas : Enables incompatible Jasmin quirks. Specifying 'all' as the exception
    handler type will implicitly catch all exceptions rather than catching a
    class named all. The ACC_SUPER flag is added to classes even if not
    specified. A SourceFile attribute based on the filename is added if no
    SourceFile attribute is specified. The classfile version defaults to
    45.3 instead of 49.0.

-r : Assembles all .j files found in the directory target (recursively)

-q : Quiet mode (only display warnings and errors)

target : Name of file to assemble. If -r is specified, this should be a
    directory.

The Krakatau assembler is intended as a replacement for Jasmin, and was
originally written due to the limitations of Jasmin. It is mostly backwards
compatible with Jasmin's syntax, though not necessarily with the extensions
introduced in JasminXT. However, Krakatau offers many new features, most
importantly the ability to directly specify constant pool references.

A tutorial on writing Java Bytecode Assembly with Krakatau is available at
https://greyhat.gatech.edu/wiki/index.php?title=Java_Assembly_Tutorial.

The assembler uses PLY, which will generate a file called "parsetab.py" in
the current directory.

=== Disassembly ===

Usage:
python Krakatau\disassemble.py [-out OUT] [-r] target

OUT : File or directory name where source files are to be written. Defaults
    to the current directory

-r : Disassembles all .class files found in the directory target (recursively)

target : Filename or jar name to disassemble. If a jar is specified, all
    classes in the jar will be disassembled. If -r is specified, this should
    be a directory.

This takes a classfile and converts it into a human readable assembly format.
Unlike Javap, this can handle even pathological classes, and the output can
be reassembled. Together with the Krakatau assembler, this tool should be
able to roundtrip any class through assembly and back into an equivalent
class with the following caveats:

* The exact format and ordering of attributes may not be preserved. The
    disassembler does not attempt to disassemble LineNumberTable,
    LocalVariableTable, or LocalVariableTypeTable attributes.

* Identical constant pool entries with distinct indexes aren't preserved.
    Normally this doesn't matter, but it can affect verification of
    InnerClasses attributes in obscure cases

=== Performance ===

You can disable the internal debugging checks by passing -O to Python. This
will make Krakatau slightly faster, so it is recommended for normal usage.

=== Java 8 ===

Java 8 is not fully supported yet. In particular, decompilation of lambdas
is not currently supported.