classfile.py is a java class file formatter / decompiler / compiler that transforms between binary java class files and a python object graph (dicts and arrays).
classfile can be used as a python module or run as a script.
usage:
classfile.py -d <classfile>
classfile.py -c <pyobjfile> <classfile>
The JVM is a great piece of technology, but I prefer not to use the Java language. classfile.py makes it possible to view and manipulate class files in python.
Info about the java class file format at:
- http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html
- http://jcp.org/aboutJava/communityprocess/final/jsr202/index.html
Round tripping should work for any class file. There are some class file attributes that are not yet supported, but when these are encountered, the attributes are written in binary format and can be read back from binary format.
Start with a Java source file or class file:
public class Example {
private static String foo = "world";
public static void main(String[] args) {
int times = 2;
for (int i = 0; i < times; i++) {
System.out.println("hello " + foo);
}
}
}
Compile and Run:
$ javac Example.java
$ java Example
hello world
hello world
Now use classfile to decompile:
$ ./classfile.py -d Example.class > Example.py
This will show the contents of the classfile as a python object:
{'01_magic': 'cafebabe',
'02_minor_version': 0,
'03_major_version': 50,
'04_constant_pool_count': 57,
'05_cp_info': [{'00_index': 0,
'00_tag_name': 'unusable at start to get numbering working',
'01_tag': 0},
{'00_index': 1,
'00_tag_name': 'CONSTANT_Class',
'01_tag': 7,
'02_name_index': 2},
{'00_index': 2,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 7,
'03_bytes': 'Example'},
{'00_index': 3,
'00_tag_name': 'CONSTANT_Class',
'01_tag': 7,
'02_name_index': 4},
{'00_index': 4,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 16,
'03_bytes': 'java/lang/Object'},
{'00_index': 5,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 3,
'03_bytes': 'foo'},
{'00_index': 6,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 18,
'03_bytes': 'Ljava/lang/String;'},
{'00_index': 7,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 8,
'03_bytes': '<clinit>'},
{'00_index': 8,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 3,
'03_bytes': '()V'},
{'00_index': 9,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 4,
'03_bytes': 'Code'},
{'00_index': 10,
'00_tag_name': 'CONSTANT_String',
'00_value': 'world',
'01_tag': 8,
'02_string_index': 11},
{'00_index': 11,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 5,
'03_bytes': 'world'},
{'00_index': 12,
'00_tag_name': 'CONSTANT_Fieldref',
'01_tag': 9,
'02_class_index': 1,
'03_name_and_type_index': 13},
{'00_index': 13,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 5,
'03_descriptor_index': 6},
{'00_index': 14,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 15,
'03_bytes': 'LineNumberTable'},
{'00_index': 15,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 18,
'03_bytes': 'LocalVariableTable'},
{'00_index': 16,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 6,
'03_bytes': '<init>'},
{'00_index': 17,
'00_tag_name': 'CONSTANT_Methodref',
'01_tag': 10,
'02_class_index': 3,
'03_name_and_type_index': 18},
{'00_index': 18,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 16,
'03_descriptor_index': 8},
{'00_index': 19,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 4,
'03_bytes': 'this'},
{'00_index': 20,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 9,
'03_bytes': 'LExample;'},
{'00_index': 21,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 4,
'03_bytes': 'main'},
{'00_index': 22,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 22,
'03_bytes': '([Ljava/lang/String;)V'},
{'00_index': 23,
'00_tag_name': 'CONSTANT_Fieldref',
'01_tag': 9,
'02_class_index': 24,
'03_name_and_type_index': 26},
{'00_index': 24,
'00_tag_name': 'CONSTANT_Class',
'01_tag': 7,
'02_name_index': 25},
{'00_index': 25,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 16,
'03_bytes': 'java/lang/System'},
{'00_index': 26,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 27,
'03_descriptor_index': 28},
{'00_index': 27,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 3,
'03_bytes': 'out'},
{'00_index': 28,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 21,
'03_bytes': 'Ljava/io/PrintStream;'},
{'00_index': 29,
'00_tag_name': 'CONSTANT_Class',
'01_tag': 7,
'02_name_index': 30},
{'00_index': 30,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 23,
'03_bytes': 'java/lang/StringBuilder'},
{'00_index': 31,
'00_tag_name': 'CONSTANT_String',
'00_value': 'hello ',
'01_tag': 8,
'02_string_index': 32},
{'00_index': 32,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 6,
'03_bytes': 'hello '},
{'00_index': 33,
'00_tag_name': 'CONSTANT_Methodref',
'01_tag': 10,
'02_class_index': 29,
'03_name_and_type_index': 34},
{'00_index': 34,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 16,
'03_descriptor_index': 35},
{'00_index': 35,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 21,
'03_bytes': '(Ljava/lang/String;)V'},
{'00_index': 36,
'00_tag_name': 'CONSTANT_Methodref',
'01_tag': 10,
'02_class_index': 29,
'03_name_and_type_index': 37},
{'00_index': 37,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 38,
'03_descriptor_index': 39},
{'00_index': 38,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 6,
'03_bytes': 'append'},
{'00_index': 39,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 45,
'03_bytes': '(Ljava/lang/String;)Ljava/lang/StringBuilder;'},
{'00_index': 40,
'00_tag_name': 'CONSTANT_Methodref',
'01_tag': 10,
'02_class_index': 29,
'03_name_and_type_index': 41},
{'00_index': 41,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 42,
'03_descriptor_index': 43},
{'00_index': 42,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 8,
'03_bytes': 'toString'},
{'00_index': 43,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 20,
'03_bytes': '()Ljava/lang/String;'},
{'00_index': 44,
'00_tag_name': 'CONSTANT_Methodref',
'01_tag': 10,
'02_class_index': 45,
'03_name_and_type_index': 47},
{'00_index': 45,
'00_tag_name': 'CONSTANT_Class',
'01_tag': 7,
'02_name_index': 46},
{'00_index': 46,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 19,
'03_bytes': 'java/io/PrintStream'},
{'00_index': 47,
'00_tag_name': 'CONSTANT_NameAndType',
'01_tag': 12,
'02_name_index': 48,
'03_descriptor_index': 35},
{'00_index': 48,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 7,
'03_bytes': 'println'},
{'00_index': 49,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 4,
'03_bytes': 'args'},
{'00_index': 50,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 19,
'03_bytes': '[Ljava/lang/String;'},
{'00_index': 51,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 5,
'03_bytes': 'times'},
{'00_index': 52,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 1,
'03_bytes': 'I'},
{'00_index': 53,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 1,
'03_bytes': 'i'},
{'00_index': 54,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 13,
'03_bytes': 'StackMapTable'},
{'00_index': 55,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 10,
'03_bytes': 'SourceFile'},
{'00_index': 56,
'00_tag_name': 'CONSTANT_Utf8',
'01_tag': 1,
'02_length': 12,
'03_bytes': 'Example.java'}],
'06_access_flags': 33,
'07_this_class': 1,
'08_super_class': 3,
'09_interfaces_count': 0,
'10_interfaces': (),
'11_fields_count': 1,
'12_field_info': [{'00_field_name': 'foo',
'01_access_flags': 10,
'02_name_index': 5,
'03_descriptor_index': 6,
'04_attributes_count': 0,
'05_attribute_info': []}],
'13_methods_count': 3,
'14_method_info': [{'00_method_name': '<clinit>()V',
'01_access_flags': 8,
'02_name_index': 7,
'03_descriptor_index': 8,
'04_attributes_count': 1,
'05_attribute_info': [{'00_attribute_name': 'Code',
'01_attribute_name_index': 9,
'02_attribute_length': 42,
'03_max_stack': 1,
'04_max_locals': 0,
'05_code_length': 6,
'06_code': [' 0: 2:ldc 10 #',
' 2: ?:putstatic 12 # fooLjava/lang/String;',
' 5: 1:return #'],
'07_exception_table_length': 0,
'08_exception_table': [],
'09_attributes_count': 2,
'10_attribute_info': [{'00_attribute_name': 'LineNumberTable',
'01_attribute_name_index': 14,
'02_attribute_length': 10,
'03_line_number_table_length': 2,
'04_line_number_table': [{'01_start_pc': 0,
'02_line_number': 2},
{'01_start_pc': 5,
'02_line_number': 1}]},
{'00_attribute_name': 'LocalVariableTable',
'01_attribute_name_index': 15,
'02_attribute_length': 2,
'03_local_variable_table_length': 0,
'04_local_variable_table': []}]}]},
{'00_method_name': '<init>()V',
'01_access_flags': 1,
'02_name_index': 16,
'03_descriptor_index': 8,
'04_attributes_count': 1,
'05_attribute_info': [{'00_attribute_name': 'Code',
'01_attribute_name_index': 9,
'02_attribute_length': 47,
'03_max_stack': 1,
'04_max_locals': 1,
'05_code_length': 5,
'06_code': [' 0: 1:aload_0 #',
' 1: ?:invokespecial 17 # <init>()V',
' 4: ?:return #'],
'07_exception_table_length': 0,
'08_exception_table': [],
'09_attributes_count': 2,
'10_attribute_info': [{'00_attribute_name': 'LineNumberTable',
'01_attribute_name_index': 14,
'02_attribute_length': 6,
'03_line_number_table_length': 1,
'04_line_number_table': [{'01_start_pc': 0,
'02_line_number': 1}]},
{'00_attribute_name': 'LocalVariableTable',
'01_attribute_name_index': 15,
'02_attribute_length': 12,
'03_local_variable_table_length': 1,
'04_local_variable_table': [{'00_name': 'LExample; this',
'01_start_pc': 0,
'02_length': 5,
'03_name_index': 19,
'04_descriptor_index': 20,
'05_index': 0}]}]}]},
{'00_method_name': 'main([Ljava/lang/String;)V',
'01_access_flags': 9,
'02_name_index': 21,
'03_descriptor_index': 22,
'04_attributes_count': 1,
'05_attribute_info': [{'00_attribute_name': 'Code',
'01_attribute_name_index': 9,
'02_attribute_length': 132,
'03_max_stack': 4,
'04_max_locals': 3,
'05_code_length': 40,
'06_code': [' 0: 4:iconst_2 #',
' 1: ?:istore_1 #',
' 2: 5:iconst_0 #',
' 3: ?:istore_2 #',
' 4: ?:goto 30 #',
' 7: 6:getstatic 23 # outLjava/io/PrintStream;',
' 10: ?:new 29 # java/lang/StringBuilder',
' 13: ?:dup #',
' 14: ?:ldc 31 #',
' 16: ?:invokespecial 33 # <init>(Ljava/lang/String;)V',
' 19: ?:getstatic 12 # fooLjava/lang/String;',
' 22: ?:invokevirtual 36 # append(Ljava/lang/String;)Ljava/lang/StringBuilder;',
' 25: ?:invokevirtual 40 # toString()Ljava/lang/String;',
' 28: ?:invokevirtual 44 # println(Ljava/lang/String;)V',
' 31: 5:iinc 2 1 #',
' 34: ?:iload_2 #',
' 35: ?:iload_1 #',
' 36: ?:if_icmplt -29 #',
' 39: 8:return #'],
'07_exception_table_length': 0,
'08_exception_table': [],
'09_attributes_count': 3,
'10_attribute_info': [{'00_attribute_name': 'LineNumberTable',
'01_attribute_name_index': 14,
'02_attribute_length': 22,
'03_line_number_table_length': 5,
'04_line_number_table': [{'01_start_pc': 0,
'02_line_number': 4},
{'01_start_pc': 2,
'02_line_number': 5},
{'01_start_pc': 7,
'02_line_number': 6},
{'01_start_pc': 31,
'02_line_number': 5},
{'01_start_pc': 39,
'02_line_number': 8}]},
{'00_attribute_name': 'LocalVariableTable',
'01_attribute_name_index': 15,
'02_attribute_length': 32,
'03_local_variable_table_length': 3,
'04_local_variable_table': [{'00_name': '[Ljava/lang/String; args',
'01_start_pc': 0,
'02_length': 40,
'03_name_index': 49,
'04_descriptor_index': 50,
'05_index': 0},
{'00_name': 'I times',
'01_start_pc': 2,
'02_length': 38,
'03_name_index': 51,
'04_descriptor_index': 52,
'05_index': 1},
{'00_name': 'I i',
'01_start_pc': 4,
'02_length': 35,
'03_name_index': 53,
'04_descriptor_index': 52,
'05_index': 2}]},
{'00_attribute_name': 'StackMapTable',
'01_attribute_name_index': 54,
'02_attribute_length': 8,
'03_info': '\x00\x02\xfd\x00\x07\x01\x01\x1a'}]}]}],
'15_attributes_count': 1,
'16_attribute_info': [{'00_attribute_name': 'SourceFile',
'00_sourcefile': 'Example.java',
'01_attribute_name_index': 55,
'02_attribute_length': 2,
'03_sourcefile_index': 56}]}
This can be modified by using a text editor, or by using python code and then converted back to a class file.
This python code will decompile, modify the code to print 'hello world' 3 times, then write back the classfile.
import classfile
cf = classfile.decompile(open('Example.class').read())
m = [m for m in cf['14_method_info'] if m['00_method_name'].startswith('main')][0]
c = [a for a in m['05_attribute_info'] if a['00_attribute_name'] == 'Code'][0]['06_code']
c[0] = c[0].replace('iconst_2', 'iconst_3')
classfile.compile(cf, 'Example.class')
Now when we run the class:
$ java Example
hello world
hello world
hello world
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