JAVA BYTECODE STRUCTURE

Each class file contains one Java type, either a class or an interface.
A class file consists of a stream of 8-bit bytes.
Multibyte data items are always stored in big-endian order, where the high bytes come first.
A class file contains a single ClassFile structure:

ClassFile {

u4 magic;

u2 minor_version;

u2 major_version;

u2 constant_pool_count;

cp_info constant_pool[constant_pool_count-1];

u2 access_flags;

u2 this_class;

u2 super_class;

u2 interfaces_count;

u2 interfaces[interfaces_count];

u2 fields_count;

field_info fields[fields_count];

u2 methods_count;

method_info methods[methods_count];

u2 attributes_count;

attribute_info attributes[attributes_count];

}

SourceFile Attribute

Attributes are used in the ClassFile, field_info, method_info, and Code_attribute structures.
All attributes have the following general format:

attribute_info {

u2 attribute_name_index;

u4 attribute_length;

u1 info[attribute_length];

}

attribute_name_index is an index to a CONSTANT_Utf8.
The only attribute used in the ClassFile is the SourceFile attribute, in version 1.0.2.
The SourceFile attribute has the format:

SourceFile_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 sourcefile_index;

}

Examples

Example 1: .java & .class -- without interfaces[ ], fields[ ], methods[ ].
Example 2: .java & .class -- with interfaces[ ], fields[ ], methods[ ].
Example 3: .class of java.lang.Object.

Constant Pool

constant_pool[0], is reserved for internal use by a Java Virtual Machine implementation. That entry is not present in the class file.
All constant_pool table entries have the following general format:

cp_info {

u1 tag;

u1 info[];

}

The valid tags and their values are:

Constant Type	Value
CONSTANT_Utf8	1
CONSTANT_Integer	3
CONSTANT_Float	4
CONSTANT_Long	5
CONSTANT_Double	6
CONSTANT_Class	7
CONSTANT_String	8
CONSTANT_Fieldref	9
CONSTANT_Methodref	10
CONSTANT_InterfaceMethodref	11
CONSTANT_NameAndType	12

CONSTANT_Integer & CONSTANT_Float

To represent four-byte numeric (int and float) constants:

CONSTANT_Integer_info {

u1 tag;

u4 bytes;

}

CONSTANT_Float_info {

u1 tag;

u4 bytes;

}

The bytes item of the CONSTANT_Float is in IEEE 754 floating-point "single format" bit layout.

CONSTANT_Long & CONSTANT_Double

To represent eight-byte numeric (long and double) constants:

CONSTANT_Long_info {

u1 tag;

u4 high_bytes;

u4 low_bytes;

}

CONSTANT_Double_info {

u1 tag;

u4 high_bytes;

u4 low_bytes;

}

All eight-byte constants take up two entries in the constant_pool table of the class file, as well as in the in-memory version of the constant pool that is constructed when a class file is read.
The high_bytes and low_bytes items of the CONSTANT_Double are in IEEE 754 floating-point "double format" bit layout.

CONSTANT_Utf8

To represent constant string values:

CONSTANT_Utf8_info {

u1 tag;

u2 length;

u1 bytes[length];

}

The strings in the CONSTANT_Utf8 are not null-terminated.
Utf character

'\u0001' to '\u007F'

bits 6-0

'\u0000' and '\u0080' to '\u07FF'

1	1	0	bits 10-6
1	0	bits 5-0

'\u0800' to '\uFFFF'

1	1	1	0	bits 15-12
1	0	bits 11-6
1	0	bits 5-0

CONSTANT_String

To represent constant objects of the type java.lang.String:

CONSTANT_String_info {

u1 tag;

u2 string_index;

}

string_index is an index to a CONSTANT_Utf8.

CONSTANT_Class

To represent a class or an interface:

CONSTANT_Class_info {

u1 tag;

u2 name_index;

}

name_index is an index to a CONSTANT_Utf8 representing a fully qualified Java class name.
For array classes, the name of the class is the descriptor of the array type.

CONSTANT_NameAndType

To represent a field or method, without indicating which class or interface type it belongs to:

CONSTANT_NameAndType_info {

u1 tag;

u2 name_index;

u2 descriptor_index;

}

name_index and descriptor_index are indices to CONSTANT_Utf8.

Signature

Field descriptors

Base type

void -- V
boolean -- Z
byte, char -- B, C
short, int, long -- S, I, J
float, double -- F, D

Object type

java.lang.Object -- Ljava/lang/Object;

Array type

array of float -- [F
array of array of double -- [[D
array of Object -- [Ljava/lang/Object;

Method descriptors

Object method1(int i, double d, Thread t) -- (IDLjava/lang/Thread;)Ljava/lang/Object;
void method2(short s) -- (S)V
boolean method3() -- ()Z

CONSTNAT_Fieldref, CONSTANT_Methodref, & CONSTANT_InterfaceMethodref

To represent fields, methods, and interface methods:

CONSTANT_Fieldref_info {

u1 tag;

u2 class_index;

u2 name_and_type_index;

}

CONSTANT_Methodref_info {

u1 tag;

u2 class_index;

u2 name_and_type_index;

}

CONSTANT_InterfaceMethodref_info {

u1 tag;

u2 class_index;

u2 name_and_type_index;

}

class_index is an index to a CONSTANT_Class.
name_and_type_index is an index to a CONSTANT_NameAndType.

Examples

Example 1: .java & .class -- for Constant Pool.
Example 2: .java & .class -- for signatures.

Fields

Each field is described by a variable-length field_info structure:

field_info {

u2 access_flags;

u2 name_index;

u2 descriptor_index;

u2 attributes_count;

attribute_info attributes[attributes_count];

}

name_index and descriptor_index are indices to CONSTANT_Utf8.
The only attribute used in the field_info is the ConstantValue attribute, in version 1.0.2.

ConstantValue Attribute

To represent the value of a ??? static ??? constant field.
The ConstantValue attribute has the format:

ConstantValue_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 constantvalue_index;

}

Example

Example 1: .java & .class

Methods

Each method is described by a variable-length method_info:

method_info {

u2 access_flags;

u2 name_index;

u2 descriptor_index;

u2 attributes_count;

attribute_info attributes[attributes_count];

}

name_index and descriptor_index are indices to CONSTANT_Utf8.
special initialization method names

<init> -- instance object initializer, i.e., constructor.
<clinit> -- class (interface) object initializer (with no arguments)

The only attributes used in the method_info are the Code and Exceptions attributes, in version 1.0.2.

Exceptions Attribute

The Exceptions attribute has the format:

Exceptions_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 number_of_exceptions;

u2 exception_index_table[number_of_exceptions];

}

To indicate which checked exceptions a method may throw.
exception_index_table[] are indices to CONSTANT_Class.
For ver 1.0.2, the above requirements are not currently enforced by the Java Virtual Machine; they are only enforced at compile time.

Example

Example 1: .java & .class

Code Attribute

Containing the Java Virtual Machine instructions and auxiliary information for a single method.
The Code attribute has the format:

Code_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 max_stack;

u2 max_locals;

u4 code_length;

u1 code[code_length];

u2 exception_table_length;

{ u2 start_pc;

u2 end_pc;

u2 handler_pc;

u2 catch_type;

} exception_table[exception_table_length];

u2 attributes_count;

attribute_info attributes[attributes_count];

}

For ver. 1.0.2, the LineNumberTable and LocalVariableTable attributes, both of which contain debugging information, are defined and used with the Code attribute.

LineNumberTable Attribute

LineNumberTable attribute is generated by javac defaultly. It can be omitted with argument "-O".
An optional variable-length attribute:

LineNumberTable_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 line_number_table_length;

{ u2 start_pc;

u2 line_number;

} line_number_table[line_number_table_length];

}

LocalVariableTable Attribute

LocalVariableTable attribute can be generated by javac with argument "-g".
An optional variable-length attribute:

LocalVariableTable_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 local_variable_table_length;

{ u2 start_pc;

u2 length;

u2 name_index;

u2 descriptor_index;

u2 index;

} local_variable_table[local_variable_table_length];

}

The scope of the given local variable is interval [start_pc, start_pc+length].
name_index and descriptor_index are indices to CONSTANT_Utf8.
The given local variable must be at index in its method's local variables. If the local variable at index is a two-word type (double or long), it occupies both index and index+1.

Examples

Example 1: .java & CodeAttribute -- instance method, with arguments, with local variables.
Example 2: .java & CodeAttribute -- static method, without argument, without local variables.
Example 3: .java & CodeAttribute -- shared local variables.

Instructions

Notations of "operand"

cp1, cp2 -- 1-byte, 2-bytes constant pool index
lv1, lv2 -- 1-byte, 2-bytes local variable index
int1, int2 -- 1-byte, 2-byte signed integer
+int1 -- 1-byte unsigned integer
offset2, offset4 -- 2-bytes, 4-bytes offset
flag1 -- 1-byte flag

Notations of "pop" and "push"

- : no effect on stack
I, L, F, D : integer, long, float, double
A, O, R : address [array or object], object only, return address (for jsr)
X : anything
[X] : address of array
1, 2, 3, 4 represent any 1-word stack item.

instructions

name	length	opcode	operand	pop	push	comment
nop	1	00		-	-	nop
getstatic	3	B2	cp2	-	X	get static variable
putstatic	3	B3	cp2	X	-	put static variable
getfield	3	B4	cp2	A	X	get instance variable
putfield	3	B5	cp2	AX	-	put instance variable
iload, lload fload, dload, aload	2	15, 16 17~19	lv1	-	X	load local variable (integer, long, float, double, object)
istore, lstore fstore, dstore, astore	2	36, 37 38~3A	lv1	X	-	store local variable
wide	0	C4				prefix of 2-bytes local variable index instruction
iload_0, iload_1 iload_2, iload_3 lload_0, lload_1 lload_2, lload_3 fload_0, fload_1 fload_2, fload_3 dload_0, dload_1 dload_2, dload_3 aload_0, aload_1 aload_2, aload_3	1	1A, 1B 1C, 1D 1E, 1F 20, 21 22, 23 24, 25 26, 27 28, 29 2A, 2B 2C, 2D		-	X	load local variables #0, #1, #2, #3
istore_0, istore_1 istore_2, istore_3 lstore_0, lstore_1 lstore_2, lstore_3 fstore_0, fstore_1 fstore_2, fstore_3 dstore_0, dstore_1 dstore_2, dstore_3 astore_0, astore_1 astore_2, astore_3	1	3B, 3C 3D, 3E 3F, 40 41, 42 43, 44 45, 46 47, 48 49, 4A 4B, 4C 4D, 4E		X	-	load local variables #0, #1, #2, #3
iaload, laload faload, daload aaload	1	2E, 2F 30, 31 32		[X]I	X	load from array
baload, caload saload	1	33, 34 35		[X]I	I	load from array (byte, char, short)
iastore, lastore fastore, dastore aastore	1	4F, 50 51, 52 53		[X]IX	-	store into array
bastore, castore sastore	1	54, 55 56		[X]II	-	store into array
aconst_null iconst_m1 iconst_0, iconst_1 iconst_2, iconst_3 iconst_4, iconst_5 lconst_0, lconst_1 fconst_0, fconst_1 fconst_2 dconst_0, dconst_1	1	01 02 03, 04 05, 06 07, 08 09, 0A 0B, 0C 0D 0E, 0F		-	X	push constant
bipush	2	10	int1	-	I	push 1-byte value
sipush	3	11	int2	-	I	push 2-bytes value
ldc	2	12	cp1	-	X	load from constant pool (1-word data)
ldc_w	3	13	cp2	-	X	load from constant pool (1-word data)
ldc2_w	3	14	cp2	-	X	load from constant pool (2-word data)
pop	1	57		1	-	pop top element
pop2	1	58		21	-	pop top two elements
dup	1	59		1	11	duplicate top element
dup_x1	1	5A		21	121	duplicate top element; put 2 down
dup_x2	1	5B		321	1321	duplicate top element; put 3 down
dup2	1	5C		21	2121	duplicate top two elements
dup2_x1	1	5D		321	21321	duplicate top two elements; put 3 down
dup2_x2	1	5E		4321	214321	duplicate top two elements; put 4 down
swap	1	5F		21	12	swap top two elements of stack
i2l, i2f, i2d l2i, l2f, l2d f2i, f2l, f2d d2i, d2l, d2f	1	85~87 88~8A 8B~8D 8E~90		X	X	type conversion
i2b, i2c, i2s	1	91~93		I	I	type conversion
iadd, ladd fadd, dadd	1	60, 61 62, 63		XX	X	addition
isub, lsub fsub, dsub	1	64, 65 66, 67		XX	X	subtraction
imul, lmul fmul, dmul	1	68, 69 6A, 6B		XX	X	multiplication
idiv, ldiv fdiv, ddiv	1	6C, 6D 6E, 6F		XX	X	division
irem, lrem frem, drem	1	70, 71 72, 73		XX	X	modulo
ineg, lneg fneg, dneg	1	74, 75 76, 77		X	X	negation
ishl, lshl	1	78, 79		XI	X	shift left
ishr, lshr	1	7A, 7B		XI	X	shift right
iushr, lushr	1	7C, 7D		XI	X	unsigned shift right
iand, land	1	7E, 7F		XX	X	boolean and
ior, lor	1	80, 81		XX	X	boolean or
ixor, lxor	1	82, 83		XX	X	boolean xor
iinc	3	84	lv1 int1	-	-	increment local variable by constant
goto	3	A7	offset2	-	-	unconditional goto
goto_w	5	C8	offset4	-	-	unconditional goto
ifeq, ifne, iflt ifge, ifgt, ifle	3	99~9B 9C~9E	offset2	I	-	conditional goto (w.r.t. 0)
if_icmpeq, if_icmpne if_icmplt, if_icmpge if_icmpgt, if_icmple	3	9F, A0 A1, A2 A3, A4	offset2	II	-	conditional goto (compare top two elements)
lcmp	1	94		LL	I	long compare
fcmpl	1	95		FF	I	float compare. -1 on incomparable
fcmpg	1	96		FF	I	float compare. 1 on incomparable
dcmpl	1	97		DD	I	double compare. -1 on incomparable
dcmpg	1	98		DD	I	double compare. 1 on incomparable
ifnull, ifnonnull	3	C6, C7	offset2	A	-	conditional goto (w.r.t. null)
if_acmpeq if_acmpne	3	A5 A6	offset2	AA	-	conditional goto (compare top two elements)
tableswitch	?	AA	?	I	-	goto (switch-case)
lookupswitch	?	AB	?	I	-	goto (switch-case)
invokevirtual	3	B6	cp2	AXX	-	call an instance method (assuming: 2 arguements)
invokespecial	3	B7	cp2	AXX	-	call <init>, private method, method of superclass (assuming: 2 arguements)
invokestatic	3	B8	cp2	XX	-	call a static method (assuming: 2 arguements)
invokeinterface	5	B9	cp2 +int1 0	AXX	-	call an interface method (assuming: 2 arguements)
ireturn, lreturn freturn, dreturn areturn	1	AC, AD AE, AF B0		X	-	return value from procedure
return	1	B1		-	-	return (void) from procedure
new	3	BB	cp2	-	A	create a new object
checkcast	3	C0	cp2	A	A	throw error if object not of given type
instanceof	3	C1	cp2	A	I	is object of given type?
newarray	2	BC	flag1	I	A	create a new array of non-objects
anewarray	3	BD	cp2	I	A	create a new array of objects
multianewarray	4	C5	cp2 +int1	II	A	create multidimensional array (assuming: 2-d)
arraylength	1	BE		[X]	I	get length of array
jsr	3	A8	offset2	-	R	jump subroutine
jsr_w	5	C9	offset4	-	R	jump subroutine
ret	2	A9	lv1	-	-	return from subroutine
athrow	1	BF		O	-	throw an exception
monitorenter	1	C2		A	-	enter a monitored region of code
monitorexit	1	C3		A	-	exit a monitored region of code
breakpoint	1	CA		-	-	call breakpoint handler
xxxunusedxxx	0	BA				was newfromname

Examples

Example 1: .java & CodeAttribute -- put/get fields
Example 2: .java & CodeAttribute -- load/store/increment local variables
Example 3: .java & CodeAttribute -- integer constants
Example 4: .java & CodeAttribute -- load constants
Example 5: .java & CodeAttribute -- operations
Example 6: .java & CodeAttribute -- control fow
Example 7: .java & CodeAttribute -- tableswitch
Example 8: .java & CodeAttribute -- lookupswitch
Example 9: .java & CodeAttribute -- invoke methods
Example 10: .java, CodeAttribute1, CodeAttribute2 & CodeAttribute3 -- invoke methods
Example 11: .java & CodeAttribute -- new, checkcast, instanceof
Example 12: .java & CodeAttribute -- array
Example 13: .java & CodeAttribute -- try-catch (without Finally)
Example 14: .java & CodeAttribute -- try-catch (with Finally)
Example 15: .java & CodeAttribute -- nested try-catch
Example 16: .java, .class & CodeAttribute -- synchronized method & statement

New Attributes

InnerClasses & Synthetic Attributes

Inner classes are implemented by the compiler, and do not require any changes to the JVM.
The InnerClasses attribute has the format:

InnerClasses_attribute {

u2 attribute_name_index;

u4 attribute_length;

u2 number_of_classes;

{

u2 inner_class_info_index; // CONSTANT_Class_info index

u2 outer_class_info_index; // CONSTANT_Class_info index

u2 inner_name_index; // CONSTANT_Utf8_info index

u2 inner_class_access_flags; // access_flags bitmask

} classes[number_of_classes]

}

The Synthetic sttribute is a zero-length attribute to indicate a field or a method is synthesized by compiler in order to implement innerclasses.
Example 1: .java & .class
Example 2: .java & .class

Deprecated Attribute

A zero-length attribute to indicate a class, a field or a method is deprecated.
Example 1: .java & .class

JAVA BYTECODE STRUCTURE

Contents

A Little Big Example

ClassFile

SourceFile Attribute

Examples

Constant Pool

CONSTANT_Integer & CONSTANT_Float

CONSTANT_Long & CONSTANT_Double

CONSTANT_Utf8

CONSTANT_String

CONSTANT_Class

CONSTANT_NameAndType

Signature

CONSTNAT_Fieldref, CONSTANT_Methodref, & CONSTANT_InterfaceMethodref

Examples

Fields

ConstantValue Attribute

Example

Methods

Exceptions Attribute

Example

Code Attribute

LineNumberTable Attribute

LocalVariableTable Attribute

Examples

Instructions

Examples

New Attributes

InnerClasses & Synthetic Attributes

Deprecated Attribute