Java Programming/Byte Code

Java Byte Code is the language to which Java source is compiled and the Java Virtual Machine understands. Unlike compiled languages that have to be specifically compiled for each different type of computers, a Java program only needs to be converted to byte code once, after which it can run on any platform for which a Java Virtual Machine exists.

Bytecode is the compiled format for Java programs. Once a Java program has been converted to bytecode, it can be transferred across a network and executed by Java Virtual Machine (JVM). Bytecode files generally have a .class extension. It is not normally necessary for a Java programmer to know byte code, but it can be useful.

Other Languages

There are a number of exciting new languages being created that also compile to Java byte code, such as Groovy.

GNAT: The GNU Ada-Compiler, is capable of compiling Ada into Java-style bytecode.; ftp://cs.nyu.edu/pub/gnat

JPython: Compiles Python to Java-style bytecode.; http://www.jpython.org/

Kawa: Compiles Scheme to Java-style bytecode.; http://www.gnu.org/software/kawa/

Example

Consider the following Java code.

 outer:
 for (int i = 2; i < 1000; i++) {
  for (int j = 2; j < i; j++) {
    if (i % j == 0)
      continue outer;
  }
  System.out.println (i);
 }

A Java compiler might translate the Java code above into byte code as follows, assuming the above was put in a method:

  Code:
   0:   iconst_2
   1:   istore_1
   2:   iload_1
   3:   sipush  1000
   6:   if_icmpge       44
   9:   iconst_2
   10:  istore_2
   11:  iload_2
   12:  iload_1
   13:  if_icmpge       31
   16:  iload_1
   17:  iload_2
   18:  irem             # remainder
   19:  ifne    25
   22:  goto    38
   25:  iinc    2, 1
   28:  goto    11
   31:  getstatic       #84; //Field java/lang/System.out:Ljava/io/PrintStream;
   34:  iload_1
   35:  invokevirtual   #85; //Method java/io/PrintStream.println:(I)V
   38:  iinc    1, 1
   41:  goto    2
   44:  return

Example 2

As an example we can write a simple Foo.java source:

public class Foo {
  public static void main(final String[] args) {
    System.out.println("This is a simple example of decompilation using javap");
    a();
    b();
  }
	
  public static void a() {
    System.out.println("Now we are calling a function...");
  }

  public static void b() {
    System.out.println("...and now we are calling b");
  }
}

Compile it and then move Foo.java to another directory or delete it if you wish. What can we do with javap and Foo.class ?

$javap Foo

produces this result:

Compiled from "Foo.java"
public class Foo extends java.lang.Object {
    public Foo();
    public static void main(java.lang.String[]);
    public static void a();
    public static void b();
}

As you can see the javac compiler doesn't strip any (public) variable name from the .class file. As a result the names of the functions, their parameters and types of return are exposed. (This is necessary in order for other classes to access them.)

Let's do a bit more, try:

$javap -c Foo

Compiled from "Foo.java"
public class Foo extends java.lang.Object{
public Foo();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc             #3; //String This is a simple example of decompilation using javap
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   invokestatic    #5; //Method a:()V
   11:  invokestatic    #6; //Method b:()V
   14:  return

public static void a();
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc             #7; //String Now we are calling a function...
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

public static void b();
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc             #8; //String ...and now we are calling b
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

}

The Java bytecodes

See Oracle's Java Virtual Machine Specification^[1] for more detailed descriptions

The manipulation of the operand stack is notated as [before]→[after], where [before] is the stack before the instruction is executed and [after] is the stack after the instruction is executed. A stack with the element 'b' on the top and element 'a' just after the top element is denoted 'a,b'.

Mnemonic	Opcode (in hex)	Other bytes	Stack [before]→[after]	Description
A
aaload	32		arrayref, index → value	loads onto the stack a reference from an array
aastore	53		arrayref, index, value →	stores a reference into an array
aconst_null	01		→ null	pushes a null reference onto the stack
aload	19	index	→ objectref	loads a reference onto the stack from a local variable #index
aload_0	2a		→ objectref	loads a reference onto the stack from local variable 0
aload_1	2b		→ objectref	loads a reference onto the stack from local variable 1
aload_2	2c		→ objectref	loads a reference onto the stack from local variable 2
aload_3	2d		→ objectref	loads a reference onto the stack from local variable 3
anewarray	bd	indexbyte1, indexbyte2	count → arrayref	creates a new array of references of length count and component type identified by the class reference index (indexbyte1 << 8 + indexbyte2) in the constant pool
areturn	b0		objectref → [empty]	returns a reference from a method
arraylength	be		arrayref → length	gets the length of an array
astore	3a	index	objectref →	stores a reference into a local variable #index
astore_0	4b		objectref →	stores a reference into local variable 0
astore_1	4c		objectref →	stores a reference into local variable 1
astore_2	4d		objectref →	stores a reference into local variable 2
astore_3	4e		objectref →	stores a reference into local variable 3
athrow	bf		objectref → [empty], objectref	throws an error or exception (notice that the rest of the stack is cleared, leaving only a reference to the Throwable)
B
baload	33		arrayref, index → value	loads a byte or Boolean value from an array
bastore	54		arrayref, index, value →	stores a byte or Boolean value into an array
bipush	10	byte	→ value	pushes a byte onto the stack as an integer value
C
caload	34		arrayref, index → value	loads a char from an array
castore	55		arrayref, index, value →	stores a char into an array
checkcast	c0	indexbyte1, indexbyte2	objectref → objectref	checks whether an objectref is of a certain type, the class reference of which is in the constant pool at index (indexbyte1 << 8 + indexbyte2)
D
d2f	90		value → result	converts a double to a float
d2i	8e		value → result	converts a double to an int
d2l	8f		value → result	converts a double to a long
dadd	63		value1, value2 → result	adds two doubles
daload	31		arrayref, index → value	loads a double from an array
dastore	52		arrayref, index, value →	stores a double into an array
dcmpg	98		value1, value2 → result	compares two doubles
dcmpl	97		value1, value2 → result	compares two doubles
dconst_0	0e		→ 0.0	pushes the constant 0.0 onto the stack
dconst_1	0f		→ 1.0	pushes the constant 1.0 onto the stack
ddiv	6f		value1, value2 → result	divides two doubles
dload	18	index	→ value	loads a double value from a local variable #index
dload_0	26		→ value	loads a double from local variable 0
dload_1	27		→ value	loads a double from local variable 1
dload_2	28		→ value	loads a double from local variable 2
dload_3	29		→ value	loads a double from local variable 3
dmul	6b		value1, value2 → result	multiplies two doubles
dneg	77		value → result	negates a double
drem	73		value1, value2 → result	gets the remainder from a division between two doubles
dreturn	af		value → [empty]	returns a double from a method
dstore	39	index	value →	stores a double value into a local variable #index
dstore_0	47		value →	stores a double into local variable 0
dstore_1	48		value →	stores a double into local variable 1
dstore_2	49		value →	stores a double into local variable 2
dstore_3	4a		value →	stores a double into local variable 3
dsub	67		value1, value2 → result	subtracts a double from another
dup	59		value → value, value	duplicates the value on top of the stack
dup_x1	5a		value2, value1 → value1, value2, value1	inserts a copy of the top value into the stack two values from the top
dup_x2	5b		value3, value2, value1 → value1, value3, value2, value1	inserts a copy of the top value into the stack two (if value2 is double or long it takes up the entry of value3, too) or three values (if value2 is neither double nor long) from the top
dup2	5c		{value2, value1} → {value2, value1}, {value2, value1}	duplicate top two stack words (two values, if value1 is not double nor long; a single value, if value1 is double or long)
dup2_x1	5d		value3, {value2, value1} → {value2, value1}, value3, {value2, value1}	duplicate two words and insert beneath third word (see explanation above)
dup2_x2	5e		{value4, value3}, {value2, value1} → {value2, value1}, {value4, value3}, {value2, value1}	duplicate two words and insert beneath fourth word
F
f2d	8d		value → result	converts a float to a double
f2i	8b		value → result	converts a float to an int
f2l	8c		value → result	converts a float to a long
fadd	62		value1, value2 → result	adds two floats
faload	30		arrayref, index → value	loads a float from an array
fastore	51		arreyref, index, value →	stores a float in an array
fcmpg	96		value1, value2 → result	compares two floats
fcmpl	95		value1, value2 → result	compares two floats
fconst_0	0b		→ 0.0f	pushes 0.0f on the stack
fconst_1	0c		→ 1.0f	pushes 1.0f on the stack
fconst_2	0d		→ 2.0f	pushes 2.0f on the stack
fdiv	6e		value1, value2 → result	divides two floats
fload	17	index	→ value	loads a float value from a local variable #index
fload_0	22		→ value	loads a float value from local variable 0
fload_1	23		→ value	loads a float value from local variable 1
fload_2	24		→ value	loads a float value from local variable 2
fload_3	25		→ value	loads a float value from local variable 3
fmul	6a		value1, value2 → result	multiplies two floats
fneg	76		value → result	negates a float
frem	72		value1, value2 → result	gets the remainder from a division between two floats
freturn	ae		value → [empty]	returns a float from method
fstore	38	index	value →	stores a float value into a local variable #index
fstore_0	43		value →	stores a float value into local variable 0
fstore_1	44		value →	stores a float value into local variable 1
fstore_2	45		value →	stores a float value into local variable 2
fstore_3	46		value →	stores a float value into local variable 3
fsub	66		value1, value2 → result	subtracts two floats
G
getfield	b4	index1, index2	objectref → value	gets a field value of an object objectref, where the field is identified by field reference in the constant pool index (index1 << 8 + index2)
getstatic	b2	index1, index2	→ value	gets a static field value of a class, where the field is identified by field reference in the constant pool index (index1 << 8 + index2)
goto	a7	branchbyte1, branchbyte2	[no change]	goes to another instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
goto_w	c8	branchbyte1, branchbyte2, branchbyte3, branchbyte4	[no change]	goes to another instruction at branchoffset (signed int constructed from unsigned bytes branchbyte1 << 24 + branchbyte2 << 16 + branchbyte3 << 8 + branchbyte4)
I
i2b	91		value → result	converts an int into a byte
i2c	92		value → result	converts an int into a character
i2d	87		value → result	converts an int into a double
i2f	86		value → result	converts an int into a float
i2l	85		value → result	converts an int into a long
i2s	93		value → result	converts an int into a short
iadd	60		value1, value2 → result	adds two ints together
iaload	2e		arrayref, index → value	loads an int from an array
iand	7e		value1, value2 → result	performs a logical and on two integers
iastore	4f		arrayref, index, value →	stores an int into an array
iconst_m1	02		→ -1	loads the int value -1 onto the stack
iconst_0	03		→ 0	loads the int value 0 onto the stack
iconst_1	04		→ 1	loads the int value 1 onto the stack
iconst_2	05		→ 2	loads the int value 2 onto the stack
iconst_3	06		→ 3	loads the int value 3 onto the stack
iconst_4	07		→ 4	loads the int value 4 onto the stack
iconst_5	08		→ 5	loads the int value 5 onto the stack
idiv	6c		value1, value2 → result	divides two integers
if_acmpeq	a5	branchbyte1, branchbyte2	value1, value2 →	if references are equal, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_acmpne	a6	branchbyte1, branchbyte2	value1, value2 →	if references are not equal, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_icmpeq	9f	branchbyte1, branchbyte2	value1, value2 →	if ints are equal, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_icmpne	a0	branchbyte1, branchbyte2	value1, value2 →	if ints are not equal, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_icmplt	a1	branchbyte1, branchbyte2	value1, value2 →	if value1 is less than value2, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_icmpge	a2	branchbyte1, branchbyte2	value1, value2 →	if value1 is greater than or equal to value2, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_icmpgt	a3	branchbyte1, branchbyte2	value1, value2 →	if value1 is greater than value2, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
if_icmple	a4	branchbyte1, branchbyte2	value1, value2 →	if value1 is less than or equal to value2, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifeq	99	branchbyte1, branchbyte2	value →	if value is 0, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifne	9a	branchbyte1, branchbyte2	value →	if value is not 0, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
iflt	9b	branchbyte1, branchbyte2	value →	if value is less than 0, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifge	9c	branchbyte1, branchbyte2	value →	if value is greater than or equal to 0, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifgt	9d	branchbyte1, branchbyte2	value →	if value is greater than 0, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifle	9e	branchbyte1, branchbyte2	value →	if value is less than or equal to 0, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifnonnull	c7	branchbyte1, branchbyte2	value →	if value is not null, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
ifnull	c6	branchbyte1, branchbyte2	value →	if value is null, branch to instruction at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2)
iinc	84	index, const	[No change]	increment local variable #index by signed byte const
iload	15	index	→ value	loads an int value from a variable #index
iload_0	1a		→ value	loads an int value from variable 0
iload_1	1b		→ value	loads an int value from variable 1
iload_2	1c		→ value	loads an int value from variable 2
iload_3	1d		→ value	loads an int value from variable 3
imul	68		value1, value2 → result	multiply two integers
ineg	74		value → result	negate int
instanceof	c1	indexbyte1, indexbyte2	objectref → result	determines if an object objectref is of a given type, identified by class reference index in constant pool (indexbyte1 << 8 + indexbyte2)
invokedynamic	ba	indexbyte1, indexbyte2	[arg1, arg2, ...] → result	invokes a dynamic method and puts the result on the stack (might be void); the method is identified by method reference index in constant pool (indexbyte1 << 8 \| indexbyte2)
invokeinterface	b9	indexbyte1, indexbyte2, count, 0	objectref, [arg1, arg2, ...] →	invokes an interface method on object objectref, where the interface method is identified by method reference index in constant pool (indexbyte1 << 8 + indexbyte2) and count is the number of arguments to pop from the stack frame including the object on which the method is being called and must always be greater than or equal to 1
invokespecial	b7	indexbyte1, indexbyte2	objectref, [arg1, arg2, ...] →	invoke instance method on object objectref requiring special handling (instance initialization method, a private method, or a superclass method), where the method is identified by method reference index in constant pool (indexbyte1 << 8 + indexbyte2)
invokestatic	b8	indexbyte1, indexbyte2	[arg1, arg2, ...] →	invoke a static method, where the method is identified by method reference index in constant pool (indexbyte1 << 8 + indexbyte2)
invokevirtual	b6	indexbyte1, indexbyte2	objectref, [arg1, arg2, ...] →	invoke virtual method on object objectref, where the method is identified by method reference index in constant pool (indexbyte1 << 8 + indexbyte2)
ior	80		value1, value2 → result	logical int or
irem	70		value1, value2 → result	logical int remainder
ireturn	ac		value → [empty]	returns an integer from a method
ishl	78		value1, value2 → result	int shift left
ishr	7a		value1, value2 → result	int shift right
istore	36	index	value →	store int value into variable #index
istore_0	3b		value →	store int value into variable 0
istore_1	3c		value →	store int value into variable 1
istore_2	3d		value →	store int value into variable 2
istore_3	3e		value →	store int value into variable 3
isub	64		value1, value2 → result	int subtract
iushr	7c		value1, value2 → result	int shift right
ixor	82		value1, value2 → result	int xor
J
jsr	a8	branchbyte1, branchbyte2	→ address	jump to subroutine at branchoffset (signed short constructed from unsigned bytes branchbyte1 << 8 + branchbyte2) and place the return address on the stack
jsr_w	c9	branchbyte1, branchbyte2, branchbyte3, branchbyte4	→ address	jump to subroutine at branchoffset (signed int constructed from unsigned bytes branchbyte1 << 24 + branchbyte2 << 16 + branchbyte3 << 8 + branchbyte4) and place the return address on the stack
L
l2d	8a		value → result	converts a long to a double
l2f	89		value → result	converts a long to a float
l2i	88		value → result	converts a long to an int
ladd	61		value1, value2 → result	add two longs
laload	2f		arrayref, index → value	load a long from an array
land	7f		value1, value2 → result	bitwise and of two longs
lastore	50		arrayref, index, value →	store a long to an array
lcmp	94		value1, value2 → result	compares two longs values
lconst_0	09		→ 0L	pushes the long 0 onto the stack
lconst_1	0a		→ 1L	pushes the long 1 onto the stack
ldc	12	index	→ value	pushes a constant #index from a constant pool (String, int, float or class type) onto the stack
ldc_w	13	indexbyte1, indexbyte2	→ value	pushes a constant #index from a constant pool (String, int, float or class type) onto the stack (wide index is constructed as indexbyte1 << 8 + indexbyte2)
ldc2_w	14	indexbyte1, indexbyte2	→ value	pushes a constant #index from a constant pool (double or long) onto the stack (wide index is constructed as indexbyte1 << 8 + indexbyte2)
ldiv	6d		value1, value2 → result	divide two longs
lload	16	index	→ value	load a long value from a local variable #index
lload_0	1e		→ value	load a long value from a local variable 0
lload_1	1f		→ value	load a long value from a local variable 1
lload_2	20		→ value	load a long value from a local variable 2
lload_3	21		→ value	load a long value from a local variable 3
lmul	69		value1, value2 → result	multiplies two longs
lneg	75		value → result	negates a long
lookupswitch	ab	<0-3 bytes padding>, defaultbyte1, defaultbyte2, defaultbyte3, defaultbyte4, npairs1, npairs2, npairs3, npairs4, match-offset pairs...	key →	a target address is looked up from a table using a key and execution continues from the instruction at that address
lor	81		value1, value2 → result	bitwise or of two longs
lrem	71		value1, value2 → result	remainder of division of two longs
lreturn	ad		value → [empty]	returns a long value
lshl	79		value1, value2 → result	bitwise shift left of a long value1 by value2 positions
lshr	7b		value1, value2 → result	bitwise shift right of a long value1 by value2 positions
lstore	37	index	value →	store a long value in a local variable #index
lstore_0	3f		value →	store a long value in a local variable 0
lstore_1	40		value →	store a long value in a local variable 1
lstore_2	41		value →	store a long value in a local variable 2
lstore_3	42		value →	store a long value in a local variable 3
lsub	65		value1, value2 → result	subtract two longs
lushr	7d		value1, value2 → result	bitwise shift right of a long value1 by value2 positions, unsigned
lxor	83		value1, value2 → result	bitwise exclusive or of two longs
M
monitorenter	c2		objectref →	enter monitor for object ("grab the lock" - start of synchronized() section)
monitorexit	c3		objectref →	exit monitor for object ("release the lock" - end of synchronized() section)
multianewarray	c5	indexbyte1, indexbyte2, dimensions	count1, [count2,...] → arrayref	create a new array of dimensions dimensions with elements of type identified by class reference in constant pool index (indexbyte1 << 8 + indexbyte2); the sizes of each dimension is identified by count1, [count2, etc]
N
new	bb	indexbyte1, indexbyte2	→ objectref	creates new object of type identified by class reference in constant pool index (indexbyte1 << 8 + indexbyte2)
newarray	bc	atype	count → arrayref	creates new array with count elements of primitive type identified by atype
nop	00		[No change]	performs no operation
P
pop	57		value →	discards the top value on the stack
pop2	58		{value2, value1} →	discards the top two values on the stack (or one value, if it is a double or long)
putfield	b5	indexbyte1, indexbyte2	objectref, value →	set field to value in an object objectref, where the field is identified by a field reference index in constant pool (indexbyte1 << 8 + indexbyte2)
putstatic	b3	indexbyte1, indexbyte2	value →	set static field to value in a class, where the field is identified by a field reference index in constant pool (indexbyte1 << 8 + indexbyte2)
R
ret	a9	index	[No change]	continue execution from address taken from a local variable #index (the asymmetry with jsr is intentional)
return	b1		→ [empty]	return void from method
S
saload	35		arrayref, index → value	load short from array
sastore	56		arrayref, index, value →	store short to array
sipush	11	byte1, byte2	→ value	pushes a signed integer (byte1 << 8 + byte2) onto the stack
swap	5f		value2, value1 → value1, value2	swaps two top words on the stack (note that value1 and value2 must not be double or long)
T
W
Unused
impdep1	fe			reserved for implementation-dependent operations within debuggers; should not appear in any class file
impdep2	ff			reserved for implementation-dependent operations within debuggers; should not appear in any class file
(no name)	cb-fd			these values are currently unassigned for opcodes and are reserved for future use
xxxunusedxxx	ba			this opcode is reserved "for historical reasons"

References

↑ Oracle's Java Virtual Machine Specification

External Links

Invoking C

Java Programming
Byte Code

Links

[1] Oracle's Java Virtual Machine Specification

[1]