Java bytecode

Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode instruction is one byte in length (hence the name), thus the number of bytecodes is limited to no more than 256. Not all 256 possible bytecode values are used, in fact Sun Microsystems, the original creators of the Java programming language, the Java virtual machine and other components of the Java Runtime Environment, have set aside a number of values to be permanently unimplemented. The bytecodes are given below.

A Java programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the IBM developerWorks journal, "Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembler helps the C or C++ programmer."[1]

It is possible to write java byte code in hand, however this method is never used in real life because nowadays the compilers are able to compile well performing code and no person is able to comprehend a piece of java bytecode of considerable size. Originally only one compiler existed, the javac compiler from Sun Microsystems, which compiles Java source code to java bytecode. But because all the specifications for Java bytecode are available other parties have supplied compilers that produce Java bytecode. E.g.:

• Jython, compiles from Python to Java bytecode
• JRuby, compiles from Ruby to Java bytecode
• Jikes, compiles from the Java programming language to Java bytecode
• Espresso, compiles from the Java programming language to Java bytecode, only Java 1.0
• Gnu Java Compiler, GCJ, compiles from the Java programming language to Java bytecode, is also able to compile to native machine code.

If executing Java bytecode in a Java virtual machine is not desirable, a developer can also compile Java source code or Java bytecode directly to native machine code with tools such as the GNU Compiler for Java.

The Java bytecodes

Please see Sun's Java Virtual Machine Specification for more detailed descriptions (link at bottom of page)

Mnemonic	Opcode (in hex)	Description
aaload	32	loads onto the stack a reference to an array
aastore	53	stores into a reference to an array
aconst_null	01	pushes a null reference onto the stack
aload	19	loads a reference onto the stack from a local variable
aload_0	2a	loads a reference from local variable 0
aload_1	2b	loads a reference from local variable 1
aload_2	2c	loads a reference from local variable 2
aload_3	2d	loads a reference from local variable 3
anewarray	bd	creates a new array of references
areturn	b0	returns a reference from a method
arraylength	be	gets the length of an array
astore	3a	stores a reference into a local variable
astore_0	4b	stores a reference into local variable 0
astore_1	4c	stores a reference into local variable 1
astore_2	4d	stores a reference into local variable 2
astore_3	4e	stores a reference into local variable 3
athrow	bf	throws an error or exception
baload	33	loads a byte or boolean value from an array
bastore	54	stores a byte or boolean value into an array
bipush	10	pushes a byte onto the stack
caload	34	loads a char from an array
castore	55	stores a char into an array
checkcast	c0	checks whether an object is of a certain type
d2f	90	converts a double to a float
d2i	8e	converts a double to an int
d2l	8f	converts a double to a long
dadd	63	adds two doubles
daload	31	loads a double from an array
dastore	52	stores a double into an array
dcmpg	98	compares two doubles
dcmpl	97	compares two doubles
dconst_0	0e	pushes the constant 0.0 onto the stack
dconst_1	0f	pushes the constant 1.0 onto the stack
ddiv	6f	divides two doubles
dload	18	loads a double from a local variable
dload_0	26	loads a double from local variable 0
dload_1	27	loads a double from local variable 1
dload_2	28	loads a double from local variable 2
dload_3	29	loads a double from local variable 3
dmul	6b	multiplies two doubles
dneg	77	negates a double
drem	73	gets the remainder from a division between two doubles
dreturn	af	returns a double from a method
dstore	39	stores a double into a local variable
dstore_0	47	stores a double into local variable 0
dstore_1	48	stores a double into local variable 1
dstore_2	49	stores a double into local variable 2
dstore_3	4a	stores a double into local variable 3
dsub	67	subtracts a double from another
dup	59	duplicates the value on top of the stack
dup_x1	5a	inserts a copy of the top value into the stack two values from the top
dup_x2	5b	inserts a copy of the top value into the stack two or three values from the top
dup2	5c
dup2_x1	5d
dup2_x2	5e
f2d	8d	converts a float to a double
f2i	8b	converts a float to an int
f2l	8c	converts a float to a long
fadd	62	adds two floats
faload	30	loads a float from an array
fastore	51	stores a float in an array
fcmpg	96	compares two floats
fcmpl	95	compares two floats
fconst_0	0b	pushes 0.0f on the stack
fconst_1	0c	pushes 1.0f on the stack
fconst_2	0d	pushes 2.0f on the stack
fdiv	6e	divides two floats
fload	17	loads a float from a local value
fload_0	22	loads a float from local variable 0
fload_1	23	loads a float from local variable 1
fload_2	24	loads a float from local variable 2
fload_3	25	loads a float from local variable 3
fmul	6a	multiplies two floats
fneg	76	negates a float
frem	72	gets the remainder from a division between two floats
freturn	ae	returns a float
fstore	38	stores a float into a local variable
fstore_0	43	stores a float into local variable 0
fstore_1	44	stores a float into local variable 1
fstore_2	45	stores a float into local variable 2
fstore_3	46	stores a float into local variable 3
fsub	66	subtracts two floats
getfield	b4	gets a field of an object
getstatic	b2	gets a static field of a class
goto	a7	goes to another instruction
goto_w	c8	goes to another instruction
i2b	91
i2c	92
i2d	87
i2f	86
i2l	85
i2s	93
iadd	60
iaload	2e
iand	7e
iastore	4f
iconst_m1	02
iconst_0	03
iconst_1	04
iconst_2	05
iconst_3	06
iconst_4	07
iconst_5	08
idiv	6c
if_acmpeq	a5
if_acmpne	a6
if_icmpeq	9f
if_icmpne	a0
if_icmplt	a1
if_icmpge	a2
if_icmpgt	a3
if_icmple	a4
ifeq	99
ifne	9a
iflt	9b
ifge	9c
ifgt	9d
ifle	9e
ifnonnull	c7
ifnull	c6
iinc	84
iload	15
iload_0	1a
iload_1	1b
iload_2	1c
iload_3	1d
imul	68
ineg	74
instanceof	c1
invokeinterface	b9
invokespecial	b7
invokestatic	b8
invokevirtual	b6
ior	80
irem	70
ireturn	ac	returns an integer from a method
ishl	78
ishr	7a
istore	36
istore_0	3b
istore_1	3c
istore_2	3d
istore_3	3e
isub	64
iushr	7c
ixor	82
jsr	a8
jsr_w	c9

This list is incomplete.

See also

• ARM9E, a CPU family with direct Java bytecode execution ability
• Common Intermediate Language (CIL), a similar bytecode specification that runs on the CLR of the .NET Framework.

External links

• Sun's Java Virtual Machine Specification