Java bytecode

Java bytecode is the instruction set of the Java virtual machine. Each bytecode is composed by one, or in some cases two bytes that represent the instruction (opcode), along with zero or more bytes for passing parameters.

Of the 255 possible byte-long opcodes, as of 2015, 198 are in use (~77%), 54 are reserved for future use, and 3 instructions (~1%) are set aside as permanently unimplemented.[1]

The Java bytecode system does not directly support floating point operations beyond 32 bits, except indirectly via bytecodes that enable use of 64-bit and 80-bit intermediate IEEE floating point maths.

Relation to Java

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 assembly helps the C or C++ programmer."[2]

Instructions fall into a number of broad groups:

There are also a few instructions for a number of more specialized tasks such as exception throwing, synchronization, etc.

Many instructions have prefixes and/or suffixes referring to the types of operands they operate on. These are as follows:

Prefix-suffix Operand type
i integer
l long
s short
b byte
c character
f float
d double
z boolean
a reference

For example, "iadd" will add two integers, while "dadd" will add two doubles. The "const", "load", and "store" instructions may also take a suffix of the form "_n", where n is a number from 0–3 for "load" and "store". The maximum n for "const" differs by type.

The "const" instructions push a value of the specified type onto the stack. For example "iconst_5" will push an integer 5, while "dconst_1" will push a double 1. There is also an "aconst_null", which pushes "null". The n for the "load" and "store" instructions specifies the location in the variable table to load from or store to. The "aload_0" instruction pushes the object in variable 0 onto the stack (this is usually the "this" object). "istore_1" stores the integer on the top of the stack into variable 1. For variables with higher numbers the suffix is dropped and operands must be used.

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:

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
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

Generation

Main article: List of JVM languages

The most common language targeting Java Virtual Machine by producing Java bytecode is Java. 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 now available, other parties have supplied compilers that produce Java bytecode. Examples of other compilers include:

Some projects provide Java assemblers to enable writing Java bytecode by hand. Assembly code may be also generated by machine, for example by a compiler targeting a Java virtual machine. Notable Java assemblers include:

Others have developed compilers, for different programming languages, in order to target the Java virtual machine, such as:

Execution

There are several machines available today, both free and commercial products.

Further information: Java virtual machine

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. Some processors can execute Java bytecode natively. Such processors are known as Java processors.

Support for dynamic languages

Main article: List of JVM languages

The Java Virtual Machine provides some support for dynamically typed languages. Most of the existing JVM instruction set is statically typed - in the sense that method calls have their signatures type-checked at compile time, without a mechanism to defer this decision to run time, or to choose the method dispatch by an alternative approach.[7]

JSR 292 (Supporting Dynamically Typed Languages on the Java™ Platform)[8] added a new invokedynamic instruction at the JVM level, to allow method invocation relying on dynamic type checking (instead of the existing statically type-checked invokevirtual instruction). The Da Vinci Machine is a prototype virtual machine implementation that hosts JVM extensions aimed at supporting dynamic languages. All JVMs supporting JSE 7 also include the invokedynamic opcode.

See also

References

External links

The Wikibook Java Programming has a page on the topic of: Java bytecode
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