Java bytecode
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Java bytecode is the
bytecode Bytecode, also termed portable code or p-code, is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normall ...
-structured
instruction set In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an '' ...
of the Java virtual machine (JVM).


Relation to Java

A
Java Java ( id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands The Greater Sunda Islands are four tropical islands situated within Southeast Asia, in the Pacific Ocean. The islands, Borneo, Java, Sulawesi and Sumatra, are internat ...
programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

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 language, assembly helps the C (programming language), C or C++ programmer."


Instruction set architecture

The JVM is both a stack machine and a register machine. Each Call stack#STACK-FRAME, frame for a method call has an "operand stack" and an array of "local variables". The operand stack is used for operands to computations and for receiving the return value of a called method, while local variables serve the same purpose as Processor register, registers and are also used to pass method arguments. The maximum size of the operand stack and local variable array, computed by the compiler, is part of the attributes of each method. Each can be independently sized from 0 to 65535 values, where each value is 32 bits. and types, which are 64 bits, take up two consecutive local variables (which need not be 64-bit aligned in the local variables array) or one value in the operand stack (but are counted as two units in the depth of the stack).


Instruction set

Each
bytecode Bytecode, also termed portable code or p-code, is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normall ...
is composed of one byte that represents the opcode, along with zero or more bytes for operands. Of the 256 possible byte-long opcodes, , 202 are in use (~79%), 51 are reserved for future use (~20%), and 3 instructions (~1%) are permanently reserved for JVM implementations to use. Two of these (impdep1 and impdep2) are to provide traps for implementation-specific software and hardware, respectively. The third is used for debuggers to implement breakpoints. Instructions fall into a number of broad groups: * Load and store (e.g. aload_0, istore) * Arithmetic and logic (e.g. ladd, fcmpl) * Type conversion (e.g. i2b, d2i) * Object creation and manipulation (new, putfield) * Operand stack management (e.g. swap, dup2) * Control transfer (e.g. ifeq, goto) * Method invocation and return (e.g. invokespecial, areturn) 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: 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 (32 bit value) with the value 5 onto the stack, while dconst_1 will push a double (64 bit floating point value) with the value 1 onto the stack. There is also an aconst_null, which pushes a reference. The ''n'' for the load and store instructions specifies the index in the local variable array to load from or store to. The aload_0 instruction pushes the object in local variable 0 onto the stack (this is usually the this (computer programming), this object). istore_1 stores the integer on the top of the stack into local variable 1. For local variables beyond 3 the suffix is dropped and operands must be used.


Example

Consider the following Java code: outer: for (int i = 2; i < 1000; i++) A Java compiler might translate the Java code above into bytecode 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

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: *Eclipse compiler for Java (ECJ) *Jikes, compiles from Java to Java bytecode (developed by
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
, implemented in C++) *Espresso, compiles from Java to Java bytecode (Java 1.0 only) *GNU Compiler for Java (GCJ), compiles from Java to Java bytecode; it can also compile to native machine code and was part of the GNU Compiler Collection (GCC) up until version 6. 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: *Jasmin (Java assembler), Jasmin, takes text descriptions for Java classes, written in a simple assembly-like syntax using Java virtual machine instruction set and generates a Java class file *Jamaica, a Macro (computer science), macro assembly language for the Java virtual machine. Java syntax is used for class or interface definition. Method bodies are specified using bytecode instructions. *Krakatau Bytecode Tools, currently contains three tools: a decompiler and disassembler for Java classfiles and an assembler to create classfiles. *Lilac, an assembler and disassembler for the Java virtual machine. Others have developed compilers, for different programming languages, to target the Java virtual machine, such as: *ColdFusion *JRuby and Jython, two scripting languages based on Ruby (programming language), Ruby and Python (programming language), Python *Groovy (programming language), Apache Groovy, optionally typed and dynamic general-purpose language, with static-typing and static compilation capabilities *Scala (programming language), Scala, a type-safe general-purpose programming language supporting object-oriented and functional programming *JGNAT and AppletMagic, compile from the language Ada (programming language), Ada to Java bytecode *Java virtual machine#C to bytecode compilers, C to Java byte-code compilers *Clojure, a functional, immutable, general-purpose programming language in the Lisp (programming language), Lisp family with a strong emphasis on concurrency *Kawa (Scheme implementation), Kawa, an implementation of the Scheme (programming language), Scheme programming language, also a dialect of Lisp (programming language), Lisp. *MIDletPascal *JavaFX Script code is compiled to Java bytecode *Kotlin (programming language), Kotlin, a statically-typed general-purpose programming language with type inference *Object Pascal source code is compiled to Java bytecode using the Free Pascal 3.0+ compiler.


Execution

There are several Java virtual machines available today to execute Java bytecode, both free and commercial products. If executing bytecode in a virtual machine is undesirable, a developer can also compile Java source code or bytecode directly to native machine code with tools such as the GNU Compiler for Java (GCJ). Some processors can execute Java bytecode natively. Such processors are termed ''Java processors''.


Support for dynamic languages

The Java virtual machine provides some support for Type system#Dynamic typing, dynamically typed languages. Most of the extant JVM instruction set is Type system#Static typing, 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 (program lifecycle phase), run time, or to choose the method dispatch by an alternative approach. Java Community Process, JSR 292 (''Supporting Dynamically Typed Languages on the Java Platform'')see JSR 292
/ref> added a new invokedynamic instruction at the JVM level, to allow method invocation relying on dynamic Type system#Type checking, type checking (instead of the extant 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 Java Platform, Standard Edition, JSE 7 also include the invokedynamic opcode.


See also

* Java bytecode instruction listings * Java class file * List of JVM languages * Java backporting tools * C to Java Virtual Machine compilers * JStik * Common Intermediate Language (CIL), Microsoft's rival to Java bytecode * ObjectWeb ASM * Byte Code Engineering Library


References


External links


Oracle's Java Virtual Machine Specification

Programming Languages for the Java Virtual Machine


* [https://web.archive.org/web/20090809232522/http://www.adaptj.com/main/stacktrace AdaptJ StackTrace – bytecode level debugging with a full control of the stack, the local variables, and the execution flow]
Java Class Unpacker – plugin for Total Commander, it lets open class files as compressed archives and see fields and methods as files. The bytecode can be viewed as text using F3
{{Java (Sun) Assembly languages Java platform, Bytecodes