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'''Java bytecode''' is the instruction set of the [[Java virtual machine]] (JVM), the language to which [[Java (programming language)|Java]] and other JVM-compatible [[source code]] is [[compiler|compiled]].<ref name="oracle jvm spec">{{Cite web|url=http://docs.oracle.com/javase/specs/jvms/se8/html/|title=Java Virtual Machine Specification|publisher=Oracle|access-date=14 November 2023}}</ref> Each instruction is represented by a single [[byte]], hence the name [[bytecode]], making it a compact form of [[data]].<ref name="JVM Book">{{Cite book|last=Lindholm|first=Tim|title=The Java Virtual Machine Specification|year=2015|publisher=Oracle|isbn=978-0133905908}}</ref>
Due to the nature of bytecode, a Java bytecode [[computer program|program]] is runnable on any machine with a compatible JVM
Java bytecode is used at [[Runtime (program lifecycle phase)|runtime]] either [[interpreter (computing)|interpreted]] by a JVM or compiled to machine code via [[Just-in-time compilation|just-in-time]] (JIT) compilation and run as a native application.
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The bytecode comprises various instruction types, including data manipulation, control transfer, object creation and manipulation, and method invocation, all integral to Java's object-oriented programming model.<ref name="oracle jvm spec"/>
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".<ref name="jvm">{{cite book |last1=Lindholm |first1=Tim |last2=Yellin |first2=Frank |last3=Bracha |first3=Gilad |last4=Buckley |first4=Alex |title=The Java Virtual Machine Specification |edition=Java SE 8 |date=2015-02-13 |url=http://docs.oracle.com/javase/specs/jvms/se8/html/}}</ref>{{rp|2.6}} <ref name="JVM Book"/> The operand stack is used for passing 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.<ref name="jvm"/>{{rp|4.7.3}} Each can be independently sized from 0 to 65535 values, where each value is 32 bits. {{code|lang="java"|long}} and {{code|lang="java"|double}} types, which are 64 bits, take up two consecutive local variables<ref name="jvm"/>{{rp|2.6.1}} (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).<ref name="jvm"/>{{rp|2.6.2}}
=== Instruction set ===
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There are also a few instructions for a number of more specialized tasks such as exception throwing, synchronization, etc.
Many instructions have [[Opcode prefix|prefixes]] and/or suffixes referring to the types of operands they operate on.<ref name="jvm"/>{{rp|2.11.1}} These are as follows:
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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<ref>{{Cite web|url=https://jasmin.sourceforge.net/|title=Jasmin Home Page|website=jasmin.sourceforge.net|accessdate=2 June 2024}}</ref>
*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.<ref>{{Cite web|url=https://www.javaworld.com/article/2072355/core-java/learn-to-speak-jamaican.html|title=Jamaica: The Java virtual machine (JVM) macro assembler<!-- Bot generated title -->|archive-url=https://web.archive.org/web/20231114000632/https://www.infoworld.com/article/2072355/learn-to-speak-jamaican.html|archive-date=14 November 2023|work=JavaWorld |accessdate=2 June 2024 |last1=Huang |first1=James Jianbo }}</ref>
*Krakatau Bytecode Tools, currently contains three tools: a decompiler and disassembler for Java classfiles and an assembler to create classfiles.<ref>{{Cite web|url=https://github.com/Storyyeller/Krakatau|title=Storyyeller/Krakatau|date=1 June 2024|accessdate=2 June 2024|via=GitHub}}</ref>
*Lilac, an assembler and disassembler for the [[Java virtual machine]].<ref>{{Cite web|url=https://lilac.sourceforge.net/|title=Lilac - a Java assembler|website=lilac.sourceforge.net|accessdate=2 June 2024}}</ref>
Others have developed compilers, for different programming languages, to target the Java virtual machine, such as:
*[[ColdFusion Markup Language|ColdFusion]]
*[[JRuby]] and [[Jython]], two [[scripting language]]s 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
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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.<ref>{{cite web
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