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Text of the page (random words):
chitecture toggle virtual machine architecture subsection 2 1 jvm languages 2 2 class loader 2 3 bytecode instructions 2 4 bytecode verifier 2 4 1 secure execution of remote code 2 5 bytecode interpreter and just in time compiler 2 6 java native interface 3 jvm in the web browser toggle jvm in the web browser subsection 3 1 javascript jvms and interpreters 3 2 transpilation to javascript 4 see also 5 notes 6 references 7 external links toggle the table of contents java virtual machine 45 languages አማርኛ العربية azərbaycanca български বাংলা bosanski català čeština deutsch español eesti فارسی suomi français עברית हिन्दी hrvatski magyar հայերեն interlingua bahasa indonesia italiano 日本語 қазақша 한국어 മലയാളം монгол bahasa melayu မြန်မာဘာသာ nederlands ਪੰਜਾਬੀ polski português русский srpskohrvatski српскохрватски simple english shqip српски srpski svenska தமிழ் türkçe українська oʻzbekcha ўзбекча tiếng việt 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects abstract wikipedia wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia virtual machine that runs java programs jvm redirects here for other uses see jvm disambiguation java virtual machine designer sun microsystems bits 32 bit 64 bit a introduced 1994 32 years ago 1994 version java se 26 3 type stack and register register encoding variable branching compare and branch endianness big open yes registers general purpose per method operand stack up to 65535 operands plus per method local variables up to 65535 overview of a java virtual machine jvm architecture based on the java virtual machine specification java se 7 edition a java virtual machine jvm is a virtual machine that enables a computer to run java programs as well as programs written in other languages referred to as jvm languages that are also compiled to java bytecode the jvm is detailed by a specification that formally describes what is required in a jvm implementation having a specification ensures interoperability of java programs across different implementations so that program authors using the java development kit jdk need not worry about idiosyncrasies of the underlying hardware platform the jvm reference implementation is developed by the openjdk project as open source code and includes a jit compiler called hotspot the commercially supported java releases available from oracle are based on the openjdk runtime eclipse openj9 is another open source jvm for openjdk jvm specification edit the java virtual machine is an abstract virtual computer defined by a specification it is a part of the java runtime environment the garbage collection algorithm used and any internal optimization of the java virtual machine instructions their translation into machine code are not specified the main reason for this omission is to not unnecessarily constrain implementers any java application can be run only inside some concrete implementation of the abstract specification of the java virtual machine 4 starting with java platform standard edition j2se 5 0 changes to the jvm specification have been developed under the java community process as jsr 924 5 as of 2006 update changes to the specification to support changes proposed to the class file format jsr 202 6 are being done as a maintenance release of jsr 924 the specification for the jvm was published as the blue book 7 whose preface states we intend that this specification should sufficiently document the java virtual machine to make possible compatible clean room implementations oracle provides tests that verify the proper operation of implementations of the java virtual machine the most commonly used java virtual machine is oracle s hotspot oracle owns the java trademark and may allow its use to certify implementation suites as fully compatible with oracle s specification garbage collectors edit see also garbage collection computer science java garbage collectors available in java openjdks virtual machine jvm include serial parallel cms concurrent mark sweep g1 garbage first zgc z garbage collector epsilon shenandoah genzgc generational zgc genshen generational shenandoah ibm metronome only in ibm openjdk sap only in sap openjdk azul c4 continuously concurrent compacting collector 8 only in azul systems openjdk java versions and their garbage collectors version default gc available gcs 6u14 serial parallel mp serial parallel cms g1 e 7u4 8 serial parallel cms g1 9 10 serial g1 11 serial parallel cms g1 epsilon e zgc e 12 13 serial parallel cms g1 epsilon e zgc e shenandoah e 14 serial parallel g1 epsilon e zgc e shenandoah e 15 20 serial parallel g1 epsilon e zgc shenandoah 21 22 serial parallel g1 epsilon e zgc shenandoah genzgc e 23 serial parallel g1 epsilon e zgc shenandoah genzgc default zgc 24 serial parallel g1 epsilon e shenandoah genzgc genshen e 25 26 serial parallel g1 epsilon e shenandoah genzgc genshen 27 g1 serial parallel g1 epsilon e shenandoah genzgc genshen 28 serial parallel g1 epsilon e shenandoah genzgc genshen default shenandoah e experimental java s design philosophy revolves around the assumption of a garbage collector unlike languages such as c and rust deterministic memory management through a delete keyword as in c is not possible even introducing such a feature is not possible due to a lack of ownership aside from using sun misc unsafe or through java lang foreign to allocate deallocate memory outside the java heap 9 10 due to java primarily using heap based allocation objects are stored as references and deletion would result in dangling pointers foo a new foo foo b a delete a system out println b virtual machine architecture edit the jvm operates on specific types of data as specified in java virtual machine specifications the data types can be divided 11 into primitive types integer and floating point values and reference types long and double types which are 64 bits are supported natively but consume two units of storage in a frame s local variables or operand stack since each unit is 32 bits boolean byte short and char types are all sign extended except char which is zero extended and operated on as 32 bit integers the same as int types the smaller types only have a few type specific instructions for loading storing and type conversion boolean is operated on as 8 bit byte values with 0 representing false and 1 representing true although boolean has been treated as a type since the java virtual machine specification second edition clarified this issue in compiled and executed code there is little difference between a boolean and a byte except for name mangling in method signatures and the type of boolean arrays boolean s in method signatures are mangled as z while byte s are mangled as b boolean arrays carry the type boolean but use 8 bits per element and the jvm has no built in capability to pack booleans into a bit array so except for the type they perform and behave the same as byte arrays in all other uses the boolean type is effectively unknown to the jvm as all instructions to operate on booleans are also used to operate on byte s the jvm has a garbage collected heap for storing objects and arrays code constants and other class data are stored in the method area the method area is logically part of the heap but implementations may treat the method area separately from the heap and for example might not garbage collect it each jvm thread also has its own call stack called a java virtual machine stack for clarity which stores frames a new frame is created each time a method is called and the frame is destroyed when that method exits each frame provides an operand stack and an array of local variables the operand stack is used for operands to run computations and for receiving the return value of a called method while local variables serve the same purpose as registers and are also used to pass method arguments thus the jvm is both a stack machine and a register machine in practice hotspot eliminates every stack besides the native thread call stack even when running in interpreted mode as its templating interpreter technically functions as a compiler the jvm uses references and stack array indexes to address data it does not use byte addressing like most physical machines do so it does not neatly fit the usual categorization of 32 bit or 64 bit machines in one sense it could be classified as a 32 bit machine since this is the size of the largest value it natively stores a 32 bit integer or floating point value or a 32 bit reference because a reference is 32 bits each program is limited to at most 2 32 unique references and therefore at most 2 32 objects however each object can be more than one byte large and potentially very large the largest possible object is an array of long of length 2 31 1 which would consume 16 gib of memory and there could potentially be 2 32 of these if there were enough memory available this results in upper bounds that are more comparable to a typical 64 bit byte addressable machine a jvm implementation can be designed to run on a processor that natively uses any bit width as long as it correctly implements the integer 8 16 32 and 64 bit and floating point 32 and 64 bit math that the jvm requires depending on the method used to implement references native pointers compressed pointers or an indirection table this can limit the number of objects to less than the theoretical maximum an implementation of the jvm on a 64 bit platform has access to a much larger address space than one on a 32 bit platform which allows for a much larger heap size and an increased maximum number of threads which is needed for certain kinds of large applications however there can be a performance hit from using a 64 bit implementation compared to a 32 bit implementation jvm languages edit main article list of jvm languages a jvm language is any language with functions that can be expressed in a valid class file which can be hosted by a jvm a class file contains jvm instructions jvm bytecode a symbol table and other ancillary information the class file format is the hardware and operating system independent binary format used to represent compiled classes and interfaces 12 there are several jvm languages both older established languages ported to jvm and much newer from scratch languages jruby and jython may be the best known ports of older languages i e ruby and python respectively of the new from scratch languages created to compile to java bytecode clojure groovy scala and kotlin may be the most popular a notable feature of the jvm languages is their language interoperability they are compatible with each other so that for example scala libraries can be used with java programs and vice versa 13 java 7 jvm implements jsr 292 supporting dynamically typed languages 14 on the java platform a new feature which supports dynamically typed languages in the jvm this feature is developed within the da vinci machine project whose mission is to extend the jvm so that it supports languages other than java 15 16 class loader edit main article java class file one of the organizational units of jvm byte code is a class a class loader implementation must be able to recognize and load anything that conforms to the java class file format any implementation is free to recognize other binary forms besides class files but it must recognize class files the class loader performs three basic activities in this strict order loading finds and imports the binary data for a type linking performs verification preparation and optionally resolution verification ensures the correctness of the imported type preparation allocates memory for class variables and initializing the memory to default values resolution transforms symbolic references from the type into direct references initialization invokes java code that initializes class variables to their proper starting values in general there are three types of class loader bootstrap class loader extension class loader and system application class loader every java virtual machine implementation must have a bootstrap class loader that is capable of loading trusted classes as well as an extension class loader or application class loader the java virtual machine specification does not specify how a class loader should locate classes bytecode instructions edit main article java bytecode the jvm has instructions for the following groups of tasks load and store arithmetic type conversion object creation and manipulation operand stack management push pop control transfer branching method invocation and return throwing exceptions monitor based concurrency the aim is binary compatibility each particular host operating system needs its own implementation of the jvm and runtime these jvms interpret the bytecode semantically the same way but the actual implementation may be different more complex than just emulating bytecode is compatibly and efficiently implementing the java core api that must be mapped to each host operating system these instructions operate on a set of common abstracted data types rather the native data types of any specific instruction set architecture bytecode verifier edit a basic philosophy of java is that it is inherently safe from the standpoint that no user program can crash the host machine or otherwise interfere inappropriately with other operations on the host machine and that it is possible to protect certain methods and data structures belonging to trusted code from access or corruption by untrusted code executing within the same jvm furthermore common programmer errors that often led to data corruption or unpredictable behavior such as accessing off the end of an array or using an uninitialized pointer are not allowed to occur several features of java combine to provide this safety including the class model the garbage collected heap and the verifier the jvm verifies all bytecode before it is executed this verification consists primarily of three types of checks branches are always to valid locations data is always initialized and references are always type safe access to private or package private data and methods is rigidly controlled the first two of these checks take place primarily during the verification step that occurs when a class is loaded and made eligible for use the third is primarily performed dynamically when data items or methods of a class are first accessed by another class the verifier permits only some bytecode sequences in valid programs e g a jump branch instruction can only target an instruction within the same method furthermore the verifier ensures that any given instruction operates on a fixed stack location 17 allowing the jit compiler to transform...
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