Java virtual machine
Designer | General-purpose | Per-method operand stack (up to 65535 operands) plus per-method local variables (up to 65535) |
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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 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
JVM specification
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.[2]
Starting with
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.
One of Oracle's JVMs is named HotSpot; the other, inherited from BEA Systems, is JRockit. Oracle owns the Java trademark and may allow its use to certify implementation suites as fully compatible with Oracle's specification.
Class loader
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.
Virtual machine architecture
The JVM operates on specific types of data as specified in Java Virtual Machine specifications. The data types can be dividedboolean
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 arraybyte
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.) However the newer JVM releases (OpenJDK HotSpot JVM) support 64-bit, so you can either have 32-bit/64-bit JVM on a 64-bit OS. The primary advantage of running Java in a 64-bit environment is the larger address space. This allows for a much larger Java heap size and an increased maximum number of Java Threads, which is needed for certain kinds of large applications; however there is a performance hit in using 64-bit JVM compared to 32-bit JVM.
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.
Bytecode instructions
The JVM has
- 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
JVM languages
A JVM language is any language with functionality that can be expressed in terms of a valid class file which can be hosted by the Java Virtual Machine. A class file contains Java Virtual Machine instructions (
There are several JVM languages, both old languages ported to JVM and completely new languages. JRuby and Jython are perhaps the most well-known ports of existing languages, i.e. Ruby and Python respectively. Of the new languages that have been created from scratch to compile to Java bytecode, Clojure, Groovy, Scala and Kotlin may be the most popular ones. A notable feature with the JVM languages is that they are compatible with each other, so that, for example, Scala libraries can be used with Java programs and vice versa.[8]
Java 7 JVM implements JSR 292: Supporting Dynamically Typed Languages[9] 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.[10][11]
Bytecode verifier
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,[12] allowing the JIT compiler to transform stack accesses into fixed register accesses. Because of this, that the JVM is a stack architecture does not imply a speed penalty for emulation on register-based architectures when using a JIT compiler. In the face of the code-verified JVM architecture, it makes no difference to a JIT compiler whether it gets named imaginary registers or imaginary stack positions that must be allocated to the target architecture's registers. In fact, code verification makes the JVM different from a classic stack architecture, of which efficient emulation with a JIT compiler is more complicated and typically carried out by a slower interpreter. Additionally, the Interpreter used by the default JVM is a special type known as a Template Interpreter, which translates bytecode directly to native, register based machine language rather than emulate a stack like a typical interpreter.[13] In many aspects the HotSpot Interpreter can be considered a JIT compiler rather than a true interpreter, meaning the stack architecture that the bytecode targets is not actually used in the implementation, but merely a specification for the intermediate representation that can well be implemented in a register based architecture. Another instance of a stack architecture being merely a specification and implemented in a register based virtual machine is the Common Language Runtime.[14]
The original specification for the bytecode verifier used natural language that was incomplete or incorrect in some respects. A number of attempts have been made to specify the JVM as a formal system. By doing this, the security of current JVM implementations can more thoroughly be analyzed, and potential security exploits prevented. It will also be possible to optimize the JVM by skipping unnecessary safety checks, if the application being run is proven to be safe.[15]
Secure execution of remote code
A virtual machine architecture allows very fine-grained control over the actions that code within the machine is permitted to take. It assumes the code is "semantically" correct, that is, it successfully passed the (formal) bytecode verifier process, materialized by a tool, possibly off-board the virtual machine. This is designed to allow safe execution of untrusted code from remote sources, a model used by
Formal proof of bytecode verifiers have been done by the Javacard industry (Formal Development of an Embedded Verifier for Java Card Byte Code[16])
Bytecode interpreter and just-in-time compiler
For each hardware architecture a different Java bytecode interpreter is needed. When a computer has a Java bytecode interpreter, it can run any Java bytecode program, and the same program can be run on any computer that has such an interpreter.
When Java bytecode is executed by an interpreter, the execution will always be slower than the execution of the same program compiled into native machine language. This problem is mitigated by just-in-time (JIT) compilers for executing Java bytecode. A JIT compiler may translate Java bytecode into native machine language while executing the program. The translated parts of the program can then be executed much more quickly than they could be interpreted. This technique gets applied to those parts of a program frequently executed. This way a JIT compiler can significantly speed up the overall execution time.
There is no necessary connection between the Java programming language and Java bytecode. A program written in Java can be compiled directly into the machine language of a real computer and programs written in other languages than Java can be compiled into Java bytecode.
Java bytecode is intended to be platform-independent and secure.[17] Some JVM implementations do not include an interpreter, but consist only of a just-in-time compiler.[18]
JVM in the web browser
At the start of the Java platform's lifetime, the JVM was marketed as a web technology for creating
The
As of June 2015[update] according to W3Techs, Java applet and Silverlight use had fallen to 0.1% each for all web sites, while Flash had fallen to 10.8%.[20]
JavaScript JVMs and interpreters
Since May 2016, JavaPoly allows users to import unmodified Java libraries, and invoke them directly from JavaScript. JavaPoly allows websites to use unmodified Java libraries, even if the user does not have Java installed on their computer.[21]
Transpilation to JavaScript
With the continuing improvements in JavaScript execution speed, combined with the increased use of mobile devices whose web browsers do not implement support for plugins, there are efforts to target those users through
Compiling the JVM bytecode, which is universal across JVM languages, allows building upon the language's existing compiler to bytecode. The main JVM bytecode to JavaScript transpilers are TeaVM,[22] the compiler contained in Dragome Web SDK,[23] Bck2Brwsr,[24] and j2js-compiler.[25]
Leading transpilers from JVM languages to JavaScript include the Java-to-JavaScript transpiler contained in Google Web Toolkit, Clojurescript (Clojure), GrooScript (Apache Groovy), Scala.js (Scala) and others.[26]
See also
- Common Language Runtime
- List of Java virtual machines
- List of JVM languages
- Comparison of Java virtual machines
- Comparison of application virtualization software
- Automated exception handling
- Java performance
- Java processor
- K virtual machine (KVM)
References
- ^ yan (2023-06-24). "JDK 20 Release Notes". Oracle Corporation. Archived from the original on 2021-07-09. Retrieved 2023-06-24.
- ^ Bill Venners, Inside the Java Virtual Machine Archived 2021-01-25 at the Wayback Machine Chapter 5
- ^ "The Java Community Process(SM) Program - JSRs: Java Specification Requests - detail JSR# 924". Jcp.org. Archived from the original on 2020-12-24. Retrieved 2015-06-26.
- ^ "The Java Community Process(SM) Program - JSRs: Java Specification Requests - detail JSR# 202". Jcp.org. Archived from the original on 2012-02-26. Retrieved 2015-06-26.
- ^ The Java Virtual Machine Specification Archived 2008-07-09 at the Wayback Machine (the first Archived 2008-10-12 at the Wayback Machine and second Archived 2011-09-25 at the Wayback Machine editions are also available online).
- ^ "Chapter 2. The Structure of the Java Virtual Machine". Archived from the original on 2021-09-15. Retrieved 2021-09-15.
- ^ "The Java Virtual Machine Specification : Java SE 7 Edition" (PDF). Docs.oracle.com. Archived (PDF) from the original on 2021-02-04. Retrieved 2015-06-26.
- ^ "Frequently Asked Questions - Java Interoperability". scala-lang.org. Archived from the original on 2020-08-09. Retrieved 2015-11-18.
- ^ "The Java Community Process(SM) Program - JSRs: Java Specification Requests - detail JSR# 292". Jcp.org. Archived from the original on 2020-12-20. Retrieved 2015-06-26.
- ^ "Da Vinci Machine project". Openjdk.java.net. Archived from the original on 2020-11-11. Retrieved 2015-06-26.
- ^ "New JDK 7 Feature: Support for Dynamically Typed Languages in the Java Virtual Machine". Oracle.com. Archived from the original on 2018-09-13. Retrieved 2015-06-26.
- ^ "The Verification process". The Java Virtual Machine Specification. Sun Microsystems. 1999. Archived from the original on 2011-03-21. Retrieved 2009-05-31.
- ^ "HotSpot Runtime Overview - Interpreter". OpenJDK. Archived from the original on 2022-05-21. Retrieved 2021-05-24.
- ^ "Why not make CLR register-based? · Issue #4775 · dotnet/runtime". GitHub. Archived from the original on 2023-04-20. Retrieved 2021-05-24.
- S2CID 14302964.
- ^ Casset, Ludovic; Burdy, Lilian; Requet, Antoine (10 April 2002). "Formal Development of an Embedded Verifier for Java Card Byte Code" (PDF). Inria - National Institute for Research in Digital Science and Technology at Côte d'Azur University. Archived (PDF) from the original on 3 October 2022.
- ^ David J. Eck, Introduction to Programming Using Java Archived 2014-10-11 at the Wayback Machine, Seventh Edition, Version 7.0, August 2014 at Section 1.3 "The Java Virtual Machine"
- ^ Oracle JRockit Introduction Archived 2015-09-06 at the Wayback Machine Release R28 at 2. "Understanding Just-In-Time Compilation and Optimization"
- ^ "Oracle deprecates the Java browser plugin, prepares for its demise". Ars Technica. 28 January 2016. Archived from the original on 8 April 2016. Retrieved 15 April 2016.
- ^ "Historical yearly trends in the usage of client-side programming languages, June 2015". W3techs.com. Retrieved 2015-06-26.
- ^ Krill, Paul (13 May 2016). "JavaPoly.js imports existing Java code and invokes it directly from JavaScript". InfoWorld. Archived from the original on 25 July 2016. Retrieved 18 July 2016.
- ^ "TeaVM project home page". Teavm.org. Archived from the original on 2015-06-27. Retrieved 2015-06-26.
- ^ "Dragome Web SDK". Dragome.com. Archived from the original on 2015-08-01. Retrieved 2015-06-26.
- ^ "Bck2Brwsr - APIDesign". Wiki.apidesign.org. Archived from the original on 2015-06-27. Retrieved 2015-06-26.
- ^ Wolfgang Kuehn (decatur). j2js-compiler Archived 2013-09-29 at the Wayback Machine GitHub
- ^ "List of languages that compile to JS · jashkenas/coffeescript Wiki · GitHub". Github.com. 2015-06-19. Archived from the original on 2020-01-31. Retrieved 2015-06-26.
- Clarifications and Amendments to the Java Virtual Machine Specification, Second Edition Archived 2006-01-10 at the Wayback Machine includes list of changes to be made to support J2SE 5.0 and JSR 45
- JSR 45 Archived 2006-02-05 at the JavaServer Pages (JSP) and SQLJthat are translated to Java