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68-513: 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. The Java virtual machine is an abstract (virtual) computer defined by a specification. It is a part of

136-517: A bool . In Java , the class BitSet creates a bit array that is then manipulated with functions named after bitwise operators familiar to C programmers. Unlike the bitset in C++, the Java BitSet does not have a "size" state (it has an effectively infinite size, initialized with 0 bits); a bit can be set or tested at any index. In addition, there is a class EnumSet , which represents

204-455: A proxy reference . This might seem a minor point, but it means that vector<bool> is not a standard STL container, which is why the use of vector<bool> is generally discouraged. Another unique STL class, bitset , creates a vector of bits fixed at a particular size at compile-time, and in its interface and syntax more resembles the idiomatic use of words as bit sets by C programmers. It also has some additional power, such as

272-456: 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

340-698: A Bit array, although this lacks support from the former module. In Perl , strings can be used as expandable bit arrays. They can be manipulated using the usual bitwise operators ( ~ | & ^ ), and individual bits can be tested and set using the vec function. In Ruby , you can access (but not set) a bit of an integer ( Fixnum or Bignum ) using the bracket operator ( [] ), as if it were an array of bits. Apple's Core Foundation library contains CFBitVector and CFMutableBitVector structures. PL/I supports arrays of bit strings of arbitrary length, which may be either fixed-length or varying. The array elements may be aligned — each element begins on

408-827: A JVM, Java applets are not restricted to the Java programming language; any language targeting the JVM may run in the plug-in. A restricted set of APIs allow applets access to the user's microphone or 3D acceleration, although applets are not able to modify the page outside its rectangular region. Adobe Flash Player , the main competing technology, works in the same way in this respect. As of June 2015 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%. 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

476-454: A Java plug-in , nor do they permit side-loading any non- Flash plug-in. The Java browser plugin was deprecated in JDK 9. The NPAPI Java browser plug-in was designed to allow the JVM to execute so-called Java applets embedded into HTML pages. For browsers with the plug-in installed, the applet is allowed to draw into a rectangular region on the page assigned to it. Because the plug-in includes

544-518: A Set of values of an enumerated type internally as a bit vector, as a safer alternative to bit fields. The .NET Framework supplies a BitArray collection class. It stores bits using an array of type int (each element in the array usually represents 32 bits). The class supports random access and bitwise operators, can be iterated over, and its Length property can be changed to grow or truncate it. Although Standard ML has no support for bit arrays, Standard ML of New Jersey has an extension,

612-546: A bit array in a straightforward manner. A bit array is the most dense storage for "random" bits, that is, where each bit is equally likely to be 0 or 1, and each one is independent. But most data are not random, so it may be possible to store it more compactly. For example, the data of a typical fax image is not random and can be compressed. Run-length encoding is commonly used to compress these long streams. However, most compressed data formats are not so easy to access randomly; also by compressing bit arrays too aggressively we run

680-429: A bit array, find first one can be used to identify the highest priority element in the queue. To expand a word-size find first one to longer arrays, one can find the first nonzero word and then run find first one on that word. The related operations find first zero , count leading zeros , count leading ones , count trailing zeros , count trailing ones , and log base 2 (see find first set ) can also be extended to

748-517: A bit is set, by zero-testing: XOR to invert or toggle a bit: NOT to invert all bits: To obtain the bit mask needed for these operations, we can use a bit shift operator to shift the number 1 to the left by the appropriate number of places, as well as bitwise negation if necessary. Given two bit arrays of the same size representing sets, we can compute their union , intersection , and set-theoretic difference using n / w simple bit operations each (2 n / w for difference), as well as

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816-671: 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. The JVM operates on specific types of data as specified in Java Virtual Machine specifications. The data types can be divided into primitive types ( integers , Floating-point, long etc.) and Reference types. The earlier JVM were only 32-bit machines. long and double types, which are 64-bits , are supported natively, but consume two units of storage in

884-638: A byte or word boundary— or unaligned — elements immediately follow each other with no padding. PL/pgSQL and PostgreSQL's SQL support bit strings as native type. There are two SQL bit types: bit( n ) and bit varying( n ) , where n is a positive integer. Hardware description languages such as VHDL , Verilog , and SystemVerilog natively support bit vectors as these are used to model storage elements like flip-flops , hardware busses and hardware signals in general. In hardware verification languages such as OpenVera, e and SystemVerilog , bit vectors are used to sample values from

952-550: A compiler. The JVM has instructions for the following groups of tasks: 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

1020-598: A convenient syntax, the bits are still accessed using bytewise operators on most machines, and they can only be defined statically (like C's static arrays, their sizes are fixed at compile-time). It is also a common idiom for C programmers to use words as small bit arrays and access bits of them using bit operators. A widely available header file included in the X11 system, xtrapbits.h, is “a portable way for systems to define bit field manipulation of arrays of bits.” A more explanatory description of aforementioned approach can be found in

1088-462: A find-first-zero operation in hardware. Bit arrays can be used for the allocation of memory pages , inodes , disk sectors, etc. In such cases, the term bitmap may be used. However, this term is frequently used to refer to raster images , which may use multiple bits per pixel . Another application of bit arrays is the Bloom filter , a probabilistic set data structure that can store large sets in

1156-505: A fixed stack location, 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

1224-512: 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

1292-412: A good representation for the posting lists of very frequent terms. If we compute the gaps between adjacent values in a list of strictly increasing integers and encode them using unary coding , the result is a bit array with a 1 bit in the n th position if and only if n is in the list. The implied probability of a gap of n is 1/2 . This is also the special case of Golomb coding where the parameter M

1360-450: A number of applications in areas where space or efficiency is at a premium. Most commonly, they are used to represent a simple group of Boolean flags or an ordered sequence of Boolean values. Bit arrays are used for priority queues , where the bit at index k is set if and only if k is in the queue; this data structure is used, for example, by the Linux kernel , and benefits strongly from

1428-457: A series of simple bit operations. We simply run such an algorithm on each word and keep a running total. Counting zeros is similar. See the Hamming weight article for examples of an efficient implementation. Vertical flipping of a one-bit-per-pixel image, or some FFT algorithms, requires flipping the bits of individual words (so b31 b30 ... b0 becomes b0 ... b30 b31 ). When this operation

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1496-433: A set of common abstracted data types rather the native data types of any specific instruction set architecture . 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 ( Java byte code ) and a symbol table, as well as other ancillary information. The class file format

1564-411: A small space in exchange for a small probability of error. It is also possible to build probabilistic hash tables based on bit arrays that accept either false positives or false negatives. Bit arrays and the operations on them are also important for constructing succinct data structures , which use close to the minimum possible space. In this context, operations like finding the n th 1 bit or counting

1632-551: A specification and implemented in a register based virtual machine is the Common Language Runtime . 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

1700-492: A stack like a typical interpreter. 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

1768-436: 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

1836-400: Is 1; this parameter is only normally selected when −log(2 − p ) / log(1 − p ) ≤ 1 , or roughly the term occurs in at least 38% of documents. The APL programming language fully supports bit arrays of arbitrary shape and size as a Boolean datatype distinct from integers. All major implementations ( Dyalog APL, APL2, APL Next, NARS2000, Gnu APL , etc.) pack the bits densely into whatever size

1904-460: Is designed to allow safe execution of untrusted code from remote sources, a model used by Java applets , and other secure code downloads. Once bytecode-verified, the downloaded code runs in a restricted " sandbox ", which is designed to protect the user from misbehaving or malicious code. As an addition to the bytecode verification process, publishers can purchase a certificate with which to digitally sign applets as safe, giving them permission to ask

1972-406: Is not available on the processor, it's still possible to proceed by successive passes, in this example on 32 bits: The find first set or find first one operation identifies the index or position of the 1-bit with the smallest index in an array, and has widespread hardware support (for arrays not larger than a word) and efficient algorithms for its computation. When a priority queue is stored in

2040-406: Is straightforward to define length , substring , lexicographical compare , concatenation , reverse operations. The implementation of some of these operations is sensitive to endianness . If we wish to find the number of 1 bits in a bit array, sometimes called the population count or Hamming weight, there are efficient branch-free algorithms that can compute the number of bits in a word using

2108-467: Is that there are only two possible values, so they can be stored in one bit. As with other arrays, the access to a single bit can be managed by applying an index to the array. Assuming its size (or length) to be n bits, the array can be used to specify a subset of the domain (e.g. {0, 1, 2, ..., n −1}), where a 1-bit indicates the presence and a 0-bit the absence of a number in the set. This set data structure uses about n / w words of space, where w

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2176-468: Is the hardware- and operating system-independent binary format used to represent compiled classes and interfaces. 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

2244-431: Is the number of bits in each machine word . Whether the least significant bit (of the word) or the most significant bit indicates the smallest-index number is largely irrelevant, but the former tends to be preferred (on little-endian machines). A finite binary relation may be represented by a bit array called a logical matrix . In the calculus of relations , these arrays are composed with matrix multiplication where

2312-443: Is the number of bits in the unit of storage, such as a byte or word , and k is some nonnegative integer. If w does not divide the number of bits to be stored, some space is wasted due to internal fragmentation . A bit array is a mapping from some domain (almost always a range of integers) to values in the set {0, 1}. The values can be interpreted as dark/light, absent/present, locked/unlocked, valid/invalid, et cetera. The point

2380-434: Is to extend the JVM so that it supports languages other than Java. 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

2448-500: 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

2516-497: The BitArray structure, in its SML/NJ Library. It is not fixed in size and supports set operations and bit operations, including, unusually, shift operations. Haskell likewise currently lacks standard support for bitwise operations, but both GHC and Hugs provide a Data.Bits module with assorted bitwise functions and operators, including shift and rotate operations and an "unboxed" array over Boolean values may be used to model

2584-584: The Fraunhofer reference implementation of the MP3 standard usually does not compare favorably to other common implementations, such as LAME , in listening tests that determine sound quality. In contrast, CPython , the reference implementation of the Python programming language , is also the implementation most widely used in production. Testing the implementation-vs-specification relationship further enhances

2652-457: The comp.lang.c faq . In C++ , although individual bool s typically occupy the same space as a byte or an integer, the STL type vector<bool> is a partial template specialization in which bits are packed as a space efficiency optimization. Since bytes (and not bits) are the smallest addressable unit in C++, the [] operator does not return a reference to an element, but instead returns

2720-477: The complement of either: If we wish to iterate through the bits of a bit array, we can do this efficiently using a doubly nested loop that loops through each word, one at a time. Only n / w memory accesses are required: Both of these code samples exhibit ideal locality of reference , which will subsequently receive large performance boost from a data cache. If a cache line is k words, only about n / wk cache misses will occur. As with character strings it

2788-412: The production 's inter-process efficiencies: A reference implementation is, in general, an implementation of a specification to be used as a definitive interpretation for that specification. During the development of the ... conformance test suite, at least one relatively trusted implementation of each interface is necessary to (1) discover errors or ambiguities in the specification, and (2) validate

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2856-485: The software development process , a reference implementation (or, less frequently, sample implementation or model implementation ) is a program that implements all requirements from a corresponding specification. The reference implementation often accompanies a technical standard , and demonstrates what should be considered the "correct" behavior of any other implementation of it. Reference implementations of algorithms, for instance cryptographic algorithms , are often

2924-503: 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

2992-419: The JVM by skipping unnecessary safety checks, if the application being run is proven to be safe. 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

3060-713: 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, the compiler contained in Dragome Web SDK, Bck2Brwsr, and j2js-compiler. 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. Reference implementation In

3128-417: 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.) However the newer JVM releases (OpenJDK HotSpot JVM) support 64-bit, so you can either have 32-bit/64-bit JVM on

3196-473: The JVM specification have been developed under the Java Community Process as JSR 924. As of 2006, changes to the specification to support changes proposed to the class file format (JSR 202) are being done as a maintenance release of JSR 924. The specification for the JVM was published as the blue book , whose preface states: We intend that this specification should sufficiently document

3264-422: 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. One of

3332-486: 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. Starting with Java Platform, Standard Edition (J2SE) 5.0, changes to

3400-414: The ability to efficiently count the number of bits that are set. The Boost C++ Libraries provide a dynamic_bitset class whose size is specified at run-time. The D programming language provides bit arrays in its standard library, Phobos, in std.bitmanip . As in C++, the [] operator does not return a reference, since individual bits are not directly addressable on most hardware, but instead returns

3468-403: The arithmetic is Boolean, and such a composition represents composition of relations . Although most machines are not able to address individual bits in memory, nor have instructions to manipulate single bits, each bit in a word can be singled out and manipulated using bitwise operations . In particular: Use OR to set a bit to one: AND to set a bit to zero: AND to determine if

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3536-495: The bit vector to be designated as dynamically resizable. The bit-vector , however, is not infinite in extent. A more restricted simple-bit-vector type exists, which explicitly excludes the dynamic characteristics. Bit vectors are represented as, and can be constructed in a more concise fashion by, the reader macro #* bits . In addition to the general functions applicable to all arrays, dedicated operations exist for bit vectors. Single bits may be accessed and modified using

3604-479: The correct functioning of the test suite. Characteristics of a Reference Implementation: Bit array A bit array (also known as bitmask , bit map , bit set , bit string , or bit vector ) is an array data structure that compactly stores bits . It can be used to implement a simple set data structure . A bit array is effective at exploiting bit-level parallelism in hardware to perform operations quickly. A typical bit array stores kw bits, where w

3672-429: The hardware models, and to represent data that is transferred to hardware during simulations. Common Lisp provides a one-dimensional bit-vector implementation as a special case of the built-in array , acting in a dual capacity as a class and a type specifier. Being a derivative of the array, it relies on the general make-array function to be configured with an element type of bit , which optionally permits

3740-511: 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. Some JVM implementations do not include an interpreter, but consist only of a just-in-time compiler. At the start of the Java platform's lifetime, the JVM was marketed as a web technology for creating Rich Web Applications . As of 2018, most web browsers and operating systems bundling web browsers do not ship with

3808-500: The machine word is. Bits may be accessed individually via the usual indexing notation (A[3]) as well as through all of the usual primitive functions and operators where they are often operated on using a special case algorithm such as summing the bits via a table lookup of bytes. The C programming language 's bit fields , pseudo-objects found in structs with size equal to some number of bits, are in fact small bit arrays; they are limited in that they cannot span words. Although they give

3876-567: 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. Java 7 JVM implements JSR 292: Supporting Dynamically Typed Languages 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

3944-408: The number of 1 bits up to a certain position become important. Bit arrays are also a useful abstraction for examining streams of compressed data, which often contain elements that occupy portions of bytes or are not byte-aligned. For example, the compressed Huffman coding representation of a single 8-bit character can be anywhere from 1 to 255 bits long. In information retrieval , bit arrays are

4012-561: 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: 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

4080-423: 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

4148-466: The result or the input of standardization processes. In this function they are often dedicated to the public domain with their source code as public domain software . Examples are the first CERN's httpd , Serpent cipher, base64 variants, and SHA-3 . The Openwall Project maintains a list of several algorithms with their reference source code in the public domain. A reference implementation may or may not be production quality. For example,

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4216-473: The risk of losing the benefits due to bit-level parallelism ( vectorization ). Thus, instead of compressing bit arrays as streams of bits, we might compress them as streams of bytes or words (see Bitmap index (compression) ). Bit arrays, despite their simplicity, have a number of marked advantages over other data structures for the same problems: However, bit arrays are not the solution to everything. In particular: Because of their compactness, bit arrays have

4284-523: 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: The first two of these checks take place primarily during

4352-416: 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

4420-505: 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

4488-461: The user does not have Java installed on their computer. 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 transpilation to JavaScript. It is possible to either transpile the source code or JVM bytecode to JavaScript. Compiling

4556-511: The user to break out of the sandbox and access the local file system, clipboard , execute external pieces of software, or network. Formal proof of bytecode verifiers have been done by the Javacard industry (Formal Development of an Embedded Verifier for Java Card Byte Code) 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

4624-440: 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

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