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has been superseded by later standards based java me configuration profile pairings execution system java jvm and bytecode main articles java software platform and java virtual machine one design goal of java is portability which means that programs written for the java platform must run similarly on any combination of hardware and operating system with adequate run time support this is achieved by compiling the java language code to an intermediate representation called jvm bytecode instead of directly to architecture specific machine code jvm bytecode instructions are analogous to machine code but they are intended to be executed by a virtual machine vm written specifically for the host hardware end users commonly use a java runtime environment jre installed on their device for standalone java applications or a web browser for java applets standard libraries provide a generic way to access host specific features such as graphics threading and networking the use of universal bytecode makes porting simple however the overhead of interpreting bytecode into machine instructions made interpreted programs almost always run more slowly than native executables just in time jit compilers that compile byte codes to machine code during runtime were introduced from an early stage java s hotspot compiler is actually two compilers in one with graalvm included in e g java 11 but removed as of java 16 allowing tiered compilation 52 java itself is platform independent and is adapted to the particular platform it is to run on by a java virtual machine jvm which translates the jvm bytecode into the platform s machine language 53 performance main article java performance programs written in java have a reputation for being slower and requiring more memory than those written in c 54 55 however java programs execution speed improved significantly with the introduction of just in time compilation in 1997 1998 for java 1 1 56 the addition of language features supporting better code analysis such as inner classes the stringbuilder class optional assertions etc and optimizations in the java virtual machine such as hotspot becoming sun s default jvm in 2000 with java 1 5 the performance was improved with the addition of the java util concurrent package including lock free implementations of the concurrentmaps and other multi core collections and it was improved further with java 1 6 non jvm some platforms offer direct hardware support for java there are micro controllers that can run jvm bytecode in hardware instead of a software java virtual machine 57 and some arm based processors could have hardware support for executing jvm bytecode through their jazelle option though support has mostly been dropped in current implementations of arm automatic memory management java uses an automatic garbage collector to manage memory in the object lifecycle the programmer determines when objects are created and the java runtime is responsible for recovering the memory once objects are no longer in use once no references to an object remain the unreachable memory becomes eligible to be freed automatically by the garbage collector something similar to a memory leak may still occur if a programmer s code holds a reference to an object that is no longer needed typically when objects that are no longer needed are stored in containers that are still in use 58 if methods for a non existent object are called a null pointer exception is thrown 59 60 one of the ideas behind java s automatic memory management model is that programmers can be spared the burden of having to perform manual memory management in some languages memory for the creation of objects is implicitly allocated on the stack or explicitly allocated and deallocated from the heap in the latter case the responsibility of managing memory resides with the programmer if the program does not deallocate an object a memory leak occurs 58 if the program attempts to access or deallocate memory that has already been deallocated the result is undefined and difficult to predict and the program is likely to become unstable or crash this can be partially remedied by the use of smart pointers but these add overhead and complexity garbage collection does not prevent logical memory leaks i e those where the memory is still referenced but never used 58 garbage collection may happen at any time ideally it will occur when a program is idle it is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object this can cause a program to stall momentarily explicit memory management is not possible in java however it is possible to manually request that the jvm do garbage collection 61 java does not support c c style pointer arithmetic 62 where object addresses can be arithmetically manipulated e g by adding or subtracting an offset this allows the garbage collector to relocate referenced objects and ensures type safety and security as in c and some other object oriented languages variables of java s primitive data types are either stored directly in fields for objects or on the stack for methods rather than on the heap as is commonly true for non primitive data types but see escape analysis this was a conscious decision by java s designers for performance reasons java contains multiple types of garbage collectors since java 9 hotspot uses the garbage first garbage collector g1gc as the default 63 however there are also several other garbage collectors that can be used to manage the heap such as the z garbage collector zgc introduced in java 11 and shenandoah gc introduced in java 12 but unavailable in oracle produced openjdk builds shenandoah is instead available in third party builds of openjdk such as eclipse temurin for most applications in java g1gc is sufficient in prior versions of java such as java 8 the parallel garbage collector was used as the default garbage collector citation needed having solved the memory management problem does not relieve the programmer of the burden of handling properly other kinds of resources like network or database connections file handles etc especially in the presence of exceptions citation needed syntax main article java syntax this dependency graph of the java core classes was created with jdeps and gephi the syntax of java is largely influenced by c and c unlike c which combines the syntax for structured generic and object oriented programming java was built almost exclusively as an object oriented language 18 all code is written inside classes and every data item is an object with the exception of the primitive data types i e integers floating point numbers boolean values and characters which are not objects for performance reasons unlike c java does not support operator overloading 64 or multiple inheritance for classes though multiple inheritance is supported for interfaces 65 java uses comments similar to those of c there are three different styles of comments a single line style marked with two slashes a multiple line style opened with and closed with and the javadoc commenting style opened with and closed with the javadoc style of commenting allows the user to run the javadoc executable to create documentation for the program and can be read by some integrated development environments ides such as eclipse to allow developers to access documentation within the ide hello world the following is an example of a hello world program in the traditional java syntax public class helloworld public static void main string args system out println hello world java 25 introduced a simplified syntax for the main class and main method which enables users to write small java programs succinctly 66 67 void main io println hello world special classes this section needs more citations please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed find sources java programming language news newspapers books scholar jstor may 2019 learn how and when to remove this message applet main article java applet java applets were programs embedded in other applications mainly in web pages displayed in web browsers the java applet api was deprecated with the release of java 9 in 2017 68 69 servlet main article java servlet java servlet technology provides web developers with a simple consistent mechanism for extending the functionality of a web server and for accessing existing business systems servlets are server side java ee components that generate responses to requests from clients most of the time this means generating html pages in response to http requests although there are a number of other standard servlet classes available for example for websocket communication the java servlet api has to some extent been superseded but still used under the hood by two standard java technologies for web services the java api for restful web services jax rs 2 0 useful for ajax json and rest services and the java api for xml web services jax ws useful for soap web services typical implementations of these apis on application servers or servlet containers use a standard servlet for handling all interactions with the http requests and responses that delegate to the web service methods for the actual business logic javaserver pages main article javaserver pages javaserver pages jsp are server side java ee components that generate responses typically html pages to http requests from clients jsps embed java code in an html page by using the special delimiters and a jsp is compiled to a java servlet a java application in its own right the first time it is accessed after that the generated servlet creates the response 70 swing application main article swing java swing is a graphical user interface library for the java se platform it is possible to specify a different look and feel through the pluggable look and feel system of swing clones of windows gtk and motif are supplied by sun apple also provides an aqua look and feel for macos where prior implementations of these looks and feels may have been considered lacking swing in java se 6 addresses this problem by using more native gui widget drawing routines of the underlying platforms 71 javafx application main article javafx javafx is a software platform for creating and delivering desktop applications as well as rich web applications that can run across a wide variety of devices javafx is intended to replace swing as the standard graphical user interface gui library for java se but since jdk 11 javafx has not been in the core jdk and is instead available as a separate module 72 javafx has support for desktop computers and web browsers on microsoft windows linux and macos javafx does not have support for native os look and feels 73 generics main article generics in java in 2004 generics were added to the java language as part of j2se 5 0 prior to the introduction of generics each variable declaration had to be of a specific type for container classes for example this is a problem because there is no easy way to create a container that accepts only specific types of objects either the container operates on all subtypes of a class or interface usually object or a different container class has to be created for each contained class generics allow compile time type checking without having to create many container classes each containing almost identical code in addition to enabling more efficient code certain runtime exceptions are prevented from occurring by issuing compile time errors if java prevented all runtime type errors classcastexception s from occurring it would be type safe in 2016 the type system of java was proven unsound in that it is possible to use generics to construct classes and methods that allow assignment of an instance of one class to a variable of another unrelated class such code is accepted by the compiler but fails at run time with a class cast exception 74 criticism main article criticism of java criticisms directed at java include the implementation of generics 75 speed 54 the handling of unsigned numbers 76 the implementation of floating point arithmetic 77 and a history of security vulnerabilities in the primary java vm implementation hotspot 78 developers have criticized the complexity and verbosity of the java persistence api jpa a standard part of java ee this has led to increased adoption of higher level abstractions like spring data jpa which aims to simplify database operations and reduce boilerplate code the growing popularity of such frameworks suggests limitations in the standard jpa implementation s ease of use for modern java development class libraries main article java class library the java class library is the standard library developed to support application development in java it is controlled by oracle in cooperation with others through the java community process program 79 companies or individuals participating in this process can influence the design and development of the apis this process has been a subject of controversy during the 2010s 80 the class library contains features such as the core libraries which include input output i o or io 81 and non blocking i o nio or io nio 82 networking 83 new user agent http client since java 11 84 reflective programming reflection concurrent computing concurrency 81 generics scripting compiler functional programming lambda streaming collection libraries that implement data structures such as lists dictionaries trees sets queues and double ended queue or stacks 85 xml processing parsing transforming validating libraries security 86 internationalization and localization libraries 87 the integration libraries which allow the application writer to communicate with external systems these libraries include the java database connectivity jdbc api for database access java naming and directory interface jndi for lookup and discovery java remote method invocation rmi and common object request broker architecture corba for distributed application development java management extensions jmx for managing and monitoring applications user interface libraries which include the heavyweight or native abstract window toolkit awt which provides gui components the means for laying out those components and the means for handling events from those components the lightweight swing libraries which are built on awt but provide non native implementations of the awt widgetry apis for audio capture processing and playback javafx a platform dependent implementation of the java virtual machine that is the means by which the bytecodes of the java libraries and third party applications are executed plugins which enable applets to be run in web browsers java web start which allows java applications to be efficiently distributed to end users across the internet licensing and documentation documentation main article javadoc javadoc is a comprehensive documentation system created by sun microsystems it provides devel...
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