One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.
Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time compilers were introduced from an early stage that compile bytecodes to machine code during runtime.
[edit]Implementations
This section is outdated. Please update this section to reflect recent events or newly available information. Please see the talk page for more information. (September 2011)
Sun Microsystems officially licensed the Java Standard Edition platform for Linux,[23] Mac OS X,[24] and Solaris. In the past Sun licensed Java to Microsoft but the license expired without renewal.[25]Because Windows does not ship with a Java software platform, a network of third-party vendors and licensees[26] develop them for Windows and other operating system/hardware platforms.
Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.[27] As a result, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows.
Platform-independent Java is essential to the Java EE strategy, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications, such as Web services, Java Servlets, and Enterprise JavaBeans, as well as with embedded systems based on OSGi, using Embedded Java environments. Through the GlassFish project, Sun is working to create a fully functional, unified open source implementation of the Java EE technologies.
Sun also distributes a superset of the JRE called the Java Development Kit (commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, Jar, and debugger.
[edit]Performance
Main article: Java performance
Programs written in Java have a reputation for being slower and requiring more memory than those written in C.[28] However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,[29] the addition of language features supporting better code analysis (such as inner classes, StringBuffer class, optional assertions, etc.), and optimizations in the Java Virtual Machine itself, such as HotSpot becoming the default for Sun's JVM in 2000. Currently (November 2011), Java 2.0 code has approximately half the performance of C code.[30]
Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software JVM, and ARM based processors can have hardware support for executing Java bytecode through its Jazelle option.
Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time compilers were introduced from an early stage that compile bytecodes to machine code during runtime.
[edit]Implementations
This section is outdated. Please update this section to reflect recent events or newly available information. Please see the talk page for more information. (September 2011)
Sun Microsystems officially licensed the Java Standard Edition platform for Linux,[23] Mac OS X,[24] and Solaris. In the past Sun licensed Java to Microsoft but the license expired without renewal.[25]Because Windows does not ship with a Java software platform, a network of third-party vendors and licensees[26] develop them for Windows and other operating system/hardware platforms.
Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.[27] As a result, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows.
Platform-independent Java is essential to the Java EE strategy, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications, such as Web services, Java Servlets, and Enterprise JavaBeans, as well as with embedded systems based on OSGi, using Embedded Java environments. Through the GlassFish project, Sun is working to create a fully functional, unified open source implementation of the Java EE technologies.
Sun also distributes a superset of the JRE called the Java Development Kit (commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, Jar, and debugger.
[edit]Performance
Main article: Java performance
Programs written in Java have a reputation for being slower and requiring more memory than those written in C.[28] However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,[29] the addition of language features supporting better code analysis (such as inner classes, StringBuffer class, optional assertions, etc.), and optimizations in the Java Virtual Machine itself, such as HotSpot becoming the default for Sun's JVM in 2000. Currently (November 2011), Java 2.0 code has approximately half the performance of C code.[30]
Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software JVM, and ARM based processors can have hardware support for executing Java bytecode through its Jazelle option.
No comments:
Post a Comment