Java Overview，javaoverview

关于Java技术：

About the Java Technology

Java technology is both a programming language and a platform.

The Java Programming Language

The Java programming language is a high-level language that can be characterized by all of the following buzzwords:

Simple Object oriented Distributed Multithreaded Dynamic

Architecture neutral Portable High performance Robust Secure

Each of the preceding buzzwords is explained in The Java Language Environment , a white paper written by James Gosling and Henry McGilton.

In the Java programming language, all source code is first written in plain text files ending with the.java extension. Those source files are then compiled into.class files by thejavac compiler. A .class file does not contain code that is native to your processor; it instead containsbytecodes — the machine language of the Java Virtual Machine¹ (Java VM). Thejava launcher tool then runs your application with an instance of the Java Virtual Machine.

An overview of the software development process.

Because the Java VM is available on many different operating systems, the same.class files are capable of running on Microsoft Windows, the Solaris™ Operating System (Solaris OS), Linux, or Mac OS. Some virtual machines, such as theJava HotSpot virtual machine, perform additional steps at runtime to give your application a performance boost. This include various tasks such as finding performance bottlenecks and recompiling (to native code) frequently used sections of code.

Through the Java VM, the same application is capable of running on multiple platforms.

The Java Platform

A platform is the hardware or software environment in which a program runs. We've already mentioned some of the most popular platforms like Microsoft Windows, Linux, Solaris OS, and Mac OS. Most platforms can be described as a combination of the operating system and underlying hardware. The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms.

The Java platform has two components:

• The Java Virtual Machine
• The Java Application Programming Interface (API)

You've already been introduced to the Java Virtual Machine; it's the base for the Java platform and is ported onto various hardware-based platforms.

The API is a large collection of ready-made software components that provide many useful capabilities. It is grouped into libraries of related classes and interfaces; these libraries are known aspackages. The next section,What Can Java Technology Do? highlights some of the functionality provided by the API.

The API and Java Virtual Machine insulate the program from the underlying hardware.

As a platform-independent environment, the Java platform can be a bit slower than native code. However, advances in compiler and virtual machine technologies are bringing performance close to that of native code without threatening portability.

The terms"Java Virtual Machine" and "JVM" mean a Virtual Machine for the Java platform.

Java Virtual Machine

The heart of the Java platform is the concept of a "virtual machine" that executesJava bytecode programs. This bytecode is the same no matter what hardware or operating system the program is running under. There is a JIT compiler within theJava Virtual Machine, or JVM. The JIT compiler translates the Java bytecode into native processor instructions at run-time and caches the native code in memory during execution.

The use of bytecode as an intermediate language permits Java programs to run on any platform that has a virtual machine available. The use of a JIT compiler means that Java applications, after a short delay during loading and once they have "warmed up" by being all or mostly JIT-compiled, tend to run about as fast as native programs. Since JRE version 1.2, Sun's JVM implementation has included ajust-in-time compiler instead of an interpreter.

Although Java programs are cross-platform or platform independent, the code of the Java Virtual Machines (JVM) that execute these programs is not. Every supported operating platform has its own JVM.

引用：

http://download.oracle.com/javase/tutorial/getStarted/intro/definition.html

http://en.wikipedia.org/wiki/Java_platform#Edition