JDK 是整个Java的核心，包括了Java运行环境（Java Runtime Envirnment），一堆Java工具和Java基础的类库(rt.jar)。不论什么Java应用服务器实质都是内置了某个版本的JDK。因此掌握 JDK是学好Java的第一步。最主流的JDK是Sun公司发布的JDK，除了Sun之外，还有很多公司和组织都开发了自己的JDK，例如IBM公司开发的JDK，BEA公司的Jrocket，还有GNU组织开发的JDK等等。其中IBM的JDK包含的JVM（Java Virtual Machine）运行效率要比Sun JDK包含的JVM高出许多。而专门运行在x86平台的Jrocket在服务端运行效率也要比Sun JDK好很多。但不管怎么说，我们还是需要先把Sun JDK掌握好。

Java Platform, Standard Edition (Java SE, formerly known as J2SE) offers a complete environment for application development and deployment on desktops and servers. Java SE is also at home in today's demanding Embedded and Real-Time environments. Java SE includes classes that support the development of Java Web Services and provides the foundation for Java Platform, Enterprise Edition (Java EE).

 下载地址：
·Java SE Development Kit (JDK) 6
·Java SE Development Kit (JDK) 6 and NetBeans IDE 5.5 Final Release
·Java SE Development Kit (JDK) 6 with Java EE
·Java Runtime Environment (JRE) 6