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。-JDK is the core of Java, including Java runtime environment (Java Runtime Envirnment), a pile of Java tools and Java-based class library (rt.jar). Java application server regardless of the substance of what is a built-in version of the JDK. Grasp the JDK are therefore the first step to learn Java. JDK most mainstream companies are Sun' s JDK, except Sun, there are a lot of companies and organizations have developed their own JDK, such as IBM to develop the JDK, BEA' s Jrocket, also developed by GNU and so on JDK . IBM' s JDK contains one of the JVM (Java Virtual Machine) operating efficiency than the Sun JDK contains much higher than the JVM. Specifically x86 platform running on the server running Jrocket efficiency much better than the Sun JDK. But in any case, we still need to better grasp the Sun JDK first.