1. Introduction The JDK, JRE and JVM

JDK (Java Development Kit) java Development Kit: JAVA core of the whole, including the Java Runtime Environment JRE (Java Runtime Envirnment), a bunch of Java tools (javac / java / jdb, etc.) and Java-based class libraries (ie Java API including rt.jar).
JRE (Java Runtime Environment): is based on running programs written in the Java language indispensable operating environment. JRE Java Runtime Environment is not a development environment, it does not contain any development tools (such as compilers and debuggers), but for users to use Java programs; JRE consists mainly JVM (Java Virtual Machine) and java base class libraries .
JVM (Java Virtual Machine): We often say that the java virtual machine, all the java programming will first be compiled into class files .class of this class file can be executed on a virtual machine.
In short:
the JDK = the JRE + compiler and other tools;
the JRE = the JVM (the Java Virtual Machine) + the Java Foundation Classes