1，jdk
install:
java_home: point to the installation directory of JDK, which is used by some java development tools such as Tomcat and Eclipse.
path: path is system variable which is used by windows. It records some location of some runnable software. It' s convenient to use some commands directly.
classpath: classpath is similar to the path but the path is used by the operating system and classpath is used by java. It tells the java execution system where we find the class we need.
bin: the bin is including some development tools like javac(java compiler), JMS(java mission control), JavaDoc.
lib: when we develop an application, we need some jar. The exe needs those jar as independence.
include: include folder include c head file which supports some interfaces of JVM tools.
2, JRE
jre：jre is including two folders. One is the bin which means JVM. Another is lib. lib is the class library needed for JVM to work. The JVM uses those libs to interpret your own classes.
3, JVM
JVM : The JVM manage the system memory and provide a portable execution environment for java-based applications.
gc: continuously identifies and eliminates unused memory in Java programs.