Basic Concepts
1. process: a dynamic concept, is running the program, can be seen as a collection of system resources.
2. Thread: included in the process, the program is the smallest unit of scheduling, shared process resources.
3. program: is a static concept, is a set of instructions and machine code.
一个程序是一个可执行的文件，而一个进程则是一个执行中的程序实例.
系统可以同时执行多个程序,并发执行,系统中可以存在一个程序多个实例.
多个进程可以并发执行同一个程序.
java应用对应一个jvm实例,一个程序对应多个jvm实例,多个jvm之间是隔离的.
一个java程序的多个线程，共享堆内存.