eixt()方法的说明：
public static void exit(int status)
Terminates the currently running Java Virtual Machine. The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.
This method calls the exit method in class Runtime. This method never returns normally.
The call System.exit(n) is effectively equivalent to the call:
Runtime.getRuntime().exit(n)
Parameters:
status – exit status.
Throws:
SecurityException – if a security manager exists and its checkExit method doesn’t allow exit with the specified status.
See Also:
Runtime.exit(int)
终止虚拟机的运行，其中，参数为0表示正常终止，参数非0为非正常终止。
[http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#exit(int)]