Design Pattern Learning Notes (ix)-singleton mode

The book "Design Pattern" is described in the singleton mode:

Guarantees that a class has only one instance and provides a global access point to access it.

This model is relatively simple, here is an example:

public class Singleton {
　private static Singleton instance;
　private Singleton(){

　}
　public static Singleton getInstance(){
　 if(instance==null)
　　instance=new Singleton();
　 return instance;
　}
}This program is not a problem running in a single thread, but it can not run in a multi-threaded environment, if you want to run in a multi-threaded environment, you must modify the following:

public class Singleton {
　private static class Instance{
　 static final Singleton instance=new Singleton();
　}
　private Singleton(){

　}
　public static Singleton getInstance(){
　 return Instance.instance;
　}
}

This can be done because the static internal class instance will only be loaded once. The single State design mode running in multithreading is also called double-checked looking mode.