Java single-Instance design pattern

What is a singleton design pattern

The singleton is only one instance, and the function of this pattern is to ensure that there is only one object for a class in the program.

The singleton model is divided into lazy and a hungry man type.

Lazy type

Class Student{static Student st;private Student () {}public static Student getinstance () {//Reference data type property in memory default value is null// If the value is null, only the object is created once if (st==null) {st = new Student ();} Return St;}} public class Test1 {public static void main (string[] args) {//Take advantage of Hascode equality is a singleton  Student tes1 = Student.getinstance (); S Ystem.out.println (Tes1.hashcode ()); Student tes2 = Student.getinstance (); System.out.println (Tes2.hashcode ());}}

Above you can see that the Hascode value of the result is the same as only one object was created

However, this pattern creates thread-safety problems in multi-threaded situations:

Class Teacher{static Teacher s;private Teacher () {}public static Teacher getinstance () {if (s = = null) {s = new Teacher ();} return s;}} Class Studentthread extends Thread{public void Run () {for (int i = 0;i<3;i++) {Teacher SS = Teacher.getinstance (); System.out.println (Ss.hashcode ());}}}  public class Test2 {public static void main (string[] args) {//Singleton mode there is a thread safety problem in multiple threads that  needs to be addressed thread safety//create thread studentthread TT1 = new Studentthread (); Studentthread tt2 = new Studentthread ();//Readiness Status Tt1.start (); Tt2.start ();}}

Hascode is not always the same if you run multiple times. This is a thread-safe issue:

Workaround you only need to set the method of the returned object to the synchronous method

Public synchronized static Teacher getinstance () {if (s = = null) {s = new Teacher ();} return s;}

A Hungry man type

A hungry man class hangle{//when creating a static property, it is assigned  and executed only once, which means that the  object is created only once, static hangle ha = new Hangle ();p ublic static Hangle GetInstance () {return ha;}} Class Singlethread extends Thread{public void Run () {for (int i = 0;i<4;i++) {hangle hh = hangle.ha; System.out.println (Hh.hashcode ());}}} public class Test3 {public static void main (string[] args) {Singlethread tt1 = new Singlethread (); Singlethread tt2 = new Singlethread ();//Readiness Status Tt1.start (); Tt2.start ();//Summary: Lazy-type thread safety issues in multithreaded environments (can solve threading problems)// And the A Hungry man-type, whether in single-threaded or multi-threaded thread-safe problems are  recommended to define a singleton mode with a hungry man//A Hungry man single-case mode than lazy-type efficiency}}

Java single-Instance design pattern