Two ways to create a singleton pattern

* What is the singleton pattern:
 * Single: only
 * example: instance
 * singleton design pattern, that is, a code pattern in which a class can only be obtained and used by one instance object in the entire system
 *
 * Design points:
 * 1. A class can only have one instance:
 * Constructor privatization
 * Second, it must create this instance by itself:
 * Contains a static variable of this class to save this unique instance
 * Third, it must provide this instance to the entire system by itself
 * The way to obtain the instance object externally:
 * 1 , direct exposure 2 , use static variable get to obtain * common two forms: hungry Chinese style, lazy style

1. Hungry mode

1. The first way

public class Singleton1 {
    public static final Singleton1 INSTANCE = new Singleton1(); private Singleton1(){ } } 

Second, the second way

Created by using enumeration, it is the simplest one in Hungry Mode

public enum  Singleton2 {
    INSTANCE } 

Three, the third way

Use scenario: suitable for complex initial instantiation

public class Singleton3(){
  private static final Singleton INSTANCE; static { INSTANCE= new Singleton3(); } private Singleton3(){ } }

Use: Load the initial value in a properties file under src

 

package com.example.springboot.test;

import java.io.IOException;
import java.util.Properties;

/**
 * @Author chenduoduo * @create 2020/4/15 10:49 * 静态代码块的饿汉式 * 使用的场景：符合复杂实例化 */ public class Singleton3 { public static final Singleton3 INSTANCE; private String info; static { try { //加载properties文件 Properties pro =new Properties(); //使用类加载器加载，可以避免线程安全问题 pro.load(Singleton3.class.getClassLoader().getResourceAsStream("singleton.properties")); INSTANCE = new Singleton3(pro.getProperty("info")); } catch (IOException e) { throw new RuntimeException(e); } } private Singleton3(String info){ this.info = info; } @Override public String toString() { return "Singleton3{" + "info='" + info + '\'' + '}'; } } 

Get value

public class SingletonTest {
    public static void main(String[] args) { Singleton3 instance = Singleton3.INSTANCE; System.out.println(instance); } }

Two, lazy mode

1. The first way

* 1 , Privatization of the constructor
 * 2 , Use static variables to save the unique instance
 * 3 , Provide a static method to get this object
 *     Suitable for single thread : Not safe under multithreading

 public static Singleton4 getInstance(){ if(instance == null){ instance = new Singleton4(); } return instance; }

Second, the second way

public class Singleton5 {

    private static Singleton5 instance; private Singleton5(){ } public static Singleton5 getInstance(){ if(instance == null){ //后面如果new过的话，就先判断一下 synchronized (Singleton5.class){//同步 try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } if(instance == null){ instance = new Singleton5(); } } } return instance; } }

Three, the third way

Static inner class, no thread safety issues

public class Singleton4{
 
  private Singleton4(){ } private class inner{ private static final Singleton4 INSTANCE = new Singleton4(); } public static singleton4 getinstance(){ return inner.INSTANCE } }

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