单例模式

饿汉模式

public class Singleton1 {
    // 私有构造
    private Singleton1() {}

    private static Singleton1 single = new Singleton1();

    // 静态工厂方法
    public static Singleton1 getInstance() {
        return single;
    }
}

懒汉模式

// 懒汉式单例
public class Singleton2 {

    // 私有构造
    private Singleton2() {}

    private static Singleton2 single = null;

    public static Singleton2 getInstance() {
        if(single == null){
            single = new Singleton2();
        }
        return single;
    }
}

线程安全懒汉模式

public class Singleton4 {
    // 私有构造
    private Singleton4() {}

    private static Singleton4 single = null;

    // 双重检查
    public static Singleton4 getInstance() {
        if (single == null) {
            synchronized (Singleton4.class) {
                if (single == null) {
                    single = new Singleton4();
                }
            }
        }
        return single;
    }
}

静态内部类,序列化对象，默认方式是多例的

package org.mlinge.s06;
 
public class MySingleton {
    
    //内部类
    private static class MySingletonHandler{
        private static MySingleton instance = new MySingleton();
    } 
    
    private MySingleton(){}
     
    public static MySingleton getInstance() { 
        return MySingletonHandler.instance;
    }
}

支持序列化

import java.io.ObjectStreamException;
import java.io.Serializable;
 
public class MySingleton implements Serializable {
     
    private static final long serialVersionUID = 1L;
 
    //内部类
    private static class MySingletonHandler{
        private static MySingleton instance = new MySingleton();
    } 
    
    private MySingleton(){}
     
    public static MySingleton getInstance() { 
        return MySingletonHandler.instance;
    }
    
    //该方法在反序列化时会被调用，该方法不是接口定义的方法，有点儿约定俗成的感觉
    protected Object readResolve() throws ObjectStreamException {
        System.out.println("调用了readResolve方法！");
        return MySingletonHandler.instance; 
    }
}
 

静态代码块

public class MySingleton{
     
    private static MySingleton instance = null;
     
    private MySingleton(){}
 
    static{
        instance = new MySingleton();
    }
    
    public static MySingleton getInstance() { 
        return instance;
    } 
}

参考博客