单例模式

一.懒汉模式

/**
 * 懒汉模式：非线程安全的
 * 所谓懒汉即用到的时候才初始化
 */
public class SingleTon {
    //实例成员私有化，防止被引用；设置为null，延迟加载
    private static SingleTon singleTon = null;
    //构造器私有化，只有通过getInstance来获取实例
    public static SingleTon getInstance(){
        if(singleTon == null){
            singleTon = new SingleTon();
        }
        return singleTon;
    }
    //构造器私有化，防止被实例化
    private SingleTon(){
        
    }
    
    public static void main(String[] args) {
        SingleTon singleTon1 = SingleTon.getInstance();
        SingleTon singleTon2 = SingleTon.getInstance();
        System.out.println(singleTon1==singleTon2);//true
    }
}

二.饿汉模式

/**
 * 饿汉模式：线程安全
 */
public class SingleTon {
    private static final SingleTon singleTon = new SingleTon();
    public static SingleTon getInstance(){
        return singleTon;
    }
    private SingleTon(){
        
    }
    public static void main(String[] args) {
        SingleTon singleTon1 = SingleTon.getInstance();
        SingleTon singleTon2 = SingleTon.getInstance();
        System.out.println(singleTon1==singleTon2);//true
    }
}

三.双重锁

/**
 * 双重锁：懒汉模式的同步优化，线程安全
 */
public class SingleTon {
    private static SingleTon singleTon = null;
    public static SingleTon getInstance(){
        if(singleTon == null){
            synchronized (SingleTon.class) {
                if(singleTon==null){
                    singleTon = new SingleTon();
                }
            }
        }
        return singleTon;
    }
    private SingleTon(){
        
    }
    public static void main(String[] args) {
        SingleTon singleTon1 = SingleTon.getInstance();
        SingleTon singleTon2 = SingleTon.getInstance();
        System.out.println(singleTon1==singleTon2);//true
    }
}

四.静态内部类

/**
 * 静态内部类：线程安全
 */
public class SingleTon {
    private static class SingleTonHolder{
        private static SingleTon singleTon = new SingleTon();
    }
    public static SingleTon getInstance(){
        return SingleTonHolder.singleTon;
    }
    private SingleTon(){
    }
    public static void main(String[] args) {
        SingleTon singleTon1 = SingleTon.getInstance();
        SingleTon singleTon2 = SingleTon.getInstance();
        System.out.println(singleTon1==singleTon2);//true
    }
}

总结：单线程模式下用第一种就行，多线程下最好用第二种或第四种。因为网上很多人说第二种也没有做到线程安全。