单例模式

1、饿汉式（非线程安全）

public class Singleton1 {
    private Singleton1() {
    }
    private static Singleton1 instance = new Singleton1();
    
    public static Singleton1 getInstance(){
        return instance;
    }
}

 

2、懒汉式（非线程安全）

public class Singleton2 {
    private Singleton2() {
    }
    private static Singleton2 instance = null;
    public static Singleton2 getInstance(){
        if (instance == null){
            instance = new Singleton2();
        }
        return instance;
    }
}

 3、懒汉式（线程安全）

public class Singleton3 {
    private Singleton3() {
    }
    private static Singleton3 instance = null;
    public synchronized static Singleton3 getInstance(){
        if (instance == null){
            instance = new Singleton3();
        }
        return instance;
    }
}

 

4、双重检查

public class Singleton4 {
    private Singleton4() {
    }
    private volatile static Singleton4 instance = null;
    public static Singleton4 getInstance(){
        if (instance == null){//1
            synchronized (Singleton4.class) {
                if (instance == null)//2
                    instance = new Singleton4();
            }
        }
        return instance;
    }
}

 5、静态私有内部类

public class Singleton5 {
    private Singleton5() {
    }
    private static class SingletonHolder{
        private static Singleton5 instance = new Singleton5();
    }
    public static Singleton5 getInstance(){
        return SingletonHolder.instance;
    }
}