单例模式一般分为：懒汉、饿汉、双重校验锁、枚举、静态内部类五种。

 

懒汉：

第一次调用时，创建对象

public class Single {

    private static Single instance;

    private Single(){};

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

饿汉：

public class Single {

    private static Single instance = new Single();

    private Single(){};

    public static Single getInstance() {
        return instance ;
    }
}

双重校验锁：

public class Single {

    private volatile static Single singleton;  //1:volatile修饰

    private Single(){}

    public static Single getSingleton() {
        if (singleton == null) {
            synchronized (Single.class) {
                if (singleton == null) {
                    singleton = new Single();
                }
            }
        }
        return singleton;
    }

}

静态内部类:

public class Single {

    private Single() {}
    public static Single getInstance() {
        return Inner.instance;
    }

    public static class Inner {
        private static final Single instance = new Single();
    }
}

枚举:

public enum Singleton {  
    INSTANCE;  
    public void whateverMethod() {  
    }  
}