单例模式

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单例模式确保某个类只有一个实例，而且自行实例化并向整个系统提供这个实例。

1.懒汉模式

public class Singleton {
    private Singleton() {}
    private static final Singleton single = new Singleton();
    //静态工厂方法
    public static Singleton getInstance() {
        return single;
    }
}

2.饿汉模式

public class Singleton {
    private Singleton() {}
    private static Singleton single=null;
    //静态工厂方法   
    public static Singleton getInstance() {
        if (single == null) {
            single = new Singleton();
        }
        return single;
    }
} 

public class Singleton {
    private Singleton() {}
    private static Singleton single=null;
    //静态工厂方法
    public static synchronized Singleton getInstance() {
        if (single == null) {
            single = new Singleton();
        }
        return single;
    }
}

public class Singleton {
    private Singleton() {}
    private static Singleton singleton =null;
    //静态工厂方法
    public static Singleton getInstance() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}

 

public class Singleton {
    private static class LazyHolder {
        private static final Singleton INSTANCE = new Singleton();
    }
    private Singleton (){}
    public static final Singleton getInstance() {
        return LazyHolder.INSTANCE;
    }
} 

public enum Singleton {
    INSTANCE;
    private Resource instance;
    Singleton() {
        instance = new Resource();
    }
    public Resource getInstance() {
        return instance;
    }
}