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单例模式的七种写法

第一种(懒汉,线程不安全):

public class Singleton {
    private static Singleton instance;
    private Singleton (){}

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

 

第二种(懒汉,线程安全):

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

 

第三种(饿汉):

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

 

第四种(饿汉,变种):

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

 

第五种(静态内部类):

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

 

第六种(枚举):

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

 

第七种(双重校验锁):

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

 

单例模式的七种写法