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;
    }
}