并发编程下的单例设计模式

/**
 * @author BNTang
 */
public class Singleton {
    /**
     * 没有延迟加载，好长时间不使用，影响性能
     */
    private static Singleton instance = new Singleton();

    private Singleton() {
    }

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

/**
 * @author BNTang
 */
public class SingletonTwo {
    private static SingletonTwo instance = null;

    private SingletonTwo() {
    }

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

/**
 * @author BNTang
 */
public class SingletonThree {
    private static volatile SingletonThree instance = null;

    private SingletonThree() {
    }

    public static SingletonThree getInstance() {
        // Double-Check-Locking, DCL
        if (instance == null) {
            synchronized (SingletonThree.class) {
                if (instance == null) {
                    instance = new SingletonThree();
                }
            }
        }
        return instance;
    }
}

memory = allocate();   // 1：分配对象的内存空间
ctorInstance(memory);  // 2：初始化对象
instance = memory;     // 3：设置 instance 指向刚分配的内存地址

memory = allocate();   // 1：分配对象的内存空间
instance = memory;     // 3：设置 instance 指向刚分配的内存地址
                       //    注意，此时对象还没有被初始化！
ctorInstance(memory);  // 2：初始化对象

/**
 * @author BNTang
 */
public class SingletonFour {
    private SingletonFour() {
        System.out.println("SingletonFour 初始化了");
    }

    /**
     * 第一次使用到的时候才去执行, 只执行一次
     */
    private static class Holder {
        private static SingletonFour instance = new SingletonFour();
    }

    public static SingletonFour getInstance() {
        return Holder.instance;
    }
}

// 该方法在反序列化时会被调用
protected Object readResolve() throws ObjectStreamException {
    System.out.println("调用了readResolve方法！");
    return Holder.instance;
}

/**
 * @author BNTang
 */
public enum SingletonFive {
    /**
     * 单值的枚举就是一个单例
     */
    INSTANCE;
    private static int a = 0;
    
    public static void add() {
        a++;
        System.out.println(a);
    }
    
    public static SingletonFive getInstance() {
        return INSTANCE;
    }
}

/**
 * @author BNTang
 */
public class SingletonSix {
    private SingletonSix() {
    }

    private enum Holder {
        INSTANCE;
        private SingletonSix instance = null;

        Holder() {
            instance = new SingletonSix();
        }

        private SingletonSix getInstance() {
            return instance;
        }
    }

    public static SingletonSix getInstance() {
        return Holder.INSTANCE.getInstance();
    }
}