【设计模式-01】Singleton单利模式

一、方式1(最常用，推荐使用)

/**
 * @description: 单例实现方式一: 饿汉式 <br>
 *     类加载到内存后，就实例化一个单例，JVM保证线程安全 <br>
 *     简单实用，推荐使用。<br>
 *     唯一缺点: 不管用到与否，类装载时就完成加载。
 * @author: flygo
 * @time: 2022/5/27 22:17
 */
public class SingletonManager01 {

  private static final SingletonManager01 INSTANCE = new SingletonManager01();

  private SingletonManager01() {}

  public static SingletonManager01 getInstance() {
    return INSTANCE;
  }

  public static void main(String[] args) {
    SingletonManager01 singletonManager01 = SingletonManager01.getInstance();
    SingletonManager01 singletonManager011 = SingletonManager01.getInstance();

    System.out.println(singletonManager01 == singletonManager011);
  }
}

二、方式2

/**
 * @description: 同第一种方式一样,静态语句块实现
 * @author: flygo
 * @time: 2022/5/27 22:24
 */
public class SingletonManager02 {

  private static final SingletonManager02 INSTANCE;

  private SingletonManager02() {}

  static {
    INSTANCE = new SingletonManager02();
  }

  public static SingletonManager02 getInstance() {
    return INSTANCE;
  }

  public static void main(String[] args) {
    SingletonManager02 singletonManager02 = SingletonManager02.getInstance();
    SingletonManager02 singletonManager021 = SingletonManager02.getInstance();

    System.out.println(singletonManager02 == singletonManager021);
  }
}

三、方式3(多线程有问题)

/**
 * @description: lazy loading 懒汉式加载，虽然达到了按需加载的目的，但带来了线程不安全的问题
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public class SingletonManager03 {
  public static SingletonManager03 INSTANCE;

  private SingletonManager03() {}

  public static SingletonManager03 getInstance() {
    if (INSTANCE == null) {
      INSTANCE = new SingletonManager03();
    }

    return INSTANCE;
  }

  public static void main(String[] args) {
    SingletonManager03 instance1 = SingletonManager03.getInstance();
    SingletonManager03 instance2 = SingletonManager03.getInstance();

    System.out.println(instance1 == instance2);
  }
}

• 验证这种方式的问题，模拟多线程模式，分析这种方式的问题

线程1执行到 instance == null时，这时实例还没实例化，线程2也刚好执行到instance == null，线程1和线程2初始化了两个不同实例对象。

为把问题模拟的更明显，在初始化之前，休眠1毫秒，模拟线程被打断，初始化不同的实例，效果更明显。

/**
 * @description: lazy loading 懒汉式加载，虽然达到了按需加载的目的，但带来了线程不安全的问题
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public class SingletonManager03 {
  public static SingletonManager03 INSTANCE;

  private SingletonManager03() {}

  public static SingletonManager03 getInstance() {
    if (INSTANCE == null) {
      try {
        Thread.sleep(1);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      INSTANCE = new SingletonManager03();
    }

    return INSTANCE;
  }

  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
      new Thread(
              () -> {
                System.out.println(SingletonManager03.getInstance().hashCode());
              })
          .start();
    }
  }
}

四、方式4

/**
 * @description: lazy loading 懒汉式加载，虽然达到了按需加载的目的，但带来了线程不安全的问题 <br>
 *     通过synchronized加锁的方式解决，同时效率下降了
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public class SingletonManager04 {
  public static SingletonManager04 INSTANCE;

  private SingletonManager04() {}

  public static synchronized SingletonManager04 getInstance() {
    if (INSTANCE == null) {
      try {
        Thread.sleep(1);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      INSTANCE = new SingletonManager04();
    }

    return INSTANCE;
  }

  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
      new Thread(
              () -> {
                System.out.println(SingletonManager04.getInstance().hashCode());
              })
          .start();
    }
  }
}

五、方式5(多线程有问题)

/**
 * @description: lazy loading 懒汉式加载，虽然达到了按需加载的目的，但带来了线程不安全的问题 <br>
 *     通过synchronized加锁的方式解决，同时效率下降了
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public class SingletonManager05 {
  public static SingletonManager05 INSTANCE;

  private SingletonManager05() {}

  public static SingletonManager05 getInstance() {
    if (INSTANCE == null) {
      // 妄图通过减少同步代码块的方式提供效率，然后并不行
      synchronized (SingletonManager05.class) {
        try {
          Thread.sleep(1);
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
        INSTANCE = new SingletonManager05();
      }
    }

    return INSTANCE;
  }

  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
      new Thread(
              () -> {
                System.out.println(SingletonManager05.getInstance().hashCode());
              })
          .start();
    }
  }
}

六、方式6

/**
 * @description: lazy loading 懒汉式加载，虽然达到了按需加载的目的，但带来了线程不安全的问题 <br>
 *     通过synchronized加锁的方式解决，同时效率下降了，增加双重检查
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public class SingletonManager06 {
  // 需要加volatile，指令重排问题
  public static volatile SingletonManager06 INSTANCE;

  private SingletonManager06() {}

  public static SingletonManager06 getInstance() {
    if (INSTANCE == null) {
      // 双重检查
      synchronized (SingletonManager06.class) {
        if (INSTANCE == null) {
          try {
            Thread.sleep(1);
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
          INSTANCE = new SingletonManager06();
        }
      }
    }

    return INSTANCE;
  }

  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
      new Thread(
              () -> {
                System.out.println(SingletonManager06.getInstance().hashCode());
              })
          .start();
    }
  }
}

七、方式7(完美的方式之一)

/**
 * @description: 静态内部类的方式 <br>
 *     JVM保证单例，加载外部类时，不会加载内部类，这样可以实现懒加载
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public class SingletonManager07 {

  private SingletonManager07() {}

  private static final class SingletonManager07Holder {
    private static final SingletonManager07 INSTANCE = new SingletonManager07();
  }

  public static SingletonManager07 getInstance() {
    return SingletonManager07Holder.INSTANCE;
  }

  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
      new Thread(
              () -> {
                System.out.println(SingletonManager07.getInstance().hashCode());
              })
          .start();
    }
  }
}

八、方式8(完美中的完美)

/**
 * @description: 不仅可以解决多线程同步，还可以解决反序列化问题 <br>
 * @author: flygo
 * @time: 2022/7/4 09:32
 */
public enum SingletonManager08 {
  INSTANCE;

  public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
      new Thread(
              () -> {
                System.out.println(SingletonManager08.INSTANCE.hashCode());
              })
          .start();
    }
  }
}

九、源码地址

GitHub - jxaufang168/Design-Patterns: 设计模式学习设计模式学习. Contribute to jxaufang168/Design-Patterns development by creating an account on GitHub.https://github.com/jxaufang168/Design-Patterns