单例模式(Singleton)

一、初始化单例类时即创建单例

• 饿汉式：（线程安全）

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

  private Singleton() {
  }

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

• 枚举类型：（线程安全）

public enum Singleton {
  INSTANCE;
}

二、按需、延迟创建单例

• 懒汉式：（线程不安全）

public class Singleton {
  private static Singleton instance = null;

  private Singleton() {
  }

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

• 懒汉式（同步锁）：（线程安全）

public class Singleton {
  private static Singleton instance = null;

  private Singleton() {
  }

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

public class Singleton {
  private static Singleton instance = null;

  private Singleton() {
  }

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

• 懒汉式（双重校验锁）：（线程安全）

public class Singleton {
  private static Singleton instance = null;

  private Singleton() {
  }

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

• 静态内部类：（线程安全）【推荐使用】

public class Singleton {
  private static class Singleton2 {
    private static Singleton instance = new Singleton();
  }

  private Singleton() {
  }

  public static Singleton getInstance() {
    return Singleton2.instance;
  }
}

单例模式的实现方式对比.png

原文：单例模式（Singleton）- 最易懂的设计模式解析