一、初始化單例類時即創(chuàng)建單例

• 餓漢式：（線程安全）

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

  private Singleton() {
  }

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

• 枚舉類型：（線程安全）

public enum Singleton {
  INSTANCE;
}

二、按需、延遲創(chuàng)建單例

• 懶漢式：（線程不安全）

public class Singleton {
  private static Singleton instance = null;

  private Singleton() {
  }

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

• 懶漢式（同步鎖）：（線程安全）

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;
  }
}

• 懶漢式（雙重校驗鎖）：（線程安全）

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;
  }
}

• 靜態(tài)內部類：（線程安全）【推薦使用】

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

  private Singleton() {
  }

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

單例模式的實現(xiàn)方式對比.png

原文：單例模式（Singleton）- 最易懂的設計模式解析