单例模式

简介

创建型模式，避免对象频繁创建，节省JVM内存占用

示例

• 饿汉模式

  @Slf4j
  public class HungrySingleton {
      private HungrySingleton() {
      }
  
      private static HungrySingleton instance = new HungrySingleton();
  
      public static HungrySingleton getInstance() {
          return instance;
      }
  
      public void sayHi(String name){
          log.info("Hi, {}", name);
      }
  
  }
  

• 懒汉模式 - double check 锁 + volatile

  @Slf4j
  public class LazySingleton_1 {
      private LazySingleton_1() {
      }
  
      private static volatile LazySingleton_1 instance;
  
      public static LazySingleton_1 getInstance() {
          if (null == instance) {
              synchronized (LazySingleton_1.class) {
                  if (null == instance) {
                      instance = new LazySingleton_1();
                  }
              }
          }
          return instance;
      }
  
      public void sayHi(String name){
          log.info("Hi, {}", name);
      }
  }
  

• 懒汉模式 - 类级内部类

  @Slf4j
  public class LazySingleton_2 {
      private LazySingleton_2() {
      }
  
      /**
       * 类级内部类
       */
      private static class LazySingletonHolder {
          private static LazySingleton_2 instance = new LazySingleton_2();
      }
  
      public static LazySingleton_2 getInstance() {
          return LazySingletonHolder.instance;
      }
  
      public void sayHi(String name){
          log.info("Hi, {}", name);
      }
  }
  

• 枚举单例（《高效java》推荐写法）

  @Slf4j
  public enum  EnumSingleton {
      INSTANCE;
  
      public void sayHi(String name){
          log.info("Hi, {}", name);
      }
  }