单例与线程安全

• 饿汉式--本身线程安全

  在类加载的时候，就已经进行实例化，无论之后用不用到。如果该类比较占内存，之后又没用到，就白白浪费了资源

  public class HungerSingleton {
  
      private static final HungerSingleton INSTANCE = new HungerSingleton();
  
      private HungerSingleton() {
      }
  
      public static HungerSingleton getInstance() {
          return INSTANCE;
      }
  
      public static void main(String[] args) {
          for (int i = 0; i < 10; i++) {
              new Thread(() -> {
                  System.out.println(HungerSingleton.getInstance());
              }).start();
          }
      }
  }
  

• 懒汉式--最简单的写法是非线程安全的

  在需要的时候再实例化

  public class LazySingleton {
  
      private static volatile LazySingleton instance = null;
  
      private LazySingleton() {
      }
  
      public static LazySingleton getInstance() {
          if (instance == null) {
              try {
                  Thread.sleep(100);
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
              synchronized (LazySingleton.class) {
                  if (instance == null) {
                      instance = new LazySingleton();
                  }
              }
          }
          return instance;
      }
  
      public static void main(String[] args) {
          for (int i = 0; i < 10; i++) {
              new Thread(() -> {
                  System.out.println(LazySingleton.getInstance());
              }).start();
          }
      }
  }