彻底玩转单例模式

彻底玩转单例模式

• 饿汉式

  public class Hungry {
      private byte[] data1 = new byte[1024*1024];
      private byte[] data2 = new byte[1024*1024];
      private byte[] data3 = new byte[1024*1024];
      private byte[] data4 = new byte[1024*1024];
      private Hungry(){
      }
      private final static Hungry HUNGRY = new Hungry();
      public static Hungry getInstance(){
          return HUNGRY;
      }
  }
  

• DCL 懒汉式

  //DCL 懒汉式
  public class LazyMan {
      private LazyMan() {
          System.out.println(Thread.currentThread().getName());
      }
  
      //private static LazyDemo lazyDemo;
      private volatile static LazyMan lazyMan;
  
      public static LazyMan getInstance() {
          //加锁，双重检测锁模式，DCL懒汉式
          if (lazyMan == null) {
              synchronized (LazyMan.class) {
                  if (lazyMan == null) {
                      lazyMan = new LazyMan(); //不是原子性操作，也有可能会出问题，所以需要加volatile
                  }
              }
          }
          //不加锁
  //        if(lazyMan == null){
  //            lazyMan = new LazyMan();
  //        }
          return lazyMan;
      }
  
      //单线程下单例OK，多线程并发会有问题,有可能不止一个实例，解决方法：加锁
      public static void main(String[] args) {
          for (int i = 0; i < 10; i++) {
              new Thread(() -> {
                  LazyMan.getInstance();
              }).start();
          }
      }
  }
  
   // 反射！
      public static void main(String[] args) throws NoSuchFieldException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
          //        LazyMan instance = LazyMan.getInstance();
          Field joker = LazyMan.class.getDeclaredField("joker");
          joker.setAccessible(true);
          Constructor<LazyMan> declaredConstructor = LazyMan.class.getDeclaredConstructor(null);
          declaredConstructor.setAccessible(true);
          LazyMan instance = declaredConstructor.newInstance();
          joker.set(instance,false);
          LazyMan instance2 = declaredConstructor.newInstance();
          System.out.println(instance);
          System.out.println(instance2);
      }
  

• 静态内部类

  public class Holder {
      public Holder() {
      }
      public static Holder getInstace(){
          return InnerClass.HOLDER;
      }
      public static class InnerClass{
          private static final Holder HOLDER = new Holder();
      }
  }
  

• 单例不安全，反射

• 枚举

• public enum EnumSingle {
      INSTANCE;
      public EnumSingle getInstance(){
          return INSTANCE;
      }
  }
  class Test {
      public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, InvocationTargetException {
          EnumSingle instance1 = EnumSingle.INSTANCE;
          Constructor<EnumSingle> declaredConstructor = EnumSingle.class.getDeclaredConstructor(String.class, int.class);
          declaredConstructor.setAccessible(true);
          EnumSingle instance2 = declaredConstructor.newInstance();
          System.out.println(instance1);
          System.out.println(instance2);
      }
  }