一篇文章教你使用枚举来实现java单例模式

目录

• 传统的单例写法解决了什么问题
• 仍然存在的问题
• 为什么枚举就没有问题
• 总结

    public synchronized static SingleClassV1 getInstance(){
        if(instance == null){
            instance = new SingleClassV1();
        }
        return instance;
    }

    private static volatile SingleClassV2 instance;
    public static SingletonV2 getInstance() {
         if(instance == null){
             synchronized (SingletonV2.class){
                 if(instance == null){
                     instance = new SingletonV2();
                 }
             }
         }
         return instance;
     }

     private static class SingletonHolder {
         private static final SingletonV3 INSTANCE = new SingletonV3();
     }
     public static final SingletonV3 getInstance() {
         return SingletonHolder.INSTANCE;
     }

    Class<&＃63;> clazzV2 = Class.forName(SingleClassV2.class.getName());
    Constructor<&＃63;> cOnstructor= clazzV2.getDeclaredConstructors()[0];
    constructor.setAccessible(true);
    Object o = constructor.newInstance();

public enum SingleClassV4 {
    INSTANCE;
    public String doSomeThing(){
        return "hello world";
    }
}

public final class git.frank.SingleClassV4 extends java.lang.Enum
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER, ACC_ENUM

  private git.frank.SingleClassV4();
    descriptor: (Ljava/lang/String;I)V
    flags: ACC_PRIVATE
    Code:
      stack=3, locals=3, args_size=3
         0: aload_0
         1: aload_1
         2: iload_2
         3: invokespecial #6                  
         // Method java/lang/Enum."":(Ljava/lang/String;I)V
         6: return
      LineNumberTable:
        line 3: 0//加入Java开发交流君样：756584822一起吹水聊天
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lgit/frank/SingleClassV4;
    Signature: #29                          // ()V

constructor.newInstance();

    public T newInstance(Object ... initargs)
        throws InstantiationException, IllegalAccessException,
               IllegalArgumentException, InvocationTargetException
    {//加入Java开发交流君样：756584822一起吹水聊天
        ...
        if ((clazz.getModifiers() & Modifier.ENUM) != 0)
            throw new IllegalArgumentException("Cannot reflectively create enum objects");
        ...
        T inst = (T) ca.newInstance(initargs);
        return inst;
    }