enum在java中的线程安全性如何?

正如@Mike所说,枚举的创建是线程安全的。但是,添加到枚举类的方法没有任何线程安全保证。尤其是方法 leaveTheBuilding 可以由多个线程并发执行。如果这个方法有副作用(改变一些变量的状态),那么你需要考虑保护它(即,使它 synchronized )或其部分。

自定义枚举定义可能不是线程安全的。例如,

RoleEnum.java:爪哇

package com.threadsafe.bad;

public enum RoleEnum {
       ADMIN(1),
       DEV(2),
       HEAD(3);

       private Integer value;
       private RoleEnum(Integer role){
              this.value=role;           
       }
       public static RoleEnum fromIntegerValue(Integer role){

              for(RoleEnum x : values()){
                     if(x.value == role ){
                           return x;
                     }
              }
              return RoleEnum.HEAD;             
       }

       Class<?> buildFromClass;
       public void setBuildFromClass(Class<?> classType){
              buildFromClass=classType;
       }
       public Class<?> getBuildFromClass(){
              return this.buildFromClass;
       }
}

Main.java版本:

package com.threadsafe.bad;

public class Main {

       public static void main(String[] args) {
              // TODO Auto-generated method stub

              Thread threadA = new Thread(){
                     public void run(){
                           System.out.println("A started");
                           RoleEnum role;
                           role=RoleEnum.fromIntegerValue(1);
                           System.out.println("A called fromIntegerValue");
                           role.setBuildFromClass(String.class);
                           System.out.println("A called setBuildFromClass and start to sleep");


                           try {
                                  Thread.sleep(10000);
                           } catch (InterruptedException e) {
                                  // TODO Auto-generated catch block
                                  e.printStackTrace();
                           }
                           System.out.println("Thread A: "+role.getBuildFromClass());
                     }
              };

              Thread threadB = new Thread(){
                     public void run(){
                           System.out.println("B started");
                           RoleEnum role;
                           role=RoleEnum.fromIntegerValue(1);
                           role.setBuildFromClass(Integer.class);
                           System.out.println("B called fromIntegerValue&setBuildFromClass and Start to sleep");
                           try {
                                  Thread.sleep(20000);
                           } catch (InterruptedException e) {
                                  // TODO Auto-generated catch block
                                  e.printStackTrace();
                           }
                           System.out.println("B waked up!");

                           System.out.println("Thread B: "+ role.getBuildFromClass());
                     }

              };

              threadA.start();
              threadB.start();


       }

}

有时输出将是:

B调用fromIntegerValue&setBuildFromClass并开始睡眠

A开始

被称为fromIntegerValue的

线程A:类java.lang.String

B醒了!

<-我们期望java.lang.Integer

有时输出将是:

A开始

一个叫做setBuildFromClass并开始睡眠

B开始

线程A:类java.lang.Integer <

B醒了!

这种技术绝对是线程安全的。一个枚举值在使用前保证只由一个线程初始化一次。但是,我不确定是加载枚举类还是第一次访问枚举值本身。使用此技术实际上比其他技术更安全,因为甚至没有方法使用反射来获取基于枚举的单例的第二个副本。

添加 同步的

下面的代码将很好地锁定打印“一”。但是当你发表评论时 同步的

import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;

public class TestEnum
{
    public static AtomicInteger count = new AtomicInteger(1);

    public static enum E
    {
        One("One"),
        Two("Two");

        String s;

        E(final String s)
        {
            this.s = s;
        }

        public void set(final String s)
        {
            this.s = s;
        }

        public String get()
        {
            return this.s;
        }
    }

    public static void main(final String[] args)
    {
        doit().start();
        doit().start();
        doit().start();
    }

    static Thread doit()
    {
        return new Thread()
        {
            @Override
            public void run()
            {
                String name = "MyThread_" + count.getAndIncrement();

                System.out.println(name + " started");

                try
                {
                    int i = 100;
                    while (--i >= 0)
                    {

                        synchronized (E.One)
                        {
                            System.out.println(E.One.get());
                            E.One.set("A");
                            Thread.sleep(new Random().nextInt(100));
                            E.One.set("B");
                            Thread.sleep(new Random().nextInt(100));
                            E.One.set("C");
                            Thread.sleep(new Random().nextInt(100));
                            E.One.set("One");
                            System.out.println(E.One.get());
                        }

                    }
                }
                catch (InterruptedException e)
                {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

                System.out.println(name + " ended");
            }
        };
    }
}