How thread-safe is enum in java?

Question

How thread-safe is enum in java? I am implementing a Singleton using enum (as per Bloch\'s Effective Java), should I worry at all about thread safety for my singleton enum?

Answer 1

Customized Enum Definition may be not thread safe. For example,

RoleEnum.java:

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:

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();


       }

}

Sometimes the output will be:

B started

B called fromIntegerValue&setBuildFromClass and Start to sleep

A started

A called fromIntegerValue

A called setBuildFromClass and start to sleep

Thread A: class java.lang.String

B waked up!

Thread B: class java.lang.String <-We expect java.lang.Integer

Sometimes the output will be:

A started

A called fromIntegerValue

A called setBuildFromClass and start to sleep

B started

B called fromIntegerValue&setBuildFromClass and Start to sleep

Thread A: class java.lang.Integer <-We expect java.lang.String

B waked up!

Thread B: class java.lang.Integer