Facing issue while using invokeAll to fetch Future object in ExecutorService

Question

Facing below error while fetching Future object by using invokeAll Method. Iam trying to implement ExecutorService to call parallelly.

ERROR>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> java.util.concurrent.ExecutionException: java.lang.UnsupportedOperationException

Code Snippet

Main Class:

ExecutorService exService = Executors.newFixedThreadPool(1);
List<CallableTask> listOfTask = new ArrayList<CallableTask>();
Map<String, String> param1= params;
param1.put("catCode", "XX278293##X");
listOfTask.add(new CallableTask(param1));

List<Future<Map<String,String>>> resultSet =  exService.invokeAll(listOfTask);
for(Future<Map<String,String>> result : resultSet){
    if(result.isDone()){
        Map<String,String> response = result.get(); <<----------Facing Error in this line
    }           
}
exService.shutdown();

Class CallableTask:

import java.util.Collections;
import java.util.Map;
import java.util.concurrent.Callable;

class CallableTask implements Callable<Map<String, String>>{
    private String name;
    private Map<String, String> params;
    public CallableTask(Map<String, String> params){    
        this.name=params.get("catCode");
        this.params =params;
    }
    @Override
    public Map<String, String> call() throws Exception {
       boolean data = false;
        //REST CALL
    Map<String, String> resultData = Collections.emptyMap();
        resultData.put("result", name+data);    
        //Getting response upto here
        return resultData;
    }
}   

thank you in advance!!!

Answer 1

In fact the exception happens here：you can't call emptymap#put :

Map<String, String> resultData = Collections.emptyMap();
resultData.put("result", name+data);

Future#get just reports the exception that occurs during the execution of CallableTask :

   FutureTask#report:

   private V report(int s) throws ExecutionException {
        Object x = outcome;  --Here: outcome is "java.lang.UnsupportedOperationException".
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x); --Status of this task is "EXCEPTIONAL"
    }