java.util.ConcurrentModificationException：对哈希图的并发访问

Question

In an implementation, i have written the following code. The try-catch block is in a method

try{
    InputVals iv = task.getInputVals();
    Map<String, String> map = iv.getAllValues();
    String a = map.get("value1");
    String b = map.get("value2");
    String x = funcxy.methodGetX();
    String y = funcxy.methodGetY();
    iv.setValue(xval, x);
    iv.setValue(yval,y);

    String []names = {"name1", "name2", "name3"}
    for(int i = 0; i<names.length; i++ ){
         iv.setValue("name"+i, names[i] );
    }
  }

When I make concurrent request few times (not always) it throws "java.util.ConcurrentModificationException: concurrent access to hashmap" error. I tried using

Map<String, String> map= new ConcurrentHashMap<String, String>();
map = iv.getAllValues();

But this didnt solve the issue. Can you help me out and let me know where I'm making the mistake. I cannot change the implementation of

InputVals iv = task.getInputVals();

Answer 1

I'm not entirely sure what you are trying to achieve, but as others have already pointed out, you likely have multiple threads trying to manipulate the map returned by iv.getAllValues() at the same time, hence throwing the exception.

Copying the map with a ConcurrentHashMap will work as you would be working off a local copy. However keep in mind that in doing so, you would be using it locally only, and consequently not need the concurrency checking that it provides. The problem with your code is that you do not actually copy the data to your new map. You would have needed to do:

Map<String, String> map= new ConcurrentHashMap<String, String>( iv.getAllValues() );

Depending on your needs when making the modifications to the map entries, the simplest & fastest would probably copy the map and work off a local copy. This will prevent any concurrency issues. Of course, if the other threads need access to your updated information, this plan does not work.

try{
    InputVals iv = task.getInputVals();
    Map<String, String> map = new HashMap<String, String>();
    // copy all map values to a local var
    map.putAll( iv.getAllValues() );
    String a = map.get("value1");
    String b = map.get("value2");
    String x = funcxy.methodGetX();
    String y = funcxy.methodGetY();
    iv.setValue(xval, x);
    iv.setValue(yval,y);

    String []names = {"name1", "name2", "name3"}
    for(int i = 0; i<names.length; i++ ){
         iv.setValue("name"+i, names[i] );
    }
  }

Short of this, you would need to ensure that any calls to the map are made in synchronized blocks. However, this can be extremely difficult and tedious if you have several different places in the code where you access this map.

Answer 2

You have to synchronize all access to the Map returned by the getInputVals(). It is being altered somewhere in a separate thread. You will need something along the lines of the code below, but what is also very important is that the lockObj must also be applied everywhere else that this map is being used, including wherever it is currently being modified (this code has not been shown).

try{
    InputVals iv = task.getInputVals();
    String a;
    String b;

    synchronized (lockOjb) {
        Map<String, String> map = iv.getAllValues();
        a = map.get("value1");
        b = map.get("value2");
    }
    String x = funcxy.methodGetX();
    String y = funcxy.methodGetY();
    iv.setValue(xval, x);
    iv.setValue(yval,y);

    String []names = {"name1", "name2", "name3"}
    for(int i = 0; i<names.length; i++ ){
         iv.setValue("name"+i, names[i] );
    }
  }

This may or may not be possible, depending on whether you have the ability to change this code. If it is already happening within a library, for instance, you may simply have to redesign your code so that it is not multithreaded, at least anywhere where this map is accessed.

Answer 3

What does your InputVals.setValue() does? How is it defined? there is no use if you define ConcurrentHashMap in the business logic as you are using the reference of original variable. You would need to take care in the actual bean itself.

If it some thing like below

Class Inputvals {
  Map<String, String> map = new HashMap <String,String>();
  public void setValue(String a,String b){
    map.put(a,b);
  }
  public Map<String,String> getAllValues(){
    return map;
  }
}

I suggest you to change as below

Class Inputvals {
  Map<String, String> map = new ConcurrentHashMap<String,String>();
  public void setValue(String a,String b){
    map.put(a,b);
  }
  ..
  ...

this should help you to resolve multi threaded access issue.

Answer 4

You may need to wrap your map in a Synchronized collection

Map<String, String> map = Collections.synchronizedMap(iv.getAllValues());

Then go ahead and access your map.