如何迭代并修改这个Arraylist？

Question

I have an arrayList which I am sharing between two threads and I am trying to iterate over and modify the list at the same time. I don't want to use the iterator's method, and I have also used the synchronized list but it still gives concurrentmodificationexception .

code is as follows :

public class testing {
  public static void main(String args[]){

  ArrayList<String> al = new ArrayList<String>();
  List<String> sal=Collections.synchronizedList(al);
   String names[] = {"amol","Robin","vikas","shanu","mahesh"};
  for(String x :names){
       al.add(x);
  }      

  Thread t1 = new Thread(new SyncArrayList(sal));
  Thread t2 = new Thread(new SyncArrayList(sal));    
  t1.setName("T1");
  t2.setName("T2");
  t1.start();
  t2.start();

  }   
}

class SyncArrayList implements Runnable
{
 List<String> unsync ;

SyncArrayList(List<String> l){
   this.unsync = l;
}

@Override
public void run() {
    displayUnsyncList();
    addNames();
  }

void displayUnsyncList(){
   synchronized(this){ 
   ListIterator<String> itr = unsync.listIterator();
  while(itr.hasNext()){
      String x = itr.next();
      System.out.println("Thread " +Thread.currentThread().getName() + " is displaying name : "+x);
   } 
  }
 }

void addNames(){
    unsync.add("preet");
   } 

}   

Answer 1

在多线程环境中，您应该考虑使用不会产生ConcurrentModificationException CopyOnWriteArrayList

Answer 2

The javadoc for Collections.synchronizedList(...) explains what you need to do:

   void displayUnsyncList() {
       synchronized (unsync) {
       ListIterator<String> itr = unsync.listIterator();
       while (itr.hasNext()) {
           String x = itr.next();
           System.out.println("Thread " + 
                              Thread.currentThread().getName() + 
                              " is displaying name : " + x);
       } 
   }

In short, your code was synchronizing on the wrong object.

Answer 3

The iterator you are using is fail-fast iterator.Fail-fast iterator read internal data structure directly . The internal data structure should not be modified while iterating through the collection.

You should use Fail-safe iterator when one or more thread is iterating over the collection, in between, one thread changes the structure of the collection.

public static void main(String args[]) {

    ArrayList<String> al = new ArrayList<String>();
    List<String> sal = Collections.synchronizedList(al);
    String names[] = { "amol", "Robin", "vikas", "shanu", "mahesh" };
    for (String x : names) {
        al.add(x);
    }

    Thread t1 = new Thread(new SyncArrayList(sal));
    Thread t2 = new Thread(new SyncArrayList(sal));
    t1.setName("T1");
    t2.setName("T2");
    t1.start();
    t2.start();

}

}

class SyncArrayList implements Runnable { List unsyn;

// List<String> unsync = Collections.synchronizedList(unsyn);

SyncArrayList(List<String> l) {
    this.unsyn = new CopyOnWriteArrayList<String>(l);
}

@Override
public void run() {
    displayUnsyncList();
    addNames();
}

void displayUnsyncList() {
    synchronized (this) {
        Iterator<String> it = unsyn.iterator();
        while (it.hasNext()) {
            System.out.println("Thread " + Thread.currentThread().getName() + " is displaying name : " + it.next());
        }
    }
}

void addNames() {
    unsyn.add("preet");
}