Java Collection与ConcurrentModificationException

在遍历Java容器(ArrayList, LinkedList, HashSet, TreeSet, HashMap, HashTable)过程中不允许向该容器中Add/Remove元素, 否则会抛出ConcurrentModificationException

但在Java 1.5版本以来, Java类库就提供了一个并发集合(Concurrent collection) CopyOnWriteArrayList, CopyOnWriteArraySet, ConcurrentHashMap, 这是专门为解决死锁和ConcurrentModificationException问题的. 通过使用并发集合可以避免任何显示的同步.

CopyOnWriteArrayList是ArrayList的一种变体, 通过重新拷贝整个底层数组, 在这个拷贝的数组中实现所有的写操作. 由于内部数组不改动, 因此迭代不需要锁定, 速度也非常快. 但如果大量使用, CopyOnWriteArrayList的性能将大受影响.

Observer:

interface SetObserver<E>{
    public void added(ObservableSet<E> set, E element);
}

package com.ai.concurrentTest;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

public class ObservableSet<E> extends HashSet<E> {

    private static final long serialVersionUID = 1L;

    public ObservableSet(){
    }
    
    private final List<SetObserver<E>> observers = new ArrayList<SetObserver<E>>();
    
    public void addObserver(SetObserver<E> observer){
        synchronized(observers){
            observers.add(observer);
        }
    }
    
    public void removeObserver(SetObserver<E> observer){
        synchronized(observers){
            observers.remove(observer);
        }
    }
    
    private void notifyElementAdded(E element){
        synchronized(observers){
            for(SetObserver<E> observer: observers){
                observer.added(this, element);
            }
        }
    }
    
    public boolean add(E element){
        boolean added = super.add(element);
        if(added)
            notifyElementAdded(element);
        return added;
    }
    
    public static void main(String[] args){
        ObservableSet<Integer> set = new ObservableSet<Integer>();
        set.addObserver(new SetObserver<Integer>() {

            @Override
            public void added(ObservableSet<Integer> set, Integer element) {
                System.out.println("Observer 1: "+element);
                
            }
        });
        set.addObserver(new SetObserver<Integer>() {

            @Override
            public void added(ObservableSet<Integer> set, Integer element) {
                System.out.println("Observer 2 and add another observer: "+element);
//                set.removeObserver(this);
                set.addObserver(new SetObserver<Integer>() {

                    @Override
                    public void added(ObservableSet<Integer> set,
                            Integer element) {
                        System.out.println("This is new observer.");
                        
                    }
                });
            }
        });
        
        for(int i=0; i<100; i++){
            set.add(i);
        }
    }
    
    
}