ArrayList thread insecurity and Solutions

What we usually say Java Medium fail-fast Mechanism （ Fast failure mechanism ）, Default means Java An error detection mechanism in a collection , When multiple threads make structural changes to the collection , It's possible to start fail-fast Mechanism , This time it will throw ConcurrentModificationException abnormal .

Why? ArrayList Thread unsafe ？Vector But thread safety ？ Let's explore how ！
ArrayList:

    public boolean add(E e) {
    
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //ensureCapacityInternal() Is to judge if the current new element is added to the list , List elementData Whether the size of the array meets , If size + 1 The length of this requirement is greater than elementData The length of this array , Then we need to expand the capacity of this array 
        elementData[size++] = e;
        return true;
    }

Vector：

    public synchronized boolean add(E e) {
    
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

Obvious ： stay ArrayList in ,elementData And size It's all global variables , But it didn't happen sychronization Synchronous processing ,elementData Shared threads are unsafe mutable Variable data .

So how to solve LIst What about thread safety ？

1. Vector：Vector list = new Vector<>();
2. Collections：List list = Collections.synchronizedList(new ArrayList());
3. CopyOnWriteArrayList：List list = new CopyOnWriteArrayList<>();

Let's see CopyOnWriteArrayList Medium add() Method , The principle is to use Copy on write technology ：

    public boolean add(E e) {
    
        final ReentrantLock lock = this.lock;
        //  First ： Lock ！
        lock.lock();
        try {
    
            Object[] elements = getArray();
            int len = elements.length;
            // 1、 Copy the content 
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            // 2、 Write new content 
            newElements[len] = e;
            // 3、 Merge 
            setArray(newElements);
            return true;
        } finally {
    
            lock.unlock();
        }
    }