简单说一下java中线程安全的List 

一、Vector

Vector是大家熟知的线程安全的List集合，不过他的性能是最差，所有的方法都是加了synchronized来同步，从而保证线程安全。

源码也是使用数组来存储数据，有以下构造方法。 

      /**
     * Constructs an empty vector with the specified initial capacity and
     * capacity increment.
     *
     * @param   initialCapacity     the initial capacity of the vector
     * @param   capacityIncrement   the amount by which the capacity is
     *                              increased when the vector overflows
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

    /**
     * Constructs an empty vector with the specified initial capacity and
     * with its capacity increment equal to zero.
     *
     * @param   initialCapacity   the initial capacity of the vector
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

    /**
     * Constructs an empty vector so that its internal data array
     * has size {@code 10} and its standard capacity increment is
     * zero.
     */
    public Vector() {
        this(10);
    }

    /**
     * Constructs a vector containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this
     *       vector
     * @throws NullPointerException if the specified collection is null
     * @since   1.2
     */
    public Vector(Collection<? extends E> c) {
        elementData = c.toArray();
        elementCount = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
    }

它的增加、修改、获取、删除等都通过synchronized来同步。

 public synchronized void insertElementAt(E obj, int index) {
        modCount++;
        if (index > elementCount) {
            throw new ArrayIndexOutOfBoundsException(index
                                                     + " > " + elementCount);
        }
        ensureCapacityHelper(elementCount + 1);
        System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
        elementData[index] = obj;
        elementCount++;
    }
 public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }
 public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }
public synchronized void removeAllElements() {
        modCount++;
        // Let gc do its work
        for (int i = 0; i < elementCount; i++)
            elementData[i] = null;

        elementCount = 0;
    }
public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        return elementData(index);
    }
 public synchronized E set(int index, E element) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

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

二、Collections.SynchronizedList

SynchronizedList是Collections类的静态内部类，它能把所有 List 接口的实现类转换成线程安全的List，比 Vector 有更好的扩展性和兼容性。

SynchronizedList的构造方法如下：

        SynchronizedList(List<E> list) {
            super(list);
            this.list = list;
        }
        SynchronizedList(List<E> list, Object mutex) {
            super(list, mutex);
            this.list = list;
        }

SynchronizedList的部分方法源码如下：

public boolean equals(Object o) {
            if (this == o)
                return true;
            synchronized (mutex) {return list.equals(o);}
        }
        public int hashCode() {
            synchronized (mutex) {return list.hashCode();}
        }

        public E get(int index) {
            synchronized (mutex) {return list.get(index);}
        }
        public E set(int index, E element) {
            synchronized (mutex) {return list.set(index, element);}
        }
        public void add(int index, E element) {
            synchronized (mutex) {list.add(index, element);}
        }
        public E remove(int index) {
            synchronized (mutex) {return list.remove(index);}
        }

        public int indexOf(Object o) {
            synchronized (mutex) {return list.indexOf(o);}
        }
        public int lastIndexOf(Object o) {
            synchronized (mutex) {return list.lastIndexOf(o);}
        }

        public boolean addAll(int index, Collection<? extends E> c) {
            synchronized (mutex) {return list.addAll(index, c);}
        }

        public ListIterator<E> listIterator() {
            return list.listIterator(); // Must be manually synched by user
        }

        public ListIterator<E> listIterator(int index) {
            return list.listIterator(index); // Must be manually synched by user
        }

        public List<E> subList(int fromIndex, int toIndex) {
            synchronized (mutex) {
                return new SynchronizedList<>(list.subList(fromIndex, toIndex),
                                            mutex);
            }
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            synchronized (mutex) {list.replaceAll(operator);}
        }
        @Override
        public void sort(Comparator<? super E> c) {
            synchronized (mutex) {list.sort(c);}
        }

        /**
         * SynchronizedRandomAccessList instances are serialized as
         * SynchronizedList instances to allow them to be deserialized
         * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
         * This method inverts the transformation.  As a beneficial
         * side-effect, it also grafts the RandomAccess marker onto
         * SynchronizedList instances that were serialized in pre-1.4 JREs.
         *
         * Note: Unfortunately, SynchronizedRandomAccessList instances
         * serialized in 1.4.1 and deserialized in 1.4 will become
         * SynchronizedList instances, as this method was missing in 1.4.
         */
        private Object readResolve() {
            return (list instanceof RandomAccess
                    ? new SynchronizedRandomAccessList<>(list)
                    : this);
        }

它所有方法都是带同步对象锁的，和 Vector 一样，它不是性能最优的。

三、CopyOnWriteArrayList

CopyOnWriteArrayList是java1.5以后才加入的新类，从命名可以理解为复制在写入的List。

它的添加时加锁的(ReentrantLock ,非synchronized同步锁)，读操作是没有加锁。

添加和删除方法部分代码实现:

 public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }
 private boolean remove(Object o, Object[] snapshot, int index) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] current = getArray();
            int len = current.length;
            if (snapshot != current) findIndex: {
                int prefix = Math.min(index, len);
                for (int i = 0; i < prefix; i++) {
                    if (current[i] != snapshot[i] && eq(o, current[i])) {
                        index = i;
                        break findIndex;
                    }
                }
                if (index >= len)
                    return false;
                if (current[index] == o)
                    break findIndex;
                index = indexOf(o, current, index, len);
                if (index < 0)
                    return false;
            }
            Object[] newElements = new Object[len - 1];
            System.arraycopy(current, 0, newElements, 0, index);
            System.arraycopy(current, index + 1,
                             newElements, index,
                             len - index - 1);
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

添加或者删除元素时，先加锁，再进行复制替换操作，最后再释放锁。 

它的优势在于，读操作是不加任和锁。这样做的好处是，在高并发情况下，读取元素时就不用加锁，写数据时才加锁，大大提升了读取性能。

看下它的 get 方法源码：

    private E get(Object[] a, int index) {
        return (E) a[index];
    }

    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        return get(getArray(), index);
    }