Concurrent

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SynchronizedMap和ConcurrentHashMap有什么区别？

ConcurrentHashMap后者具有更高的并发

SynchronizedMap锁的是整个对象

ConcurrentHashMap锁的是段

...     
   /** 
     * Maps the specified key to the specified value in this table. 
     * Neither the key nor the value can be null. 
     * 
     * <p>The value can be retrieved by calling the {@code get} method 
     * with a key that is equal to the original key. 
     * 
     * @param key key with which the specified value is to be associated 
     * @param value value to be associated with the specified key 
     * @return the previous value associated with {@code key}, or 
     *         {@code null} if there was no mapping for {@code key} 
     * @throws NullPointerException if the specified key or value is null 
     */  
    public V put(K key, V value) {  
        return putVal(key, value, false);  
    }  
  
    /** Implementation for put and putIfAbsent */  
    final V putVal(K key, V value, boolean onlyIfAbsent) {  
        if (key == null || value == null) throw new NullPointerException();  
        int hash = spread(key.hashCode());  
        int binCount = 0;  
        for (Node<K,V>[] tab = table;;) {  
            Node<K,V> f; int n, i, fh;  
            if (tab == null || (n = tab.length) == 0)  
                tab = initTable();  
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {  
                if (casTabAt(tab, i, null,  
                             new Node<K,V>(hash, key, value, null)))  
                    break;                   // no lock when adding to empty bin  
            }  
            else if ((fh = f.hash) == MOVED)  
                tab = helpTransfer(tab, f);  
            else {  
                V oldVal = null;  
                synchronized (f) {  
                    if (tabAt(tab, i) == f) {  
                        if (fh >= 0) {  
                            binCount = 1;  
                            for (Node<K,V> e = f;; ++binCount) {  
                                K ek;  
                                if (e.hash == hash &&  
                                    ((ek = e.key) == key ||  
                                     (ek != null && key.equals(ek)))) {  
                                    oldVal = e.val;  
                                    if (!onlyIfAbsent)  
                                        e.val = value;  
                                    break;  
                                }  
                                Node<K,V> pred = e;  
                                if ((e = e.next) == null) {  
                                    pred.next = new Node<K,V>(hash, key,  
                                                              value, null);  
                                    break;  
                                }  
                            }  
                        }  
                        else if (f instanceof TreeBin) {  
                            Node<K,V> p;  
                            binCount = 2;  
                            if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,  
                                                           value)) != null) {  
                                oldVal = p.val;  
                                if (!onlyIfAbsent)  
                                    p.val = value;  
                            }  
                        }  
                    }  
                }  
                if (binCount != 0) {  
                    if (binCount >= TREEIFY_THRESHOLD)  
                        treeifyBin(tab, i);  
                    if (oldVal != null)  
                        return oldVal;  
                    break;  
                }  
            }  
        }  
        addCount(1L, binCount);  
        return null;  
    }  
...  

Note:

ConcurrentHashMap 的结构示意图

ConcurrentHashMap 在默认并发级别会创建包含 16 个 Segment 对象的数组。每个 Segment 的成员对象 table 包含若干个散列表的桶。每个桶是由 HashEntry 链接起来的一个链表。如果键能均匀散列，每个 Segment 大约守护整个散列表中桶总数的 1/16。

CopyOnWriteArrayList可以用于什么应用场景？

多读少写

...      
   /** 
     * Appends the specified element to the end of this list. 
     * 
     * @param e element to be appended to this list 
     * @return {@code true} (as specified by {@link Collection#add}) 
     */  
    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();  
        }  
    }  
  
    /** 
     * Inserts the specified element at the specified position in this 
     * list. Shifts the element currently at that position (if any) and 
     * any subsequent elements to the right (adds one to their indices). 
     * 
     * @throws IndexOutOfBoundsException {@inheritDoc} 
     */  
    public void add(int index, E element) {  
        final ReentrantLock lock = this.lock;  
        lock.lock();  
        try {  
            Object[] elements = getArray();  
            int len = elements.length;  
            if (index > len || index < 0)  
                throw new IndexOutOfBoundsException("Index: "+index+  
                                                    ", Size: "+len);  
            Object[] newElements;  
            int numMoved = len - index;  
            if (numMoved == 0)  
                newElements = Arrays.copyOf(elements, len + 1);  
            else {  
                newElements = new Object[len + 1];  
                System.arraycopy(elements, 0, newElements, 0, index);  
                System.arraycopy(elements, index, newElements, index + 1,  
                                 numMoved);  
            }  
            newElements[index] = element;  
            setArray(newElements);  
        } finally {  
            lock.unlock();  
        }  
    }  
...