AndroidPN客户端的阻塞读写(2)

      AndroidPN客户端分别启动了读和写线程之后，如果有某种写必须等待结果的又如何处理呢？比如客户端向服务端发个消息，要求等待服务端返回后再对本地逻辑做处理。想象一下客户端要求修改密码，PacketWriter把请求write出去后，成功或失败信息肯定是从PacketReader线程中获得，那么如何把二者关联起来呢，看看asmack的做法。

     PacketCollector类

public class PacketCollector {

    private PacketFilter packetFilter;
    private ArrayBlockingQueue<Packet> resultQueue;
    private Connection connection;
    private boolean cancelled = false;

    /**
     * Creates a new packet collector. If the packet filter is <tt>null</tt>, then
     * all packets will match this collector.
     *
     * @param conection the connection the collector is tied to.
     * @param packetFilter determines which packets will be returned by this collector.
     */
    protected PacketCollector(Connection conection, PacketFilter packetFilter) {
    	this(conection, packetFilter, SmackConfiguration.getPacketCollectorSize());
    }

    /**
     * Creates a new packet collector. If the packet filter is <tt>null</tt>, then
     * all packets will match this collector.
     *
     * @param conection the connection the collector is tied to.
     * @param packetFilter determines which packets will be returned by this collector.
     * @param maxSize the maximum number of packets that will be stored in the collector.
     */
    protected PacketCollector(Connection conection, PacketFilter packetFilter, int maxSize) {
        this.connection = conection;
        this.packetFilter = packetFilter;
        this.resultQueue = new ArrayBlockingQueue<Packet>(maxSize);
    }

    /**
     * Explicitly cancels the packet collector so that no more results are
     * queued up. Once a packet collector has been cancelled, it cannot be
     * re-enabled. Instead, a new packet collector must be created.
     */
    public void cancel() {
        // If the packet collector has already been cancelled, do nothing.
        if (!cancelled) {
            cancelled = true;
            connection.removePacketCollector(this);
        }
    }

    /**
     * Returns the packet filter associated with this packet collector. The packet
     * filter is used to determine what packets are queued as results.
     *
     * @return the packet filter.
     */
    public PacketFilter getPacketFilter() {
        return packetFilter;
    }

    /**
     * Polls to see if a packet is currently available and returns it, or
     * immediately returns <tt>null</tt> if no packets are currently in the
     * result queue.
     *
     * @return the next packet result, or <tt>null</tt> if there are no more
     *      results.
     */
    public Packet pollResult() {
    	return resultQueue.poll();
    }

    /**
     * Returns the next available packet. The method call will block (not return)
     * until a packet is available.
     *
     * @return the next available packet.
     */
    public Packet nextResult() {
        try {
			return resultQueue.take();
		}
		catch (InterruptedException e) {
			throw new RuntimeException(e);
		}
    }

    /**
     * Returns the next available packet. The method call will block (not return)
     * until a packet is available or the <tt>timeout</tt> has elapased. If the
     * timeout elapses without a result, <tt>null</tt> will be returned.
     *
     * @param timeout the amount of time to wait for the next packet (in milleseconds).
     * @return the next available packet.
     */
    public Packet nextResult(long timeout) {
    	try {
			return resultQueue.poll(timeout, TimeUnit.MILLISECONDS);
		}
		catch (InterruptedException e) {
			throw new RuntimeException(e);
		}
    }

    /**
     * Processes a packet to see if it meets the criteria for this packet collector.
     * If so, the packet is added to the result queue.
     *
     * @param packet the packet to process.
     */
    protected void processPacket(Packet packet) {
        if (packet == null) {
            return;
        }
        
        if (packetFilter == null || packetFilter.accept(packet)) {
        	while (!resultQueue.offer(packet)) {
        		// Since we know the queue is full, this poll should never actually block.
        		resultQueue.poll();
        	}
        }
    }
}

 这个类相当赞，最喜欢这种简单的东西了，我觉得我可以照抄呵呵，基本上不需要看其它代码就能猜asmack是怎么干的了，看看流程

 

 

1.首先，如果有需要，比如要修改密码了，就将PacketCollect注册到connection上

   /**
     * Creates a new packet collector for this connection. A packet filter determines
     * which packets will be accumulated by the collector. A PacketCollector is
     * more suitable to use than a {@link PacketListener} when you need to wait for
     * a specific result.
     * 
     * @param packetFilter the packet filter to use.
     * @return a new packet collector.
     */
    public PacketCollector createPacketCollector(PacketFilter packetFilter) {
        PacketCollector collector = new PacketCollector(this, packetFilter);
        // Add the collector to the list of active collectors.
        collectors.add(collector);
        return collector;
    }

 PacketFilter是个过滤器，对于修改密码而言，我只对服务端返回的密码修改结果感兴趣，其它无视，那么它只将相关的接收到的packet放进PacketCollector的ArrayBlockingQueue<Packet>中。注意下processPacket这个方法，队列满了就开始丢弃之前的packet.

 

 

2.发起修改密码请求到服务端，其实就是通过PacketWriter线程write一个packet给服务器。

 

3.坐等服务端返回，可用nextResult()方法，是一直阻塞，还是阻塞一定时间，你自己来定，一般应该是阻塞几秒就差不多了。

 

4.最后无论有没服务端回应，cancel掉这个PacketCollector,即从connection中将之删除。

 

来看一段AccounterManager中源码：

    public void createAccount(String username, String password, Map<String, String> attributes)
            throws XMPPException
    {
        if (!supportsAccountCreation()) {
            throw new XMPPException("Server does not support account creation.");
        }
        Registration reg = new Registration();
        reg.setType(IQ.Type.SET);
        reg.setTo(connection.getServiceName());
        attributes.put("username",username);
        attributes.put("password",password);
        reg.setAttributes(attributes);
        PacketFilter filter = new AndFilter(new PacketIDFilter(reg.getPacketID()),
                new PacketTypeFilter(IQ.class));
        PacketCollector collector = connection.createPacketCollector(filter);
        connection.sendPacket(reg);
        IQ result = (IQ)collector.nextResult(SmackConfiguration.getPacketReplyTimeout());
        // Stop queuing results
        collector.cancel();
        if (result == null) {
            throw new XMPPException("No response from server.");
        }
        else if (result.getType() == IQ.Type.ERROR) {
            throw new XMPPException(result.getError());
        }
    }

 这个过程还是相当清楚的。

总结下，asmack写线程和读线程之间，通过分发指定消息类型到指定的同步队列，利用同步队列的阻塞，来实现发出消息和收到消息的关联。