Tomcat源码分析（三）------ 可携带状态的线程池

最近想实现一个可携带状态的线程池，具体需求就是池中的线程被用来处理某种信息，而此信息可视为线程所依赖的外部状态。如果用简单的线程池来实现，线程初始化时就得赋予某些信息，使得线程无法被再次利用。在看老版Tomcat的源码时，找到了答案，其实现思路主要是利用了线程的等待和唤起，HttpProcessor的实现正好基于此思路，时序图如下所示：

初始化HttpProcessor线程时，没法赋予所需的Socket对象，因为如果在初始化阶段就赋予Socket会导致此线程没法回收用来处理其他Socket。因此，在HttpProcessor的run阶段，先把线程给wait住，具体在await方法里体现，代码如下所示：

/**
 * Await a newly assigned Socket from our Connector, or <code>null</code>
 * if we are supposed to shut down.
 */
private synchronized Socket await() {

    // Wait for the Connector to provide a new Socket
    while (!available) {
        try {
            wait();
        } catch (InterruptedException e) {
        }
    }

    // Notify the Connector that we have received this Socket
    Socket socket = this.socket;
    available = false;
    notifyAll();

    if ((debug >= 1) && (socket != null))
        log("  The incoming request has been awaited");

    return (socket);

}

当HttpConnector调用HttpProcessor.assign(socket)方法时，会给此线程赋予Socket对象，并唤起此线程，使其继续执行，assign方法的源码如下所示：

/**
 * Process an incoming TCP/IP connection on the specified socket.  Any
 * exception that occurs during processing must be logged and swallowed.
 * <b>NOTE</b>:  This method is called from our Connector's thread.  We
 * must assign it to our own thread so that multiple simultaneous
 * requests can be handled.
 *
 * @param socket TCP socket to process
 */
synchronized void assign(Socket socket) {

    // Wait for the Processor to get the previous Socket
    while (available) {
        try {
            wait();
        } catch (InterruptedException e) {
        }
    }

    // Store the newly available Socket and notify our thread
    this.socket = socket;
    available = true;
    notifyAll();

    if ((debug >= 1) && (socket != null))
        log(" An incoming request is being assigned");

}

线程被唤起和赋予socket对象后，继续执行核心的process方法，HttpProcessor.run的完整源码如下所示：

/**
 * The background thread that listens for incoming TCP/IP connections and
 * hands them off to an appropriate processor.
 */
public void run() {

    // Process requests until we receive a shutdown signal
    while (!stopped) {

        // Wait for the next socket to be assigned
        Socket socket = await();
        if (socket == null)
            continue;

        // Process the request from this socket
        try {
            process(socket);
        } catch (Throwable t) {
            log("process.invoke", t);
        }

        // Finish up this request
        connector.recycle(this);

    }

    // Tell threadStop() we have shut ourselves down successfully
    synchronized (threadSync) {
        threadSync.notifyAll();
    }

}