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：
[java] view plain copy /** * 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：
[java] view plain copy /** * 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：
[java] view plain copy /** * 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(); } }
