Apache MINA 2 是一个开发高性能和高可伸缩性网络应用程序的网络应用框架。它提供了一个抽象的事件驱动的异步 API，可以使用 TCP/IP、UDP/IP、串口和虚拟机内部的管道等传输方式。
首先，mina server端acceptor启动方法：
1、NioSocketAcceptor.bind(InetSocketAddress)或者NioSocketAcceptor.bind(SocketAddress...)方法。
例如：
acceptor.bind(new InetSocketAddress(1234));
mina底层的调用链：
NioSocketAcceptor.bind(InetSocketAddress)-->
AbstractIoAcceptor.bind(SocketAddress localAddress) -->
AbstractIoAcceptor.bind(Iterable<? extends SocketAddress> localAddresses)-->
AbstractPollingIoAcceptor.bindInternal(List<? extends SocketAddress> localAddresses)-->
AbstractPollingIoAcceptor.startupAcceptor()
1、创建线程Acceptor线程-->
acceptor线程启动run()
1、初始化acceptor端的Selector,即NioSocketAcceptor.Selector
2、NioSocketAcceptor.open(SocketAddress localAddress)
// Register the channel within the selector for ACCEPT event
channel.register(selector, SelectionKey.OP_ACCEPT);
acceptor工作流程：
一、IoService类图
如上图所示，IOService层根据不同的角色又分为IOAcceptor(服务端左半部分)和IOConnector (客户端右半部分)，分别用于接受连接与请求连接操作。
二、服务端
2.1、IoAcceptor接口
IoAcceptor相当于是对ServerSocketChannel的封装，最重要的两个操作是绑定（解绑）与接受连接，IoAcceptor接口中有多个重载的bind()方法,当然呼应有多个重载的unbind()方法。
public interface IoAcceptor extends IoService {
void bind() throws IOException;
void bind(SocketAddress localAddress) throws IOException;
void bind(SocketAddress firstLocalAddress, SocketAddress... addresses) throws IOException;
void bind(Iterable<? extends SocketAddress> localAddresses) throws IOException;
void unbind();
void unbind(SocketAddress localAddress);
void unbind(SocketAddress firstLocalAddress, SocketAddress... otherLocalAddresses);
void unbind(Iterable<? extends SocketAddress> localAddresses);
}
2.2、IoAcceptor的实现类AbstractIOAcceptor
IoAcceptor其方法的实现在抽象类AbstractIOAcceptor的bind方法中，这个方法在做了参数检查等操作后，将真正的绑定操作交给abstract bindInternal()来完成。对于bindInternal有基于TCP/IP,UDP/IP,VMPipe三种实现类，分别对应
- AbstractPollingIoAcceptor对应TCP/IP
- AbstractPollingConnectionlessIoAcceptor对应UDP/IP
- VmPipeAcceptor对应串口和虚拟机内部的管道
以TCP/IP为例来看绑定过程，见AbstractPollingIoAcceptor.java的源码：
protected final Set<SocketAddress> bindInternal(List<? extends SocketAddress> localAddresses) throws Exception {
// Create a bind request as a Future operation. When the selector
// have handled the registration, it will signal this future.
AcceptorOperationFuture request = new AcceptorOperationFuture(localAddresses);
// adds the Registration request to the queue for the Workers
// to handle
registerQueue.add(request);
// creates the Acceptor instance and has the local
// executor kick it off.
startupAcceptor();
// As we just started the acceptor, we have to unblock the select()
// in order to process the bind request we just have added to the
// registerQueue.
try {
lock.acquire();
// Wait a bit to give a chance to the Acceptor thread to do the select()
Thread.sleep(10);
wakeup();
} finally {
lock.release();
}
// Now, we wait until this request is completed.
request.awaitUninterruptibly();
if (request.getException() != null) {
throw request.getException();
}
// Update the local addresses.
// setLocalAddresses() shouldn't be called from the worker thread
// because of deadlock.
Set<SocketAddress> newLocalAddresses = new HashSet<SocketAddress>();
for (H handle : boundHandles.values()) {
newLocalAddresses.add(localAddress(handle));
}
return newLocalAddresses;
主要做了以下几件事情：
1、将绑定请求放入registerQueue中
2、启动Acceptor，从Acceptor类的run方法可以看到，这一步会阻塞在Acceptor选择器的选择操作中
3、调用wakeup让选择器返回
4、等待请求处理完成，这一步会阻塞在ready变量中，当ready变量为true时才会返回，当接受连接后ready会被设置为true.
现在重点看一下AbstractPollingIoAcceptor$Acceptor的run方法：
public void run() {
assert (acceptorRef.get() == this);
int nHandles = 0;
// Release the lock
lock.release();
while (selectable) {
try {
// Detect if we have some keys ready to be processed
// The select() will be woke up if some new connection
// have occurred, or if the selector has been explicitly
// woke up
int selected = select();
// this actually sets the selector to OP_ACCEPT,
// and binds to the port on which this class will
// listen on
nHandles += registerHandles();
// Now, if the number of registred handles is 0, we can
// quit the loop: we don't have any socket listening
// for incoming connection.
if (nHandles == 0) {
acceptorRef.set(null);
if (registerQueue.isEmpty() && cancelQueue.isEmpty()) {
assert (acceptorRef.get() != this);
break;
}
if (!acceptorRef.compareAndSet(null, this)) {
assert (acceptorRef.get() != this);
break;
}
assert (acceptorRef.get() == this);
}
if (selected > 0) {
// We have some connection request, let's process
// them here.
processHandles(selectedHandles());
}
// check to see if any cancellation request has been made.
nHandles -= unregisterHandles();
} catch (ClosedSelectorException cse) {
// If the selector has been closed, we can exit the loop
break;
} catch (Throwable e) {
ExceptionMonitor.getInstance().exceptionCaught(e);
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
ExceptionMonitor.getInstance().exceptionCaught(e1);
}
}
}
// Cleanup all the processors, and shutdown the acceptor.
if (selectable && isDisposing()) {
selectable = false;
try {
if (createdProcessor) {
processor.dispose();
}
} finally {
try {
synchronized (disposalLock) {
if (isDisposing()) {
destroy();
}
}
} catch (Exception e) {
ExceptionMonitor.getInstance().exceptionCaught(e);
} finally {
disposalFuture.setDone();
}
}
}
}
(1)、selector被wakeup唤醒后，调用registerHandles方法从registerQueue中取出请求依次调用open方法
private int registerHandles() {
for (;;) {
// The register queue contains the list of services to manage
// in this acceptor.
AcceptorOperationFuture future = registerQueue.poll();
if (future == null) {
return 0;
}
// We create a temporary map to store the bound handles,
// as we may have to remove them all if there is an exception
// during the sockets opening.
Map<SocketAddress, H> newHandles = new ConcurrentHashMap<SocketAddress, H>();
List<SocketAddress> localAddresses = future.getLocalAddresses();
try {
// Process all the addresses
for (SocketAddress a : localAddresses) {
H handle = open(a);
newHandles.put(localAddress(handle), handle);
}
// Everything went ok, we can now update the map storing
// all the bound sockets.
boundHandles.putAll(newHandles);
// and notify.
future.setDone();
return newHandles.size();
} catch (Exception e) {
// We store the exception in the future
future.setException(e);
} finally {
// Roll back if failed to bind all addresses.
if (future.getException() != null) {
for (H handle : newHandles.values()) {
try {
close(handle);
} catch (Exception e) {
ExceptionMonitor.getInstance().exceptionCaught(e);
}
}
// TODO : add some comment : what is the wakeup() waking up ?
wakeup();
}
}
}
}
open方法完成了ServerSocket的绑定和注册(NioSocketAcceptor.open(SocketAddress localAddress)方法如下)
protected ServerSocketChannel open(SocketAddress localAddress) throws Exception {
// Creates the listening ServerSocket
ServerSocketChannel channel = ServerSocketChannel.open();
boolean success = false;
try {
// This is a non blocking socket channel
channel.configureBlocking(false);
// Configure the server socket,
ServerSocket socket = channel.socket();
// Set the reuseAddress flag accordingly with the setting
socket.setReuseAddress(isReuseAddress());
// and bind.
socket.bind(localAddress, getBacklog());
// Register the channel within the selector for ACCEPT event
channel.register(selector, SelectionKey.OP_ACCEPT);
success = true;
} finally {
if (!success) {
close(channel);
}
}
return channel;
}
(2)、从(1)中可以知道selector上注册了ServerSocketChannel的OP_ACCEPT键，注册后nHandles==0,selected==0，进行下一次循环，同样是阻塞在select方法上
(3)、当连接到来时，select方法返回，selected>0,执行processHandles方法
private void processHandles(Iterator<H> handles) throws Exception {
while (handles.hasNext()) {
H handle = handles.next();
handles.remove();
// Associates a new created connection to a processor,
// and get back a session
S session = accept(processor, handle);
if (session == null) {
continue;
}
initSession(session, null, null);
// add the session to the SocketIoProcessor
session.getProcessor().add(session);
}
}
该方法在完成真正的接受连接操作后，创建session并扔到processor中，后续的工作交给processor来完成。每个session中其实有一个SocketChannel，这个socketChannel实际上是被注册到了processor的selector上。注册代码在NioProcessor类中可以找到
总结一下：Acceptor线程专门负责接受连接，在其上有一个selector，轮询是否有连接建立上来，当有连接建立上来，调用ServerSocketChannel.accept方法来接受连接，这个方法返回一个session对象，然后将这个session对象加入processor中，由processor遍历每个session来完成真正的IO操作。processor上也有一个selector与一个Processor线程,selector用于轮询session，Processor线程处理每个session的IO操作。