NIO框架之MINA源代码解析（一）：背景
MINA的底层还是利用了jdk提供了nio功能，mina仅仅是对nio进行封装。包含MINA用的线程池都是jdk直接提供的。
MINA的server端主要有accept、processor、session三部分组成的。当中accept主要负责在指定的port监听。若有新连接则建立一个新的session。processor则负责处理session相应的发送数据和接收数据并调用上层处理；session则缓存当前连接数据。
MINA採用了线程懒启动的技术，即最少启动线程。在MINA server启动的时候，仅仅有一个线程-accept,而且accept线程仅仅有一个，在指定的port进行监听（能够同一时候监听多个port，mina能够绑定多port）。
1、acceptor
先看下acceptor的主要类图吧。
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mina server的启动入口是在NioSocketAcceptor.bind（InetSocketAddress）或者NioSocketAcceptor.bind（SocketAddress...）方法， acceptor.bind(new InetSocketAddress(1234));
然后会调用AbstractPollingIoAcceptor.bindInternal（List<? extends SocketAddress>）方法，在bindInternal方法里面会调用startupAcceptor()方法提交一个accept线程到线程池里面（仅仅提交一次），并初始化acceptor端的Selector。就这样一个acceptor线程启动了。
acceptor端业务相对简单，相当于在当前Selector里面监听acceptor事件，处理新连接并新建一个session放到相应的processor里面。
acceptor 代码。非常easy。
private class Acceptor implements Runnable {
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 在通道里面注冊连接事件
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.处理连接
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();
}
}
}
}
2、processor
processor顾名思义，就是进行IO处理，处理当前session的数据读写，并进行业务处理。
在mina server初始化的时候，会初始化一个processor池，通过NioSocketAcceptor的构造器传入池的大小，默认是当前处理器的个数+1。
processor池里面有一个jdk提供的 线程池 - Executors.newCachedThreadPool()。各个processor线程会引用此线程池，即每一个processor线程都在这个线程池里面运行。
在mina server实际处理时，每一个processor相当于一个线程，轮流处理当前的session队列里面的数据（每一个processor里面的session相当于顺序处理，共享一个线程）。
每一个processor有一个Selector对象。
processor类图
processor端的处理逻辑相对有点复杂。看以下的流程图。
1、把新加入进来的session注冊到当前processor的Selector里面的read事件，并初始化session；
2、推断当前Selector是否有读写事件。
3、若第2步有读事件时。进入步骤4，若没有的话，直接到第6步。
4、处理当前读事件，并把处理后的数据放入到flush队列里面。
5、把第4步运行的结果flush到client；
6、处理session。比方session idle时间等。
7、又一次运行第1步，循环运行。
processor端代码。
private class Processor implements Runnable {
public void run() {
assert (processorRef.get() == this);
int nSessions = 0;
lastIdleCheckTime = System.currentTimeMillis();
for (;;) {
try {
// This select has a timeout so that we can manage
// idle session when we get out of the select every
// second. (note : this is a hack to avoid creating
// a dedicated thread).
long t0 = System.currentTimeMillis();
int selected = select(SELECT_TIMEOUT);
long t1 = System.currentTimeMillis();
long delta = (t1 - t0);
if ((selected == 0) && !wakeupCalled.get() && (delta < 100)) {
// Last chance : the select() may have been
// interrupted because we have had an closed channel.
if (isBrokenConnection()) {
LOG.warn("Broken connection");
// we can reselect immediately
// set back the flag to false
wakeupCalled.getAndSet(false);
continue;
} else {
LOG.warn("Create a new selector. Selected is 0, delta = " + (t1 - t0));
// Ok, we are hit by the nasty epoll
// spinning.
// Basically, there is a race condition
// which causes a closing file descriptor not to be
// considered as available as a selected channel, but
// it stopped the select. The next time we will
// call select(), it will exit immediately for the same
// reason, and do so forever, consuming 100%
// CPU.
// We have to destroy the selector, and
// register all the socket on a new one.
registerNewSelector();
}
// Set back the flag to false
wakeupCalled.getAndSet(false);
// and continue the loop
continue;
}
// Manage newly created session first 处理新加入进来的session，并注冊到当前processor的Selector读事件
nSessions += handleNewSessions();
updateTrafficMask();
// Now, if we have had some incoming or outgoing events,
// deal with them
if (selected > 0) {
//LOG.debug("Processing ..."); // This log hurts one of the MDCFilter test...处理读事件。并把结果放入flush队列里面
process();
}
// Write the pending requests 把flush队列里面的session的处理完的数据发送给client
long currentTime = System.currentTimeMillis();
flush(currentTime);
// And manage removed sessions
nSessions -= removeSessions();
// Last, not least, send Idle events to the idle sessions
notifyIdleSessions(currentTime);
// Get a chance to exit the infinite loop if there are no
// more sessions on this Processor
if (nSessions == 0) {
processorRef.set(null);
if (newSessions.isEmpty() && isSelectorEmpty()) {
// newSessions.add() precedes startupProcessor
assert (processorRef.get() != this);
break;
}
assert (processorRef.get() != this);
if (!processorRef.compareAndSet(null, this)) {
// startupProcessor won race, so must exit processor
assert (processorRef.get() != this);
break;
}
assert (processorRef.get() == this);
}
// Disconnect all sessions immediately if disposal has been
// requested so that we exit this loop eventually.
if (isDisposing()) {
for (Iterator<S> i = allSessions(); i.hasNext();) {
scheduleRemove(i.next());
}
wakeup();
}
} catch (ClosedSelectorException cse) {
// If the selector has been closed, we can exit the loop
break;
} catch (Throwable t) {
ExceptionMonitor.getInstance().exceptionCaught(t);
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
ExceptionMonitor.getInstance().exceptionCaught(e1);
}
}
}
try {
synchronized (disposalLock) {
if (disposing) {
doDispose();
}
}
} catch (Throwable t) {
ExceptionMonitor.getInstance().exceptionCaught(t);
} finally {
disposalFuture.setValue(true);
}
}
}
3、session
session做为一个连接的详细对象，缓存当前连接用户的一些信息。
session类图
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session对象是绑定在SelectableChannel的一个attach。
class NioProcessor
@Override
protected void init(NioSession session) throws Exception {
SelectableChannel ch = (SelectableChannel) session.getChannel();
ch.configureBlocking(false);
session.setSelectionKey(ch.register(selector, SelectionKey.OP_READ, session));//chanel 注冊读事件，并把session当做一个attach辅导SelectableChannel里面。
}