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之前在做项目的时候使用到了Netty这个网络框架，对于Java中的IO模型有了进一步的了解，熟悉的NIO非阻塞的模式。而Netty就是对于Java NIO 的高级封装。这篇文章就是个人根据Netty4.1.6的源码，进行了总结。
Netty组件
NioEventLoop
对于Netty中的NioEventLoop这个组件来说，它就是类似于写的普通网络编程中的通过创建一个新的线程Thread来实现对于客户端的监听，这个监听如果放到主线程中会导致主线程阻塞，所以在实现的时候通过创建要给新线程的方式来实现。从这个角度上理解，这个NioEventLoop组件，主要做的两件事情，第一，就是建立客户端和服务器端段的连接，保证连接正常。第二，实现客户端和服务器端的数据的交互。
但是在使用Thread实现上面两个功能的时候要保证监听过程处于一个持续监听的状态，也就是说在其中要实现一个死循环。通过这个死循环来持续监听连接在Netty中也提供了这样的一个方法。那就是下面这个方法
@Override
protected void run() {
//实现一个死循环不断去绑定对应的监听状态
for (;;) {
try {
//根据不同的装响应实现不同的操作
switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
case SelectStrategy.CONTINUE:
continue;
case SelectStrategy.SELECT:
select(wakenUp.getAndSet(false));
// 'wakenUp.compareAndSet(false, true)' is always evaluated
// before calling 'selector.wakeup()' to reduce the wake-up
// overhead. (Selector.wakeup() is an expensive operation.)
//
// However, there is a race condition in this approach.
// The race condition is triggered when 'wakenUp' is set to
// true too early.
//
// 'wakenUp' is set to true too early if:
// 1) Selector is waken up between 'wakenUp.set(false)' and
// 'selector.select(...)'. (BAD)
// 2) Selector is waken up between 'selector.select(...)' and
// 'if (wakenUp.get()) { ... }'. (OK)
//
// In the first case, 'wakenUp' is set to true and the
// following 'selector.select(...)' will wake up immediately.
// Until 'wakenUp' is set to false again in the next round,
// 'wakenUp.compareAndSet(false, true)' will fail, and therefore
// any attempt to wake up the Selector will fail, too, causing
// the following 'selector.select(...)' call to block
// unnecessarily.
//
// To fix this problem, we wake up the selector again if wakenUp
// is true immediately after selector.select(...).
// It is inefficient in that it wakes up the selector for both
// the first case (BAD - wake-up required) and the second case
// (OK - no wake-up required).
if (wakenUp.get()) {
selector.wakeup();
}
default:
// fallthrough
}
cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
//开始处理每个链接
if (ioRatio == 100) {
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
runAllTasks();
}
} else {
final long ioStartTime = System.nanoTime();
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
final long ioTime = System.nanoTime() - ioStartTime;
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
}
} catch (Throwable t) {
handleLoopException(t);
}
// Always handle shutdown even if the loop processing threw an exception.
try {
if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
return;
}
}
} catch (Throwable t) {
handleLoopException(t);
}
}
}
从代码中可以看出来真正实现对于数据读写操作就是从processSelectedKeys()方法开始的，而这个方法processSelectedKeys()。主要的作用是什么呢？
//调用上面的处理方法
private void processSelectedKeys() {
//如果SelectionKey为空
if (selectedKeys != null) {
processSelectedKeysOptimized(selectedKeys.flip());
} else {
processSelectedKeysPlain(selector.selectedKeys());
}
}
private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
for (int i = 0;; i ++) {
final SelectionKey k = selectedKeys[i];
if (k == null) {
break;
}
// null out entry in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
selectedKeys[i] = null;
final Object a = k.attachment();
if (a instanceof AbstractNioChannel) {
processSelectedKey(k, (AbstractNioChannel) a);
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
processSelectedKey(k, task);
}
if (needsToSelectAgain) {
// null out entries in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
for (;;) {
i++;
if (selectedKeys[i] == null) {
break;
}
selectedKeys[i] = null;
}
selectAgain();
// Need to flip the optimized selectedKeys to get the right reference to the array
// and reset the index to -1 which will then set to 0 on the for loop
// to start over again.
//
// See https://github.com/netty/netty/issues/1523
selectedKeys = this.selectedKeys.flip();
i = -1;
}
}
}
private void processSelectedKeysPlain(Set<SelectionKey> selectedKeys) {
// check if the set is empty and if so just return to not create garbage by
// creating a new Iterator every time even if there is nothing to process.
// See https://github.com/netty/netty/issues/597
if (selectedKeys.isEmpty()) {
return;
}
Iterator<SelectionKey> i = selectedKeys.iterator();
for (;;) {
final SelectionKey k = i.next();
final Object a = k.attachment();
i.remove();
if (a instanceof AbstractNioChannel) {
processSelectedKey(k, (AbstractNioChannel) a);
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
processSelectedKey(k, task);
}
if (!i.hasNext()) {
break;
}
if (needsToSelectAgain) {
selectAgain();
selectedKeys = selector.selectedKeys();
// Create the iterator again to avoid ConcurrentModificationException
if (selectedKeys.isEmpty()) {
break;
} else {
i = selectedKeys.iterator();
}
}
}
}
可以看到上面的processSelectedKeys()方法通过对于SelectionKey的判断实现了两种不同的逻辑。但是这两个逻辑最为核心的操作就是AbstractNioChannel类的存在，可以继续跟进源码会发现这个类其实就是一个Channel，在Java NIO中我们知道一个Channel就类似于普通网络编程中的一个Socket。所以说这里的AbstractNioChannel是对于Channel在Netty NIO实现的基础上做了进一步的封装。
Channel
在上面提到的概念就是Channel可以理解为一个Socket那么既然是用来做客户端和服务器端的数据传递工作，首先需要搞清楚的是Channel是什么时候创建？第二点就是Channel底层实现也是通过Socket来实现的又对Socket做了哪些优化呢？
在NioEventLoop类中有如下的一个方法
private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
//获取到unsafe
final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
if (!k.isValid()) {
final EventLoop eventLoop;
try {
eventLoop = ch.eventLoop();
} catch (Throwable ignored) {
// If the channel implementation throws an exception because there is no event loop, we ignore this
// because we are only trying to determine if ch is registered to this event loop and thus has authority
// to close ch.
return;
}
// Only close ch if ch is still registerd to this EventLoop. ch could have deregistered from the event loop
// and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
// still healthy and should not be closed.
// See https://github.com/netty/netty/issues/5125
if (eventLoop != this || eventLoop == null) {
return;
}
// close the channel if the key is not valid anymore
unsafe.close(unsafe.voidPromise());
return;
}
try {
int readyOps = k.readyOps();
// We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
// the NIO JDK channel implementation may throw a NotYetConnectedException.
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
// remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
// See https://github.com/netty/netty/issues/924
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
// Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
if ((readyOps & SelectionKey.OP_WRITE) != 0) {
// Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
ch.unsafe().forceFlush();
}
// Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
// to a spin loop
//这里有一个对于OP_ACCEPT的判断。
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
if (!ch.isOpen()) {
// Connection already closed - no need to handle write.
return;
}
}
} catch (CancelledKeyException ignored) {
unsafe.close(unsafe.voidPromise());
}
}
在上面这个方法中有一块逻辑是指定注意的
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
if (!ch.isOpen()) {
// Connection already closed - no need to handle write.
return;
}
}
在上面这段逻辑中，可以看到在SelectionKey判断的时候会有一个OP_ACCEPT事件，这里就会有一个unsafe.read的方法。可进入NioMessageUnsafe中查看read方法如下。而这个NioMessageUnsafe就是对于连接处理的一个类。
private final class NioMessageUnsafe extends AbstractNioUnsafe {
private final List<Object> readBuf = new ArrayList<Object>();
@Override
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline();
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
allocHandle.incMessagesRead(localRead);
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i));
}
readBuf.clear();
allocHandle.readComplete();
pipeline.fireChannelReadComplete();
if (exception != null) {
closed = closeOnReadError(exception);
pipeline.fireExceptionCaught(exception);
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}
在上面逻辑中值得关注一个方法便是doReadMessages(readBuf)方法，查看这个方法的实现会发现，是由NioServerSocketChannel实现。到这里终于是接近Java NIO的底层了。
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = javaChannel().accept();
try {
if (ch != null) {
buf.add(new NioSocketChannel(this, ch));
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}
关于Java底层NIO的实现,而这里所对应的SocketChannel就是对应的NIO模型中的SocketChannel。通过获取到对应的ServerSocketChannel，进行的accept操作。在获取到对象之后将其添加到一个Object的列表中。而这个列表理解为获取到了一个Channel的列表。Netty直接把一个SocketChannel封装成了一个NioSocketChannel。然后通过一个List进行返回，最后只需要通过这个NioSocketChannel进行数据的读写操作就可以了。
@Override
protected ServerSocketChannel javaChannel() {
return (ServerSocketChannel) super.javaChannel();
}
ByteBuf
在Java实现的NIO编程中有一个ByteBuffer，这个ByteBuffer是对IO流的封装，在Netty中还有就是对于ByteBuffer的封装ByteBuf类，在这个类中定义了很多的read和write方法， 而这些方法就是对一普通IO中的输入输出流。如下图
Pipline
Pipeline，对应的普通IO编程中的对于数据的处理操作。那么Netty是什么时候将Pipeline加入到对应的连接处理过程中的呢？深究一下会发现在AbstractChannel的构造方法中调用了newChannelPipeline方法。而这个方法创建了一个默认的ChannelPipeline。也就是说我们也可以自己实现这个Pipeline。
protected AbstractChannel(Channel parent) {
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}
protected DefaultChannelPipeline newChannelPipeline() {
return new DefaultChannelPipeline(this);
}
通过下面的操作，最终将逻辑处理操作加载到Channel中。
protected DefaultChannelPipeline(Channel channel) {
this.channel = ObjectUtil.checkNotNull(channel, "channel");
succeededFuture = new SucceededChannelFuture(channel, null);
voidPromise = new VoidChannelPromise(channel, true);
tail = new TailContext(this);
head = new HeadContext(this);
head.next = tail;
tail.prev = head;
}
ChannelHandler
对应业务逻辑处理块，在ChannelPipeline中有很多的add方法、remove方法其实这些方法就是实现了对于处理逻辑的动态的处理。这里使用的一个策略模式 使用者可以根据不同的处理逻辑实现不同的Handler，但是整个的处理逻辑是不变的就是实现对于数据的读写操作。这里看一个比较常用的方法addLast()。无论传入什么样的Handler都不会改变这个方法的实现逻辑。而这方法就是将对应的处理逻辑加入到整个处理的最后。
@Override
public final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) {
if (handlers == null) {
throw new NullPointerException("handlers");
}
for (ChannelHandler h: handlers) {
if (h == null) {
break;
}
addLast(executor, null, h);
}
return this;
}
总结
- NioEventLoop组件
- Channel组件
- ByteBuf组件
- Pipeline组件
- ChannelHandler组件