在上一篇文章中，我们分析了processSelectedKey这个方法中的accept过程，本文将分析一下work线程中的read过程。
private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) { final NioUnsafe unsafe = ch.unsafe(); //检查该SelectionKey是否有效，如果无效，则关闭channel if (!k.isValid()) { // close the channel if the key is not valid anymore unsafe.close(unsafe.voidPromise()); return; } try { int readyOps = k.readyOps(); // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead // to a spin loop // 如果准备好READ或ACCEPT则触发unsafe.read() ,检查是否为0，如上面的源码英文注释所说：解决JDK可能会产生死循环的一个bug。 if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { unsafe.read(); if (!ch.isOpen()) {//如果已经关闭，则直接返回即可，不需要再处理该channel的其他事件 // Connection already closed - no need to handle write. return; } } // 如果准备好了WRITE则将缓冲区中的数据发送出去，如果缓冲区中数据都发送完成，则清除之前关注的OP_WRITE标记 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(); } // 如果是OP_CONNECT，则需要移除OP_CONNECT否则Selector.select(timeout)将立即返回不会有任何阻塞，这样可能会出现cpu 100% 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(); } } catch (CancelledKeyException ignored) { unsafe.close(unsafe.voidPromise()); } }
该方法主要是对SelectionKey k进行了检查，有如下几种不同的情况
1）OP_ACCEPT，接受客户端连接
2）OP_READ, 可读事件, 即 Channel 中收到了新数据可供上层读取。
3）OP_WRITE, 可写事件, 即上层可以向 Channel 写入数据。
4）OP_CONNECT, 连接建立事件, 即 TCP 连接已经建立, Channel 处于 active 状态。
本篇博文主要来看下当work 线程 selector检测到OP_READ事件时，内部干了些什么。
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { unsafe.read(); if (!ch.isOpen()) {//如果已经关闭，则直接返回即可，不需要再处理该channel的其他事件 // Connection already closed - no need to handle write. return; } }
从代码中可以看到，当selectionKey发生的事件是SelectionKey.OP_READ，执行unsafe的read方法。注意这里的unsafe是NioByteUnsafe的实例
为什么说这里的unsafe是NioByteUnsafe的实例呢？在上篇博文Netty源码分析：accept中我们知道Boss NioEventLoopGroup中的NioEventLoop只负责accpt客户端连接，然后将该客户端注册到Work NioEventLoopGroup中的NioEventLoop中，即最终是由work线程对应的selector来进行read等时间的监听，即work线程中的channel为SocketChannel，SocketChannel的unsafe就是NioByteUnsafe的实例
下面来看下NioByteUnsafe中的read方法
@Override public void read() { final ChannelConfig config = config(); if (!config.isAutoRead() && !isReadPending()) { // ChannelConfig.setAutoRead(false) was called in the meantime removeReadOp(); return; } final ChannelPipeline pipeline = pipeline(); final ByteBufAllocator allocator = config.getAllocator(); final int maxMessagesPerRead = config.getMaxMessagesPerRead(); RecvByteBufAllocator.Handle allocHandle = this.allocHandle; if (allocHandle == null) { this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle(); } ByteBuf byteBuf = null; int messages = 0; boolean close = false; try { int totalReadAmount = 0; boolean readPendingReset = false; do { //1、分配缓存 byteBuf = allocHandle.allocate(allocator); int writable = byteBuf.writableBytes();//可写的字节容量 //2、将socketChannel数据写入缓存 int localReadAmount = doReadBytes(byteBuf); if (localReadAmount <= 0) { // not was read release the buffer byteBuf.release(); close = localReadAmount < 0; break; } if (!readPendingReset) { readPendingReset = true; setReadPending(false); } //3、触发pipeline的ChannelRead事件来对byteBuf进行后续处理 pipeline.fireChannelRead(byteBuf); byteBuf = null; if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) { // Avoid overflow. totalReadAmount = Integer.MAX_VALUE; break; } totalReadAmount += localReadAmount; // stop reading if (!config.isAutoRead()) { break; } if (localReadAmount < writable) { // Read less than what the buffer can hold, // which might mean we drained the recv buffer completely. break; } } while (++ messages < maxMessagesPerRead); pipeline.fireChannelReadComplete(); allocHandle.record(totalReadAmount); if (close) { closeOnRead(pipeline); close = false; } } catch (Throwable t) { handleReadException(pipeline, byteBuf, t, close); } finally { if (!config.isAutoRead() && !isReadPending()) { removeReadOp(); } } } }
下面一一介绍比较重要的代码
allocHandler的实例化过程
allocHandle负责自适应调整当前缓存分配的大小，以防止缓存分配过多或过少，先看allocHandler的实例化过程
RecvByteBufAllocator.Handle allocHandle = this.allocHandle; if (allocHandle == null) { this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle(); }
其中， config.getRecvByteBufAllocator()得到的是一个 AdaptiveRecvByteBufAllocator实例DEFAULT。
public static final AdaptiveRecvByteBufAllocator DEFAULT = new AdaptiveRecvByteBufAllocator();
而AdaptiveRecvByteBufAllocator中的newHandler()方法的代码如下：
@Override public Handle newHandle() { return new HandleImpl(minIndex, maxIndex, initial); } HandleImpl(int minIndex, int maxIndex, int initial) { this.minIndex = minIndex; this.maxIndex = maxIndex; index = getSizeTableIndex(initial); nextReceiveBufferSize = SIZE_TABLE[index]; }
其中，上面方法中所用到参数：minIndex maxIndex initial是什么意思呢？含义如下：minIndex是最小缓存在SIZE_TABLE中对应的下标。maxIndex是最大缓存在SIZE_TABLE中对应的下标，initial为初始化缓存大小。
AdaptiveRecvByteBufAllocator的相关常量字段
public class AdaptiveRecvByteBufAllocator implements RecvByteBufAllocator { static final int DEFAULT_MINIMUM = 64; static final int DEFAULT_INITIAL = 1024; static final int DEFAULT_MAXIMUM = 65536; private static final int INDEX_INCREMENT = 4; private static final int INDEX_DECREMENT = 1; private static final int[] SIZE_TABLE;
上面这些字段的具体含义说明如下：
1）、SIZE_TABLE：按照从小到大的顺序预先存储可以分配的缓存大小。
从16开始，每次累加16，直到496，接着从512开始，每次增大一倍，直到溢出。SIZE_TABLE初始化过程如下。
static { List<Integer> sizeTable = new ArrayList<Integer>(); for (int i = 16; i < 512; i += 16) { sizeTable.add(i); } for (int i = 512; i > 0; i <<= 1) { sizeTable.add(i); } SIZE_TABLE = new int[sizeTable.size()]; for (int i = 0; i < SIZE_TABLE.length; i ++) { SIZE_TABLE[i] = sizeTable.get(i); } }
2）、DEFAULT_MINIMUM：最小缓存（64），在SIZE_TABLE中对应的下标为3。
3）、DEFAULT_MAXIMUM ：最大缓存（65536），在SIZE_TABLE中对应的下标为38。
4）、DEFAULT_INITIAL ：初始化缓存大小，第一次分配缓存时，由于没有上一次实际收到的字节数做参考，需要给一个默认初始值。
5）、INDEX_INCREMENT：上次预估缓存偏小，下次index的递增值。
6）、INDEX_DECREMENT ：上次预估缓存偏大，下次index的递减值。
构造函数：
private AdaptiveRecvByteBufAllocator() { this(DEFAULT_MINIMUM, DEFAULT_INITIAL, DEFAULT_MAXIMUM); } public AdaptiveRecvByteBufAllocator(int minimum, int initial, int maximum) { if (minimum <= 0) { throw new IllegalArgumentException("minimum: " + minimum); } if (initial < minimum) { throw new IllegalArgumentException("initial: " + initial); } if (maximum < initial) { throw new IllegalArgumentException("maximum: " + maximum); } int minIndex = getSizeTableIndex(minimum); if (SIZE_TABLE[minIndex] < minimum) { this.minIndex = minIndex + 1; } else { this.minIndex = minIndex; } int maxIndex = getSizeTableIndex(maximum); if (SIZE_TABLE[maxIndex] > maximum) { this.maxIndex = maxIndex - 1; } else { this.maxIndex = maxIndex; } this.initial = initial; }
该构造函数对参数进行了有效性检查，然后初始化了如下3个字段，这3个字段就是上面用于产生allocHandle对象所要用到的参数。
private final int minIndex; private final int maxIndex; private final int initial;
其中，getSizeTableIndex函数的代码如下，该函数的功能为：找到SIZE_TABLE中的元素刚好大于或等于size的位置。
private static int getSizeTableIndex(final int size) { for (int low = 0, high = SIZE_TABLE.length - 1;;) { if (high < low) { return low; } if (high == low) { return high; } int mid = low + high >>> 1; int a = SIZE_TABLE[mid]; int b = SIZE_TABLE[mid + 1]; if (size > b) { low = mid + 1; } else if (size < a) { high = mid - 1; } else if (size == a) { return mid; } else { //这里的情况就是 a < size <= b 的情况 return mid + 1; } } }
byteBuf = allocHandle.allocate(allocator);
申请一块指定大小的内存
AdaptiveRecvByteBufAllocator#HandlerImpl
@Override public ByteBuf allocate(ByteBufAllocator alloc) { return alloc.ioBuffer(nextReceiveBufferSize); }
直接调用了ioBuffer方法，继续看
AbstractByteBufAllocator.java
@Override public ByteBuf ioBuffer(int initialCapacity) { if (PlatformDependent.hasUnsafe()) { return directBuffer(initialCapacity); } return heapBuffer(initialCapacity); }
ioBuffer函数中主要逻辑为：看平台是否支持unsafe，选择使用直接物理内存还是堆上内存。先看 heapBuffer
AbstractByteBufAllocator.java
@Override public ByteBuf heapBuffer(int initialCapacity) { return heapBuffer(initialCapacity, Integer.MAX_VALUE); } @Override public ByteBuf heapBuffer(int initialCapacity, int maxCapacity) { if (initialCapacity == 0 && maxCapacity == 0) { return emptyBuf; } validate(initialCapacity, maxCapacity); return newHeapBuffer(initialCapacity, maxCapacity); }
这里的newHeapBuffer有两种实现：至于具体用哪一种，取决于我们对系统属性io.netty.allocator.type的设置，如果设置为： “pooled”，则缓存分配器就为：PooledByteBufAllocator，进而利用对象池技术进行内存分配。如果不设置或者设置为其他，则缓存分配器为：UnPooledByteBufAllocator，则直接返回一个UnpooledHeapByteBuf对象。
UnpooledByteBufAllocator.java
@Override protected ByteBuf newHeapBuffer(int initialCapacity, int maxCapacity) { return new UnpooledHeapByteBuf(this, initialCapacity, maxCapacity); }
PooledByteBufAllocator.java
@Override protected ByteBuf newHeapBuffer(int initialCapacity, int maxCapacity) { PoolThreadCache cache = threadCache.get(); PoolArena<byte[]> heapArena = cache.heapArena; ByteBuf buf; if (heapArena != null) { buf = heapArena.allocate(cache, initialCapacity, maxCapacity); } else { buf = new UnpooledHeapByteBuf(this, initialCapacity, maxCapacity); } return toLeakAwareBuffer(buf); }
再看directBuffer
AbstractByteBufAllocator.java
@Override public ByteBuf directBuffer(int initialCapacity) { return directBuffer(initialCapacity, Integer.MAX_VALUE); } @Override public ByteBuf directBuffer(int initialCapacity, int maxCapacity) { if (initialCapacity == 0 && maxCapacity == 0) { return emptyBuf; } validate(initialCapacity, maxCapacity);//参数的有效性检查 return newDirectBuffer(initialCapacity, maxCapacity); }
与newHeapBuffer一样，这里的newDirectBuffer方法也有两种实现：至于具体用哪一种，取决于我们对系统属性io.netty.allocator.type的设置，如果设置为： “pooled”，则缓存分配器就为：PooledByteBufAllocator，进而利用对象池技术进行内存分配。如果不设置或者设置为其他，则缓存分配器为：UnPooledByteBufAllocator。这里主要看下UnpooledByteBufAllocator. newDirectBuffer的内部实现
UnpooledByteBufAllocator.java
@Override protected ByteBuf newDirectBuffer(int initialCapacity, int maxCapacity) { ByteBuf buf; if (PlatformDependent.hasUnsafe()) { buf = new UnpooledUnsafeDirectByteBuf(this, initialCapacity, maxCapacity); } else { buf = new UnpooledDirectByteBuf(this, initialCapacity, maxCapacity); } return toLeakAwareBuffer(buf); }
UnpooledUnsafeDirectByteBuf是如何实现缓存管理的？对Nio的ByteBuffer进行了封装，通过ByteBuffer的allocateDirect方法实现缓存的申请。
protected UnpooledUnsafeDirectByteBuf(ByteBufAllocator alloc, int initialCapacity, int maxCapacity) { super(maxCapacity); //省略了部分参数检查的代码 this.alloc = alloc; setByteBuffer(allocateDirect(initialCapacity)); }
protected ByteBuffer allocateDirect(int initialCapacity) { return ByteBuffer.allocateDirect(initialCapacity); } private void setByteBuffer(ByteBuffer buffer) { ByteBuffer oldBuffer = this.buffer; if (oldBuffer != null) { if (doNotFree) { doNotFree = false; } else { freeDirect(oldBuffer); } } this.buffer = buffer; memoryAddress = PlatformDependent.directBufferAddress(buffer); tmpNioBuf = null; capacity = buffer.remaining(); }
上面代码的主要逻辑为：
1、先利用ByteBuffer的allocateDirect方法分配了大小为initialCapacity的缓存
2、然后判断将旧缓存给free掉
3、最后将新缓存赋给字段buffer上
其中：memoryAddress = PlatformDependent.directBufferAddress(buffer) 获取buffer的address字段值，指向缓存地址。
capacity = buffer.remaining() 获取缓存容量。
接下来看toLeakAwareBuffer(buf)方法
protected static ByteBuf toLeakAwareBuffer(ByteBuf buf) { ResourceLeak leak; switch (ResourceLeakDetector.getLevel()) { case SIMPLE: leak = AbstractByteBuf.leakDetector.open(buf); if (leak != null) { buf = new SimpleLeakAwareByteBuf(buf, leak); } break; case ADVANCED: case PARANOID: leak = AbstractByteBuf.leakDetector.open(buf); if (leak != null) { buf = new AdvancedLeakAwareByteBuf(buf, leak); } break; } return buf; }
方法toLeakAwareBuffer(buf)对申请的buf又进行了一次包装。
上面一长串的分析，得到了缓存后，回到AbstractNioByteChannel.read方法，继续看。
doReadBytes方法
下面看下doReadBytes方法：将socketChannel数据写入缓存。
NioSocketChannel.java
@Override protected int doReadBytes(ByteBuf byteBuf) throws Exception { return byteBuf.writeBytes(javaChannel(), byteBuf.writableBytes()); }
将Channel中的数据读入缓存byteBuf中。继续看
WrappedByteBuf.java
@Override public int writeBytes(ScatteringByteChannel in, int length) throws IOException { return buf.writeBytes(in, length); }
AbstractByteBuf.java
@Override public int writeBytes(ScatteringByteChannel in, int length) throws IOException { ensureAccessible(); ensureWritable(length); int writtenBytes = setBytes(writerIndex, in, length); if (writtenBytes > 0) { writerIndex += writtenBytes; } return writtenBytes; }
这里的setBytes方法有不同的实现，这里看下UnpooledUnsafeDirectByteBuf的setBytes的实现。
UnpooledUnsafeDirectByteBuf.java
@Override public int setBytes(int index, ScatteringByteChannel in, int length) throws IOException { ensureAccessible(); ByteBuffer tmpBuf = internalNioBuffer(); tmpBuf.clear().position(index).limit(index + length); try { return in.read(tmpBuf); } catch (ClosedChannelException ignored) { return -1;//当Channel 已经关闭，则返回-1. } } private ByteBuffer internalNioBuffer() { ByteBuffer tmpNioBuf = this.tmpNioBuf; if (tmpNioBuf == null) { this.tmpNioBuf = tmpNioBuf = buffer.duplicate(); } return tmpNioBuf; }
最终底层采用ByteBuffer实现read操作，无论是PooledByteBuf、还是UnpooledXXXBuf，里面都将底层数据结构BufBuffer/array转换为ByteBuffer 来实现read操作。即无论是UnPooledXXXBuf还是PooledXXXBuf里面都有一个ByteBuffer tmpNioBuf，这个tmpNioBuf才是真正用来存储从管道Channel中读取出的内容的。到这里就完成了将channel的数据读入到了缓存Buf中。
我们具体来看看 in.read(tmpBuf); FileChannel和SocketChannel的read最后都是依赖的IOUtil来实现，代码如下
public int read(ByteBuffer dst) throws IOException { ensureOpen(); if (!readable) throw new NonReadableChannelException(); synchronized (positionLock) { int n = 0; int ti = -1; try { begin(); ti = threads.add(); if (!isOpen()) return 0; do { n = IOUtil.read(fd, dst, -1, nd); } while ((n == IOStatus.INTERRUPTED) && isOpen()); return IOStatus.normalize(n); } finally { threads.remove(ti); end(n > 0); assert IOStatus.check(n); } } }
最后目的就是将SocketChannel中的数据读出存放到ByteBuffer dst中，我们看看 IOUtil.read(fd, dst, -1, nd)
static int read(FileDescriptor var0, ByteBuffer var1, long var2, NativeDispatcher var4) throws IOException { if (var1.isReadOnly()) { throw new IllegalArgumentException("Read-only buffer"); //如果最终承载数据的buffer是DirectBuffer，则直接将数据读入到堆外内存中 } else if (var1 instanceof DirectBuffer) { return readIntoNativeBuffer(var0, var1, var2, var4); } else { // 分配临时的堆外内存 ByteBuffer var5 = Util.getTemporaryDirectBuffer(var1.remaining()); int var7; try { // Socket I/O 操作会将数据读入到堆外内存中 int var6 = readIntoNativeBuffer(var0, var5, var2, var4); var5.flip(); if (var6 > 0) { // 将堆外内存的数据拷贝到堆内存中（用户定义的缓存，在jvm中分配内存） var1.put(var5); } var7 = var6; } finally { // 里面会调用DirectBuffer.cleaner().clean()来释放临时的堆外内存 Util.offerFirstTemporaryDirectBuffer(var5); } return var7; } }
2、如果缓存内存是堆内存，则先申请一块和缓存同大小的临时 DirectByteBuffer var5。
3、将内核缓存中的数据读到堆外缓存var5，底层由NativeDispatcher的read实现。
4、把堆外缓存var5的数据拷贝到堆内存var1（用户定义的缓存，在jvm中分配内存）。
private static int readIntoNativeBuffer(FileDescriptor filedescriptor, ByteBuffer bytebuffer, long l, NativeDispatcher nativedispatcher, Object obj) throws IOException { int i = bytebuffer.position(); int j = bytebuffer.limit(); //如果断言开启，buffer的position大于limit，则抛出断言错误 if(!$assertionsDisabled && i > j) throw new AssertionError(); //获取需要读的字节数 int k = i > j ? 0 : j - i; if(k == 0) return 0; int i1 = 0; //从输入流读取k个字节到buffer if(l != -1L) i1 = nativedispatcher.pread(filedescriptor, ((DirectBuffer)bytebuffer).address() + (long)i, k, l, obj); else i1 = nativedispatcher.read(filedescriptor, ((DirectBuffer)bytebuffer).address() + (long)i, k); //重新定位buffer的position if(i1 > 0) bytebuffer.position(i + i1); return i1; }
回到AbstractNioByteChannel.read方法，继续看。
@Override public void read() { //... try { int totalReadAmount = 0; boolean readPendingReset = false; do { byteBuf = allocHandle.allocate(allocator); int writable = byteBuf.writableBytes(); int localReadAmount = doReadBytes(byteBuf); if (localReadAmount <= 0) { // not was read release the buffer byteBuf.release(); close = localReadAmount < 0; break; } if (!readPendingReset) { readPendingReset = true; setReadPending(false); } pipeline.fireChannelRead(byteBuf); byteBuf = null; if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) { // Avoid overflow. totalReadAmount = Integer.MAX_VALUE; break; } totalReadAmount += localReadAmount; // stop reading if (!config.isAutoRead()) { break; } if (localReadAmount < writable) { // Read less than what the buffer can hold, // which might mean we drained the recv buffer completely. break; } } while (++ messages < maxMessagesPerRead); pipeline.fireChannelReadComplete(); allocHandle.record(totalReadAmount); if (close) { closeOnRead(pipeline); close = false; } } catch (Throwable t) { handleReadException(pipeline, byteBuf, t, close); } finally { if (!config.isAutoRead() && !isReadPending()) { removeReadOp(); } } } }
int localReadAmount = doReadBytes(byteBuf);
1、如果返回0，则表示没有读取到数据，则退出循环。
2、如果返回-1，表示对端已经关闭连接，则退出循环。
3、否则，表示读取到了数据，数据读入缓存后，触发pipeline的ChannelRead事件，byteBuf作为参数进行后续处理，这时自定义Inbound类型的handler就可以进行业务处理了。Pipeline的事件处理在我之前的博文中有详细的介绍。处理完成之后，再一次从Channel读取数据，直至退出循环。
4、循环次数超过maxMessagesPerRead时，即只能在管道中读取maxMessagesPerRead次数据，既是还没有读完也要退出。在上篇博文中，Boss线程接受客户端连接也用到了此变量，即当boss线程 selector检测到OP_ACCEPT事件后一次只能接受maxMessagesPerRead个客户端连接