Netty源码分析 (七)----- read过程 源码分析

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)) != || readyOps == ) {
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) != ) {
// 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) != ) {
// 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)) !=  || readyOps == ) {
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 = ;
boolean close = false;
try {
int totalReadAmount = ;
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 <= ) {
// not was read release the buffer
byteBuf.release();
close = localReadAmount < ;
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 = ;
int ti = -;
try {
begin();
ti = threads.add();
if (!isOpen())
return ;
do {
n = IOUtil.read(fd, dst, -, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
return IOStatus.normalize(n);
} finally {
threads.remove(ti);
end(n > );
assert IOStatus.check(n);
}
}
}

最后目的就是将SocketChannel中的数据读出存放到ByteBuffer dst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 > ) {
// 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;
}
}
通过上述实现可以看出,基于channel的数据读取步骤如下:
1、如果缓存内存是DirectBuffer,就直接将Channel中的数据读取到堆外内存
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)。
5、会调用DirectBuffer.cleaner().clean()来释放创建的临时的堆外内存
如果AbstractNioByteChannel.read中第一步创建的是堆外内存,则会直接将数据读入到堆外内存,并不会先创建临时堆外内存,再将数据读入到堆外内存,最后将堆外内存拷贝到堆内存
简单的说,如果使用堆外内存,则只会复制一次数据,如果使用堆内存,则会复制两次数据
我们来看看readIntoNativeBuffer
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 ? : j - i;
if(k == )
return ;
int i1 = ;
//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 > )
bytebuffer.position(i + i1);
return i1;
}
这个函数就是将内核缓冲区中的数据读取到堆外缓存DirectBuffer

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

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