Java网络编程——NIO处理写事件(SelectionKey.OP_WRITE)
Posted 胡玉洋
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在前面NIO的例子中,在服务端,有对连接事件(SelectionKey.OP_ACCEPT)的处理,也有对读事件(SelectionKey.OP_READ)的处理,但是没有对写事件(SelectionKey.OP_WRITE)进行处理,原因就是写事件有些特殊,在这里单独记录一下。
网上有一些例子都是在服务端读完数据后直接给客户端SocketChannel对应的SelectionKey注册上写事件(SelectionKey.OP_WRITE),写完数据后也不注销写事件:
@Slf4j
public class Nioserver
public static void main(String[] args) throws Exception
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.bind(new InetSocketAddress(8080));
Selector selector = Selector.open();
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
while (true)
int select = selector.select();
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectionKeys.iterator();
while (iterator.hasNext())
SelectionKey selectionKey = iterator.next();
if (selectionKey.isAcceptable())
ServerSocketChannel serverSocket = (ServerSocketChannel) selectionKey.channel();
SocketChannel socketChannel = serverSocket.accept();
log.info("receive connection from client. client:", socketChannel.getRemoteAddress());
socketChannel.configureBlocking(false);
socketChannel.register(selector, SelectionKey.OP_READ);
else if (selectionKey.isReadable())
ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
socketChannel.read(byteBuffer);
String message = new String(byteBuffer.array()).trim();
byteBuffer.clear();
log.info("receive message from client. client: message length:", socketChannel.getRemoteAddress(),message.length());
socketChannel.register(selectionKey.selector(), SelectionKey.OP_WRITE);
selectionKey.attach(message);
else if (selectionKey.isWritable())
SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
String response = (String) selectionKey.attachment();
ByteBuffer byteBuffer = ByteBuffer.allocate(response.length());
byteBuffer.put(response.getBytes());
byteBuffer.flip();
socketChannel.write(byteBuffer);
log.info("send message to client. client: message length:", socketChannel.getRemoteAddress(), response.length());
iterator.remove();
但实际上根本不能这么用,即使在服务端向客户端写完数据后,也会不断触发写事件(selector.select()返回, selectionKey.isWritable()返回true )。
什么时候会触发写事件呢?
在前面的文章 《Java网络编程——NIO的阻塞IO模式、非阻塞IO模式、IO多路复用模式的使用》 中简单提到SelectionKey.OP_WRITE事件表示已经可以向通道写数据了(通道目前可以用于写操作),那什么时候才算“可以向通道写数据”呢?
如果有channel在Selector上注册了SelectionKey.OP_WRITE事件,在调用selector.select();时,系统会检查内核写缓冲区是否可写(当写缓冲区已满、channel调用了shutdownOutPut等情况,内核缓冲区不可写),如果可写,selector.select();会立即返回写事件。
把服务端处理读事件的代码优化如下:
@Slf4j
public class NIOSelectorNonblockingWriteServer
private final static int MESSAGE_LENGTH = 1024 * 1024 * 100;
public static void main(String[] args) throws Exception
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.bind(new InetSocketAddress("127.0.0.1", 8080), 50);
Selector selector = Selector.open();
SelectionKey serverSocketKey = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
while (true)
int count = selector.select();
log.info("select event count:" + count);
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectionKeys.iterator();
while (iterator.hasNext())
SelectionKey selectionKey = iterator.next();
// 有客户端请求建立连接
if (selectionKey.isAcceptable())
handleAccept(selectionKey);
// 有客户端发送数据
else if (selectionKey.isReadable())
handleRead(selectionKey);
// 可以向客户端发送数据
else if (selectionKey.isWritable())
handleWrite(selectionKey);
iterator.remove();
private static void handleAccept(SelectionKey selectionKey) throws IOException
ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();
SocketChannel socketChannel = serverSocketChannel.accept();
if (Objects.nonNull(socketChannel))
log.info("receive connection from client. client:", socketChannel.getRemoteAddress());
// 设置客户端Channel为非阻塞模式,否则在执行socketChannel.read()时会阻塞
socketChannel.configureBlocking(false);
Selector selector = selectionKey.selector();
socketChannel.register(selector, SelectionKey.OP_READ);
private static void handleRead(SelectionKey selectionKey) throws IOException
SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
ByteBuffer readBuffer = ByteBuffer.allocate(MESSAGE_LENGTH);
int length = 0;
while (length < MESSAGE_LENGTH)
length += socketChannel.read(readBuffer);
log.info("receive message from client. client: message length:", socketChannel.getRemoteAddress(), readBuffer.position());
ByteBuffer writeBuffer = ByteBuffer.allocate(readBuffer.position());
readBuffer.flip();
writeBuffer.put(readBuffer);
// 读完数据后,为 SelectionKey 注册可写事件
if (!isInterest(selectionKey, SelectionKey.OP_WRITE))
selectionKey.interestOps(selectionKey.interestOps() + SelectionKey.OP_WRITE);
writeBuffer.flip();
selectionKey.attach(writeBuffer);
// 服务端可能是为每个Channel维护一块缓冲区,当向某个Channel写数据时缓冲区满了,还可以向其他Channel写数据
private static void handleWrite(SelectionKey selectionKey) throws IOException
SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
ByteBuffer writeBuffer = (ByteBuffer) selectionKey.attachment();
int writeLength = socketChannel.write(writeBuffer);
log.info("send message to client. client: message length:", socketChannel.getRemoteAddress(), writeLength);
if (!writeBuffer.hasRemaining())
// 写完数据后,要把写事件取消,否则当写缓冲区有剩余空间时,会一直触发写事件
selectionKey.interestOps(selectionKey.interestOps() - SelectionKey.OP_WRITE);
// socketChannel.shutdownOutput(); // channel调用shutdownOutput()后,会停止触发写事件
// 判断 SelectionKey 对某个事件是否感兴趣
private static boolean isInterest(SelectionKey selectionKey, int event)
int interestSet = selectionKey.interestOps();
boolean isInterest = (interestSet & event) == event;
return isInterest;
在服务端读取完客户端消息后,会先判断当前SelectionKey是否已经注册了写事件,如果没有则为其注册写事件。在服务端向客户端写完数据后,会取消写事件。
在上面判断SelectionKey是否已经注册了某个事件时,判断条件是 selectionKey.interestOps() & SelectionKey.OP_WRITE
,selectionKey.interestOps()就是已经注册的事件,SelectionKey中可以只用1个整形数字来表示多个注册的事件(interestOps变量),SelectionKey.OP_READ=1(二进制为 00000001),SelectionKey.OP_WRITE=4(二进制为 00000100),SelectionKey.OP_CONNECT=8(二进制为 00001000),SelectionKey.OP_ACCEPT=16(二进制为 00010000)。当注册某个事件时,会把对应事件对应的整数(单个事件对应的整数或者多个事件对应的整数和)赋值给interestOps变量,比如注册读事件时,interestOps=1;注册读事件+写事件时,interestOps=1+4 。因此可以通过按位与的算法来判断当前SelectionKey是否注册过某个事件。
为什么不在处理完读事件(selectionKey.isReadable())后,直接写数据,而要重新注册一个读事件呢?
那我们就继续看这个例子:
@Slf4j
public class NIOSelectorBlockingWriteServer
private final static int MESSAGE_LENGTH = 1024 * 1024 * 100;
public static void main(String[] args) throws Exception
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.bind(new InetSocketAddress("127.0.0.1", 8080), 50);
Selector selector = Selector.open();
SelectionKey serverSocketKey = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
while (true)
int count = selector.select();
log.info("select event count:" + count);
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectionKeys.iterator();
while (iterator.hasNext())
SelectionKey selectionKey = iterator.next();
// 有客户端请求建立连接
if (selectionKey.isAcceptable())
handleAccept(selectionKey);
// 有客户端发送数据
else if (selectionKey.isReadable())
handleRead(selectionKey);
iterator.remove();
private static void handleAccept(SelectionKey selectionKey) throws IOException
ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();
SocketChannel socketChannel = serverSocketChannel.accept();
if (Objects.nonNull(socketChannel))
log.info("receive connection from client. client:", socketChannel.getRemoteAddress());
// 设置客户端Channel为非阻塞模式,否则在执行socketChannel.read()时会阻塞
socketChannel.configureBlocking(false);
Selector selector = selectionKey.selector();
socketChannel.register(selector, SelectionKey.OP_READ);
private static void handleRead(SelectionKey selectionKey) throws IOException
SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
ByteBuffer readBuffer = ByteBuffer.allocate(MESSAGE_LENGTH);
int length = 0;
while (length < MESSAGE_LENGTH)
length += socketChannel.read(readBuffer);
log.info("receive message from client. client: message length:", socketChannel.getRemoteAddress(), readBuffer.position());
ByteBuffer writeBuffer = ByteBuffer.allocate(readBuffer.position());
readBuffer.flip();
writeBuffer.put(readBuffer);
writeBuffer.flip();
while (writeBuffer.hasRemaining())
int writeLength = socketChannel.write(writeBuffer);
log.info("send message to client. client: message length:", socketChannel.getRemoteAddress(), writeLength);
- Debug模式启动服务端NIOSelectorBlockingWriteServer,在最后一行
log.info("send message to client. client ……
打上断点 - Debug模式运行上面客户端NIOClient的例子,向服务端发送100MB的数据,在
ByteBuffer readBuffer = ByteBuffer.allocate(MESSAGE_LENGTH);
打上断点,运行到断点时,当服务端调用了socketChannel.write(writeBuffer);
向客户端发送的数据还没来得及被客户端接收完时(这时候客户端卡在断点,并没有开始接收服务端的数据),数据会在服务端写缓冲区积压,在极限情况下,当服务写端缓冲区写满时,再调用socketChannel.write(writeBuffer);
就写不进去了(返回0)。所以如果在处理完读事件后直接发送数据,遇到服务端写缓冲区满的情况时,会直接阻塞当前线程(比如这个例子中会不断执行int writeLength = socketChannel.write(writeBuffer);
并返回0),无法及时处理其他客户端请求。 - 这时再起一个线程,以Debug模式运行NIOClient,当执行
socketChannel.write(writeBuffer);
向服务端写数据时,可能会阻塞(如果发送的数据量太大导致服务端接收缓存写满,此时服务端线程又在死循环,所以无法读取客户端发来的数据),如果这里不阻塞,执行到socketChannel.read(readBuffer)
就会阻塞(因为服务端一直没有读取到该客户端发送的数据,肯定也还没执行向客户端发送数据的代码)。这里的客户端是以阻塞模式运行的,即使把客户端的SocketChannel设置为非阻塞模式,也是无法及时收到服务端返回的数据的。
所以在传输数据量较大,需要向客户端回写数据时,最好注册一个写事件,避免服务端缓冲区写满时导致线程阻塞,而无法及时处理其他事件的情况。这样才能体现NIO多路复用模式的特点,才能可以让一个线程同时为多个客户端服务。
转载请注明出处——胡玉洋 《Java网络编程——NIO处理写事件(SelectionKey.OP_WRITE)》
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