一、前言
公司最近要基于Netty构建一个TCP通讯框架, 因Netty是基于NIO的,为了更好的学习和使用Netty,特意去翻了之前记录的NIO的资料,以及重新实现了一遍NIO的网络通讯,不试不知道,一试发现好多细节没注意,导致客户端和服务端通讯的时候出现了一些非常莫名其妙的问题,这边我记录下耗了我一晚上的问题~
二、正文
废话不多说,先上问题代码~
服务端:
package com.nio.server;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
public class NIOServer {
private static Selector selector;
private static ServerSocketChannel serverSocketChannel;
private static ByteBuffer bf = ByteBuffer.allocate(1024);
public static void main(String[] args) throws Exception{
init();
while(true){
selector.select();
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while(it.hasNext()){
SelectionKey key = it.next();
if(key.isAcceptable()){
System.out.println("连接准备就绪");
ServerSocketChannel server = (ServerSocketChannel)key.channel();
System.out.println("等待客户端连接中........................");
SocketChannel channel = server.accept();
channel.configureBlocking(false);
channel.register(selector,SelectionKey.OP_READ);
}
else if(key.isReadable()){
System.out.println("读准备就绪,开始读.......................");
SocketChannel channel = (SocketChannel)key.channel();
System.out.println("客户端的数据如下:");
int readLen = 0;
bf.clear();
StringBuffer sb = new StringBuffer();
while((readLen=channel.read(bf))>0){
sb.append(new String(bf.array()));
bf.clear();
}
if(-1==readLen){
channel.close();
}
channel.write(ByteBuffer.wrap(("客户端,你传过来的数据是:"+sb.toString()).getBytes()));
}
it.remove();
}
}
}
private static void init() throws Exception{
selector = Selector.open();
serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.socket().bind(new InetSocketAddress(8080));
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
}
}
客户端:
package com.nio.client;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
public class NIOClient {
private static Selector selector;
public static void main(String[]args) throws Exception{
selector = Selector.open();
SocketChannel sc = SocketChannel.open();
sc.configureBlocking(false);
sc.connect(new InetSocketAddress("127.0.0.1",8080));
sc.register(selector,SelectionKey.OP_READ);
ByteBuffer bf = ByteBuffer.allocate(1024);
bf.put("Hi,server,i'm client".getBytes());
if(sc.finishConnect()){
bf.flip();
while(bf.hasRemaining()){
sc.write(bf);
}
while(true){
selector.select();
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while(it.hasNext()){
SelectionKey key = it.next();
if(key.isReadable()){
bf.clear();
SocketChannel othersc = (SocketChannel)key.channel();
othersc.read(bf);
System.out.println("服务端返回的数据:"+new String(bf.array()));
}
}
selector.selectedKeys().clear();
}
}
}
}
服务端运行结果:
客户端运行结果:
这边我们可以看到,客户端输出了两次,笔者调试的时候发现,服务端只往客户端写过一次数据,但是客户端却打印了两次数据,而且两次的数据不一样,挺诡异的!然后我就各种查,折磨了我一夜,今早一来,又想着怎么解决问题,不经意间发现了一篇文章:java nio使用的是水平触发还是边缘触发?,文章中指出Nio的Selector.select()是“水平触发”(也叫“条件触发”),只要条件一直满足,那么就会一直触发,至此我如醍醐灌顶:是不是我通道里面的数据第一次没有读取干净?导致客户端触发了多次读取?后来验证之后,发现确实是这个问题,读者可以看我服务器端的代码,我返回的是数据字节数是:"服务端返回的数据:".length()+bf.array().length=26+1024,而客户端只是将这个数据读入1024大小的ByteBuffer中,还有26字节没有读取干净,所以就触发了第二次的读事件!!!
既然问题找到了,现在就是要解决如何将通道内的数据读取干净了,修改之后的代码如下, 特别注意红色部分:
服务端:
package com.nio.server;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
public class NIOServer {
private static Selector selector;
private static ServerSocketChannel serverSocketChannel;
private static ByteBuffer bf = ByteBuffer.allocate(1024);
public static void main(String[] args) throws Exception{
init();
while(true){
selector.select();
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while(it.hasNext()){
SelectionKey key = it.next();
if(key.isAcceptable()){
System.out.println("连接准备就绪");
ServerSocketChannel server = (ServerSocketChannel)key.channel();
System.out.println("等待客户端连接中........................");
SocketChannel channel = server.accept();
channel.configureBlocking(false);
channel.register(selector,SelectionKey.OP_READ);
}
else if(key.isReadable()){
System.out.println("读准备就绪,开始读.......................");
SocketChannel channel = (SocketChannel)key.channel();
System.out.println("客户端的数据如下:");
int readLen = 0;
bf.clear();
StringBuffer sb = new StringBuffer();
while((readLen=channel.read(bf))>0){
bf.flip();
byte [] temp = new byte[readLen];
bf.get(temp,0,readLen);
sb.append(new String(temp));
bf.clear();
}
if(-1==readLen){
channel.close();
} System.out.println(sb.toString());
channel.write(ByteBuffer.wrap(("客户端,你传过来的数据是:"+sb.toString()).getBytes()));
}
it.remove();
}
}
}
private static void init() throws Exception{
selector = Selector.open();
serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.socket().bind(new InetSocketAddress(8080));
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
}
}
客户端:
package com.nio.client;
import java.io.ByteArrayOutputStream;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
public class NIOClient {
private static Selector selector;
public static void main(String[]args) throws Exception{
selector = Selector.open();
SocketChannel sc = SocketChannel.open();
sc.configureBlocking(false);
sc.connect(new InetSocketAddress("127.0.0.1",8080));
sc.register(selector,SelectionKey.OP_READ);
ByteBuffer bf = ByteBuffer.allocate(1024);
bf.put("Hi,server,i'm client".getBytes());
if(sc.finishConnect()){
bf.flip();
while(bf.hasRemaining()){
sc.write(bf);
}
while(true){
selector.select();
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while(it.hasNext()){
SelectionKey key = it.next();
if(key.isReadable()){
ByteArrayOutputStream bos = new ByteArrayOutputStream();
bf.clear();
SocketChannel othersc = (SocketChannel)key.channel();
while(othersc.read(bf)>0){
bf.flip();
while(bf.hasRemaining()){
bos.write(bf.get());
}
bf.clear();
};
System.out.println("服务端返回的数据:"+bos.toString());
}
}
selector.selectedKeys().clear();
}
}
}
}
客户端的输出:
三、参考链接
https://www.zhihu.com/question/22524908
四、联系本人
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