今天看了javaNIO的教学视频，感觉终于理解了NIO的基本知识，在这里做一下记录。
NIO关键的四个类：ServerSocketChannel、SocketChannel、Selector、ByteBuffer。
我们就走个流程来理解吧:
对应的代码如下：
package com.pt.nio;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.util.Iterator;
import java.util.Set;
public class Reactor implements Runnable {
public int id = 100001;
public int bufferSize = 2048;
@Override
public void run() {
// TODO Auto-generated method stub
init();
}
public void init() {
try {
// 创建通道和选择器
ServerSocketChannel socketChannel = ServerSocketChannel.open();
Selector selector = Selector.open();
InetSocketAddress inetSocketAddress = new InetSocketAddress(
InetAddress.getLocalHost(), 4700);
socketChannel.socket().bind(inetSocketAddress);
// 设置通道非阻塞 绑定选择器
socketChannel.configureBlocking(false);
socketChannel.register(selector, SelectionKey.OP_ACCEPT).attach(
id++);
System.out.println("Server started .... port:4700");
listener(selector);
} catch (Exception e) {
// TODO: handle exception
}
}
public void listener(Selector in_selector) {
try {
while (true) {
Thread.sleep(1*1000);
in_selector.select(); // 阻塞 直到有就绪事件为止
Set<SelectionKey> readySelectionKey = in_selector
.selectedKeys();
Iterator<SelectionKey> it = readySelectionKey.iterator();
while (it.hasNext()) {
SelectionKey selectionKey = it.next();
// 判断是哪个事件
if (selectionKey.isAcceptable()) {// 客户请求连接
System.out.println(selectionKey.attachment()
+ " - 接受请求事件");
// 获取通道 接受连接,
// 设置非阻塞模式（必须），同时需要注册 读写数据的事件，这样有消息触发时才能捕获
ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey
.channel();
serverSocketChannel
.accept()
.configureBlocking(false)
.register(
in_selector,
SelectionKey.OP_READ
| SelectionKey.OP_WRITE).attach(id++);
System.out
.println(selectionKey.attachment() + " - 已连接");
// 下面这种写法是有问题的 不应该在serverSocketChannel上面注册
/*
* serverSocketChannel.configureBlocking(false);
* serverSocketChannel.register(in_selector,
* SelectionKey.OP_READ);
* serverSocketChannel.register(in_selector,
* SelectionKey.OP_WRITE);
*/
}
if (selectionKey.isReadable()) {// 读数据
System.out.println(selectionKey.attachment()
+ " - 读数据事件");
SocketChannel clientChannel=(SocketChannel)selectionKey.channel();
ByteBuffer receiveBuf = ByteBuffer.allocate(bufferSize);
clientChannel.read(receiveBuf);
System.out.println(selectionKey.attachment()
+ " - 读取数据：" + getString(receiveBuf));
}
if (selectionKey.isWritable()) {// 写数据
System.out.println(selectionKey.attachment()
+ " - 写数据事件");
SocketChannel clientChannel = (SocketChannel) selectionKey.channel();
ByteBuffer sendBuf = ByteBuffer.allocate(bufferSize);
String sendText = "hello\n";
sendBuf.put(sendText.getBytes());
sendBuf.flip(); //写完数据后调用此方法
clientChannel.write(sendBuf);
}
if (selectionKey.isConnectable()) {
System.out.println(selectionKey.attachment()
+ " - 连接事件");
}
// 必须removed 否则会继续存在，下一次循环还会进来,
// 注意removed 的位置，针对一个.next() remove一次
it.remove();
}
}
} catch (Exception e) {
// TODO: handle exception
System.out.println("Error - " + e.getMessage());
e.printStackTrace();
}
}
/**
* ByteBuffer 转换 String
* @param buffer
* @return
*/
public static String getString(ByteBuffer buffer)
{
String string = "";
try
{
for(int i = 0; i<buffer.position();i++){
string += (char)buffer.get(i);
}
return string;
}
catch (Exception ex)
{
ex.printStackTrace();
return "";
}
}
}
NIO服务器端其实个人最不能理解的地方就是死循环那一段里面的内容，监听各种事件，然后又注册其他的事件，怎么ACCEPT完了又注册READ、WRITE，然后又去执行READ、WRITE的监听操作……乱七八糟的。其实，主要还是没有理解这些事件监听的对象和流畅。
一开始，其实只有我们的ServerSocketChannel监听着OPT_ACCEPT事件的发生，然后死循环中，当Selector执行select()方法发现有客户端连接进来了，于是出发OPT_ACCEPT事件，此时ServerSocketChannel就去执行accept获取回来了连接到服务器的SocketChannel。之后我们再往Selector注册的其实是这些连接进来的客户端SocketChannel，对他们设置感兴趣的READ或WRITE事件。之后等下一轮循环再来时，如果有READ或WRITE事件发生，则这些SocketChannel就被挑选出来触发读写操作，而我们的ServerSocketChannel还是只监听OPT_ACCEPT操作，一直接收着进来的客户端请求而不做其他的读写操作。就这样一直循环下去……