处理消息边界

为什么要处理边界

因为会存在半包和粘包的问题

1.客户端和服务端约定一个固定长度

• 优点：简单

• 缺点：可能造成浪费

2.客户端与服务端约定一个固定分割符

*缺点 效率低

3.先发送长度，再发送数据

TLV格式： type类型，length长度，Value数据，类型和长度已知的情况下，就可以方便获取消息大小

http1.1是TLV格式
http2.0是LTV格式

4.自动扩容解决消息边界问题

第一次read事件未能读完全部的输入，那么会产生第二个读事件，那么在第一次读的时候进行扩容，
并复制之前的内容至新的buffer中，
在第二个读事件触发以后使用扩容后的buffer，读取剩余的数据

buffer应当和各自的channel绑定，如何绑定，需要用到附件attachment，
attachment需要在注册时放到selectionKey中。

// 绑定附件
SelectionKey scKey = channel.register(selector,0,byteBuffer);// 获取附件
scKey.attachment();// 指定新的附件（覆盖附件）
scKey.attach(bytebuffer);

示例代码：

package com.ysf;import java.io.IOException;
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.nio.charset.Charset;
import java.util.Iterator;public class BorderSelectorServer {/*** 读取到 '\n'时打印* @param byteBuffer 缓冲字节*/public static void handle(ByteBuffer byteBuffer) {byteBuffer.flip();for (int i = 0; i < byteBuffer.limit(); i++) {if (byteBuffer.get(i) == '\n') {int length = i + 1 - byteBuffer.position();ByteBuffer allocate = ByteBuffer.allocate(length);for (int j=0;j<length;j++){allocate.put(byteBuffer.get());}allocate.flip();System.out.println(Charset.defaultCharset().decode(allocate));}}byteBuffer.compact();}public static void main(String[] args) throws IOException {// 声明一个选择器Selector selector = Selector.open();// 声明一个serverServerSocketChannel ssc = ServerSocketChannel.open();ssc.bind(new InetSocketAddress(11027));ssc.configureBlocking(false);// 注册这个server到selectorSelectionKey sscKey = ssc.register(selector, 0, null);// 添加sscKey关心的事件，因为是serverChannel,所以应当关心accept事件sscKey.interestOps(SelectionKey.OP_ACCEPT);while (true) {selector.select();Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();while (iterator.hasNext()) {SelectionKey key = iterator.next();iterator.remove();if (key.isAcceptable()) {ServerSocketChannel serverChannel = (ServerSocketChannel) key.channel();SocketChannel socketChannel = serverChannel.accept();socketChannel.configureBlocking(false);// 声明一个buffer缓冲区和socketChannel绑定ByteBuffer byteBuffer = ByteBuffer.allocate(16);SelectionKey scKey = socketChannel.register(selector, 0, byteBuffer);scKey.interestOps(SelectionKey.OP_READ);} else if (key.isReadable()) {// 当客户端异常断开链接是需要处理IOExceptiontry {SocketChannel channel = (SocketChannel) key.channel();ByteBuffer byteBuffer = (ByteBuffer) key.attachment();int read = channel.read(byteBuffer);if (read == -1) {// 客户端close()了key.cancel();}else{// 调用处理逻辑handle(byteBuffer);if (byteBuffer.position() == byteBuffer.limit()){// buffer满了，需要扩容ByteBuffer bufferEx = ByteBuffer.allocate(byteBuffer.capacity() * 2);byteBuffer.flip();bufferEx.put(byteBuffer);key.attach(bufferEx);}}}catch (IOException e){
//                        e.printStackTrace();key.cancel();}}}}}
}