1.NIO簡稱:
有人稱之為New I/O,因為相對于之前的I/O是新增的。這是官方叫法。但是,更多的人喜歡稱之為非阻塞I/O(Non-block I/O)。
2.與Socket類和ServerSocket類相對應,NIO提供了SocketChannel和ServerSocketChannel兩種不同套接字通道實作。支援阻塞和非阻塞兩種方式,阻塞模式使用簡單,但是性能和可靠性都不好,非阻塞模式正好相反,開發人員可以根據需求進行開發。
3.緩沖區 Buffer概念:
NIO類庫中加入了Buffer對象,在面向流的I/O中,可以将資料直接寫入或者将資料直接讀到Stream對象中。在NIO類庫中,所有資料都是用緩沖區處理,在讀取資料時,它是直接讀到緩沖區中的,在寫入資料時,寫入到緩沖區中。緩沖區實質上是一個數組。最常用的是ByteBuffer緩沖區。
4.通道Channel概念:
Channel是一個通道,就像自來水管一樣,網絡資料通過Channel進行讀取和寫入,通道與流的不同之處在于通道是雙向的,流隻在一個方向移動,而通道可以用于讀寫,或者兩者同時進行。
5.多路複用器 Selector概念:
多路複用器Selector 是Java NIO 程式設計的基礎,多路複用器提供選擇已經就緒的任務的能力。Selector 會不斷輪詢注冊在其上的Channel,如果某個Channel上發生讀或者寫事件,這個Channel就處于就緒狀态,會被Selector 輪詢出來,然後通過SelectionKey擷取就緒的Channel集合,然後進行後續的IO操作。
一個多路複用器Selector可以同時輪詢多個Channel,由于 JDK使用了epoll() 代替傳統的select實作,是以它并沒有最大連接配接句柄1024/2048的限制。這也意味着隻需要一個線程的負責Selector的輪詢,就可以接入成千上萬的用戶端,這是個非常巨大的進步。
6.NIO建立TimeServer.java源碼:
package com.pats.file.nio;
public class TimeServer {
public static void main(String[] args) {
int port = 8080;
if(args != null&& args.length > 0 ) {
port = Integer.valueOf(args[0]);
}
MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port);
new Thread(timeServer,"NIO-MultiplexerTimeServer-001").start();
}
}
MultiplexerTimeServer.java
package com.pats.file.nio;
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.util.Date;
import java.util.Iterator;
import java.util.Set;
public class MultiplexerTimeServer implements Runnable{
private Selector selector;
private ServerSocketChannel serverSocketChannel;
private volatile boolean stop;
/**
* 初始化多路複用器,綁定監聽端口;
*/
public MultiplexerTimeServer(int port){
try {
selector = Selector.open();
serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
serverSocketChannel.socket().bind(new InetSocketAddress(port),1024);
serverSocketChannel.register(selector,SelectionKey.OP_ACCEPT);
System.out.println("The time server is start in port : " + port);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void stop() {
this.stop = true;
}
@Override
public void run() {
while(!stop){
try {
selector.select(1000);
Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectedKeys.iterator();
SelectionKey key = null;
while(iterator.hasNext()) {
key = iterator.next();
iterator.remove();
try {
handleInput(key);
} catch (Exception e) {
if(key != null) {
key.cancel();
if(key.channel() != null) {
key.channel().close();
}
}
}
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
//:多路複用器關閉後所有注冊在上面的Channel和Pipe等資源都會被自動去注冊并關閉,是以不需要重複釋放資源
if( selector != null) {
try {
selector.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
private void handleInput(SelectionKey key) throws IOException {
if(key.isValid()) {
//:處理新接入的請求消息
if(key.isAcceptable()) {
//accept the new connection
ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
SocketChannel sc = ssc.accept();
sc.configureBlocking(false);
//add the new cinnection to the selector
sc.register(selector, SelectionKey.OP_READ);
}
if(key.isReadable()) {
//read to data
SocketChannel sc = (SocketChannel) key.channel();
ByteBuffer readBuffer = ByteBuffer.allocate(1024);
int readBytes = sc.read(readBuffer);
if(readBytes>0) {
readBuffer.flip();
byte[] bytes = new byte[readBuffer.remaining()];
readBuffer.get(bytes);
String body = new String(bytes,"UTF-8");
System.out.println("The time server receive order : "+ body);
String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body)? new Date(System.currentTimeMillis()).toString() : "BAD ORDER";
doWrite(sc,currentTime);
}else if(readBytes < 0) {
//:對端鍊路關閉
key.cancel();
sc.close();
} else {
;//:讀到0位元組 忽略
}
}
}
}
private void doWrite(SocketChannel channel, String response) throws IOException {
if(response != null && response.trim().length() > 0) {
byte[] bytes = response.getBytes();
ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
writeBuffer.put(bytes);
writeBuffer.flip();
channel.write(writeBuffer);
}
}
}
由于SocketChannel 是異步非阻塞的,并不能夠保證一次性将需要發送的位元組數組全部發送完畢,此時會出現寫半包問題。我們需要注冊寫操作,不斷輪詢Selector将沒有發送完的ByteBuffer發送完畢,可以通過ByteBuffer的hasRemain()方法判斷消息是否發送完成。此處未示範如何處理寫半包問題。
7. NIO建立TimeClient.java 源碼:
package com.pats.file.nio;
public class TimeClient {
public static void main(String[] args) {
int port = 8080;
if(args!=null && args.length > 0) {
port = Integer.valueOf(args[0]);
}
new Thread(new TimeClientHandle("127.0.0.1", port),"TimeClient-001").start();
}
}
TimeClientHandle.java
package com.pats.file.nio;
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.SocketChannel;
import java.util.Iterator;
import java.util.Set;
public class TimeClientHandle implements Runnable{
private String host;
private int port;
private Selector selector;
private SocketChannel socketChannel;
private volatile boolean stop;
public TimeClientHandle(String host, int port) {
this.host = host == null ? "127.0.0.1" : host;
this.port = port;
try {
selector = Selector.open();
socketChannel = SocketChannel.open();
socketChannel.configureBlocking(false);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
System.exit(1);
}
}
@Override
public void run() {
try {
doConnect();
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
while(!stop) {
try {
selector.select(1000);
Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectedKeys.iterator();
SelectionKey key = null;
while(iterator.hasNext()) {
key = iterator.next();
iterator.remove();
try {
handleInput(key);
} catch (Exception e) {
if(key != null) {
key.cancel();
if(key.channel() != null) {
key.channel().close();
}
}
}
}
} catch (Exception e) {
System.exit(1);
}
}
//:多路複用器關閉後所有注冊在上面的Channel和Pipe等資源都會被自動去注冊并關閉,是以不需要重複釋放資源
if( selector != null) {
try {
selector.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
private void handleInput(SelectionKey key) throws IOException {
if(key.isValid()) {
//:判斷是否連接配接成功
SocketChannel sc = (SocketChannel) key.channel();
if(key.isConnectable()) {
if(sc.finishConnect()) {
sc.register(selector, SelectionKey.OP_READ);
doWrite(sc);
}else {
System.exit(1);
}
}
if(key.isReadable()) {
ByteBuffer readBuffer = ByteBuffer.allocate(1024);
int readBytes = sc.read(readBuffer);
if(readBytes > 0) {
readBuffer.flip();
byte[] bytes = new byte[readBuffer.remaining()];
readBuffer.get(bytes);
String body = new String(bytes,"UTF-8");
System.out.println("Now time is : "+ body);
this.stop = true;
}else if(readBytes < 0) {
//:對端鍊路關閉
key.cancel();
sc.close();
}else {
;//:讀到0位元組 忽略
}
}
}
}
private void doConnect() throws IOException {
//:如果直接連接配接成功,則注冊到多路複用器上,發送請求消息,讀應答
if(socketChannel.connect(new InetSocketAddress(host, port))) {
socketChannel.register(selector, SelectionKey.OP_READ);
doWrite(socketChannel);
}else {
socketChannel.register(selector, SelectionKey.OP_CONNECT);
}
}
private void doWrite(SocketChannel sc) throws IOException {
byte[] req = "QUERY TIME ORDER".getBytes();
ByteBuffer writeBuffer = ByteBuffer.allocate(req.length);
writeBuffer.put(req);
writeBuffer.flip();
sc.write(writeBuffer);
if(!writeBuffer.hasRemaining()) {
System.out.println("Send order 2 server succeed.");
}
}
}
8.運作代碼如下:
服務端:
用戶端:
9.總結:
1.NIO編碼比同步阻塞BIO編碼難度大很多,以上并沒有考慮半包讀,半包寫的問題。
2.用戶端發起的連接配接操作是異步的,可以通過在多路複用器注冊 OP_CONNECT 等待後續結果,不需要像之前的用戶端那樣被同步阻塞。
3.SocketChannel 的讀寫操作都是異步的,如果沒有可讀可寫的資料它不會同步等待,直接傳回,這樣I/O通信線程就可以處理其它的鍊路,不需要同步等待這個鍊路可用。
4.線程模型的優化,JDK的Selector 在linux作業系統上通過epoll實作,沒有連接配接句柄數限制,隻受限于作業系統的最大句柄數或者對單個線程的句柄數限制,這意味着一個Selector 可以同時處理成千上萬個用戶端連接配接,而且性能不會随着用戶端的增加而線性下降。