IO模型
IO模型就是说用什么样的通道进行数据的发送和接收，Java共支持3种网络编程IO模式：BIO，NIO，AIO
BIO(Blocking IO)
同步阻塞模型，一个客户端连接对应一个处理线程
BIO代码示例：
服务端代码
package com.tuling.bio;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
public class SocketServer {
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(9000);
while (true) {
System.out.println("等待连接。。");
//阻塞方法
Socket socket = serverSocket.accept();
System.out.println("有客户端连接了。。");
new Thread(new Runnable() {
@Override
public void run() {
try {
handler(socket);
} catch (IOException e) {
e.printStackTrace();
}
}
}).start();
//handler(socket);
}
}
private static void handler(Socket socket) throws IOException {
System.out.println("thread id = " + Thread.currentThread().getId());
byte[] bytes = new byte[1024];
System.out.println("准备read。。");
//接收客户端的数据，阻塞方法，没有数据可读时就阻塞
int read = socket.getInputStream().read(bytes);
System.out.println("read完毕。。");
if (read != -1) {
System.out.println("接收到客户端的数据：" + new String(bytes, 0, read));
System.out.println("thread id = " + Thread.currentThread().getId());
}
socket.getOutputStream().write("HelloClient".getBytes());
socket.getOutputStream().flush();
}
}客户端代码
package com.tuling.bio;
import java.io.IOException;
import java.net.Socket;
public class SocketClient {
public static void main(String[] args) throws IOException {
Socket socket = new Socket("127.0.0.1", 9000);
//向服务端发送数据
socket.getOutputStream().write("HelloServer".getBytes());
socket.getOutputStream().flush();
System.out.println("向服务端发送数据结束");
byte[] bytes = new byte[1024];
//接收服务端回传的数据
socket.getInputStream().read(bytes);
System.out.println("接收到服务端的数据：" + new String(bytes));
socket.close();
}
}缺点：
1、IO代码里read操作是阻塞操作，如果连接不做数据读写操作会导致线程阻塞，浪费资源
2、如果线程很多，会导致服务器线程太多，压力太大，比如C10K问题
应用场景：
BIO 方式适用于连接数目比较小且固定的架构， 这种方式对服务器资源要求比较高， 但程序简单易理解。
NIO(Non Blocking IO)
同步非阻塞，服务器实现模式为一个线程可以处理多个请求(连接)，客户端发送的连接请求都会注册到多路复用器selector上，多路复用器轮询到连接有IO请求就进行处理，JDK1.4开始引入。
应用场景：
NIO方式适用于连接数目多且连接比较短（轻操作） 的架构， 比如聊天服务器， 弹幕系统， 服务器间通讯，编程比较复杂
NIO非阻塞代码示例：
NioServer
package com.tuling.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.Iterator;
public class NIOServer {
//public static ExecutorService pool = Executors.newFixedThreadPool(10);
public static void main(String[] args) throws IOException {
// 创建一个在本地端口进行监听的服务Socket通道.并设置为非阻塞方式
ServerSocketChannel ssc = ServerSocketChannel.open();
//必须配置为非阻塞才能往selector上注册，否则会报错，selector模式本身就是非阻塞模式
ssc.configureBlocking(false);
ssc.socket().bind(new InetSocketAddress(9000));
// 创建一个选择器selector
Selector selector = Selector.open();
// 把ServerSocketChannel注册到selector上，并且selector对客户端accept连接操作感兴趣
ssc.register(selector, SelectionKey.OP_ACCEPT);
while (true) {
System.out.println("等待事件发生。。");
// 轮询监听channel里的key，select是阻塞的，accept()也是阻塞的
int select = selector.select();
System.out.println("有事件发生了。。");
// 有客户端请求，被轮询监听到
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while (it.hasNext()) {
SelectionKey key = it.next();
//删除本次已处理的key，防止下次select重复处理
it.remove();
handle(key);
}
}
}
private static void handle(SelectionKey key) throws IOException {
if (key.isAcceptable()) {
System.out.println("有客户端连接事件发生了。。");
ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
//NIO非阻塞体现：此处accept方法是阻塞的，但是这里因为是发生了连接事件，所以这个方法会马上执行完，不会阻塞
//处理完连接请求不会继续等待客户端的数据发送
SocketChannel sc = ssc.accept();
sc.configureBlocking(false);
//通过Selector监听Channel时对读事件感兴趣
sc.register(key.selector(), SelectionKey.OP_READ);
} else if (key.isReadable()) {
System.out.println("有客户端数据可读事件发生了。。");
SocketChannel sc = (SocketChannel) key.channel();
ByteBuffer buffer = ByteBuffer.allocate(1024);
//NIO非阻塞体现:首先read方法不会阻塞，其次这种事件响应模型，当调用到read方法时肯定是发生了客户端发送数据的事件
int len = sc.read(buffer);
if (len != -1) {
System.out.println("读取到客户端发送的数据：" + new String(buffer.array(), 0, len));
}
ByteBuffer bufferToWrite = ByteBuffer.wrap("HelloClient".getBytes());
sc.write(bufferToWrite);
key.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE);
} else if (key.isWritable()) {
SocketChannel sc = (SocketChannel) key.channel();
System.out.println("write事件");
// NIO事件触发是水平触发
// 使用Java的NIO编程的时候，在没有数据可以往外写的时候要取消写事件，
// 在有数据往外写的时候再注册写事件
key.interestOps(SelectionKey.OP_READ);
//sc.close();
}
}
}nioClient
package com.tuling.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;
public class NioClient {
//通道管理器
private Selector selector;
/**
* 启动客户端测试
*
* @throws IOException
*/
public static void main(String[] args) throws IOException {
NioClient client = new NioClient();
client.initClient("127.0.0.1", 9000);
client.connect();
}
/**
* 获得一个Socket通道，并对该通道做一些初始化的工作
*
* @param ip 连接的服务器的ip
* @param port 连接的服务器的端口号
* @throws IOException
*/
public void initClient(String ip, int port) throws IOException {
// 获得一个Socket通道
SocketChannel channel = SocketChannel.open();
// 设置通道为非阻塞
channel.configureBlocking(false);
// 获得一个通道管理器
this.selector = Selector.open();
// 客户端连接服务器,其实方法执行并没有实现连接，需要在listen（）方法中调
//用channel.finishConnect() 才能完成连接
channel.connect(new InetSocketAddress(ip, port));
//将通道管理器和该通道绑定，并为该通道注册SelectionKey.OP_CONNECT事件。
channel.register(selector, SelectionKey.OP_CONNECT);
}
/**
* 采用轮询的方式监听selector上是否有需要处理的事件，如果有，则进行处理
*
* @throws IOException
*/
public void connect() throws IOException {
// 轮询访问selector
while (true) {
selector.select();
// 获得selector中选中的项的迭代器
Iterator<SelectionKey> it = this.selector.selectedKeys().iterator();
while (it.hasNext()) {
SelectionKey key = (SelectionKey) it.next();
// 删除已选的key,以防重复处理
it.remove();
// 连接事件发生
if (key.isConnectable()) {
SocketChannel channel = (SocketChannel) key.channel();
// 如果正在连接，则完成连接
if (channel.isConnectionPending()) {
channel.finishConnect();
}
// 设置成非阻塞
channel.configureBlocking(false);
//在这里可以给服务端发送信息哦
ByteBuffer buffer = ByteBuffer.wrap("HelloServer".getBytes());
channel.write(buffer);
//在和服务端连接成功之后，为了可以接收到服务端的信息，需要给通道设置读的权限。
channel.register(this.selector, SelectionKey.OP_READ); // 获得了可读的事件
} else if (key.isReadable()) {
read(key);
}
}
}
}
/**
* 处理读取服务端发来的信息 的事件
*
* @param key
* @throws IOException
*/
public void read(SelectionKey key) throws IOException {
//和服务端的read方法一样
// 服务器可读取消息:得到事件发生的Socket通道
SocketChannel channel = (SocketChannel) key.channel();
// 创建读取的缓冲区
ByteBuffer buffer = ByteBuffer.allocate(1024);
int len = channel.read(buffer);
if (len != -1) {
System.out.println("客户端收到信息：" + new String(buffer.array(), 0, len));
}
}
}总结：
如果连接数太多的话，会有大量的无效遍历，假如有10000个连接，其中只有1000个连接有写数据，但是由于其他9000个连接并没有断开，我们还是要每次轮询遍历一万次，其中有十分之九的遍历都是无效的，这显然不是一个让人很满意的状态。
NIO引入多路复用器代码示例：
package com.tuling.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.Iterator;
import java.util.Set;
public class NioSelectorServer {
public static void main(String[] args) throws IOException, InterruptedException {
// 创建NIO ServerSocketChannel
ServerSocketChannel serverSocket = ServerSocketChannel.open();
serverSocket.socket().bind(new InetSocketAddress(9000));
// 设置ServerSocketChannel为非阻塞
serverSocket.configureBlocking(false);
// 打开Selector处理Channel，即创建epoll
Selector selector = Selector.open();
// 把ServerSocketChannel注册到selector上，并且selector对客户端accept连接操作感兴趣
serverSocket.register(selector, SelectionKey.OP_ACCEPT);
System.out.println("服务启动成功");
while (true) {
// 阻塞等待需要处理的事件发生
selector.select();
// 获取selector中注册的全部事件的 SelectionKey 实例
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectionKeys.iterator();
// 遍历SelectionKey对事件进行处理
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
// 如果是OP_ACCEPT事件，则进行连接获取和事件注册
if (key.isAcceptable()) {
ServerSocketChannel server = (ServerSocketChannel) key.channel();
SocketChannel socketChannel = server.accept();
socketChannel.configureBlocking(false);
// 这里只注册了读事件，如果需要给客户端发送数据可以注册写事件
socketChannel.register(selector, SelectionKey.OP_READ);
System.out.println("客户端连接成功");
} else if (key.isReadable()) {
// 如果是OP_READ事件，则进行读取和打印
SocketChannel socketChannel = (SocketChannel) key.channel();
ByteBuffer byteBuffer = ByteBuffer.allocate(128);
int len = socketChannel.read(byteBuffer);
// 如果有数据，把数据打印出来
if (len > 0) {
System.out.println("接收到消息：" + new String(byteBuffer.array()));
} else if (len == -1) {
// 如果客户端断开连接，关闭Socket
// System.out.println("客户端断开连接"); socketChannel.close();
}
}
// 从事件集合里删除本次处理的key，防止下次select重复处理
iterator.remove();
}
}
}
}NIO 有三大核心组件：
Channel(通道)， Buffer(缓冲区)，Selector(多路复用器)
1、channel 类似于流，每个 channel 对应一个 buffer缓冲区，buffer 底层就是个数组
2、channel 会注册到 selector 上，由 selector 根据 channel 读写事件的发生将其交由某个空闲的线程处理
3、NIO 的 Buffer 和 channel 都是既可以读也可以写
NIO底层在JDK1.4版本是用linux的内核函数select()或poll()来实现，跟上面的NioServer代码类似，selector每次都会轮询所有的sockchannel看下哪个channel有读写事件，有的话就处理，没有就继续遍历，JDK1.5开始引入了epoll基于事件响应机制来优化NIO。
NioSelectorServer 代码里如下几个方法非常重要，我们从Hotspot与Linux内核函数级别来理解下
Selector.open() //创建多路复用器 socketChannel.register(selector, SelectionKey.OP_READ) //将channel注册到多路复用器上 selector.select() //阻塞等待需要处理的事件发生
总结：NIO整个调用流程就是Java调用了操作系统的内核函数来创建Socket，获取到Socket的文件描述符，再创建一个Selector对象，对应操作系统的Epoll描述符，将获取到的Socket连接的文件描述符的事件绑定到Selector对应的Epoll文件描述符上，进行事件的异步通知，这样就实现了使用一条线程，并且不需要太多的无效的遍历，将事件处理交给了操作系统内核(操作系统中断程序实现)，大大提高了效率。
Epoll函数详解
int epoll_create(int size);
创建一个epoll实例，并返回一个非负数作为文件描述符，用于对epoll接口的所有后续调用。参数size代表可能会容纳size个描述符，但size不是一个最大值，只是提示操作系统它的数量级，现在这个参数基本上已经弃用了。
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
使用文件描述符epfd引用的epoll实例，对目标文件描述符fd执行op操作。
参数epfd表示epoll对应的文件描述符，参数fd表示socket对应的文件描述符。
参数op有以下几个值：
EPOLL_CTL_ADD：注册新的fd到epfd中，并关联事件event；
EPOLL_CTL_MOD：修改已经注册的fd的监听事件；
EPOLL_CTL_DEL：从epfd中移除fd，并且忽略掉绑定的event，这时event可以为null；
参数event是一个结构体
struct epoll_event { __uint32_t events; /* Epoll events */ epoll_data_t data; /* User data variable */ }; typedef union epoll_data { void *ptr; int fd; __uint32_t u32; __uint64_t u64; } epoll_data_t;
events有很多可选值，这里只举例最常见的几个：
EPOLLIN ：表示对应的文件描述符是可读的；
EPOLLOUT：表示对应的文件描述符是可写的；
EPOLLERR：表示对应的文件描述符发生了错误；
成功则返回0，失败返回-1
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);
等待文件描述符epfd上的事件。
epfd是Epoll对应的文件描述符，events表示调用者所有可用事件的集合，maxevents表示最多等到多少个事件就返回，timeout是超时时间。
I/O多路复用底层主要用的Linux 内核·函数（select，poll，epoll）来实现，windows不支持epoll实现，windows底层是基于winsock2的select函数实现的(不开源)
| select |
poll |
epoll(jdk 1.5及以上) |
|
| 操作方式 |
遍历 |
遍历 |
回调 |
| 底层实现 |
数组 |
链表 |
哈希表 |
| IO效率 |
每次调用都进行线性遍历，时间复杂度为O(n) |
每次调用都进行线性遍历，时间复杂度为O(n) |
事件通知方式，每当有IO事件就绪，系统注册的回调函数就会被调用，时间复杂度O(1) |
| 最大连接 |
有上限 |
无上限 |
无上限 |
Redis线程模型
Redis就是典型的基于epoll的NIO线程模型(nginx也是)，epoll实例收集所有事件(连接与读写事件)，由一个服务端线程连续处理所有事件命令。
Redis底层关于epoll的源码实现在redis的src源码目录的ae_epoll.c文件里，感兴趣可以自行研究。
AIO(NIO 2.0)
异步非阻塞， 由操作系统完成后回调通知服务端程序启动线程去处理， 一般适用于连接数较多且连接时间较长的应用
应用场景：
AIO方式适用于连接数目多且连接比较长(重操作)的架构，JDK7 开始支持
AIO代码示例：
package com.tuling.aio;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousServerSocketChannel;
import java.nio.channels.AsynchronousSocketChannel;
import java.nio.channels.CompletionHandler;
public class AIOServer {
public static void main(String[] args) throws Exception {
final AsynchronousServerSocketChannel serverChannel =
AsynchronousServerSocketChannel.open().bind(new InetSocketAddress(9000));
serverChannel.accept(null, new CompletionHandler<AsynchronousSocketChannel, Object>() {
@Override
public void completed(AsynchronousSocketChannel socketChannel, Object attachment) {
try {
// 再此接收客户端连接，如果不写这行代码后面的客户端连接连不上服务端
serverChannel.accept(attachment, this);
System.out.println(socketChannel.getRemoteAddress());
ByteBuffer buffer = ByteBuffer.allocate(1024);
socketChannel.read(buffer, buffer, new CompletionHandler<Integer, ByteBuffer>() {
@Override
public void completed(Integer result, ByteBuffer buffer) {
buffer.flip();
System.out.println(new String(buffer.array(), 0, result));
socketChannel.write(ByteBuffer.wrap("HelloClient".getBytes()));
}
@Override
public void failed(Throwable exc, ByteBuffer buffer) {
exc.printStackTrace();
}
});
} catch (IOException e) {
e.printStackTrace();
}
}
@Override
public void failed(Throwable exc, Object attachment) {
exc.printStackTrace();
}
});
Thread.sleep(Integer.MAX_VALUE);
}
}package com.tuling.aio;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousSocketChannel;
public class AIOClient {
public static void main(String... args) throws Exception {
AsynchronousSocketChannel socketChannel = AsynchronousSocketChannel.open();
socketChannel.connect(new InetSocketAddress("127.0.0.1", 9000)).get();
socketChannel.write(ByteBuffer.wrap("HelloServer".getBytes()));
ByteBuffer buffer = ByteBuffer.allocate(512);
Integer len = socketChannel.read(buffer).get();
if (len != -1) {
System.out.println("客户端收到信息：" + new String(buffer.array(), 0, len));
}
}
}/*
* Copyright (c) 2007, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.nio.channels;
import java.nio.channels.spi.AsynchronousChannelProvider;
import java.io.IOException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
/**
* A grouping of asynchronous channels for the purpose of resource sharing.
*
* <p> An asynchronous channel group encapsulates the mechanics required to
* handle the completion of I/O operations initiated by {@link AsynchronousChannel
* asynchronous channels} that are bound to the group. A group has an associated
* thread pool to which tasks are submitted to handle I/O events and dispatch to
* {@link CompletionHandler completion-handlers} that consume the result of
* asynchronous operations performed on channels in the group. In addition to
* handling I/O events, the pooled threads may also execute other tasks required
* to support the execution of asynchronous I/O operations.
*
* <p> An asynchronous channel group is created by invoking the {@link
* #withFixedThreadPool withFixedThreadPool} or {@link #withCachedThreadPool
* withCachedThreadPool} methods defined here. Channels are bound to a group by
* specifying the group when constructing the channel. The associated thread
* pool is <em>owned</em> by the group; termination of the group results in the
* shutdown of the associated thread pool.
*
* <p> In addition to groups created explicitly, the Java virtual machine
* maintains a system-wide <em>default group</em> that is constructed
* automatically. Asynchronous channels that do not specify a group at
* construction time are bound to the default group. The default group has an
* associated thread pool that creates new threads as needed. The default group
* may be configured by means of system properties defined in the table below.
* Where the {@link java.util.concurrent.ThreadFactory ThreadFactory} for the
* default group is not configured then the pooled threads of the default group
* are {@link Thread#isDaemon daemon} threads.
*
* <table border summary="System properties">
* <tr>
* <th>System property</th>
* <th>Description</th>
* </tr>
* <tr>
* <td> {@code java.nio.channels.DefaultThreadPool.threadFactory} </td>
* <td> The value of this property is taken to be the fully-qualified name
* of a concrete {@link java.util.concurrent.ThreadFactory ThreadFactory}
* class. The class is loaded using the system class loader and instantiated.
* The factory's {@link java.util.concurrent.ThreadFactory#newThread
* newThread} method is invoked to create each thread for the default
* group's thread pool. If the process to load and instantiate the value
* of the property fails then an unspecified error is thrown during the
* construction of the default group. </td>
* </tr>
* <tr>
* <td> {@code java.nio.channels.DefaultThreadPool.initialSize} </td>
* <td> The value of the {@code initialSize} parameter for the default
* group (see {@link #withCachedThreadPool withCachedThreadPool}).
* The value of the property is taken to be the {@code String}
* representation of an {@code Integer} that is the initial size parameter.
* If the value cannot be parsed as an {@code Integer} it causes an
* unspecified error to be thrown during the construction of the default
* group. </td>
* </tr>
* </table>
*
* <a name="threading"></a><h2>Threading</h2>
*
* <p> The completion handler for an I/O operation initiated on a channel bound
* to a group is guaranteed to be invoked by one of the pooled threads in the
* group. This ensures that the completion handler is run by a thread with the
* expected <em>identity</em>.
*
* <p> Where an I/O operation completes immediately, and the initiating thread
* is one of the pooled threads in the group then the completion handler may
* be invoked directly by the initiating thread. To avoid stack overflow, an
* implementation may impose a limit as to the number of activations on the
* thread stack. Some I/O operations may prohibit invoking the completion
* handler directly by the initiating thread (see {@link
* AsynchronousServerSocketChannel#accept(Object,CompletionHandler) accept}).
*
* <a name="shutdown"></a><h2>Shutdown and Termination</h2>
*
* <p> The {@link #shutdown() shutdown} method is used to initiate an <em>orderly
* shutdown</em> of a group. An orderly shutdown marks the group as shutdown;
* further attempts to construct a channel that binds to the group will throw
* {@link ShutdownChannelGroupException}. Whether or not a group is shutdown can
* be tested using the {@link #isShutdown() isShutdown} method. Once shutdown,
* the group <em>terminates</em> when all asynchronous channels that are bound to
* the group are closed, all actively executing completion handlers have run to
* completion, and resources used by the group are released. No attempt is made
* to stop or interrupt threads that are executing completion handlers. The
* {@link #isTerminated() isTerminated} method is used to test if the group has
* terminated, and the {@link #awaitTermination awaitTermination} method can be
* used to block until the group has terminated.
*
* <p> The {@link #shutdownNow() shutdownNow} method can be used to initiate a
* <em>forceful shutdown</em> of the group. In addition to the actions performed
* by an orderly shutdown, the {@code shutdownNow} method closes all open channels
* in the group as if by invoking the {@link AsynchronousChannel#close close}
* method.
*
* @since 1.7
*
* @see AsynchronousSocketChannel#open(AsynchronousChannelGroup)
* @see AsynchronousServerSocketChannel#open(AsynchronousChannelGroup)
*/
public abstract class AsynchronousChannelGroup {
private final AsynchronousChannelProvider provider;
/**
* Initialize a new instance of this class.
*
* @param provider
* The asynchronous channel provider for this group
*/
protected AsynchronousChannelGroup(AsynchronousChannelProvider provider) {
this.provider = provider;
}
/**
* Returns the provider that created this channel group.
*
* @return The provider that created this channel group
*/
public final AsynchronousChannelProvider provider() {
return provider;
}
/**
* Creates an asynchronous channel group with a fixed thread pool.
*
* <p> The resulting asynchronous channel group reuses a fixed number of
* threads. At any point, at most {@code nThreads} threads will be active
* processing tasks that are submitted to handle I/O events and dispatch
* completion results for operations initiated on asynchronous channels in
* the group.
*
* <p> The group is created by invoking the {@link
* AsynchronousChannelProvider#openAsynchronousChannelGroup(int,ThreadFactory)
* openAsynchronousChannelGroup(int,ThreadFactory)} method of the system-wide
* default {@link AsynchronousChannelProvider} object.
*
* @param nThreads
* The number of threads in the pool
* @param threadFactory
* The factory to use when creating new threads
*
* @return A new asynchronous channel group
*
* @throws IllegalArgumentException
* If {@code nThreads <= 0}
* @throws IOException
* If an I/O error occurs
*/
public static AsynchronousChannelGroup withFixedThreadPool(int nThreads,
ThreadFactory threadFactory)
throws IOException
{
return AsynchronousChannelProvider.provider()
.openAsynchronousChannelGroup(nThreads, threadFactory);
}
/**
* Creates an asynchronous channel group with a given thread pool that
* creates new threads as needed.
*
* <p> The {@code executor} parameter is an {@code ExecutorService} that
* creates new threads as needed to execute tasks that are submitted to
* handle I/O events and dispatch completion results for operations initiated
* on asynchronous channels in the group. It may reuse previously constructed
* threads when they are available.
*
* <p> The {@code initialSize} parameter may be used by the implementation
* as a <em>hint</em> as to the initial number of tasks it may submit. For
* example, it may be used to indicate the initial number of threads that
* wait on I/O events.
*
* <p> The executor is intended to be used exclusively by the resulting
* asynchronous channel group. Termination of the group results in the
* orderly {@link ExecutorService#shutdown shutdown} of the executor
* service. Shutting down the executor service by other means results in
* unspecified behavior.
*
* <p> The group is created by invoking the {@link
* AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int)
* openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide
* default {@link AsynchronousChannelProvider} object.
*
* @param executor
* The thread pool for the resulting group
* @param initialSize
* A value {@code >=0} or a negative value for implementation
* specific default
*
* @return A new asynchronous channel group
*
* @throws IOException
* If an I/O error occurs
*
* @see java.util.concurrent.Executors#newCachedThreadPool
*/
public static AsynchronousChannelGroup withCachedThreadPool(ExecutorService executor,
int initialSize)
throws IOException
{
return AsynchronousChannelProvider.provider()
.openAsynchronousChannelGroup(executor, initialSize);
}
/**
* Creates an asynchronous channel group with a given thread pool.
*
* <p> The {@code executor} parameter is an {@code ExecutorService} that
* executes tasks submitted to dispatch completion results for operations
* initiated on asynchronous channels in the group.
*
* <p> Care should be taken when configuring the executor service. It
* should support <em>direct handoff</em> or <em>unbounded queuing</em> of
* submitted tasks, and the thread that invokes the {@link
* ExecutorService#execute execute} method should never invoke the task
* directly. An implementation may mandate additional constraints.
*
* <p> The executor is intended to be used exclusively by the resulting
* asynchronous channel group. Termination of the group results in the
* orderly {@link ExecutorService#shutdown shutdown} of the executor
* service. Shutting down the executor service by other means results in
* unspecified behavior.
*
* <p> The group is created by invoking the {@link
* AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int)
* openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide
* default {@link AsynchronousChannelProvider} object with an {@code
* initialSize} of {@code 0}.
*
* @param executor
* The thread pool for the resulting group
*
* @return A new asynchronous channel group
*
* @throws IOException
* If an I/O error occurs
*/
public static AsynchronousChannelGroup withThreadPool(ExecutorService executor)
throws IOException
{
return AsynchronousChannelProvider.provider()
.openAsynchronousChannelGroup(executor, 0);
}
/**
* Tells whether or not this asynchronous channel group is shutdown.
*
* @return {@code true} if this asynchronous channel group is shutdown or
* has been marked for shutdown.
*/
public abstract boolean isShutdown();
/**
* Tells whether or not this group has terminated.
*
* <p> Where this method returns {@code true}, then the associated thread
* pool has also {@link ExecutorService#isTerminated terminated}.
*
* @return {@code true} if this group has terminated
*/
public abstract boolean isTerminated();
/**
* Initiates an orderly shutdown of the group.
*
* <p> This method marks the group as shutdown. Further attempts to construct
* channel that binds to this group will throw {@link ShutdownChannelGroupException}.
* The group terminates when all asynchronous channels in the group are
* closed, all actively executing completion handlers have run to completion,
* and all resources have been released. This method has no effect if the
* group is already shutdown.
*/
public abstract void shutdown();
/**
* Shuts down the group and closes all open channels in the group.
*
* <p> In addition to the actions performed by the {@link #shutdown() shutdown}
* method, this method invokes the {@link AsynchronousChannel#close close}
* method on all open channels in the group. This method does not attempt to
* stop or interrupt threads that are executing completion handlers. The
* group terminates when all actively executing completion handlers have run
* to completion and all resources have been released. This method may be
* invoked at any time. If some other thread has already invoked it, then
* another invocation will block until the first invocation is complete,
* after which it will return without effect.
*
* @throws IOException
* If an I/O error occurs
*/
public abstract void shutdownNow() throws IOException;
/**
* Awaits termination of the group.
* <p> This method blocks until the group has terminated, or the timeout
* occurs, or the current thread is interrupted, whichever happens first.
*
* @param timeout
* The maximum time to wait, or zero or less to not wait
* @param unit
* The time unit of the timeout argument
*
* @return {@code true} if the group has terminated; {@code false} if the
* timeout elapsed before termination
*
* @throws InterruptedException
* If interrupted while waiting
*/
public abstract boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;
}
BIO、 NIO、 AIO 对比：
为什么Netty使用NIO而不是AIO？
在Linux系统上，AIO的底层实现仍使用Epoll，没有很好实现AIO，因此在性能上没有明显的优势，而且被JDK封装了一层不容易深度优化，Linux上AIO还不够成熟。Netty是异步非阻塞框架，Netty在NIO上做了很多异步的封装。
同步异步与阻塞非阻塞(段子)
老张爱喝茶，废话不说，煮开水。
出场人物：老张，水壶两把（普通水壶，简称水壶；会响的水壶，简称响水壶）。
1 老张把水壶放到火上，立等水开。（同步阻塞）
老张觉得自己有点傻
2 老张把水壶放到火上，去客厅看电视，时不时去厨房看看水开没有。（同步非阻塞）
老张还是觉得自己有点傻，于是变高端了，买了把会响笛的那种水壶。水开之后，能大声发出嘀~~~~的噪音。
3 老张把响水壶放到火上，立等水开。（异步阻塞）
老张觉得这样傻等意义不大
4 老张把响水壶放到火上，去客厅看电视，水壶响之前不再去看它了，响了再去拿壶。（异步非阻塞）
老张觉得自己聪明了。
所谓同步异步，只是对于水壶而言。
普通水壶，同步；响水壶，异步。
虽然都能干活，但响水壶可以在自己完工之后，提示老张水开了。这是普通水壶所不能及的。
同步只能让调用者去轮询自己（情况2中），造成老张效率的低下。
所谓阻塞非阻塞，仅仅对于老张而言。
立等的老张，阻塞；看电视的老张，非阻塞。