2.4 多路复用javaAPI
在上面我们简单java代码实现了多路复用，是一个单线程版的。讲上面的epoll代码复制到linux服务器中，使用strace追踪系统调用。
javaAPI会根据系统类型自动选择最优的多路复用器，因此在linux下默认就是使用epoll模型，也可以通过启动参数指定多路复用器模型。
# 因为默认是epoll，所以系统调用起名为epoll开头
strace -ff -o epoll java SingletonMultiplexingIO
# 再指定poll 起名为poll开头的
strace -ff -o poll java -Djava.nio.channels.spi.SelectorProvider=sun.nio.ch.PollSelectorProvider SingletonMultiplexingIO
执行完上面两条命令后就可以得到如下文件：
total 2160
-rw-rw-r-- 1 xz xz 13911 Jun 22 17:13 epoll.32474
-rw-rw-r-- 1 xz xz 526239 Jun 22 17:13 epoll.32475
-rw-rw-r-- 1 xz xz 62970 Jun 22 17:13 epoll.32476
-rw-rw-r-- 1 xz xz 1033 Jun 22 17:13 epoll.32477
-rw-rw-r-- 1 xz xz 1157 Jun 22 17:13 epoll.32478
-rw-rw-r-- 1 xz xz 2183 Jun 22 17:13 epoll.32479
-rw-rw-r-- 1 xz xz 28151 Jun 22 17:13 epoll.32480
-rw-rw-r-- 1 xz xz 20128 Jun 22 17:13 epoll.32481
-rw-rw-r-- 1 xz xz 1033 Jun 22 17:13 epoll.32482
-rw-rw-r-- 1 xz xz 1214432 Jun 22 17:13 epoll.32483
-rw-rw-r-- 1 xz xz 1935 Jun 22 17:13 epoll.32589
-rw-rw-r-- 1 xz xz 13661 Jun 22 17:13 poll.32595
-rw-rw-r-- 1 xz xz 9633 Jun 22 17:13 poll.32596
-rw-rw-r-- 1 xz xz 13954 Jun 22 17:13 poll.32602
-rw-rw-r-- 1 xz xz 206806 Jun 22 17:13 poll.32603
-rw-rw-r-- 1 xz xz 2121 Jun 22 17:13 poll.32604
-rw-rw-r-- 1 xz xz 1033 Jun 22 17:13 poll.32605
-rw-rw-r-- 1 xz xz 1157 Jun 22 17:13 poll.32606
-rw-rw-r-- 1 xz xz 2342 Jun 22 17:13 poll.32607
-rw-rw-r-- 1 xz xz 6577 Jun 22 17:13 poll.32608
-rw-rw-r-- 1 xz xz 3897 Jun 22 17:13 poll.32609
-rw-rw-r-- 1 xz xz 1033 Jun 22 17:13 poll.32610
-rw-rw-r-- 1 xz xz 10674 Jun 22 17:13 poll.32611
-rw-rw-r-- 1 xz xz 1935 Jun 22 17:13 poll.32612
-rw-rw-r-- 1 xz xz 3317 Jun 22 17:07 SingletonMultiplexingIO.class
-rw-rw-r-- 1 xz xz 3520 Jun 22 17:06 SingletonMultiplexingIO.java
查看epoll.32475:
...
# 首先创建一个socket套接字，得到文件描述符4
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 4
...
# 绑定端口9000到文件描述符4上，并监听文件描述符4
bind(4, {sa_family=AF_INET, sin_port=htons(9000), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
listen(4, 20)
...
# 设置非阻塞
fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK) = 0
...
# 打印创建socket成功
write(1, "create socket 9000 successful", 29) = 29
# 建立一个管道，占用文件描述符5，6，这两个文件描述符先不管
...
pipe([5, 6]) = 0
...
# 创建epoll，得到文件描述符7 这个7就是epoll在内核空间开辟的红黑树对应的文件描述符
epoll_create(256) = 7
...
# 调用epoll_ctl,将5添加到epoll中，这个不管
epoll_ctl(7, EPOLL_CTL_ADD, 5, {EPOLLIN, {u32=5, u64=4613014228473741317}}) = 0
write(1, "create multiplexing successful", 30) = 30
...
write(1, "register socket to epoll success"..., 35) = 35
...
# 将4通过 epoll_ctl 添加（EPOLL_CTL_ADD)到epoll中。这里注意，我们代码中，
# 打印: register socket to epoll success 这句话位置是在socket创建完成后，selector.select()代码执行前
# 但是在实际的调用中，注册4到epoll中在那句话后面打印的，因此实际执行中，在第一次执行 selector.select()时才注册 serverSocket到epoll中，其实是懒加载
epoll_ctl(7, EPOLL_CTL_ADD, 4, {EPOLLIN, {u32=4, u64=5340325089890009092}}) = 0
# selector.select(50) 对应的就是下面的epoll_wait
epoll_wait(7, {}, 8192, 50) = 0
epoll_wait(7, {}, 8192, 50) = 0
...
然后再看poll.32596:
...
# 首先创建一个socket套接字，得到文件描述符4
socket(PF_INET6, SOCK_STREAM, IPPROTO_IP) = 4
...
# 绑定端口9000到文件描述符4上，并监听文件描述符4
bind(4, {sa_family=AF_INET, sin_port=htons(9000), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
listen(4, 20)
...
# 设置非阻塞
fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK) = 0
...
write(1, "create socket 9000 successful", 29) = 29
pipe([5, 6]) = 0
write(1, "create multiplexing successful", 30) = 30
write(1, "register socket to epoll success"..., 35) = 35
poll([{fd=5, events=POLLIN}, {fd=4, events=POLLIN}], 2, 50) = 0 (Timeout)
通过上面两个文件，可以很详细的看出两种方式的系统调用的区别。
然后我们来看如何使用java代码实现多路复用器：
单线程多路复用器
也就是上面我们做测试用的多路复用器：
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;
/**
* @author shuai.zhao@going-link.com
* @date 2021/6/15
*/
public class SingletonMultiplexingIO {
private Selector selector;
public void init() {
try {
// 创建服务端 socket 系统调用
// socket(...) = 4
ServerSocketChannel ssc = ServerSocketChannel.open();
// bind(4, 9000)
// listen(4, 20)
ssc.bind(new InetSocketAddress(9000), 20);
// 设置为非阻塞
// fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK) = 0
ssc.configureBlocking(false);
System.out.println("create socket 9000 successful");
// 创建多路复用器, 如果是epoll 则相当于调用 epoll_create
selector = Selector.open();
System.out.println("create multiplexing successful");
// 向多路复用器注册fd
// 代码虽然写在这里，但是实际调用是在下面第一次调用 selector.select()
// 原因是懒加载
ssc.register(selector, SelectionKey.OP_ACCEPT);
System.out.println("register socket to epoll successful");
} catch (IOException e) {
e.printStackTrace();
}
}
public void start() {
init();
System.out.println("服务端启动了");
while (true) {
try {
// selector.select()
// 如果使用 select/poll 模型的话，那么就相当于调用select/poll
// 如果使用epoll，就相当于调用epoll_wait
while (selector.select(50) > 0) {
// 获取有状态的fd
// poll的话就是传一堆fd过去，筛选出有状态的fd
// epoll就是返回链表中的fd
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = selectionKeys.iterator();
// 迭代处理每一个fd
while (iterator.hasNext()) {
SelectionKey selectionKey = iterator.next();
// 每处理一个fd就从集合中remove掉
iterator.remove();
if (selectionKey.isAcceptable()) {
// 是可接收的fd
acceptHandler(selectionKey);
} else if (selectionKey.isReadable()) {
// 是可读的fd
readHandler(selectionKey);
}
// else if (selectionKey.isWritable()) {
// 暂时先不考虑此状态，但是要知道有这个状态
// }
}
}
} catch (Exception ignore) {
}
}
}
public void acceptHandler(SelectionKey selectionKey) throws IOException {
ServerSocketChannel ssc = (ServerSocketChannel) selectionKey.channel();
SocketChannel client = ssc.accept();
client.configureBlocking(false);
ByteBuffer buffer = ByteBuffer.allocateDirect(4096);
// 将这个可读的fd注册到红黑树中
// epoll_ctl( fd, EPOLL_CTL_ADD...
client.register(selector, SelectionKey.OP_READ, buffer);
System.out.println("新客户端：" + client.getRemoteAddress());
}
public void readHandler(SelectionKey selectionKey) throws IOException {
SocketChannel client = (SocketChannel) selectionKey.channel();
ByteBuffer buffer = (ByteBuffer) selectionKey.attachment();
buffer.clear();
while (true) {
int read = client.read(buffer);
System.out.println("read = " + read);
if (read > 0) {
buffer.flip();
while (buffer.hasRemaining()) {
// 读到什么东西就写回去什么
client.write(buffer);
}
buffer.clear();
} else if (read == 0) {
break;
} else {
client.close();
}
}
}
public static void main(String[] args) {
SingletonMultiplexingIO smio = new SingletonMultiplexingIO();
smio.start();
}
}
来看上面这种单线程的多路复用器代码，这个版本的实现只有一个线程，也就是说所有的fd都是在一个线程中处理的，这样如果有一个线程执行时间耗时较长的话(在readHandler中处理时间过长)就会导致其他的fd都会被阻塞住往后延迟。因此我们可以很自然的想到，在处理读写事件时，使用多线程去处理。也就是下面这种方式：
基于多线程的多路复用器V1(version 1)
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.*;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* @author shuai.zhao@going-link.com
* @date 2021/6/22
*/
public class MultiplexingIOThreads {
/**
* 线程池，处理事件
*/
private ExecutorService executor = Executors.newFixedThreadPool(5);
private Selector selector;
/**
* 初始化
*/
private void init() {
try {
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.bind(new InetSocketAddress(9000), 20);
ssc.configureBlocking(false);
this.selector = Selector.open();
ssc.register(this.selector, SelectionKey.OP_ACCEPT);
} catch (IOException ignore) {
}
}
private void start() {
this.init();
try {
while (true) {
while (selector.select(50) > 0) {
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> ski = selectionKeys.iterator();
while (ski.hasNext()) {
SelectionKey selectionKey = ski.next();
ski.remove();
if (selectionKey.isAcceptable()) {
acceptHandler(selectionKey);
} else if (selectionKey.isReadable()) {
// selectionKey.cancel() 取消key
selectionKey.cancel();
System.out.println("**selectionKey cancel**");
readHandler(selectionKey);
} else if (selectionKey.isWritable()) {
selectionKey.cancel();
writeHandler(selectionKey);
}
}
}
}
} catch (Exception ignore) {
}
}
private void writeHandler(SelectionKey selectionKey) {
executor.execute(() -> {
System.out.println("writeHandler ...");
SocketChannel sc = (SocketChannel) selectionKey.channel();
ByteBuffer buffer = (ByteBuffer) selectionKey.attachment();
buffer.flip();
try {
sc.write(buffer);
} catch (IOException ignore) {
}
// 数据写完，将此fd重新置为可读状态
buffer.clear();
try {
sc.register(selector, SelectionKey.OP_READ, buffer);
} catch (ClosedChannelException e) {
e.printStackTrace();
}
});
}
private void readHandler(SelectionKey selectionKey) {
// 另起一个线程处理读事件
executor.execute(() -> {
System.out.println("readHandler ...");
SocketChannel sc = (SocketChannel) selectionKey.channel();
ByteBuffer buffer = (ByteBuffer) selectionKey.attachment();
buffer.clear();
try {
while (true) {
// 可以读到数据
if (sc.read(buffer) > 0) {
// 反转buffer
buffer.flip();
// 读客户端发送的数据
byte[] bytes = new byte[buffer.limit()];
buffer.get(bytes);
System.out.println("client write： = " + new String(bytes));
buffer.clear();
buffer.put("successful".getBytes());
// 读取完数据后，将此链接注册为可写事件
sc.register(selector, SelectionKey.OP_WRITE, buffer);
} else {
break;
}
}
} catch (IOException e) {
e.printStackTrace();
}
});
}
private void acceptHandler(SelectionKey selectionKey) throws IOException {
ServerSocketChannel ssc = (ServerSocketChannel) selectionKey.channel();
SocketChannel sc = ssc.accept();
sc.configureBlocking(false);
ByteBuffer buffer = ByteBuffer.allocateDirect(4096);
sc.register(selector, SelectionKey.OP_READ, buffer);
}
public static void main(String[] args) {
MultiplexingIOThreads instance = new MultiplexingIOThreads();
instance.start();
}
}
追踪启动上面的进程，然后使用nc连接服务端，主要是为了查看selectionKey.cancel()这个方法的系统调用：
# readhandler开辟新线程
clone(child_stack=0x7f5d048d3fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f5d048d49d0, tls=0x7f5d048d4700, child_tidptr=0x7f5d048d49d0) = 2
7579
futex(0x7f5d1404c354, FUTEX_WAIT_PRIVATE, 39, NULL) = 0
futex(0x7f5d1404c328, FUTEX_WAIT_PRIVATE, 2, NULL) = 0
futex(0x7f5d1404c328, FUTEX_WAKE_PRIVATE, 1) = 0
# 下一次执行epoll_wait即select()前先执行删除key操作
epoll_ctl(7, EPOLL_CTL_DEL, 8, {0, {u32=8, u64=1958317697552875528}}) = 0
futex(0x7f5d14107154, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0x7f5d14107150, {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1}) = 1
futex(0x7f5d14107128, FUTEX_WAKE_PRIVATE, 1) = 0
# 执行select()
epoll_wait(7, {}, 8192, 50)
关于
selectionKey.cancel()方法：
cancel()方法被调用时，会在下一次调用select()方法时将key从红黑树中删除，通过epoll_ctl(7, EPOLL_CTL_DELETE....)删除。
为什么要调用cancel()？
在之前的版本中，数据的输入和输出操作都是在同一个线程一个方法中进行，我们读完数据立即就执行了写操作，因此当一个方法执行完时，就代表这个key中的数据已经被读完了，但是现在使用多线程处理读写操作，因此可能出现的情况就是：当启动了一个子线程，但是因为需要读的数据非常大，比较耗时，子线程还在读数据是，主线程进入了下一次循环，仍然找到了这个fd，那么就会重新开劈子进程，重新读取fd数据，出现重复读数据的情况。因此当使用这种多线程读时，就需要调用cancel()去删除这个key，这样下次就不会再读到它了。
但是因为
-
线程池大小难控制，多线程间线程切换会影响效率，多线程会有线程安全性问题
-
cancel()实际调用的是epoll_ctl(DELETE)也是属于系统调用，IO模型的进化 BIO->NIO->多路复用，本就是为了减少 系统调用，因此这种方式违背了初心，不可取。
所以就进化出了新的多线程模型。
基于多线程的多路复用器V2
由于上面版本的多线程的问题，提出的解决方案就是：
开辟多个selector，每一个selector内部是单线程的，多个selector是并行的。同时利用了多线程和单线程的优点。
代码实现：
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;
/**
* @author shuai.zhao@going-link.com
* @date 2021/6/30
*/
public class MultiplexingIOThreads_V2 {
private Selector selector1;
private Selector selector2;
private Selector selector3;
void init() {
try {
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.bind(new InetSocketAddress(9000));
ssc.configureBlocking(false);
selector1 = Selector.open();
selector2 = Selector.open();
selector3 = Selector.open();
ssc.register(selector1, SelectionKey.OP_ACCEPT);
ssc.register(selector2, SelectionKey.OP_ACCEPT);
ssc.register(selector3, SelectionKey.OP_ACCEPT);
} catch (IOException ignore) {
}
}
public static void main(String[] args) {
MultiplexingIOThreads_V2 mt_v2 = new MultiplexingIOThreads_V2();
mt_v2.init();
new SelectorThread(mt_v2.selector1, "selector1").start();
new SelectorThread(mt_v2.selector2, "selector2").start();
new SelectorThread(mt_v2.selector3, "selector3").start();
}
/**
* 每一个selector对应一个单线程
*/
static class SelectorThread extends Thread{
private Selector selector;
public SelectorThread(Selector selector, String threadName) {
super(threadName);
this.selector = selector;
}
@Override
public void run() {
try {
while (true) {
while (selector.select(50) > 0) {
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> ski = selectionKeys.iterator();
while (ski.hasNext()) {
SelectionKey selectionKey = ski.next();
ski.remove();
if (selectionKey.isAcceptable()) {
acceptHandler(selectionKey);
} else if (selectionKey.isReadable()) {
readHandler(selectionKey);
}
}
}
}
} catch (Exception ignore) {
}
}
public void acceptHandler(SelectionKey selectionKey) throws IOException {
ServerSocketChannel ssc = (ServerSocketChannel) selectionKey.channel();
SocketChannel client = ssc.accept();
client.configureBlocking(false);
ByteBuffer buffer = ByteBuffer.allocateDirect(4096);
// 将这个可读的fd注册到红黑树中
// epoll_ctl( fd, EPOLL_CTL_ADD...
client.register(selector, SelectionKey.OP_READ, buffer);
System.out.println(this.getName() + "新客户端：" + client.getRemoteAddress());
}
public void readHandler(SelectionKey selectionKey) throws IOException {
SocketChannel client = (SocketChannel) selectionKey.channel();
ByteBuffer buffer = (ByteBuffer) selectionKey.attachment();
buffer.clear();
while (true) {
int read = client.read(buffer);
System.out.println("read = " + read);
if (read > 0) {
buffer.flip();
while (buffer.hasRemaining()) {
// 读到什么东西就写回去什么
client.write(buffer);
}
buffer.clear();
} else {
break;
}
}
}
}
}
首先看这一版本的多线程实现。多个selector线程共同处理客户端的连接请求。通过前面的C10K去验证此服务端。会发现一个问题：
因为多个线程的优先级是一样的，因此在客户端连接进来时，是随机分配给每个selector的，一个是不可控问题，可能有的selector注册的特别多，特别忙，有的selector却很闲。我们希望是多个selector协同处理所有的fd。因此我们做一下改动：
package com.gouxiazhi.io;
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.Arrays;
import java.util.Iterator;
import java.util.Scanner;
import java.util.Set;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
/**
* @author shuai.zhao@going-link.com
* @date 2021/6/30
*/
public class MultiplexingIOThreads_V3 {
private Selector selector1;
void init() {
try {
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.bind(new InetSocketAddress(9000));
ssc.configureBlocking(false);
selector1 = Selector.open();
ssc.register(selector1, SelectionKey.OP_ACCEPT);
} catch (IOException ignore) {
}
}
public static void main(String[] args) throws IOException {
MultiplexingIOThreads_V3 mt_v3 = new MultiplexingIOThreads_V3();
mt_v3.init();
new SelectorThread("boss", 2, mt_v3.selector1).start();
new SelectorThread("worker1").start();
new SelectorThread("worker2").start();
}
/**
* 每一个selector对应一个单线程
*/
static class SelectorThread extends Thread {
/**
* 统计当前boss所接收的客户端总数
*/
private static AtomicInteger idx = new AtomicInteger();
/**
* 下边有几个小弟
*/
private static int workers;
/**
* 一个小弟分一个任务链
*/
private volatile static LinkedBlockingQueue<SocketChannel>[] queues;
/**
* 干活的小弟
*/
private Selector selector;
/**
* 小弟编号
*/
private int id;
/**
* boss线程构造器
*
* @param name 线程名
* @param workers 小弟数量
* @param selector boss
*/
public SelectorThread(String name, int workers, Selector selector) {
super(name);
this.selector = selector;
SelectorThread.workers = workers;
queues = new LinkedBlockingQueue[workers];
for (int i = 0; i < workers; i++) {
queues[i] = new LinkedBlockingQueue<>();
}
}
public SelectorThread(String name) throws IOException {
super(name);
this.selector = Selector.open();
this.id = idx.getAndIncrement() % workers;
}
@Override
public void run() {
System.out.println(this.getName() + "start running");
try {
while (true) {
// 收集key
while (selector.select(50) > 0) {
Set<SelectionKey> selectionKeys = selector.selectedKeys();
Iterator<SelectionKey> ski = selectionKeys.iterator();
while (ski.hasNext()) {
SelectionKey selectionKey = ski.next();
ski.remove();
// boss线程会出现
if (selectionKey.isAcceptable()) {
acceptHandler(selectionKey);
}
// worker线程会出现
else if (selectionKey.isReadable()) {
readHandler(selectionKey);
}
}
}
if (!queues[id].isEmpty()) {
SocketChannel sc = queues[id].take();
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(4096);
sc.register(this.selector, SelectionKey.OP_READ, byteBuffer);
System.out.println(this.getName() + "新客户端：" + sc.getRemoteAddress());
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
public void acceptHandler(SelectionKey selectionKey) throws IOException {
ServerSocketChannel ssc = (ServerSocketChannel) selectionKey.channel();
SocketChannel client = ssc.accept();
client.configureBlocking(false);
// 给小弟的链表添加工作任务
int num = idx.getAndIncrement() % workers;
queues[num].add(client);
}
public void readHandler(SelectionKey selectionKey) throws IOException {
SocketChannel client = (SocketChannel) selectionKey.channel();
ByteBuffer buffer = (ByteBuffer) selectionKey.attachment();
buffer.clear();
try {
while (true) {
int read = client.read(buffer);
System.out.println(this.getName() + "read = " + read);
if (read > 0) {
buffer.flip();
while (buffer.hasRemaining()) {
// 读到什么东西就写回去什么
client.write(buffer);
}
buffer.clear();
} else {
break;
}
}
} catch (Exception ignore) {
}
}
}
}
在上面这个版本中我们实现了多线程协作处理，其实已经有了netty的雏形，IO笔记至此完。