高阶流程控制语句（四）: 实战 delimited continuation
30 January 2012
前面大量篇幅介绍了很多理论知识，这次介绍一下 delimited continuation 的一个应用，使用 Scala 的 reset/shift 实现一个单线程的 Echo Server
首先看一下下面这段代码：
object Echo extends App {
val server = ServerSocketChannel.open
server.socket.bind(new InetSocketAddress(12345))
while (true) {
server.accept match {
case c: SocketChannel =>
println("Accept: " + c)
while (c.isOpen && c.isConnected) {
val bb = ByteBuffer.allocateDirect(1024)
c.read(bb) match {
case count if count > 0 =>
println("Read: " + c + " count: " + count)
bb.flip
while (bb.hasRemaining) {
c.write(bb) match {
case count if count > 0 =>
println("Write: " + c + " count: " + count)
case count if count == 0 =>
println("WriteBlock: " + c)
case _ =>
println("WriteError: " + c)
bb.clear
c.close
}
}
case count if count == 0 =>
println("ReadBlock: " + c)
case _ =>
println("ReadError: " + c)
c.close
}
}
case null =>
println("AcceptBlock")
}
}
} |
这看起来像是一个 Echo Server，不过写出这种代码的哥们估计第二天就会被炒鱿鱼了。众所周知，实现单线程的多 TCP 连接程序，肯定要使用 Select/Epoll 这些多路 IO 复用机制，如果不使用这些机制，就会像上面这段程序一样，只能处理一条连接。但是这段程序并不是一无是处，它*逻辑清晰，简单易懂*。几十行代码一气呵成，把需要的逻辑都包含了：接受连接，读取数据，返回数据。后来我想尝试用 Java NIO 写了一下 Echo Server，不过在我动手前，我的脑袋已经被各种回调扭成麻花了，如果没有 mina netty 这种框架，写网络程序无异于自杀。
回到上面那段有问题的程序，将错就错，先使用多线程，让它能跑起来：
object EchoMultiThread extends App {
val server = ServerSocketChannel.open
server.socket.bind(new InetSocketAddress(12345))
while (true) {
server.accept match {
case c: SocketChannel =>
(new Thread(new Runnable { // <============================= 1
override def run { // <==================================== 2
println("Accept: " + c)
while (c.isOpen && c.isConnected) {
val bb = ByteBuffer.allocateDirect(1024)
c.read(bb) match {
case count if count > 0 =>
println("Read: " + c + " count: " + count)
bb.flip
while (bb.hasRemaining) {
c.write(bb) match {
case count if count > 0 =>
println("Write: " + c + " count: " + count)
case count if count == 0 =>
println("WriteBlock: " + c)
case _ =>
println("WriteError: " + c)
bb.clear
c.close
}
}
case count if count == 0 =>
println("ReadBlock: " + c)
case _ =>
println("ReadError: " + c)
c.close
}
}
} // // <================================================== 3
})).start // <=============================================== 4
case null =>
println("AcceptBlock")
}
}
} |
上面加了四行代码，把这段程序变成多线程，跑起来了，不过离我们的目标还有一段距离，我们需要使用单线程和 Java NIO 把这段程序跑起来。分析一下这段程序，主线程阻塞在外层 while 循环，所有子线程阻塞在里面的 while 循环，如果我们能够使用 reset/shift 把这两个 while 循环变成可中断的，那么问题不就解决了么？
先把这段程序中，需要使用线程的部分用 reset 包起来
object Echo extends App {
val server = ServerSocketChannel.open
server.socket.bind(new InetSocketAddress(12345))
server.configureBlocking(false)
reset { // <=================================== here it is the main thread
while (true) {
server.accept match {
case c: SocketChannel =>
reset { // <=========================== here it is child threads
println("Accept: " + c)
c.configureBlocking(false)
while (c.isOpen && c.isConnected) {
val bb = ByteBuffer.allocateDirect(1024)
c.read(bb) match {
case count if count > 0 =>
println("Read: " + c + " count: " + count)
bb.flip
while (bb.hasRemaining) {
c.write(bb) match {
case count if count > 0 =>
println("Write: " + c + " count: " + count)
case count if count == 0 =>
println("WriteBlock: " + c)
case _ =>
println("WriteError: " + c)
bb.clear
c.close
}
}
case count if count == 0 =>
println("ReadBlock: " + c)
case _ =>
println("ReadError: " + c)
c.close
}
}
}
case null =>
println("AcceptBlock")
}
}
}
} |
然后找到需要阻塞的地方 shift 出去，shift 的时候把 continuation 函数注册到 Selector 里面:
object Echo extends App {
val selector = Selector.open
val server = ServerSocketChannel.open
server.socket.bind(new InetSocketAddress(12345))
server.configureBlocking(false)
reset {
while (true) {
server.accept match {
case c: SocketChannel =>
reset {
println("Accept: " + c)
c.configureBlocking(false)
while (c.isOpen && c.isConnected) {
val bb = ByteBuffer.allocateDirect(1024)
c.read(bb) match {
case count if count > 0 =>
println("Read: " + c + " count: " + count)
bb.flip
while (bb.hasRemaining) {
c.write(bb) match {
case count if count > 0 =>
println("Write: " + c + " count: " + count)
case count if count == 0 =>
println("WriteBlock: " + c)
// <=============================== May block here
shift[Unit, Unit, Unit] { cont =>
c.register(selector, SelectionKey.OP_WRITE, cont)
}
case _ =>
println("WriteError: " + c)
bb.clear
c.close
}
}
case count if count == 0 =>
println("ReadBlock: " + c)
// <===================================== May block here
shift[Unit, Unit, Unit] { cont =>
c.register(selector, SelectionKey.OP_READ, cont)
}
case _ =>
println("ReadError: " + c)
c.close
}
}
}
case null =>
println("AcceptBlock")
// <============================================= May block here
shift[Unit, Unit, Unit] { cont =>
server.register(selector, SelectionKey.OP_ACCEPT, cont)
}
}
}
}
} |
最后，加上 Selector 的 select 部分，select 成功后调用 shift 时存放在 attachment 中的 continuation 函数。为了编译通过，必须保证所有的 match/case 条件分支语句返回值类型相同，需要加入一些 shiftUnit 语句：
object Echo extends App {
val selector = Selector.open
val server = ServerSocketChannel.open
server.socket.bind(new InetSocketAddress(12345))
server.configureBlocking(false)
reset {
while (true) {
server.accept match {
case c: SocketChannel =>
reset {
println("Accept: " + c)
c.configureBlocking(false)
while (c.isOpen && c.isConnected) {
val bb = ByteBuffer.allocateDirect(1024)
c.read(bb) match {
case count if count > 0 =>
println("Read: " + c + " count: " + count)
bb.flip
while (bb.hasRemaining) {
c.write(bb) match {
case count if count > 0 =>
println("Write: " + c + " count: " + count)
shiftUnit[Unit, Unit, Unit]()
case count if count == 0 =>
println("WriteBlock: " + c)
shift[Unit, Unit, Unit] { cont =>
c.register(selector, SelectionKey.OP_WRITE, cont)
}
case _ =>
println("WriteError: " + c)
bb.clear
c.close
shiftUnit[Unit, Unit, Unit]()
}
}
case count if count == 0 =>
println("ReadBlock: " + c)
shift[Unit, Unit, Unit] { cont =>
c.register(selector, SelectionKey.OP_READ, cont)
}
case _ =>
println("ReadError: " + c)
c.close
shiftUnit[Unit, Unit, Unit]()
}
}
}
shiftUnit[Unit, Unit, Unit]()
case null =>
println("AcceptBlock")
shift[Unit, Unit, Unit] { cont =>
server.register(selector, SelectionKey.OP_ACCEPT, cont)
}
}
shiftUnit[Unit, Unit, Unit]()
}
}
val keys = selector.selectedKeys
while (true) {
selector.select
keys foreach { k =>
k.interestOps(0)
k.attachment.asInstanceOf[Function1[Unit, Unit]].apply(Unit)
}
keys.clear
}
} |
大功告成，怎么样，感觉不错吧，这样一个逻辑非常清晰的 Echo Server 就做完了，所有麻花一样的回调过程都隐藏在 reset/shift 的过程中了。源代码在 这里 ，有兴趣可以研究一下。
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