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< a class = "header" href = "#使用消息传递在线程间传送数据" name = "使用消息传递在线程间传送数据" > < h2 > 使用消息传递在线程间传送数据< / h2 > < / a >
< blockquote >
< p > < a href = "https://github.com/rust-lang/book/blob/master/second-edition/src/ch16-02-message-passing.md" > ch16-02-message-passing.md< / a >
< br >
commit da15de39eaabd50100d6fa662c653169254d9175< / p >
< / blockquote >
< p > 最近人气正在上升的一个并发方式是< strong > 消息传递< / strong > （ < em > message passing< / em > ），这里线程或 actor 通过发送包含数据的消息来沟通。这个思想来源于口号：< / p >
< blockquote >
< p > Do not communicate by sharing memory; instead, share memory by
communicating.< / p >
< p > 不要共享内存来通讯；而是要通讯来共享内存。< / p >
< p > --< a href = "http://golang.org/doc/effective_go.html" > Effective Go< / a > < / p >
< / blockquote >
< p > 实现这个目标的主要工具是< strong > 通道< / strong > （ < em > channel< / em > ） ， （ ） （ ） ， < / p >
< p > 我们将编写一个例子使用一个线程生成值并向通道发送他们。主线程会接收这些值并打印出来。< / p >
< p > 首先，如列表 16-6 所示，先创建一个通道但不做任何事：< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > use std::sync::mpsc;
fn main() {
let (tx, rx) = mpsc::channel();
# tx.send(()).unwrap();
}
< / code > < / pre >
< p > < span class = "caption" > Listing 16-6: Creating a channel and assigning the two
halves to < code > tx< / code > and < code > rx< / code > < / span > < / p >
< p > < code > mpsc::channel< / code > 函数创建一个新的通道。< code > mpsc< / code > 是< strong > 多个生产者，单个消费者< / strong > （ < em > multiple producer, single consumer< / em > ）的缩写。简而言之，可以有多个产生值的< strong > 发送端< / strong > ，但只能有一个消费这些值的< strong > 接收端< / strong > 。现在我们以一个单独的生产者开始，不过一旦例子可以工作了就会增加多个生产者。< / p >
< p > < code > mpsc::channel< / code > 返回一个元组：第一个元素是发送端，而第二个元素是接收端。由于历史原因，很多人使用< code > tx< / code > 和< code > rx< / code > 作为< strong > 发送者< / strong > 和< strong > 接收者< / strong > 的缩写，所以这就是我们将用来绑定这两端变量的名字。这里使用了一个< code > let< / code > 语句和模式来解构了元组。第十八章会讨论< code > let< / code > 语句中的模式和解构。< / p >
< p > 让我们将发送端移动到一个新建线程中并发送一个字符串，如列表 16-7 所示：< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > use std::thread;
use std::sync::mpsc;
fn main() {
let (tx, rx) = mpsc::channel();
thread::spawn(move || {
let val = String::from(" hi" );
tx.send(val).unwrap();
});
}
< / code > < / pre >
< p > < span class = "caption" > Listing 16-7: Moving < code > tx< / code > to a spawned thread and sending
" hi" < / span > < / p >
< p > 正如上一部分那样使用< code > thread::spawn< / code > 来创建一个新线程。并使用一个< code > move< / code > 闭包来将< code > tx< / code > 移动进闭包这样新建线程就是其所有者。< / p >
< p > 通道的发送端有一个< code > send< / code > 方法用来获取需要放入通道的值。< code > send< / code > 方法返回一个< code > Result< T, E> < / code > 类型，因为如果接收端被丢弃了，将没有发送值的目标，所以发送操作会出错。在这个例子中，我们简单的调用< code > unwrap< / code > 来忽略错误，不过对于一个真实程序，需要合理的处理它。第九章是你复习正确错误处理策略的好地方。< / p >
< p > 在列表 16-8 中，让我们在主线程中从通道的接收端获取值：< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > use std::thread;
use std::sync::mpsc;
fn main() {
let (tx, rx) = mpsc::channel();
thread::spawn(move || {
let val = String::from(" hi" );
tx.send(val).unwrap();
});
let received = rx.recv().unwrap();
println!(" Got: {}" , received);
}
< / code > < / pre >
< p > < span class = "caption" > Listing 16-8: Receiving the value " hi" in the main thread
and printing it out< / span > < / p >
< p > 通道的接收端有两个有用的方法：< code > recv< / code > 和< code > try_recv< / code > 。这里，我们使用了< code > recv< / code > ，它是 < em > receive< / em > 的缩写。这个方法会阻塞执行直到从通道中接收一个值。一旦发送了一个值，< code > recv< / code > 会在一个< code > Result< T, E> < / code > 中返回它。当通道发送端关闭，< code > recv< / code > 会返回一个错误。< code > try_recv< / code > 不会阻塞；相反它立刻返回一个< code > Result< T, E> < / code > 。< / p >
< p > 如果运行列表 16-8 中的代码，我们将会看到主线程打印出这个值：< / p >
< pre > < code > Got: hi
< / code > < / pre >
< a class = "header" href = "#通道与所有权如何交互" name = "通道与所有权如何交互" > < h3 > 通道与所有权如何交互< / h3 > < / a >
< p > 现在让我们做一个试验来看看通道与所有权如何在一起工作：我们将尝试在新建线程中的通道中发送完< code > val< / code > 之后再使用它。尝试编译列表 16-9 中的代码：< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust,ignore" > use std::thread;
use std::sync::mpsc;
fn main() {
let (tx, rx) = mpsc::channel();
thread::spawn(move || {
let val = String::from(" hi" );
tx.send(val).unwrap();
println!(" val is {}" , val);
});
let received = rx.recv().unwrap();
println!(" Got: {}" , received);
}
< / code > < / pre >
< p > < span class = "caption" > Listing 16-9: Attempting to use < code > val< / code > after we have sent
it down the channel< / span > < / p >
< p > 这里尝试在通过< code > tx.send< / code > 发送< code > val< / code > 到通道中之后将其打印出来。这是一个坏主意：一旦将值发送到另一个线程后，那个线程可能会在我们在此使用它之前就修改或者丢弃它。这会由于不一致或不存在的数据而导致错误或意外的结果。< / p >
< p > 尝试编译这些代码， < / p >
< pre > < code > error[E0382]: use of moved value: `val`
--> src/main.rs:10:31
|
9 | tx.send(val).unwrap();
| --- value moved here
10 | println!(" val is {}" , val);
| ^^^ value used here after move
|
= note: move occurs because `val` has type `std::string::String`, which does
not implement the `Copy` trait
< / code > < / pre >
< p > 我们的并发错误会造成一个编译时错误！< code > send< / code > 获取其参数的所有权并移动这个值归接收者所有。这个意味着不可能意外的在发送后再次使用这个值；所有权系统检查一切是否合乎规则。< / p >
< p > 在这一点上，消息传递非常类似于 Rust 的单所有权系统。消息传递的拥护者出于相似的原因支持消息传递，就像 Rustacean 们欣赏 Rust 的所有权一样：单所有权意味着特定类型问题的消失。如果一次只有一个线程可以使用某些内存，就没有出现数据竞争的机会。< / p >
< a class = "header" href = "#发送多个值并观察接收者的等待" name = "发送多个值并观察接收者的等待" > < h3 > 发送多个值并观察接收者的等待< / h3 > < / a >
< p > 列表 16-8 中的代码可以编译和运行，不过这并不是很有趣：通过它难以看出两个独立的线程在一个通道上相互通讯。列表 16-10 则有一些改进会证明这些代码是并发执行的：新建线程现在会发送多个消息并在每个消息之间暂定一段时间。< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > use std::thread;
use std::sync::mpsc;
use std::time::Duration;
fn main() {
let (tx, rx) = mpsc::channel();
thread::spawn(move || {
let vals = vec![
String::from(" hi" ),
String::from(" from" ),
String::from(" the" ),
String::from(" thread" ),
];
for val in vals {
tx.send(val).unwrap();
thread::sleep(Duration::new(1, 0));
}
});
for received in rx {
println!(" Got: {}" , received);
}
}
< / code > < / pre >
< p > < span class = "caption" > Listing 16-10: Sending multiple messages and pausing
between each one< / span > < / p >
< p > 这一次，在新建线程中有一个字符串 vector 希望发送到主线程。我们遍历他们，单独的发送每一个字符串并通过一个< code > Duration< / code > 值调用< code > thread::sleep< / code > 函数来暂停一秒。< / p >
< p > 在主线程中，不再显式的调用< code > recv< / code > 函数：而是将< code > rx< / code > 当作一个迭代器。对于每一个接收到的值，我们将其打印出来。当通道被关闭时，迭代器也将结束。< / p >
< p > 当运行列表 16-10 中的代码时，将看到如下输出，每一行都会暂停一秒：< / p >
< pre > < code > Got: hi
Got: from
Got: the
Got: thread
< / code > < / pre >
< p > 在主线程中并没有任何暂停或位于< code > for< / code > 循环中用于等待的代码，所以可以说主线程是在等待从新建线程中接收值。< / p >
< a class = "header" href = "#通过克隆发送者来创建多个生产者" name = "通过克隆发送者来创建多个生产者" > < h3 > 通过克隆发送者来创建多个生产者< / h3 > < / a >
< p > 差不多在本部分的开头，我们提到了< code > mpsc< / code > 是 < em > multiple producer, single consumer< / em > 的缩写。可以扩展列表 16-11 中的代码来创建都向同一接收者发送值的多个线程。这可以通过克隆通道的发送端在来做到，如列表 16-11 所示：< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > # use std::thread;
# use std::sync::mpsc;
# use std::time::Duration;
#
# fn main() {
// ...snip...
let (tx, rx) = mpsc::channel();
let tx1 = tx.clone();
thread::spawn(move || {
let vals = vec![
String::from(" hi" ),
String::from(" from" ),
String::from(" the" ),
String::from(" thread" ),
];
for val in vals {
tx1.send(val).unwrap();
thread::sleep(Duration::new(1, 0));
}
});
thread::spawn(move || {
let vals = vec![
String::from(" more" ),
String::from(" messages" ),
String::from(" for" ),
String::from(" you" ),
];
for val in vals {
tx.send(val).unwrap();
thread::sleep(Duration::new(1, 0));
}
});
// ...snip...
#
# for received in rx {
# println!(" Got: {}" , received);
# }
# }
< / code > < / pre >
< p > < span class = "caption" > Listing 16-11: Sending multiple messages and pausing
between each one< / span > < / p >
< p > 这一次，在创建新线程之前，我们对通道的发送端调用了< code > clone< / code > 方法。这会给我们一个可以传递给第一个新建线程的发送端句柄。我们会将原始的通道发送端传递给第二个新建线程，这样每个线程将向通道的接收端发送不同的消息。< / p >
< p > 如果运行这些代码，你< strong > 可能< / strong > 会看到这样的输出：< / p >
< pre > < code > Got: hi
Got: more
Got: from
Got: messages
Got: for
Got: the
Got: thread
Got: you
< / code > < / pre >
< p > 虽然你可能会看到这些以不同的顺序出现。这依赖于你的系统！这也就是并发既有趣又困难的原因。如果你拿< code > thread::sleep< / code > 做实验，在不同的线程中提供不同的值，就会发现他们的运行更加不确定并每次都会产生不同的输出。< / p >
< p > 现在我们见识过了通道如何工作，再看看共享内存并发吧。< / p >
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< / a >
< / div >
< a href = "ch16-01-threads.html" class = "nav-chapters previous" title = "You can navigate through the chapters using the arrow keys" >
< i class = "fa fa-angle-left" > < / i >
< / a >
< a href = "ch16-03-shared-state.html" class = "nav-chapters next" title = "You can navigate through the chapters using the arrow keys" >
< i class = "fa fa-angle-right" > < / i >
< / a >
< / div >
<!-- Local fallback for Font Awesome -->
< script >
if ($(".fa").css("font-family") !== "FontAwesome") {
$('< link rel = "stylesheet" type = "text/css" href = "_FontAwesome/css/font-awesome.css" > ').prependTo('head');
}
< / script >
<!-- Livereload script (if served using the cli tool) -->
< script src = "highlight.js" > < / script >
< script src = "book.js" > < / script >
< / body >
< / html >
