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<main>
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<h1 id="日志门面-log"><a class="header" href="#日志门面-log">日志门面 log</a></h1>
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<p>就如同 slf4j 是 Java 的日志门面库，<a href="https://github.com/rust-lang/log">log</a> 也是 Rust 的日志门面库( 这不是我自己编的，官方用语: logging facade )，它目前由官方积极维护，因此大家可以放心使用。</p>
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<p>使用方式很简单，只要在 <code>Cargo.toml</code> 中引入即可：</p>
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<pre><code class="language-toml">[dependencies]
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log = "0.4"
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</code></pre>
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<blockquote>
|
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|
<p>日志门面不是说排场很大的意思，而是指相应的日志 API 已成为事实上的标准，会被其它日志框架所使用。通过这种统一的门面，开发者就可以不必再拘泥于日志框架的选择，未来大不了再换一个日志框架就是</p>
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</blockquote>
|
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<p>既然是门面，<code>log</code> 自然定义了一套统一的日志特征和 API，将日志的操作进行了抽象。</p>
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<h2 id="log-特征"><a class="header" href="#log-特征">Log 特征</a></h2>
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<p>例如，它定义了一个 <code>Log</code> 特征：</p>
|
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>pub trait Log: Sync + Send {
|
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fn enabled(&self, metadata: &Metadata<'_>) -> bool;
|
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fn log(&self, record: &Record<'_>);
|
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fn flush(&self);
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}
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<span class="boring">}</span></code></pre>
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<ul>
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|
|
<li><code>enabled</code> 用于判断某条带有元数据的日志是否能被记录，它对于 <code>log_enabled!</code> 宏特别有用</li>
|
|
|
<li><code>log</code> 会记录 <code>record</code> 所代表的日志</li>
|
|
|
<li><code>flush</code> 会将缓存中的日志数据刷到输出中，例如标准输出或者文件中</li>
|
|
|
</ul>
|
|
|
<h2 id="日志宏"><a class="header" href="#日志宏">日志宏</a></h2>
|
|
|
<p><code>log</code> 还为我们提供了一整套标准的宏，用于方便地记录日志。看到 <code>trace!</code>、<code>debug!</code>、<code>info!</code>、<code>warn!</code>、<code>error!</code>，大家是否感觉眼熟呢？是的，它们跟上一章节提到的日志级别几乎一模一样，唯一的区别就是这里乱入了一个 <code>trace!</code>，它比 <code>debug!</code> 的日志级别还要低，记录的信息还要详细。可以说，你如果想巨细无遗地了解某个流程的所有踪迹，它就是不二之选。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use log::{info, trace, warn};
|
|
|
|
|
|
pub fn shave_the_yak(yak: &mut Yak) {
|
|
|
trace!("Commencing yak shaving");
|
|
|
|
|
|
loop {
|
|
|
match find_a_razor() {
|
|
|
Ok(razor) => {
|
|
|
info!("Razor located: {}", razor);
|
|
|
yak.shave(razor);
|
|
|
break;
|
|
|
}
|
|
|
Err(err) => {
|
|
|
warn!("Unable to locate a razor: {}, retrying", err);
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面的例子使用 <code>trace!</code> 记录了一条可有可无的信息：准备开始剃须，然后开始寻找剃须刀，找到后就用 <code>info!</code> 记录一条可能事后也没人看的信息：找到剃须刀；没找到的话，就记录一条 <code>warn!</code> 信息，这条信息就有一定价值了，不仅告诉我们没找到的原因，还记录了发生的次数，有助于事后定位问题。</p>
|
|
|
<p>可以看出，这里使用日志级别的方式和我们上一章节所述基本相符。</p>
|
|
|
<p>除了以上常用的，<code>log</code> 还提供了 <code>log!</code> 和 <code>log_enabled!</code> 宏，后者用于确定一条消息在当前模块中，对于给定的日志级别是否能够被记录</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use log::Level::Debug;
|
|
|
use log::{debug, log_enabled};
|
|
|
|
|
|
// 判断能否记录 Debug 消息
|
|
|
if log_enabled!(Debug) {
|
|
|
let data = expensive_call();
|
|
|
// 下面的日志记录较为昂贵，因此我们先在前面判断了是否能够记录，能，才继续这里的逻辑
|
|
|
debug!("expensive debug data: {} {}", data.x, data.y);
|
|
|
}
|
|
|
if log_enabled!(target: "Global", Debug) {
|
|
|
let data = expensive_call();
|
|
|
debug!(target: "Global", "expensive debug data: {} {}", data.x, data.y);
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>而 <code>log!</code> 宏就简单的多，它是一个通用的日志记录方式，因此需要我们手动指定日志级别：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use log::{log, Level};
|
|
|
|
|
|
let data = (42, "Forty-two");
|
|
|
let private_data = "private";
|
|
|
|
|
|
log!(Level::Error, "Received errors: {}, {}", data.0, data.1);
|
|
|
log!(target: "app_events", Level::Warn, "App warning: {}, {}, {}",
|
|
|
data.0, data.1, private_data);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h2 id="日志输出在哪里"><a class="header" href="#日志输出在哪里">日志输出在哪里？</a></h2>
|
|
|
<p>我不知道有没有同学尝试运行过上面的代码，但是我知道，就算你们运行了，也看不到任何输出。</p>
|
|
|
<p>为什么？原因很简单，<code>log</code> 仅仅是日志门面库，<strong>它并不具备完整的日志库功能！</strong>，因此你无法在控制台中看到任何日志输出，这种情况下，说实话，远不如一个 <code>println!</code> 有用！</p>
|
|
|
<p>但是别急，让我们看看该如何让 <code>log</code> 有用起来。</p>
|
|
|
<h2 id="使用具体的日志库"><a class="header" href="#使用具体的日志库">使用具体的日志库</a></h2>
|
|
|
<p><code>log</code> 包这么设计，其实是有很多好处的。</p>
|
|
|
<h3 id="rust-库的开发者"><a class="header" href="#rust-库的开发者">Rust 库的开发者</a></h3>
|
|
|
<p>最直接的好处就是，如果你是一个 Rust 库开发者，那你自己或库的用户肯定都不希望这个库绑定任何具体的日志库，否则用户想使用 <code>log1</code> 来记录日志，你的库却使用了 <code>log2</code>，这就存在很多问题了！</p>
|
|
|
<p>因此，<strong>作为库的开发者，你只要在库中使用门面库即可</strong>，将具体的日志库交给用户去选择和绑定。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use log::{info, trace, warn};
|
|
|
pub fn deal_with_something() {
|
|
|
// 开始处理
|
|
|
|
|
|
// 记录一些日志
|
|
|
trace!("a trace log");
|
|
|
info!("a info long: {}", "abc");
|
|
|
warn!("a warning log: {}, retrying", err);
|
|
|
|
|
|
// 结束处理
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h3 id="应用开发者"><a class="header" href="#应用开发者">应用开发者</a></h3>
|
|
|
<p>如果是应用开发者，那你的应用运行起来，却看不到任何日志输出，这种场景想想都捉急。此时就需要去选择一个具体的日志库了。</p>
|
|
|
<p>目前来说，已经有了不少日志库实现，官方也<a href="https://github.com/rust-lang/log#in-executables">推荐了一些</a>
|
|
|
，大家可以根据自己的需求来选择，不过 <a href="https://docs.rs/env_logger/*/env_logger/">env_logger</a> 是一个相当不错的选择。</p>
|
|
|
<p><code>log</code> 还提供了 <a href="https://docs.rs/log/0.4.8/log/fn.set_logger.html">set_logger</a> 函数用于设置日志库，<a href="https://docs.rs/log/0.4.8/log/fn.set_max_level.html">set_max_level</a> 用于设置最大日志级别，但是如果你选了具体的日志库，它往往会提供更高级的 API，无需我们手动调用这两个函数，例如下面的 <code>env_logger</code> 就是如此。</p>
|
|
|
<h4 id="env_logger"><a class="header" href="#env_logger">env_logger</a></h4>
|
|
|
<p>修改 <code>Cargo.toml</code> , 添加以下内容:</p>
|
|
|
<pre><code class="language-toml"># in Cargo.toml
|
|
|
|
|
|
[dependencies]
|
|
|
log = "0.4.0"
|
|
|
env_logger = "0.9"
|
|
|
</code></pre>
|
|
|
<p>在 <code>src/main.rs</code> 中添加如下代码：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use log::{debug, error, log_enabled, info, Level};
|
|
|
|
|
|
fn main() {
|
|
|
// 注意，env_logger 必须尽可能早的初始化
|
|
|
env_logger::init();
|
|
|
|
|
|
debug!("this is a debug {}", "message");
|
|
|
error!("this is printed by default");
|
|
|
|
|
|
if log_enabled!(Level::Info) {
|
|
|
let x = 3 * 4; // expensive computation
|
|
|
info!("the answer was: {}", x);
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>在运行程序时，可以通过环境变量来设定日志级别:</p>
|
|
|
<pre><code class="language-shell">$ RUST_LOG=error ./main
|
|
|
[2017-11-09T02:12:24Z ERROR main] this is printed by default
|
|
|
</code></pre>
|
|
|
<p>我们还可以为单独一个模块指定日志级别:</p>
|
|
|
<pre><code class="language-shell">$ RUST_LOG=main=info ./main
|
|
|
[2017-11-09T02:12:24Z ERROR main] this is printed by default
|
|
|
[2017-11-09T02:12:24Z INFO main] the answer was: 12
|
|
|
</code></pre>
|
|
|
<p>还能为某个模块开启所有日志级别：</p>
|
|
|
<pre><code class="language-shell">$ RUST_LOG=main ./main
|
|
|
[2017-11-09T02:12:24Z DEBUG main] this is a debug message
|
|
|
[2017-11-09T02:12:24Z ERROR main] this is printed by default
|
|
|
[2017-11-09T02:12:24Z INFO main] the answer was: 12
|
|
|
</code></pre>
|
|
|
<p>需要注意的是，如果文件名包含 <code>-</code>，你需要将其替换成下划线来使用，原因是 Rust 的模块和包名不支持使用 <code>-</code>。</p>
|
|
|
<pre><code class="language-shell">$ RUST_LOG=my_app ./my-app
|
|
|
[2017-11-09T02:12:24Z DEBUG my_app] this is a debug message
|
|
|
[2017-11-09T02:12:24Z ERROR my_app] this is printed by default
|
|
|
[2017-11-09T02:12:24Z INFO my_app] the answer was: 12
|
|
|
</code></pre>
|
|
|
<p>默认情况下，<code>env_logger</code> 会输出到标准错误 <code>stderr</code>，如果你想要输出到标准输出 <code>stdout</code>，可以使用 <code>Builder</code> 来改变日志对象( target ):</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::env;
|
|
|
use env_logger::{Builder, Target};
|
|
|
|
|
|
let mut builder = Builder::from_default_env();
|
|
|
builder.target(Target::Stdout);
|
|
|
|
|
|
builder.init();
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>默认</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span> if cfg!(debug_assertions) {
|
|
|
eprintln!("debug: {:?} -> {:?}",
|
|
|
record, fields);
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h3 id="日志库开发者"><a class="header" href="#日志库开发者">日志库开发者</a></h3>
|
|
|
<p>对于这类开发者而言，自然要实现自己的 <code>Log</code> 特征咯:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use log::{Record, Level, Metadata};
|
|
|
struct SimpleLogger;
|
|
|
impl log::Log for SimpleLogger {
|
|
|
fn enabled(&self, metadata: &Metadata) -> bool {
|
|
|
metadata.level() <= Level::Info
|
|
|
}
|
|
|
fn log(&self, record: &Record) {
|
|
|
if self.enabled(record.metadata()) {
|
|
|
println!("{} - {}", record.level(), record.args());
|
|
|
}
|
|
|
}
|
|
|
fn flush(&self) {}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>除此之外，我们还需要像 <code>env_logger</code> 一样包装下 <code>set_logger</code> 和 <code>set_max_level</code>:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use log::{SetLoggerError, LevelFilter};
|
|
|
static LOGGER: SimpleLogger = SimpleLogger;
|
|
|
pub fn init() -> Result<(), SetLoggerError> {
|
|
|
log::set_logger(&LOGGER)
|
|
|
.map(|()| log::set_max_level(LevelFilter::Info))
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h2 id="更多示例"><a class="header" href="#更多示例">更多示例</a></h2>
|
|
|
<p>关于 <code>log</code> 门面库和具体的日志库还有更多的使用方式，详情请参见锈书的<a href="https://rusty.course.rs/devtools/log.html">开发者工具</a>一章。</p>
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