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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>
<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>
<p>使用方式很简单，只要在 <code>Cargo.toml</code> 中引入即可：</p>
<pre><code class="language-toml">[dependencies]
log = &quot;0.4&quot;
</code></pre>
<blockquote>
<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>
</blockquote>
<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>
<h2 id="log-特征"><a class="header" href="#log-特征">Log 特征</a></h2>
<p>例如，它定义了一个 <code>Log</code> 特征：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub trait Log: Sync + Send {
fn enabled(&amp;self, metadata: &amp;Metadata&lt;'_&gt;) -&gt; bool;
fn log(&amp;self, record: &amp;Record&lt;'_&gt;);
fn flush(&amp;self);
}
<span class="boring">}</span></code></pre></pre>
<ul>
<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><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: &amp;mut Yak) {
trace!(&quot;Commencing yak shaving&quot;);
loop {
match find_a_razor() {
Ok(razor) =&gt; {
info!(&quot;Razor located: {}&quot;, razor);
yak.shave(razor);
break;
}
Err(err) =&gt; {
warn!(&quot;Unable to locate a razor: {}, retrying&quot;, err);
}
}
}
}
<span class="boring">}</span></code></pre></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><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!(&quot;expensive debug data: {} {}&quot;, data.x, data.y);
}
if log_enabled!(target: &quot;Global&quot;, Debug) {
let data = expensive_call();
debug!(target: &quot;Global&quot;, &quot;expensive debug data: {} {}&quot;, data.x, data.y);
}
<span class="boring">}</span></code></pre></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><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, &quot;Forty-two&quot;);
let private_data = &quot;private&quot;;
log!(Level::Error, &quot;Received errors: {}, {}&quot;, data.0, data.1);
log!(target: &quot;app_events&quot;, Level::Warn, &quot;App warning: {}, {}, {}&quot;,
data.0, data.1, private_data);
<span class="boring">}</span></code></pre></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><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!(&quot;a trace log&quot;);
info!(&quot;a info long: {}&quot;, &quot;abc&quot;);
warn!(&quot;a warning log: {}, retrying&quot;, err);
// 结束处理
}
<span class="boring">}</span></code></pre></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 = &quot;0.4.0&quot;
env_logger = &quot;0.9&quot;
</code></pre>
<p>在 <code>src/main.rs</code> 中添加如下代码：</p>
<pre><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!(&quot;this is a debug {}&quot;, &quot;message&quot;);
error!(&quot;this is printed by default&quot;);
if log_enabled!(Level::Info) {
let x = 3 * 4; // expensive computation
info!(&quot;the answer was: {}&quot;, x);
}
}</code></pre></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><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></pre>
<p>默认</p>
<pre><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!(&quot;debug: {:?} -&gt; {:?}&quot;,
record, fields);
}
<span class="boring">}</span></code></pre></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><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(&amp;self, metadata: &amp;Metadata) -&gt; bool {
metadata.level() &lt;= Level::Info
}
fn log(&amp;self, record: &amp;Record) {
if self.enabled(record.metadata()) {
println!(&quot;{} - {}&quot;, record.level(), record.args());
}
}
fn flush(&amp;self) {}
}
<span class="boring">}</span></code></pre></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><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() -&gt; Result&lt;(), SetLoggerError&gt; {
log::set_logger(&amp;LOGGER)
.map(|()| log::set_max_level(LevelFilter::Info))
}
<span class="boring">}</span></code></pre></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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