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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 = "0.4"
< / 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， < / 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 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(& self, metadata: & Metadata< '_> ) -> bool;
fn log(& self, record: & Record< '_> );
fn flush(& self);
}
< span class = "boring" > }< / span > < / code > < / 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 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， ， < 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::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 >
< / main >
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