查看“SLF4J 入门”的源代码

https://www.bilibili.com/video/BV1iJ411H74S?p=22

简单日志门面（Simple Logging Facade For Java）SLF4J 主要是为了给 Java 日志访问提供一套标准、规范的 API 框架，其主要意义在于提供接口，具体的实现可以交由其他日志框架，

例如 log4j 和 logback 等。当然 slf4j 自己也提供了功能较为简单的实现，但是一般很少用到。对于一般的 Java 项目而言，日志框架会选择 '''<big>slf4j-api</big>''' 作为门面，配上具体的实现框架（log4j、logback等），

中间使用桥接器完成桥接。

SLF4J 是目前市面上最流行的日志门面。现在的项目中，基本上都是使用 SLF4J 作为我们的日志系统。SLF4J 日志门面主要提供两大功能：

# 日志框架的绑定
# 日志框架的桥接

=== 入门示例 ===

==== pom.xml ====
<syntaxhighlight lang="xml">
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>io.github.jihch</groupId>
    <artifactId>slf4j-demo</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <!-- slf4j 日志门面 -->
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.27</version>
        </dependency>

        <!-- slf4j 内置的简单实现 -->
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-simple</artifactId>
            <version>1.7.21</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
    </dependencies>

</project>
</syntaxhighlight>

==== SLF4JTest.java ====
<syntaxhighlight lang="java">
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SLF4JTest {

    public static final Logger LOGGER = LoggerFactory.getLogger(SLF4JTest.class);

    @Test
    public void test() {
        LOGGER.error("error");
        LOGGER.warn("warn");
        LOGGER.info("info");
        LOGGER.debug("debug");
        LOGGER.trace("trace");
    }

}
</syntaxhighlight><syntaxhighlight lang="console">
[main] ERROR io.github.jihch.SLF4JTest - error
[main] WARN io.github.jihch.SLF4JTest - warn
[main] INFO io.github.jihch.SLF4JTest - info
</syntaxhighlight>需要注意的是区分引入的 Logger 类是 org.slf4j 包下的，不是 log4j 里 org.apache.log4j 包下的；

然后 SLF4J 里没有日志级别 fatal；

从控制台的输出能看出来 SLF4J 中默认日志级别也是 INFO




=== 占位符示例 ===

==== SLF4JTest.java ====
<syntaxhighlight lang="java">
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SLF4JTest {

    public static final Logger LOGGER = LoggerFactory.getLogger(SLF4JTest.class);

    @Test
    public void test() {
        // 日志输出
        LOGGER.error("error");
        LOGGER.warn("warn");
        LOGGER.info("info");
        LOGGER.debug("debug");
        LOGGER.trace("trace");

        // 使用占位符输出日志信息
        String name = "jihch";
        Integer age = 14;
        LOGGER.info("用户：{}，{}", name, age);
    }

}
</syntaxhighlight><syntaxhighlight lang="console">
[main] ERROR io.github.jihch.SLF4JTest - error
[main] WARN io.github.jihch.SLF4JTest - warn
[main] INFO io.github.jihch.SLF4JTest - info
[main] INFO io.github.jihch.SLF4JTest - 用户：jihch，14
</syntaxhighlight>







=== 将系统异常信息输出 ===

==== SLF4JTest.java ====
<syntaxhighlight lang="java">
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SLF4JTest {

    public static final Logger LOGGER = LoggerFactory.getLogger(SLF4JTest.class);

    @Test
    public void test() {
        // 日志输出
        LOGGER.error("error");
        LOGGER.warn("warn");
        LOGGER.info("info");
        LOGGER.debug("debug");
        LOGGER.trace("trace");

        // 使用占位符输出日志信息
        String name = "jihch";
        Integer age = 14;
        LOGGER.info("用户：{}，{}", name, age);

        // 将系统的异常信息输出
        try {
            int i = 1/0;
        } catch (Exception e) {
//            e.printStackTrace();
            LOGGER.error("出现异常：", e);
        }

    }

}
</syntaxhighlight><syntaxhighlight lang="console">
[main] ERROR io.github.jihch.SLF4JTest - error
[main] WARN io.github.jihch.SLF4JTest - warn
[main] INFO io.github.jihch.SLF4JTest - info
[main] INFO io.github.jihch.SLF4JTest - 用户：jihch，14
[main] ERROR io.github.jihch.SLF4JTest - 出现异常：
java.lang.ArithmeticException: / by zero
	at io.github.jihch.SLF4JTest.test(SLF4JTest.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
	at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)

Process finished with exit code 0

</syntaxhighlight>