SLF4J

www.slf4j.org/
核心依赖 slf4j-api
日志门面，虽然底层的日志实现变了，但原代码不必改变

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slf4j-core: 日志门面

<dependency>
     <groupId>org.slf4j</groupId>
     <artifactId>slf4j-api</artifactId>
     <version>1.7.32</version>
 </dependency>

2. 三类

unbound
logback 和 simple（包括 nop）在 sl4j 时间线之后，完全遵循 sl4j 的实现
log4j, JUL 在 sl4j 之间就出现，适配桥接技术

2.1 simple

自带的简单日志实现
slf4j-simple.jar

<!-- 加上 slf4j-api 依赖-->
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-simple</artifactId>
    <version>1.7.32</version>
    <scope>test</scope>
</dependency>

2.2 SLF4J bind JUL

JUL 是 Java 原生日志框架，无需导入依赖
需绑定一个适配器 slf4j-jdk14

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-jdk14</artifactId>
    <version>1.7.32</version>
</dependency>

2.3 SLF4J bind Log4j

需要绑定一个适配器 slf4j-log4j12
log4j.jar
log4j.properpties

<!-- 加上 slf4j-api 依赖-->
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-log4j12</artifactId>
    <version>1.7.32</version>
</dependency>
<dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
</dependency>

2.4 SLF4J bind Logback

jar logback-classic, logback-core
pom.xml 因依赖传递性，只需导入 logback-classic

<!-- 加上 slf4j-api 依赖-->
<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-classic</artifactId>
    <version>1.2.3</version>
</dependency>

2.5 SLF4J bind Log4j 2

log4j-slf4j-impl 适配器
slf4j借助log4j-slf4j-impl适配层，连接到log4j-api上，log4j-api再连接到log4j-core上

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-slf4j-impl</artifactId>
    <version>2.17.1</version>
</dependency>
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-api</artifactId>
    <version>2.17.1</version>
</dependency>
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>2.17.1</version>
</dependency>

3. 示例

SLF4J: Class path contains multiple SLF4J bindings. 表明同时出现了多个日志实现，默认是先导入的先实现

import org.slf4j.LoggerFactory;
private static final Logger logger = LoggerFactory.getLogger(Demo.class);
private Logger logger = LoggerFactory.getLogger(Demo.class.getName());
// protected 子类可以直接使用
protected final Logger log = LoggerFactory.getLogger(this.getClass());

5. SLF4J 源码分析

// LoggerFactory#getLogger(java.lang.Class<?>)
Logger logger = getLogger(clazz.getName());
public static Logger getLogger(String name) {
  ILoggerFactory iLoggerFactory = getILoggerFactory();
  return iLoggerFactory.getLogger(name);
}
private final static void performInitialization() {
  bind();
  if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
    versionSanityCheck();
  }
}
// org.slf4j.LoggerFactory#bind
StaticLoggerBinder.getSingleton();