一、概述
java日志框架较多，其中主流的slf4j和commons-logging是日志接口，log4j、log4j2和logback是真正的日志实现库。
二、具体库单独使用
2.1 log4j
<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>
classpath下配置文件log4j.properties
log4j.rootLogger=INFO,console
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.target=System.out
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} [%p] %c: %m%n
使用：
import org.apache.log4j.Logger;
...
Logger LOGGER = Logger.getLogger(Main.class);
2.2 log4j2
<dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-core</artifactId><version>2.12.1</version>
</dependency>
classpath下log4j2.properties
rootLogger.level= info
rootLogger.appenderRef.stdout.ref= STDOUTappender.console.type= Console
appender.console.name= STDOUT
appender.console.layout.type= PatternLayout
appender.console.layout.pattern= %d{yyyy-MM-dd HH:mm:ss} [%p] %c: %m%n
使用：
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
...
Logger LOGGER = LogManager.getLogger(Main.class);
2.3 logback
<dependency><groupId>ch.qos.logback</groupId><artifactId>logback-classic</artifactId><version>1.2.3</version>
</dependency>
classpath下logback.xml
<?xml version="1.0" encoding="UTF-8"?>
<configuration><appender name="console" class="ch.qos.logback.core.ConsoleAppender"><encoder><pattern>%d{yyyy-MM-dd HH:mm:ss} [%p] %c: %m%n</pattern></encoder></appender><root level="debug"><appender-ref ref="console" /></root>
</configuration>
使用：
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
...
Logger LOGGER = LoggerFactory.getLogger(Main.class);
值得注意的是，logback自身已经实现了slf4j接口
三、具体库实现slf4j标准
3.1 log4j
<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId><version>1.7.29</version>
</dependency>
使用：
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
...
static final Logger LOGGER = LoggerFactory.getLogger(Main.class);
3.2 log4j2
<dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-core</artifactId><version>2.12.1</version>
</dependency>
<dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-slf4j-impl</artifactId><version>2.9.0</version>
</dependency>
使用：
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
...
static final Logger LOGGER = LoggerFactory.getLogger(Main.class);
3.3 logback
logback自身已经实现了slf4j接口
四、多依赖项目的日志统一
项目可能有很多依赖，各个依赖有可能有着各不相同的日志实现方式。比如我们有五个依赖，他们分别是：
- 独立log4j
- 独立log4j2
- slf4j化log4j
- slf4j化log4j2
- slf4j化logback
4.1 各依赖都实现了slf4j，指定具体库
项目project有三个依赖，分别是log4j-test、log4j2-test、logback-test，而且它们都已经实现了slf4j接口。那么此时项目project会发现slf4j有多个binding：
解决办法：比如只用logback来打印，那么需要把log4j和log4j2的桥接给剔除掉
<dependency><groupId>org.example</groupId><artifactId>log4j-test</artifactId><version>1.0-SNAPSHOT</version><exclusions><exclusion><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.example</groupId><artifactId>log4j2-test</artifactId><version>1.0-SNAPSHOT</version><exclusions><exclusion><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-slf4j-impl</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.example</groupId><artifactId>logback-test</artifactId><version>1.0-SNAPSHOT</version></dependency>
4.2 有的依赖实现了slf4j，有的依赖使用的是具体库，指定一种具体库
同样，项目project还是统一使用logback。
解决办法：用log4j-over-slf4j替换log4j，用log4j-to-slf4j替换log4j2，并在项目project中提出log4j和log4j2。
<dependency><groupId>org.example</groupId><artifactId>log4j-test</artifactId><version>1.0-SNAPSHOT</version><exclusions><exclusion><groupId>log4j</groupId><artifactId>log4j</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.slf4j</groupId><artifactId>log4j-over-slf4j</artifactId><version>1.7.29</version></dependency><dependency><groupId>org.example</groupId><artifactId>log4j2-test</artifactId><version>1.0-SNAPSHOT</version><exclusions><exclusion><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-core</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-to-slf4j</artifactId><version>2.12.1</version></dependency><dependency><groupId>org.example</groupId><artifactId>logback-test</artifactId><version>1.0-SNAPSHOT</version></dependency>
