为什么要用SLF4J+Logback 替换commons-logging+log4j？

1. SLF4J是编译时绑定到具体的日志框架，性能优于采用运行时搜寻的方式的commons-logging
2. 不需要使用logger.isDebugEnabled()来解决日志因为字符拼接产生的性能问题

   1. logger.info("my name is {}", "medusar");
      
      logger.info("my name is " + "medusar");

   2. 在效率上，第一行比第二行更高，因为如果当前日志级别是ERROR,第一行不会进行字符串拼接，而第二行，无论日志级别是什么，都会先进行字符串拼接。

   3. 所以为了解决这个问题，commons-logging等框架提供了下面的方式：

      if (log.isDebugEnabled()){
      
              log.debug("dddd"+"eee");
      
          }
      

1. import org.apache.commons.logging.Log;
   
   import org.apache.commons.logging.LogFactory;
   
   public class XXXService {
   
       private static final Log log = LogFactory.getLog(XXXService.class);
   
       public void doSomething(){
   
           log.info("begin dosomething....");
   
       }
   
   }

基于slf4j的日志使用

1. import org.slf4j.Logger;
   
   import org.slf4j.LoggerFactory;
   
   public class XXXService {
   
       private static final Logger logger = LoggerFactory.getLogger(XXXService.class);
   
       public void doSomething() {
   
           logger.info("begin dosomething...");
   
       }
   
   }

1.  <dependency>
   
       <groupId>org.springframework</groupId>
   
       <artifactId>spring-core</artifactId>
   
       <exclusions>
   
           <exclusion>
   
               <groupId>commons-logging</groupId> 
               <artifactId>commons-logging</artifactId>
   
           </exclusion>
   
       </exclusions>
   
   </dependency>

2. 以后开发的时候，日志记录方法的调用，不应该直接调用日志的实现类，而是调用日志抽象层里面的方法；给系统里面导入slf4j的jar和logback的实现jar

3. import org.slf4j.Logger;
   
   import org.slf4j.LoggerFactory;
   
   public class HelloWorld {
   
    public static void main(String[] args) {
   
       Logger logger = LoggerFactory.getLogger(HelloWorld.class);
   
       logger.info("Hello World"); }
   
    }

指定文件中日志输出的格式

1. # 在控制台输出的日志格式 logging.pattern.console=%d{yyyy-MM-dd} [%thread] %-5level %logger{} - %msg%n
   
    # 指定文件中日志输出的格式 logging.pattern.file=%d{yyyy-MM-dd} === [%thread] === %-5level === %logger{} === - %msg%n

2. <!-- 日志输出格式：            
   1. %d表示日期时间，
   2. %thread表示线程名，
   3. %-5level：级别从左显示5个字符宽度
   4. %logger{50} 表示logger名字最长50个字符，否则按照句点分割。
   5. %msg：日志消息，
   6. %n是换行符-->

切换日志框架（无意义，slf4j+logback已经是最佳实现）

1. <dependencies>
   
       <dependency>
   
           <groupId>org.springframework.boot</groupId>
   
           <artifactId>spring-boot-starter-web</artifactId>
   
           <!--排除转换包-->
   
               <exclusions>
   
                   <exclusion>
   
                       <artifactId>logback-classic</artifactId>
   
                       <groupId>ch.qos.logback</groupId>
   
                   </exclusion>
   
                   <exclusion>
   
                       <artifactId>log4j-over-slf4j</artifactId>
   
                       <groupId>org.slf4j</groupId> </exclusion>
   
               </exclusions>
   
           </dependency>
   
           <!--添加slf4j依赖-->
   
           <dependency>
   
               <groupId>org.slf4j</groupId>
   
               <artifactId>slf4j-log4j12</artifactId>
   
           </dependency>
   
               <dependency>
   
                   <groupId>org.springframework.boot</groupId>
   
                   <artifactId>spring-boot-starter-test</artifactId>
   
                   <scope>test</scope>
   
       </dependency>
   
   </dependencies>

1. Log Level: ERROR, WARN, INFO, DEBUG, or TRACE.
2. Logback does not have a FATAL level. It is mapped to ERROR.
3. The default log configuration echoes messages to the console as they are written. By default, ERROR-level, WARN-level, and INFO-level messages are logged. You can also enable a “debug” mode by starting your application with a --debug flag.

4. $ java -jar myapp.jar --debug

5. Alternatively, you can enable a “trace” mode by starting your application with a --trace flag (or trace=true in your application.properties).

6. All the supported logging systems can have the logger levels set in the Spring Environment (for example, in application.properties) by using logging.level.<logger-name>=<level> where level is one of TRACE, DEBUG, INFO, WARN, ERROR, FATAL, or OFF. The root logger can be configured by using logging.level.root.

   The following example shows potential logging settings in application.properties:

7. logging.level.root=WARN
   
   logging.level.org.springframework.web=DEBUG
   
   logging.level.org.hibernate=ERROR

1. By default, Spring Boot logs only to the console and does not write log files. If you want to write log files in addition to the console output, you need to set a logging.file or logging.path property (for example, in your application.properties).
2. Log files rotate when they reach 10 MB and, as with console output, ERROR-level, WARN-level, and INFO-level messages are logged by default. Size limits can be changed using the logging.file.max-size property. Previously rotated files are archived indefinitely unless the logging.file.max-history property has been set.
3. Logging properties are independent of the actual logging infrastructure. As a result, specific configuration keys (such as logback.configurationFile for Logback) are not managed by spring Boot.
4. 

Log Groups

1. It’s often useful to be able to group related loggers together so that they can all be configured at the same time. For example, you might commonly change the logging levels for all Tomcat related loggers, but you can’t easily remember top level packages.

   To help with this, Spring Boot allows you to define logging groups in your Spring Environment. For example, here’s how you could define a “tomcat” group by adding it to your application.properties:

   logging.group.tomcat=org.apache.catalina, org.apache.coyote, org.apache.tomcat

   Once defined, you can change the level for all the loggers in the group with a single line:

   logging.level.tomcat=TRACE

   Spring Boot includes the following pre-defined logging groups that can be used out-of-the-box:

   Name	Loggers
   web	org.springframework.core.codec, org.springframework.http, org.springframework.web
   sql	org.springframework.jdbc.core, org.hibernate.SQL

Custom Log Configuration

1. The various logging systems can be activated by including the appropriate libraries on the classpath and can be further customized by providing a suitable configuration file in the root of the classpath or in a location specified by the following Spring Environment property: logging.config.
2. Since logging is initialized before the ApplicationContext is created, it is not possible to control logging from @PropertySources in Spring @Configuration files. The only way to change the logging system or disable it entirely is via System properties.
3. Depending on your logging system, the following files are loaded:

4. Logging System	Customization
   Logback	logback-spring.xml, logback-spring.groovy, logback.xml, or logback.groovy
   Log4j2	log4j2-spring.xml or log4j2.xml
   JDK (Java Util Logging)	logging.properties

5. When possible, we recommend that you use the -spring variants for your logging configuration (for example, logback-spring.xml rather than logback.xml). If you use standard configuration locations, Spring cannot completely control log initialization.

6. To help with the customization, some other properties are transferred from the Spring Environment to System properties, as described in the following table:

   1. Spring Environment	System Property	Comments
      logging.exception-conversion-word	LOG_EXCEPTION_CONVERSION_WORD	The conversion word used when logging exceptions.
      logging.file	LOG_FILE	If defined, it is used in the default log configuration.
      logging.file.max-size	LOG_FILE_MAX_SIZE	Maximum log file size (if LOG_FILE enabled). (Only supported with the default Logback setup.)
      logging.file.max-history	LOG_FILE_MAX_HISTORY	Maximum number of archive log files to keep (if LOG_FILE enabled). (Only supported with the default Logback setup.)
      logging.path	LOG_PATH	If defined, it is used in the default log configuration.
      logging.pattern.console	CONSOLE_LOG_PATTERN	The log pattern to use on the console (stdout). (Only supported with the default Logback setup.)
      logging.pattern.dateformat	LOG_DATEFORMAT_PATTERN	Appender pattern for log date format. (Only supported with the default Logback setup.)
      logging.pattern.file	FILE_LOG_PATTERN	The log pattern to use in a file (if LOG_FILE is enabled). (Only supported with the default Logback setup.)
      logging.pattern.level	LOG_LEVEL_PATTERN	The format to use when rendering the log level (default %5p). (Only supported with the default Logback setup.)
      PID	PID	The current process ID (discovered if possible and when not already defined as an OS environment variable).