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一、使用默认配置
当你没有配置任何日志打印设置，看看SpringBoot默认打印方式如何？
在一个服务方法中这样写：
使用的是commons-logging，被整在spring-jcl模块中去了。
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
在类中声明一个log变量。
private static final Log log = LogFactory.getLog(StudentService.class);
服务方法中调用日志打印方法。
log.trace("trace info...");
log.debug("debug info...");
log.info("info info...");
log.warn("warn info...");
log.error("error info...");
log.fatal("fatal info...");
最后在控制台的输出效果：
2019-03-02 08:40:55.708 INFO 4904 --- [nio-8080-exec-1] com.yale.StudentService : info info...
2019-03-02 08:40:55.708 WARN 4904 --- [nio-8080-exec-1] com.yale.StudentService : warn info...
2019-03-02 08:40:55.708 ERROR 4904 --- [nio-8080-exec-1] com.yale.StudentService : error info...
2019-03-02 08:40:55.708 ERROR 4904 --- [nio-8080-exec-1] com.yale.StudentService : fatal info...
可以看出，trace和debug并没有打印。默认级别是info。
二、使用application.yml配置
在你开发的时候想用debug，就需要在application.yml中配置了。
logging:
file: target/app.log
level:
ROOT: WARN
com.yale: DEBUG
设置了三点：
1.日志文件路径：target/app.log；
2.规定了ROOT的级别：WARN；
3.指定包com.yale的级别：DEBUG。
重新启动应用之后，调用服务，
在控制台输出：
2019-03-02 09:07:57.904 DEBUG 4516 --- [nio-8080-exec-1] com.yale.StudentService : debug info...
2019-03-02 09:07:57.905 INFO 4516 --- [nio-8080-exec-1] com.yale.StudentService : info info...
2019-03-02 09:07:57.905 WARN 4516 --- [nio-8080-exec-1] com.yale.StudentService : warn info...
2019-03-02 09:07:57.905 ERROR 4516 --- [nio-8080-exec-1] com.yale.StudentService : error info...
2019-03-02 09:07:57.905 ERROR 4516 --- [nio-8080-exec-1] com.yale.StudentService : fatal info...
很明显，debug信息也打印出来了！
但是打开app.log文件，去发现只有这些：
2019-03-02 09:03:36.099 INFO 4904 --- [restartedMain] com.yale.Application : Starting Application on USER-20160707JH with PID 4904 (D:\idea_workspace\springboot_test\springboot-rest\target\classes started by Administrator in D:\idea_workspace\springboot_test\springboot-rest)
2019-03-02 09:03:36.100 DEBUG 4904 --- [restartedMain] com.yale.Application : Running with Spring Boot v2.0.0.RELEASE, Spring v5.0.4.RELEASE
2019-03-02 09:03:36.100 INFO 4904 --- [restartedMain] com.yale.Application : No active profile set, falling back to default profiles: default
2019-03-02 09:03:37.412 INFO 4904 --- [restartedMain] com.yale.Application : Started Application in 1.423 seconds (JVM running for 1395.999)
然后再调用一次服务，
在控制台输出：
2019-03-02 09:07:57.904 DEBUG 4516 --- [nio-8080-exec-1] com.yale.StudentService : debug info...
2019-03-02 09:07:57.905 INFO 4516 --- [nio-8080-exec-1] com.yale.StudentService : info info...
2019-03-02 09:07:57.905 WARN 4516 --- [nio-8080-exec-1] com.yale.StudentService : warn info...
2019-03-02 09:07:57.905 ERROR 4516 --- [nio-8080-exec-1] com.yale.StudentService : error info...
2019-03-02 09:07:57.905 ERROR 4516 --- [nio-8080-exec-1] com.yale.StudentService : fatal info...
2019-03-02 09:08:50.085 DEBUG 4516 --- [nio-8080-exec-4] com.yale.StudentService : debug info...
2019-03-02 09:08:50.086 INFO 4516 --- [nio-8080-exec-4] com.yale.StudentService : info info...
2019-03-02 09:08:50.086 WARN 4516 --- [nio-8080-exec-4] com.yale.StudentService : warn info...
2019-03-02 09:08:50.086 ERROR 4516 --- [nio-8080-exec-4] com.yale.StudentService : error info...
2019-03-02 09:08:50.086 ERROR 4516 --- [nio-8080-exec-4] com.yale.StudentService : fatal info...
两次打印输出都在。
这个时候，app.log文件神奇的将上一次的也打印出来了：
2019-03-02 09:03:36.099 INFO 4904 --- [restartedMain] com.yale.Application : Starting Application on USER-20160707JH with PID 4904 (D:\idea_workspace\springboot_test\springboot-rest\target\classes started by Administrator in D:\idea_workspace\springboot_test\springboot-rest)
2019-03-02 09:03:36.100 DEBUG 4904 --- [restartedMain] com.yale.Application : Running with Spring Boot v2.0.0.RELEASE, Spring v5.0.4.RELEASE
2019-03-02 09:03:36.100 INFO 4904 --- [restartedMain] com.yale.Application : No active profile set, falling back to default profiles: default
2019-03-02 09:03:37.412 INFO 4904 --- [restartedMain] com.yale.Application : Started Application in 1.423 seconds (JVM running for 1395.999)
2019-03-02 09:03:47.852 INFO 4516 --- [restartedMain] com.yale.Application : Starting Application on USER-20160707JH with PID 4516 (D:\idea_workspace\springboot_test\springboot-rest\target\classes started by Administrator in D:\idea_workspace\springboot_test\springboot-rest)
2019-03-02 09:03:47.854 DEBUG 4516 --- [restartedMain] com.yale.Application : Running with Spring Boot v2.0.0.RELEASE, Spring v5.0.4.RELEASE
2019-03-02 09:03:47.856 INFO 4516 --- [restartedMain] com.yale.Application : No active profile set, falling back to default profiles: default
2019-03-02 09:03:51.816 INFO 4516 --- [restartedMain] com.yale.Application : Started Application in 4.505 seconds (JVM running for 5.027)
2019-03-02 09:07:57.904 DEBUG 4516 --- [http-nio-8080-exec-1] com.yale.StudentService : debug info...
2019-03-02 09:07:57.905 INFO 4516 --- [http-nio-8080-exec-1] com.yale.StudentService : info info...
2019-03-02 09:07:57.905 WARN 4516 --- [http-nio-8080-exec-1] com.yale.StudentService : warn info...
2019-03-02 09:07:57.905 ERROR 4516 --- [http-nio-8080-exec-1] com.yale.StudentService : error info...
2019-03-02 09:07:57.905 ERROR 4516 --- [http-nio-8080-exec-1] com.yale.StudentService : fatal info...
2019-03-02 09:08:50.085 DEBUG 4516 --- [http-nio-8080-exec-4] com.yale.StudentService : debug info...
2019-03-02 09:08:50.086 INFO 4516 --- [http-nio-8080-exec-4] com.yale.StudentService : info info...
2019-03-02 09:08:50.086 WARN 4516 --- [http-nio-8080-exec-4] com.yale.StudentService : warn info...
2019-03-02 09:08:50.086 ERROR 4516 --- [http-nio-8080-exec-4] com.yale.StudentService : error info...
2019-03-02 09:08:50.086 ERROR 4516 --- [http-nio-8080-exec-4] com.yale.StudentService : fatal info...
所以说，第一次使用文件记录日志，第一时间没有打印任何日志的时候，千万不要着急，可以重新调用记录日志的方法，再回过头来看看！