承接上一篇关于spring 5.x的日志体系，本篇看看Mybatis的日志体系及实现，Mybatis版本基于3.x。
关于mybatis的官方文档比较友好，分门别类，各有论述，如mybatis官方文档详见https://mybatis.org/mybatis-3/#，mybatis与spring的官方文档详见http://mybatis.org/spring/index.html，与springboot相关的参见http://mybatis.org/spring-boot-starter/mybatis-spring-boot-autoconfigure/#，官方文档写的很完备，也适合初学者。
Mybatis环境搭建及日志展示
研究mybatis日志，有必要写一个demo，该有的数据库（Mysql）相关的jar包依赖都应该有，pom文件如下。
1 <dependency> 2 <groupId>org.mybatis</groupId> 3 <artifactId>mybatis</artifactId> 4 <version>3.4.6</version> 5 </dependency> 6 7 <dependency> 8 <groupId>org.mybatis</groupId> 9 <artifactId>mybatis-spring</artifactId> 10 <version>1.3.2</version> 11 </dependency> 12 13 <dependency> 14 <groupId>mysql</groupId> 15 <artifactId>mysql-connector-java</artifactId> 16 version>8.0.13</version> 17 </dependency> 18 19 <dependency> 20 <groupId>org.springframework</groupId> 21 <artifactId>spring-jdbc</artifactId> 22 <version>5.1.3.RELEASE</version> 23 </dependency> 24 25 <dependency> 26 <groupId>org.springframework</groupId> 27 <artifactId>spring-context</artifactId> 28 <version>5.1.3.RELEASE</version> 29 </dependency>
配置mybatis环境：java config风格获取一个SqlSessionFactoryBean和动态动态注入一个数据源dataSource，连接本地的数据库company，表为user。
1 @Configuration 2 @ComponentScan("com.mystyle") 3 @MapperScan("com.mystyle.dao") 4 public class MybatisConfig { 5 @Bean 6 @Autowired 7 public SqlSessionFactoryBean sqlSessionFactory(DataSource dataSource) throws Exception { 8 SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean(); 9 sqlSessionFactoryBean.setDataSource(dataSource); 10 // org.apache.ibatis.session.Configuration configuration = new org.apache.ibatis.session.Configuration(); 11 // configuration.setLogImpl(Log4jImpl.class); 12 // sqlSessionFactoryBean.setConfiguration(configuration); 13 return sqlSessionFactoryBean; 14 } 15 16 @Bean 17 public DataSource dataSource() { 18 DriverManagerDataSource driverManagerDataSource = new DriverManagerDataSource(); 19 driverManagerDataSource.setUsername("root"); 20 driverManagerDataSource.setPassword("12345678"); 21 driverManagerDataSource.setUrl("jdbc:mysql://localhost:3306/company?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone=UTC"); 22 driverManagerDataSource.setDriverClassName("com.mysql.cj.jdbc.Driver"); 23 return driverManagerDataSource; 24 25 } 26 27 }
注：注释的代码请先忽略，后续会用到。
写一个UserDao，查询出user表中的user信息
public interface UserDao { @Select("select * from user") List<Map<String,String>> queryUsers(); }
运行完成之后，可以看到console成功打印出了user的信息，但是除此之外，并未看到我们想要的打印出sql语句等信息。
Mybatis日志代码跟踪
在MybatisConfig的sqlSessionFactory方法中，new出了一个SqlSessionFactoryBean，在SqlSessionFactoryBean类中可以看到一个关于Logger的私有对象的声明并初始化。
public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> { private static final Log LOGGER = LogFactory.getLog(SqlSessionFactoryBean.class);
在LogFactory类中，
1 public final class LogFactory { 2 3 /** 4 * Marker to be used by logging implementations that support markers 5 */ 6 public static final String MARKER = "MYBATIS"; 7 8 private static Constructor<? extends Log> logConstructor; 9 10 static {
//1.启动线程 11 tryImplementation(new Runnable() { 12 @Override 13 public void run() {
//2。加载日志类，返回构造器 14 useSlf4jLogging(); 15 } 16 }); 17 tryImplementation(new Runnable() { 18 @Override 19 public void run() { 20 useCommonsLogging(); 21 } 22 }); 23 tryImplementation(new Runnable() { 24 @Override 25 public void run() { 26 useLog4J2Logging(); 27 } 28 }); 29 tryImplementation(new Runnable() { 30 @Override 31 public void run() { 32 useLog4JLogging(); 33 } 34 }); 35 tryImplementation(new Runnable() { 36 @Override 37 public void run() { 38 useJdkLogging(); 39 } 40 }); 41 tryImplementation(new Runnable() { 42 @Override 43 public void run() { 44 useNoLogging(); 45 } 46 }); 47 } 48 49 private LogFactory() { 50 // disable construction 51 } 52 53 public static Log getLog(Class<?> aClass) { 54 return getLog(aClass.getName()); 55 } 56 57 public static Log getLog(String logger) { 58 try { 59 return logConstructor.newInstance(logger); 60 } catch (Throwable t) { 61 throw new LogException("Error creating logger for logger " + logger + ". Cause: " + t, t); 62 } 63 } 64 65 public static synchronized void useCustomLogging(Class<? extends Log> clazz) { 66 setImplementation(clazz); 67 } 68 69 public static synchronized void useSlf4jLogging() { 70 setImplementation(org.apache.ibatis.logging.slf4j.Slf4jImpl.class); 71 } 72 73 public static synchronized void useCommonsLogging() { 74 setImplementation(org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl.class); 75 } 76 77 public static synchronized void useLog4JLogging() { 78 setImplementation(org.apache.ibatis.logging.log4j.Log4jImpl.class); 79 } 80 81 public static synchronized void useLog4J2Logging() { 82 setImplementation(org.apache.ibatis.logging.log4j2.Log4j2Impl.class); 83 } 84 85 public static synchronized void useJdkLogging() { 86 setImplementation(org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl.class); 87 } 88 89 public static synchronized void useStdOutLogging() { 90 setImplementation(org.apache.ibatis.logging.stdout.StdOutImpl.class); 91 } 92 93 public static synchronized void useNoLogging() { 94 setImplementation(org.apache.ibatis.logging.nologging.NoLoggingImpl.class); 95 } 96 97 private static void tryImplementation(Runnable runnable) { 98 if (logConstructor == null) { 99 try { 100 runnable.run(); 101 } catch (Throwable t) { 102 // ignore 103 } 104 } 105 } 106 107 private static void setImplementation(Class<? extends Log> implClass) { 108 try { 109 Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
//3. 试图去实例化一个实现类的日志对象 110 Log log = candidate.newInstance(LogFactory.class.getName()); 111 if (log.isDebugEnabled()) { 112 log.debug("Logging initialized using '" + implClass + "' adapter."); 113 } 114 logConstructor = candidate; 115 } catch (Throwable t) { 116 throw new LogException("Error setting Log implementation. Cause: " + t, t); 117 } 118 } 119 120 }
LogFactory在加载时，有一个静态代码块，会依次去加载各种日志产品的实现，请其中的顺序是 SLF4J > Apache Commons Logging > Log4j 2 > Log4j > JDK logging。
以SLF4J为例说明(其余类似)：
- 执行tryImplementation方法：如果构造器logConstructor（LogFactory中的私有变量）不为空，启动一个线程去执行其中的run 方法。
- 具体是去执行useSlf4jLogging方法中的setImplementation方法，该方法会传入SLF4J的日志实现类的class对象：class org.apache.ibatis.logging.slf4j.Slf4jImpl。
- 试图去实例化一个实现类的日志对象，由于此时并未加入slf4j相关jar包依赖，所以会抛一个异常出去。
在默认情况下，即是用户未指定得情况下，class org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl也就是Apache Commons Logging会默认实例化成功，spring默认使用此日志实现，由于存在该实现依赖，所以会被实例化成功，并返回其构造器。SqlSessionFactoryBean中调用的getLog方法实际是返回了一个基于JCL实现的日志对象。
用户指定Mybatis日志实现类
假设用户需要使得Mybatis使用的日志是基于log4j的，同样，首先需要在pom中先添加基于log4j的相关依赖，log4j.properties配置文件添加，在sqlSessionFactoryBean实例化时通过configuration去手动指定logimpl为log4J。
1 public void setLogImpl(Class<? extends Log> logImpl) { 2 if (logImpl != null) { 3 this.logImpl = logImpl; 4 LogFactory.useCustomLogging(this.logImpl); 5 } 6 }
实际内部是去调用了LogFactory的useCustomLogging方法。
1 public static synchronized void useCustomLogging(Class<? extends Log> clazz) { 2 setImplementation(clazz); 3 }
其余和上述流程一样，把factory中的日志实现类更新为用户所指定的实现类。
日志打印效果
1 DEBUG - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter. 2 DEBUG - Creating a new SqlSession 3 DEBUG - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@4716be8b] was not registered for synchronization because synchronization is not active 4 DEBUG - JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@3faf2e7d] will not be managed by Spring 5 DEBUG - ==> Preparing: select * from user 6 DEBUG - ==> Parameters: 7 DEBUG - <== Total: 7 8 DEBUG - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@4716be8b] 9 [{eventId=1, count_num=1, name=tom, id=1}, {eventId=2, count_num=2, name=jack, id=2}, {eventId=7, count_num=2, name=david, id=3}, {eventId=3, count_num=2, name=david, id=4}, {eventId=4, count_num=0, name=david, id=5}, {eventId=5, count_num=1, name=david, id=6}, {eventId=6, count_num=0, name=lily, id=7}] 10 Disconnected from the target VM, address: '127.0.0.1:53155', transport: 'socket'
可以看到日志打印出的格式以及内容都是我们所想要的。