springboot默认的日志框架是logback，只需要添加logback的配置即可打印；但若是换成了log4j2，则需要添加如下配置；如何将springboot日志换成log4j2，参考springboot换log4j2写日志源码分析本篇文章；使用log4j2打印mybatis日志，只需要下面两步即可；
添加gradle引用如下：
compile("org.mybatis.spring.boot:mybatis-spring-boot-starter:2.1.4")
compile("mysql:mysql-connector-java")
1，springboot日志换成log4j2后，只需要在resources下面添加mybatis-config.xml文件 ，并添加以下配置：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<settings>
<setting name="logImpl" value="LOG4J2"/>
</settings>
</configuration>
2，在application.xml中添加如下配置：
mybatis:
configLocation: classpath:mybatis-config.xml
接下来从mybatis源码分析一下为什么需要在mybatis-config.xml加该配置，其中涉及到的一个很重要的类是org.apache.ibatis.logging.LogFactory，重要逻辑代码如下：
public final class LogFactory {
/**
* Marker to be used by logging implementations that support markers.
*/
public static final String MARKER = "MYBATIS";
private static Constructor<? extends Log> logConstructor;
static {
tryImplementation(LogFactory::useSlf4jLogging);
tryImplementation(LogFactory::useCommonsLogging);
tryImplementation(LogFactory::useLog4J2Logging);
tryImplementation(LogFactory::useLog4JLogging);
tryImplementation(LogFactory::useJdkLogging);
tryImplementation(LogFactory::useNoLogging);
}
private static void tryImplementation(Runnable runnable) {
if (logConstructor == null) {
try {
runnable.run();
} catch (Throwable t) {
// ignore
}
}
}
private LogFactory() {
// disable construction
}
public static Log getLog(Class<?> clazz) {
return getLog(clazz.getName());
}
public static Log getLog(String logger) {
try {
return logConstructor.newInstance(logger);
} catch (Throwable t) {
throw new LogException("Error creating logger for logger " + logger + ". Cause: " + t, t);
}
}
public static synchronized void useCustomLogging(Class<? extends Log> clazz) {
setImplementation(clazz);
}
public static synchronized void useSlf4jLogging() {
setImplementation(org.apache.ibatis.logging.slf4j.Slf4jImpl.class);
}
public static synchronized void useCommonsLogging() {
setImplementation(org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl.class);
}
private static void setImplementation(Class<? extends Log> implClass) {
try {
Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
Log log = candidate.newInstance(LogFactory.class.getName());
if (log.isDebugEnabled()) {
log.debug("Logging initialized using '" + implClass + "' adapter.");
}
logConstructor = candidate;
} catch (Throwable t) {
throw new LogException("Error setting Log implementation. Cause: " + t, t);
}
}
}
如上所示，tryImplementation方法中，会判断当logConstructor == null才会执行，当表态构造中执行完useSlf4jLogging后，静态构造里面其它方法都不会再执行了，logConstructor直接就给赋值org.apache.ibatis.logging.slf4j.Slf4jImpl.class为该类的实例了。所以必须添加配置，才能使logConstructor的值发生改变
当添加完配置时：org.apache.ibatis.session.Configuration中setLogImpl方法会重新设置上面logConstructor的值，源码如下：
public void setLogImpl(Class<? extends Log> logImpl) {
if (logImpl != null) {
this.logImpl = logImpl;
LogFactory.useCustomLogging(this.logImpl);
}
}
那么配置为什么是<setting name="logImpl" value="LOG4J2"/>呢，org.apache.ibatis.builder.xml.XMLConfigBuilder该类中loadCustomLogImpl给出了答案，代码如下：
private void loadCustomLogImpl(Properties props) {
Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
configuration.setLogImpl(logImpl);
}
从上面代码可以看出，配置中name值必须为logImpl，否则读取不到值，而loadCustomLogImpl方法的参数来自该类的parseConfiguration方法，代码如下：
private void parseConfiguration(XNode root) {
try {
// issue #117 read properties first
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
从第5，7行就可以看出来，配置是从settings中读取出来的
至于为什么值是LOG4J2(不区分大小写，小写也可以，上面resolveClass方法会把取到的值转为小写)，从org.apache.ibatis.session.Configuration的构造函数中就可以得出，如下代码所示：
public Configuration() {
typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);
typeAliasRegistry.registerAlias("JNDI", JndiDataSourceFactory.class);
typeAliasRegistry.registerAlias("POOLED", PooledDataSourceFactory.class);
typeAliasRegistry.registerAlias("UNPOOLED", UnpooledDataSourceFactory.class);
typeAliasRegistry.registerAlias("PERPETUAL", PerpetualCache.class);
typeAliasRegistry.registerAlias("FIFO", FifoCache.class);
typeAliasRegistry.registerAlias("LRU", LruCache.class);
typeAliasRegistry.registerAlias("SOFT", SoftCache.class);
typeAliasRegistry.registerAlias("WEAK", WeakCache.class);
typeAliasRegistry.registerAlias("DB_VENDOR", VendorDatabaseIdProvider.class);
typeAliasRegistry.registerAlias("XML", XMLLanguageDriver.class);
typeAliasRegistry.registerAlias("RAW", RawLanguageDriver.class);
typeAliasRegistry.registerAlias("SLF4J", Slf4jImpl.class);
typeAliasRegistry.registerAlias("COMMONS_LOGGING", JakartaCommonsLoggingImpl.class);
typeAliasRegistry.registerAlias("LOG4J", Log4jImpl.class);
typeAliasRegistry.registerAlias("LOG4J2", Log4j2Impl.class);
typeAliasRegistry.registerAlias("JDK_LOGGING", Jdk14LoggingImpl.class);
typeAliasRegistry.registerAlias("STDOUT_LOGGING", StdOutImpl.class);
typeAliasRegistry.registerAlias("NO_LOGGING", NoLoggingImpl.class);
typeAliasRegistry.registerAlias("CGLIB", CglibProxyFactory.class);
typeAliasRegistry.registerAlias("JAVASSIST", JavassistProxyFactory.class);
languageRegistry.setDefaultDriverClass(XMLLanguageDriver.class);
languageRegistry.register(RawLanguageDriver.class);
}
那么为什么要在application.xml中添加配置呢？springboot在启动时，会读取mybatis配置(下面即是配置类)，若没有配置，则会用使用上面源码逻辑中所写的logback打印日志，代码如下所示：
配置中configLocation字段即是在application.xml中配置的mybatis xml config的路径；配置类为：org.mybatis.spring.boot.autoconfigure.MybatisProperties
@ConfigurationProperties(prefix = MybatisProperties.MYBATIS_PREFIX)
public class MybatisProperties {
public static final String MYBATIS_PREFIX = "mybatis";
private static final ResourcePatternResolver resourceResolver = new PathMatchingResourcePatternResolver();
/**
* Location of MyBatis xml config file.
*/
private String configLocation;
/**
* Locations of MyBatis mapper files.
*/
private String[] mapperLocations;
}
总结：mybatis使用log4j2打印日志，需要添加配置mybatis-config.xml，在settings中指定 logImpl = LOG4J2；并且在application.xml中添加配置引用即可
