settings标签也是一个很重要的标签，虽然我们在使用的时候，没怎么配置settings标签里面的内容。好像一开始为了看sql语句，我们在settings标签里面配置了日志。
<settings>
<setting name="logImpl" value="SLF4J"/>
</settings>其他的好像就没干什么了。其实settings标签里面的配置，还是跟mybatis的全局对象Configuration有关。直接看下解析这个标签的代码
XMLConfigBuilder:
private Properties settingsAsProperties(XNode context) {
if (context == null) {
return new Properties();
}
//将settings标签里面的子标签解析出来，并且放到一个Properties对象中
Properties props = context.getChildrenAsProperties();
// Check that all settings are known to the configuration class
//localReflectorFactory->DefaultReflectorFactory
MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
//这个for循环就是在判断settings标签内部的元素是不是在Configuration对象中，就看Configuration是否有set方法
for (Object key : props.keySet()) {
//如果settings标签里面配置的key在Configuration中找不到对应的方法，那么报错
if (!metaConfig.hasSetter(String.valueOf(key))) {
throw new BuilderException("The setting " + key + " is not known. Make sure you spelled it correctly (case sensitive).");
}
}
return props;
}这段代码也不难，就是将settings里面配置的标签转化为Prop对象。然后循环遍历key，如果key作为Configuration对象里面成员变量，如果这个成员变量没有对应的set方法，则报错。所以这里我们推断，settings里面配置的key和value，最终通过Configuration里面的set方法设置给对应的成员变量。比如上面我们配置的日志，就设置给了Configuration里面的日志对象。
Configuration里面的成员变量非常多，对应的set方法也非常多，所以settings里面的配置也非常多。那么接下来我们继续看看他把settings标签解析完后，获取Prop对象，又干了什么，看这个方法。
XMLConfigBuilder#
private void settingsElement(Properties props) {
//configuration就是全局配置对象，看看有这么多成员变量需要set的，如果你没有在settings里面配，还给了默认值。
configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));
configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));
configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));
configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));
configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));
configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));
configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));
//设置mybatis默认的执行器，不配就是 SIMPLE
configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));
configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));
configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));
configuration.setDefaultResultSetType(resolveResultSetType(props.getProperty("defaultResultSetType")));
configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));
configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));
configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));
configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));
configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));
configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));
configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));
configuration.setDefaultEnumTypeHandler(resolveClass(props.getProperty("defaultEnumTypeHandler")));
configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));
configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));
configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));
configuration.setLogPrefix(props.getProperty("logPrefix"));
configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
configuration.setShrinkWhitespacesInSql(booleanValueOf(props.getProperty("shrinkWhitespacesInSql"), false));
configuration.setDefaultSqlProviderType(resolveClass(props.getProperty("defaultSqlProviderType")));
}是不是挺吓人的。。。。至于Configuration里面那么多对象干嘛的，这是后面要研究的事情，而且这么多，我们肯定得挑几个重点的看看。
好的，本节告一段落。