深入浅出Mybatis系列（二）---配置简介（mybatis源码篇）

上篇文章《深入浅出Mybatis系列（一）---Mybatis源码分析环境准备》， 将Mybatis源码准备好了。本次，将简单介绍一下Mybatis的配置文件：

我们以 SqlSessionFactoryBuilder 去创建 SqlSessionFactory,  那么，我们就先从SqlSessionFactoryBuilder入手， 咱们先看看源码是怎么实现的：

public class SqlSessionFactoryBuilder {

//Reader读取mybatis配置文件，传入构造方法
//除了Reader外，其实还有对应的inputStream作为参数的构造方法，
//这也体现了mybatis配置的灵活性
public SqlSessionFactory build(Reader reader) {
return build(reader, null, null);
}

public SqlSessionFactory build(Reader reader, String environment) {
return build(reader, environment, null);
}

//mybatis配置文件 + properties, 此时mybatis配置文件中可以不配置properties，也能使用${}形式
public SqlSessionFactory build(Reader reader, Properties properties) {
return build(reader, null, properties);
}

//通过XMLConfigBuilder解析mybatis配置，然后创建SqlSessionFactory对象
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
//下面看看这个方法的源码
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}

public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}

}

通过源码，我们可以看到SqlSessionFactoryBuilder 通过XMLConfigBuilder 去解析我们传入的mybatis的配置文件， 下面就接着看看 XMLConfigBuilder 部分源码：

/**
* mybatis 配置文件解析
*/
public class XMLConfigBuilder extends BaseBuilder {
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}

private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
super(new Configuration());
ErrorContext.instance().resource("SQL Mapper Configuration");
this.configuration.setVariables(props);
this.parsed = false;
this.environment = environment;
this.parser = parser;
}

//外部调用此方法对mybatis配置文件进行解析
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//从根节点configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}

//此方法就是解析configuration节点下的子节点
//由此也可看出，我们在configuration下面能配置的节点为以下10个节点
private void parseConfiguration(XNode root) {
try {
propertiesElement(root.evalNode("properties")); //issue #117 read properties first
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
settingsElement(root.evalNode("settings"));
environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
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);
}
}
}

解析完成后

我们copy root 的value来看看解析了哪些内容

<configuration>
<properties resource="org/apache/ibatis/databases/blog/blog-derby.properties"/>
<settings>
<setting name="cacheEnabled" value="true"/>
<setting name="lazyLoadingEnabled" value="false"/>
<setting name="multipleResultSetsEnabled" value="true"/>
<setting name="useColumnLabel" value="true"/>
<setting name="useGeneratedKeys" value="false"/>
<setting name="defaultExecutorType" value="SIMPLE"/>
<setting name="defaultStatementTimeout" value="25"/>
</settings>
<typeAliases>
<typeAlias alias="Author" type="org.apache.ibatis.domain.blog.Author"/>
<typeAlias alias="Blog" type="org.apache.ibatis.domain.blog.Blog"/>
<typeAlias alias="Comment" type="org.apache.ibatis.domain.blog.Comment"/>
<typeAlias alias="Post" type="org.apache.ibatis.domain.blog.Post"/>
<typeAlias alias="Section" type="org.apache.ibatis.domain.blog.Section"/>
<typeAlias alias="Tag" type="org.apache.ibatis.domain.blog.Tag"/>
</typeAliases>
<typeHandlers>
<typeHandler jdbcType="VARCHAR" handler="org.apache.ibatis.builder.CustomStringTypeHandler" javaType="String"/>
</typeHandlers>
<objectFactory type="org.apache.ibatis.builder.ExampleObjectFactory">
<property name="objectFactoryProperty" value="100"/>
</objectFactory>
<plugins>
<plugin interceptor="org.apache.ibatis.builder.ExamplePlugin">
<property name="pluginProperty" value="100"/>
</plugin>
</plugins>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC">
<property name="" value=""/>
</transactionManager>
<dataSource type="UNPOOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="org/apache/ibatis/builder/AuthorMapper.xml"/>
<mapper resource="org/apache/ibatis/builder/BlogMapper.xml"/>
<mapper resource="org/apache/ibatis/builder/CachedAuthorMapper.xml"/>
<mapper resource="org/apache/ibatis/builder/PostMapper.xml"/>
<mapper resource="org/apache/ibatis/builder/NestedBlogMapper.xml"/>
</mappers>
</configuration>

通过以上源码，我们就能看出，在mybatis的配置文件中：

1. configuration节点为根节点。

2. 在configuration节点之下，我们可以配置10个子节点， 分别为：properties、typeAliases、plugins、objectFactory、objectWrapperFactory、settings、environments、databaseIdProvider、typeHandlers、mappers。

本篇文章就先只介绍这些内容，接下来的文章将依次分析解析这个10个节点中比较重要的几个节点的源码，看看在解析这些节点的时候，到底做了些什么。