Mybatis源码分析之--配置加载

本文对mybatis的配置不做讲解，只从源码层面来分析mybatis的加载过程。

我们在使用mybatis时，一般流程如下，实例化一个sqlSessionFactory，然后通过sqlSessionFactory来获取一个sqlSession。

InputStream inputStream=null;
            try{
                
                inputStream= Resources.getResourceAsStream("mybatis-config.xml");  //配置文件
                sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);

                SqlSession sqlSession=sqlSessionFactory.openSession()

}catch(Exception e){ e.printStackTrace(); }

先看下sqlSessionFactory的build过程，首先将mybatis-config.xml作为一个输入流传入给build()方法，build的代码如下：

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
        SqlSessionFactory var5;
        try {
            XMLConfigBuilder e = new XMLConfigBuilder(inputStream, environment, properties); //将配置文件加载为XMLConfig
            var5 = this.build(e.parse()); //构建SqlSessionFactory
        } catch (Exception var14) {
            throw ExceptionFactory.wrapException("Error building SqlSession.", var14);
        } finally {
            ErrorContext.instance().reset();

            try {
                inputStream.close();
            } catch (IOException var13) {
                ;
            }

        }

        return var5;
    }

//XMConfigBuilder中使用的XPathParser，XPathParser中使用的createDocument方法来将DOM加载到XPathParser中
private Document createDocument(InputSource inputSource) {
        try {
            DocumentBuilderFactory e = DocumentBuilderFactory.newInstance();
            e.setValidating(this.validation);
            e.setNamespaceAware(false);
            e.setIgnoringComments(true);
            e.setIgnoringElementContentWhitespace(false);
            e.setCoalescing(false);
            e.setExpandEntityReferences(true);
            DocumentBuilder builder = e.newDocumentBuilder();
            builder.setEntityResolver(this.entityResolver);
            builder.setErrorHandler(new ErrorHandler() {
                public void error(SAXParseException exception) throws SAXException {
                    throw exception;
                }

                public void fatalError(SAXParseException exception) throws SAXException {
                    throw exception;
                }

                public void warning(SAXParseException exception) throws SAXException {
                }
            });
            return builder.parse(inputSource);//将inputSource加载成Document，最后存储到XpathParser中
        } catch (Exception var4) {
            throw new BuilderException("Error creating document instance.  Cause: " + var4, var4);
        }
    }

再从最开始的var5 = this.build(e.parse())看，先看一下e.parse()方法：

public Configuration parse() {
        if(this.parsed) {
            throw new BuilderException("Each XMLConfigBuilder can only be used once.");
        } else {
            this.parsed = true;
            this.parseConfiguration(this.parser.evalNode("/configuration"));//解析configuration标签下的内容到Configuration对象中
            return this.configuration;
        }
    }

    private void parseConfiguration(XNode root) {
//分别解析各个标签下内容到Configuration对象中
        try {
            this.propertiesElement(root.evalNode("properties")); //properties标签
            Properties e = this.settingsAsProperties(root.evalNode("settings"));//settings标签
            this.loadCustomVfs(e);
            this.typeAliasesElement(root.evalNode("typeAliases"));
            this.pluginElement(root.evalNode("plugins"));
            this.objectFactoryElement(root.evalNode("objectFactory"));
            this.objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
            this.reflectorFactoryElement(root.evalNode("reflectorFactory"));
            this.settingsElement(e);
            this.environmentsElement(root.evalNode("environments"));//environments标签
            /this.databaseIdProviderElement(root.evalNode("databaseIdProvider"));
            this.typeHandlerElement(root.evalNode("typeHandlers"));
            this.mapperElement(root.evalNode("mappers")); //Mappers对象
        } catch (Exception var3) {
            throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + var3, var3);
        }
    }

这里解析mybatis-config.xml中的所有标签出来，并加载到Configuration对象中，mybatis-config.xml的文件如下：

<configuration>

    <properties resource="jdbc.properties"/>   <!-- 属性文件，也可以直接在本文件中写properties属性 -->

    <settings>
        <setting name="cacheEnabled" value="true"/>
    </settings>

    <typeAliases>
       <package name="com.mymvc.bean"></package>
              <package name="com.java1234.model"></package>
       
    </typeAliases>
    <environments default="development">  <!-- 默认开发环境 -->
        <environment >  <!-- 开发环境 -->
            <transactionManager type="JDBC" />  <!-- 事务管理器，JDBC和MANAGED（tomcat不支持） -->
            <dataSource type="POOLED">  <!-- 有UNPOOLED，POOLED，JNDI三种 -->
                <property name="driver" value="${jdbc.driverClassName}" />
                <property name="url" value="${jdbc.url}" />
                <property name="username" value="${jdbc.username}" />
                <property name="password" value="${jdbc.password}" />
            </dataSource>
        </environment>
        
        <environment >  <!-- 测试环境 -->
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driverClassName}" />
                <property name="url" value="${jdbc.url}" />
                <property name="username" value="${jdbc.username}" />
                <property name="password" value="${jdbc.password}" />
            </dataSource>
        </environment>
    </environments>
    
    <mappers>
      <package name="com.java1234.mappers" />
    </mappers>

    </configuration>

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