Mybatis初始化加载过程(一)

案例

先看下一次完成的Mybatis查询过程示例代码：

String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = session.getMapper(UserMapper.class);  
//调用代理对象方法  
User user = userMapper.findUserById(27);  
//关闭session  
session.close();

本文先重点分析，以下代码背后的原理

1 2	SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); SqlSession sqlSession = sqlSessionFactory.openSession();

SqlSessionFactory创建

MyBatis初始化基本过程：

SqlSessionFactoryBuilder根据传入的数据流生成Configuration对象，然后根据Configuration对象创建默认的SqlSessionFactory实例。
MyBatis通过加载XML配置文件，将配置文信息组装成内部的Configuration对象,Configuration对象的组织结构和XML配置文件的组织结构几乎完全。

由此SqlSessionFactory的创建，有以下几个步骤：

调用SqlSessionFactoryBuilder对象的build(inputStream)方法；

SqlSessionFactoryBuilder会根据输入流inputStream等信息创建XMLConfigBuilder对象;

SqlSessionFactoryBuilder调用XMLConfigBuilder对象的parse()方法获取Configuration对象；

SqlSessionFactoryBuilder根据Configuration对象创建一个DefaultSqlSessionFactory对象；

SqlSessionFactoryBuilder返回 DefaultSqlSessionFactory对象。

SqlSessionFactoryBuilder相关的代码如下所示：

public SqlSessionFactory build(InputStream inputStream)
{
    return build(inputStream, null, null);
}
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties)
{
    try
    {
        //2. 创建XMLConfigBuilder对象用来解析XML配置文件，生成Configuration对象
        XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
        //3. 将XML配置文件内的信息解析成Java对象Configuration对象
        Configuration config = parser.parse();
        //4. 根据Configuration对象创建出SqlSessionFactory对象
        return build(config);
    }
    catch (Exception e)
    {
        throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    }
    finally
    {
        ErrorContext.instance().reset();
        try
        {
            inputStream.close();
        }
        catch (IOException e)
        {
            // Intentionally ignore. Prefer previous error.
        }
    }
}
//从此处可以看出，MyBatis内部通过Configuration对象来创建SqlSessionFactory,用户也可以自己通过API构造好Configuration对象，调用此方法创建SqlSessionFactory
public SqlSessionFactory build(Configuration config)
{
    return new DefaultSqlSessionFactory(config);
}

上述的初始化过程中，涉及到了以下几个对象：

SqlSessionFactoryBuilder ： SqlSessionFactory的构造器，用于创建SqlSessionFactory，采用了Builder设计模式

Configuration ：该对象是mybatis-config.xml文件中所有mybatis配置信息

SqlSessionFactory：SqlSession工厂类，以工厂形式创建SqlSession对象，采用了Factory工厂设计模式

生成Configuration对象过程

XMLConfigBuilder会将XML配置文件的信息转换为Document对象，而XML配置定义文件DTD转换成XMLMapperEntityResolver对象，然后将二者封装到XpathParser对象中，XpathParser的作用是提供根据Xpath表达式获取基本的DOM节点Node信息的操作。如下图所示：

之后XMLConfigBuilder调用parse()方法：会从XPathParser中取出节点对应的Node对象，然后解析此Node节点的子Node：properties, settings, typeAliases,typeHandlers, objectFactory, objectWrapperFactory, plugins, environments,databaseIdProvider, mappers

public Configuration parse()
   {
       if (parsed)
       {
           throw new BuilderException("Each XMLConfigBuilder can only be used once.");
       }
       parsed = true;
       //源码中没有这一句，只有 parseConfiguration(parser.evalNode("/configuration"));
       //为了让读者看得更明晰，源码拆分为以下两句
       XNode configurationNode = parser.evalNode("/configuration");
       parseConfiguration(configurationNode);
       return configuration;
   }
   /*
   解析 "/configuration"节点下的子节点信息，然后将解析的结果设置到Configuration对象中
   */
 private void parseConfiguration(XNode root) {
   try {
     //1.首先处理properties 节点	
     propertiesElement(root.evalNode("properties")); //issue #117 read properties first
     //2.处理typeAliases
     typeAliasesElement(root.evalNode("typeAliases"));
     //3.处理插件
     pluginElement(root.evalNode("plugins"));
     //4.处理objectFactory
     objectFactoryElement(root.evalNode("objectFactory"));
     //5.objectWrapperFactory
     objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
     //6.settings
     settingsElement(root.evalNode("settings"));
     //7.处理environments
     environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
     //8.database
     databaseIdProviderElement(root.evalNode("databaseIdProvider"));
     //9. typeHandlers
     typeHandlerElement(root.evalNode("typeHandlers"));
     //10 mappers
     mapperElement(root.evalNode("mappers"));
   } catch (Exception e) {
     throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
   }
 }

在上述代码中，还有一个非常重要的地方，就是解析XML配置文件子节点的方法mapperElements(root.evalNode(“mappers”)), 它将解析我们配置的Mapper.xml配置文件。

然后将这些值解析出来设置到Configuration对象中。

创建SqlSession对象

在DefaultSqlSessionFactory中openSessionFromDataSource()方法返回DefaultSqlSession对象。

/**
   * 通常一系列openSession方法最终都会调用本方法
   * @param execType 
   * @param level
   * @param autoCommit
   * @return
   */
  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      //通过Confuguration对象去获取Mybatis相关配置信息, Environment对象包含了数据源和事务的配置
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //之前说了，从表面上来看，咱们是用sqlSession在执行sql语句， 实际呢，其实是通过excutor执行， excutor是对于Statement的封装
      final Executor executor = configuration.newExecutor(tx, execType);
      //关键看这儿，创建了一个DefaultSqlSession对象
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

设计模式应用

在创建SqlSessionFactory和Environment对象时用到了builder设计模式。

由于构造时参数不定，可以为其创建一个构造器Builder，将SqlSessionFactory的构建过程和表示分开

数据库连接环境Environment对象的创建

private void environmentsElement(XNode context) throws Exception {
  if (context != null) {
    
    for (XNode child : context.getChildren()) {
      String id = child.getStringAttribute("id");
      
        //使用了Environment内置的构造器Builder，传递id 事务工厂和数据源
        Environment.Builder environmentBuilder = new Environment.Builder(id)
            .transactionFactory(txFactory)
            .dataSource(dataSource);
        configuration.setEnvironment(environmentBuilder.build());
      }
    }
}

public final class Environment {
  private final String id;
  private final TransactionFactory transactionFactory;
  private final DataSource dataSource;
 
  public Environment(String id, TransactionFactory transactionFactory, DataSource dataSource) {
    if (id == null) {
      throw new IllegalArgumentException("Parameter 'id' must not be null");
    }
    if (transactionFactory == null) {
        throw new IllegalArgumentException("Parameter 'transactionFactory' must not be null");
    }
    this.id = id;
    if (dataSource == null) {
      throw new IllegalArgumentException("Parameter 'dataSource' must not be null");
    }
    this.transactionFactory = transactionFactory;
    this.dataSource = dataSource;
  }
 
  public static class Builder {
      private String id;
      private TransactionFactory transactionFactory;
      private DataSource dataSource;
 
    public Builder(String id) {
      this.id = id;
    }
 
    public Builder transactionFactory(TransactionFactory transactionFactory) {
      this.transactionFactory = transactionFactory;
      return this;
    }
 
    public Builder dataSource(DataSource dataSource) {
      this.dataSource = dataSource;
      return this;
    }
 
    public String id() {
      return this.id;
    }
 
    public Environment build() {
      return new Environment(this.id, this.transactionFactory, this.dataSource);
    }
 
  }
 
  public String getId() {
    return this.id;
  }
 
  public TransactionFactory getTransactionFactory() {
    return this.transactionFactory;
  }
 
  public DataSource getDataSource() {
    return this.dataSource;
  }
 
}

在下文我们将分析在获得sqlSession对象之后是怎样获取Mapper对象的，并执行SQL语句的（没有Mapper接口实现类的情况）。

案例

SqlSessionFactory创建

生成Configuration对象过程

创建SqlSession对象

设计模式应用

参考资料