MyBatis源碼解析3-MapperFactoryBean

org.mybatis.spring.mapper.MapperFactoryBean
是我們本篇文章需要研究到核心類。這個類的源碼在mybatis-spring.jar中，不在mybatis.jar中

比如我們數據庫操作到interface爲 GreetMapper

public interface GreetMapper {
	String list();
}

MapperFactoryBean會生成一個GreetMapper到代理對象. 這個對象是實現了GreetMapper接口的。
我們下面來看看MapperFactoryBean.java

package org.mybatis.spring.mapper;
import static org.springframework.util.Assert.notNull;
import org.apache.ibatis.executor.ErrorContext;
import org.apache.ibatis.session.Configuration;
import org.mybatis.spring.SqlSessionTemplate;
import org.mybatis.spring.support.SqlSessionDaoSupport;
import org.springframework.beans.factory.FactoryBean;

/**
 * BeanFactory that enables injection of MyBatis mapper interfaces. It can be set up with a SqlSessionFactory or a
 * pre-configured SqlSessionTemplate.
 * <p>
 * Sample configuration:
 *
 * <pre class="code">
 * {@code
 *   <bean id="baseMapper" class="org.mybatis.spring.mapper.MapperFactoryBean" abstract="true" lazy-init="true">
 *     <property name="sqlSessionFactory" ref="sqlSessionFactory" />
 *   </bean>
 *
 *   <bean id="oneMapper" parent="baseMapper">
 *     <property name="mapperInterface" value="my.package.MyMapperInterface" />
 *   </bean>
 *
 *   <bean id="anotherMapper" parent="baseMapper">
 *     <property name="mapperInterface" value="my.package.MyAnotherMapperInterface" />
 *   </bean>
 * }
 * </pre>
 * <p>
 * Note that this factory can only inject <em>interfaces</em>, not concrete classes.
 *
 * @author Eduardo Macarron
 *
 * @see SqlSessionTemplate
 */
public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  private Class<T> mapperInterface;

  private boolean addToConfig = true;

  public MapperFactoryBean() {
    // intentionally empty
  }

  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  protected void checkDaoConfig() {
    super.checkDaoConfig();

    notNull(this.mapperInterface, "Property 'mapperInterface' is required");

    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        configuration.addMapper(this.mapperInterface);
      } catch (Exception e) {
        logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
        throw new IllegalArgumentException(e);
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public Class<T> getObjectType() {
    return this.mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public boolean isSingleton() {
    return true;
  }

  // ------------- mutators --------------

  /**
   * Sets the mapper interface of the MyBatis mapper
   *
   * @param mapperInterface
   *          class of the interface
   */
  public void setMapperInterface(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * Return the mapper interface of the MyBatis mapper
   *
   * @return class of the interface
   */
  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  /**
   * If addToConfig is false the mapper will not be added to MyBatis. This means it must have been included in
   * mybatis-config.xml.
   * <p>
   * If it is true, the mapper will be added to MyBatis in the case it is not already registered.
   * <p>
   * By default addToConfig is true.
   *
   * @param addToConfig
   *          a flag that whether add mapper to MyBatis or not
   */
  public void setAddToConfig(boolean addToConfig) {
    this.addToConfig = addToConfig;
  }

  /**
   * Return the flag for addition into MyBatis config.
   *
   * @return true if the mapper will be added to MyBatis in the case it is not already registered.
   */
  public boolean isAddToConfig() {
    return addToConfig;
  }
}

主要看getObject() 和isSingleton()
isSingleton()返回true說明這個getObject只會被調用一次。
getObject()

  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

查看getMapper() 選擇實現類是SqlSessionTemplate

SqlSessionTemplate#getMapper()

  @Override
  public <T> T getMapper(Class<T> type) {
    return getConfiguration().getMapper(type, this);
  }

繼續查看getMapper的定義 Configuration.java

    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        return this.mapperRegistry.getMapper(type, sqlSession);
    }

這時候可以定位到MapperRegistry.java中到getMapper(type, sqlSession)的定義
而裏面又會調用mapperProxyFactory.newInstance(sqlSession);
org.apache.ibatis.binding.MapperProxyFactory#newInstance()

//Step 1
    public T newInstance(SqlSession sqlSession) {
        MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        return this.newInstance(mapperProxy);
    }

//Step 2
protected T newInstance(MapperProxy<T> mapperProxy) {
        return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
    }

Proxy.newProxyInstance() 說明開始創建代理對象。 注意這裏涉及到InvocationHandler接口的實現類MapperProxy
這個代理對象最終成爲MapperFactoryBean.getBean()到返回值，並且被單例形式註冊到Spring容器。

當我們使用

@Autowired
private GreetMapper greetMapper;

public void greet() {
	List list = greetMapper.list();
}

greetMapper.list()執行的時候。 就會執行InvocationHandler接口的實現類MapperProxy裏到invoke方法。 整個JDK動態代理過程就完成了。

在invoke方法裏，會根據事先解析好到sql語句進行數據庫操作,並返回數據庫操作結果.

連載列表
MyBatis源碼解析1-全局熟悉源碼

MyBatis源碼解析2-MapperRegistry

MyBatis源碼解析3-MapperFactoryBean