Mybatis源碼閱讀之一

    Spring中使用Mybatis-spring。

    Spring版本4.3.11，Mybatis-spring版本是1.3.2，Mybatis版本是3.4.6。

    使用示例以下圖1，源碼地址：

                                                               圖1

    咱們來看MapperScannerConfigurer的來實現，它的類繼承圖以下圖2

                                                             圖2

    MapperScannerConfigurer實現了BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法，以下List-1所示：

    List-1

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
        this.processPropertyPlaceHolders();
    }

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    scanner.setAddToConfig(this.addToConfig);
    scanner.setAnnotationClass(this.annotationClass);
    scanner.setMarkerInterface(this.markerInterface);
    scanner.setSqlSessionFactory(this.sqlSessionFactory);
    scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
    scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
    scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
    scanner.setResourceLoader(this.applicationContext);
    scanner.setBeanNameGenerator(this.nameGenerator);
    scanner.registerFilters();
    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ",; \t\n"));
}

    List-1中，基本上是給ClassPathMapperScanner設置屬性，再來看ClassPathMapperScanner，以下圖3所示:

                   

                                                                        圖3

    List-1中調用的scan方法在ClassPathBeanDefinitionScanner中，以下List-2，doScan方法本來在ClassPathBeanDefinitionScanner中，不過被ClassPathMapperScanner類覆寫了，

    List-2

public int scan(String... basePackages) {
    int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
    this.doScan(basePackages);
    if (this.includeAnnotationConfig) {
        AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
    }

    return this.registry.getBeanDefinitionCount() - beanCountAtScanStart;
}

    List-3

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
    if (beanDefinitions.isEmpty()) {
        this.logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
    } else {
        this.processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
}

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    Iterator var3 = beanDefinitions.iterator();

    while(var3.hasNext()) {
        BeanDefinitionHolder holder = (BeanDefinitionHolder)var3.next();
        GenericBeanDefinition definition = (GenericBeanDefinition)holder.getBeanDefinition();
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + definition.getBeanClassName() + "' mapperInterface");
        }

        definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName());
        definition.setBeanClass(this.mapperFactoryBean.getClass());
        definition.getPropertyValues().add("addToConfig", this.addToConfig);
        boolean explicitFactoryUsed = false;
        if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
            definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
            explicitFactoryUsed = true;
        } else if (this.sqlSessionFactory != null) {
            definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
            explicitFactoryUsed = true;
        }

        if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
            if (explicitFactoryUsed) {
                this.logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
            }

            definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
            explicitFactoryUsed = true;
        } else if (this.sqlSessionTemplate != null) {
            if (explicitFactoryUsed) {
                this.logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
            }

            definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
            explicitFactoryUsed = true;
        }

        if (!explicitFactoryUsed) {
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
            }

            definition.setAutowireMode(2);
        }
    }

}

    List-3的doScan方法，先調用父方法的doScan方法獲取BeanDefinition，以後調用processBeanDefinitions方法。有沒有想過一個問題，咱們定義的Mapper 類是interface，是不能被實例化的，它是怎麼被注入Spring中讓咱們使用的？List-3的processBeanDefinitions方法中設置BeanClass爲MapperFactoryBean，答案就在這個MapperFactoryBean中。

    注意List-3的末尾處有個"definition.setAutowireMode(2);"，這是設置注入的類型，哪2是哪一種類型呢，來看AbstractBeanDefinition，以下List-4，2是Autowire_by_name。

    List-4

public abstract class AbstractBeanDefinition extends BeanMetadataAttributeAccessor implements BeanDefinition, Cloneable {
    public static final String SCOPE_DEFAULT = "";
    public static final int AUTOWIRE_NO = 0;
    public static final int AUTOWIRE_BY_NAME = 1;
    public static final int AUTOWIRE_BY_TYPE = 2;
    public static final int AUTOWIRE_CONSTRUCTOR = 3;
    /** @deprecated */
    @Deprecated
    public static final int AUTOWIRE_AUTODETECT = 4;
    public static final int DEPENDENCY_CHECK_NONE = 0;
    public static final int DEPENDENCY_CHECK_OBJECTS = 1;
    public static final int DEPENDENCY_CHECK_SIMPLE = 2;
    public static final int DEPENDENCY_CHECK_ALL = 3;
......

    咱們繼續看MapperFactoryBean，

                

                                                                  圖4

    如圖4所示，看到FactoryBean，若是瞭解Spring IOC，那個應該意識到了。在Spring IOC中若是遇到FactoryBean，會調用它的getObject方法，將其結果做爲值注入到IOC，FactoryBean的getObjectType則是返會類型。以下List-5所示：

    List-5

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {
    private Class<T> mapperInterface;
    private boolean addToConfig = true;

    public MapperFactoryBean() {
    }

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

    ......

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

    public Class<T> getObjectType() {
        return this.mapperInterface;
    }

    public boolean isSingleton() {
        return true;
    }
......

    List-5的getObject()中，調用父類SqlSessionDaoSupport的方法getSqlSession()，來看下SqlSessionDaoSupport，以下List-6

    List-6

public abstract class SqlSessionDaoSupport extends DaoSupport {
    private SqlSession sqlSession;
    private boolean externalSqlSession;

    public SqlSessionDaoSupport() {
    }

    public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
        if (!this.externalSqlSession) {
            this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
        }

    }

    public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
        this.sqlSession = sqlSessionTemplate;
        this.externalSqlSession = true;
    }

    public SqlSession getSqlSession() {
        return this.sqlSession;
    }

    protected void checkDaoConfig() {
        Assert.notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
    }
}

    Spring會調用List-6中的方法setSqlSessionFactory方法，爲何這麼說，List-3中"definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));"，這樣設置了以後Spring會調用setSqlSessionFactory方法。這樣List-6中，就引用了咱們在圖1中定義的sqlSessionFactory，以後構造出SqlSessionTemplate。

    接下來就是分析SqlSessionTemplate。

    到目前爲止，咱們知道了Mybatis是如何融入到Spring容器中的。不過仍是遺留了不少問題，好比最重要的，List-5中SqlSessionTemplate的.getMapper(this.mapperInterface)方法底層上是如何實現的。