Mybatis原始碼閱讀之一
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。
使用示例如下圖1，原始碼地址：
圖1
我們來看MapperScannerConfigurer的來實現，它的類繼承圖如下圖2
圖2
MapperScannerConfigurer實現了BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法，如下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設定屬性，再來看ClassPathMapperScanner，如下圖3所示:
圖3
List-1中呼叫的scan方法在ClassPathBeanDefinitionScanner中，如下List-2，doScan方法原本在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，先呼叫父方法的doScan方法獲取BeanDefinition，之後呼叫processBeanDefinitions方法。有沒有想過一個問題，我們定義的Mapper 類是interface，是不能被例項化的，它是怎麼被注入Spring中讓我們使用的？List-3的processBeanDefinitions方法中設定BeanClass為MapperFactoryBean，答案就在這個MapperFactoryBean中。
注意List-3的末尾處有個"definition.setAutowireMode(2);"，這是設定注入的型別，哪2是哪種型別呢，來看AbstractBeanDefinition，如下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;
......我們繼續看MapperFactoryBean，
圖4
如圖4所示，看到FactoryBean，如果瞭解Spring IOC，那個應該意識到了。在Spring IOC中如果遇到FactoryBean，會呼叫它的getObject方法，將其結果作為值注入到IOC，FactoryBean的getObjectType則是返會型別。如下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，呼叫父類SqlSessionDaoSupport的方法getSqlSession()，來看下SqlSessionDaoSupport，如下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會呼叫List-6中的方法setSqlSessionFactory方法，為什麼這麼說，List-3中"definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));"，這樣設定了之後Spring會呼叫setSqlSessionFactory方法。這樣List-6中，就引用了我們在圖1中定義的sqlSessionFactory，之後構造出SqlSessionTemplate。
接下來就是分析SqlSessionTemplate。
到目前為止，我們知道了Mybatis是如何融入到Spring容器中的。不過還是遺留了很多問題，比如最重要的，List-5中SqlSessionTemplate的.getMapper(this.mapperInterface)方法底層上