声明解决方案是基于Mybatis源码，进行二次开发实现。
问题领导最近跟我提了一个需求，是有关于实现类Mybatis的@Select、@Insert注解的功能。其是基于interface层面，不存在任何的接口实现类。因而在实现的过程中，首先要解决的是如何动态实现接口的实例化。其次是如何将使接口根据注解实现相应的功能。
我们先来看看Mybatis是如何实现Dao类的扫描的。
MapperScannerConfigurer.java
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
if (this.processPropertyPlaceHolders) {
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, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}ClassPathMapperScanner是Mybatis继承ClassPathBeanDefinitionScanner类而来的。这里对于ClassPathMapperScanner的配置参数来源于我们在使用Mybatis时的配置而来，是不是还记得在使用Mybatis的时候要配置basePackage的参数呢？
接着我们就顺着scanner.scan()方法，进入查看一下里面的实现。
ClassPathBeanDefinitionScanner.java
public int scan(String... basePackages) {
int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
doScan(basePackages);
// Register annotation config processors, if necessary.
if (this.includeAnnotationConfig) {
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}这里关键的代码是doScan(basePackages);，那么我们在进去看一下。可能你会看到的是Spring源码的实现方法，但这里Mybatis也实现了自己的一套，我们看一下Mybatis的实现。
ClassPathMapperScanner.java
public Set doScan(String... basePackages) {
Set beanDefinitions = super.doScan(basePackages);
if (beanDefinitions.isEmpty()) {
logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
} else {
for (BeanDefinitionHolder holder : beanDefinitions) {
GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();
if (logger.isDebugEnabled()) {
logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()
+ "' and '" + definition.getBeanClassName() + "' mapperInterface");
}
// the mapper interface is the original class of the bean
// but, the actual class of the bean is MapperFactoryBean
definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
definition.setBeanClass(MapperFactoryBean.class);
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) {
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) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
explicitFactoryUsed = true;
}
if (!explicitFactoryUsed) {
if (logger.isDebugEnabled()) {
logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
}
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}
return beanDefinitions;
} definition.setBeanClass(MapperFactoryBean.class);这行代码是非常关键的一句，由于在Spring中存在两种自动实例化的方式，一种是我们常用的本身的接口实例化类进行接口实例化，还有一种就是这里的自定义实例化。而这里的setBeanClass方法就是在BeanDefinitionHolder中进行配置。在Spring进行实例化的时候进行处理。
那么我们在看一下MapperFactoryBean.class
MapperFactoryBean.java
public class MapperFactoryBean extends SqlSessionDaoSupport implements FactoryBean {
private Class mapperInterface;
private boolean addToConfig = true;
/**
* Sets the mapper interface of the MyBatis mapper
*
* @param mapperInterface class of the interface
*/
public void setMapperInterface(Class mapperInterface) {
this.mapperInterface = mapperInterface;
}
/**
* If addToConfig is false the mapper will not be added to MyBatis. This means
* it must have been included in mybatis-config.xml.
*
* If it is true, the mapper will be added to MyBatis in the case it is not already
* registered.
*
* By default addToCofig is true.
*
* @param addToConfig
*/
public void setAddToConfig(boolean addToConfig) {
this.addToConfig = addToConfig;
}
/**
* {@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 (Throwable t) {
logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", t);
throw new IllegalArgumentException(t);
} finally {
ErrorContext.instance().reset();
}
}
}
/**
* {@inheritDoc}
*/
public T getObject() throws Exception {
return getSqlSession().getMapper(this.mapperInterface);
}
/**
* {@inheritDoc}
*/
public Class getObjectType() {
return this.mapperInterface;
}
/**
* {@inheritDoc}
*/
public boolean isSingleton() {
return true;
}
在该类中其实现了FactoryBean接口，看过Spring源码的人，我相信对其都有很深的印象，其在Bean的实例化中起着很重要的作用。在该类中我们要关注的是getObject方法，我们之后将动态实例化的接口对象放到Spring实例化列表中，这里就是入口，也是我们的起点。不过要特别说明的是mapperInterface的值是如何被赋值的，可能会有疑问，我们再来看看上面的ClassPathMapperScanner.java我们在配置MapperFactoryBean.class的上面存在一行 definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());其在之后在Spring的PostProcessorRegistrationDelegate类的populateBean方法中进行属性配置，会将其依靠反射的方式将其注入到MapperFactoryBean.class中。
而且definition.getPropertyValues().add中添加的值是注入到MapperFactoryBean对象中去的。这一点需要说明一下。
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