做JAVA开发的小伙伴都知道，接口几乎都由实现类实现其功能，使用接口作变量引用实现类作变量实例。然而有部分接口我们在源代码中却找不到其实现类，mybatis的mapper接口便是如此。那么，他们是怎么实现其功能的呢，那就是动态代理。
什么是动态代理这里就不做解释了，不了解的朋友可以参考一下设计模式。
mybatis的动态代理过程：
初始化SqlSessionFactory解析mapper.xml的namespace属性的时候，将MapperProxyFactory代理工厂存入mapper缓存中，源代码的调取过程如下：
org.mybatis.spring.SqlSessionFactoryBean#buildSqlSessionFactory ->
if (this.mapperLocations.length == 0) {
LOGGER.warn(() -> "Property 'mapperLocations' was specified but matching resources are not found.");
} else {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'");
}
}
org.apache.ibatis.builder.xml.XMLMapperBuilder#parse ->
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}
parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}
org.apache.ibatis.builder.xml.XMLMapperBuilder#bindMapperForNamespace ->
private void bindMapperForNamespace() {
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
if (boundType != null) {
if (!configuration.hasMapper(boundType)) {
configuration.addLoadedResource("namespace:" + namespace);
configuration.addMapper(boundType);
}
}
}
}
org.apache.ibatis.session.Configuration#addMapper ->
public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}
org.apache.ibatis.binding.MapperRegistry#addMapper ->
public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
knownMappers.put(type, new MapperProxyFactory<>(type));
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
注册bean(mapper)的时候会调用doGetObjectFromFactoryBean，这个时候FactoryBean<?>传入的是MapperFactoryBean对象，然后获取前面存入knownMappers里面的MapperProxyFactory代理工厂，用代理工创建一个Mapper代理实例给容器注册
org.springframework.beans.factory.support.FactoryBeanRegistrySupport#doGetObjectFromFactoryBean ->
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
} else {
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}
org.mybatis.spring.mapper.MapperFactoryBean#getObject ->
public T getObject() throws Exception {
return getSqlSession().getMapper(this.mapperInterface);
}
org.mybatis.spring.SqlSessionTemplate#getMapper ->
public <T> T getMapper(Class<T> type) {
return getConfiguration().getMapper(type, this);
}
org.apache.ibatis.session.Configuration#getMapper ->
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
org.apache.ibatis.binding.MapperRegistry#getMapper ->
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
所以，注册到spring容器的mapper其实是MapperProxy，我们调用mapper接口的时候就会自动装配动态生成的MapperProxy实例实现mapper的功能。
第一次写博客，内容写得比较粗糙，有什么意见或者建议请建议博主Qq，谢谢。