我们先Mapper接口的调用方式,见<MyBatis框架中Mapper映射配置的使用及原理解析(一) 配置与使用>的示例:
public void findUserById() {
SqlSessionFactory sqlSessionFactory = getSessionFactory();
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
User user = userMapper.selectByPrimaryKey(1l);
System.out.println(user.getId() + " / " + user.getName());
}
sqlsession.getMapper(UserMapper.class) 也就是调用DefaultSqlSession的对应方法:
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
继续跟踪Configuration对象对应源码:
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
我们在回忆,在 <MyBatis框架中Mapper映射配置的使用及原理解析(四) 解析Mapper接口映射xml文件> 一文,我们知道在读取mapper xml文件后会调用org.apache.ibatis.builder.xml.XMLMapperBuilder类的bindMapperForNamespace()方法,绑定到命名空间:
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)) {//判断是否存在
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource("namespace:" + namespace);//添加资源标识
configuration.addMapper(boundType); //Mapper接口添加到Configuration
}
}
}
}
继续跟踪configuration.addMapper(boundType)方法:
public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}
我们发现添加和获取Mapper实例都使用到了同一个类MapperRegistry,在Configuration中的声明如下:
protected MapperRegistry mapperRegistry = new MapperRegistry(this);
下面贴出MapperRegistry的源代码:
package org.apache.ibatis.binding; import org.apache.ibatis.builder.annotation.MapperAnnotationBuilder;
import org.apache.ibatis.io.ResolverUtil;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession; import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set; public class MapperRegistry { private Configuration config;
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>(); public MapperRegistry(Configuration config) {
this.config = config;
} @SuppressWarnings("unchecked")
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);
}
} public <T> boolean hasMapper(Class<T> type) {
return knownMappers.containsKey(type);
} 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<T>(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
} /**
* @since 3.2.2
*/
public Collection<Class<?>> getMappers() {
return Collections.unmodifiableCollection(knownMappers.keySet());
} /**
* @since 3.2.2
*/
public void addMappers(String packageName, Class<?> superType) {
ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
for (Class<?> mapperClass : mapperSet) {
addMapper(mapperClass);
}
} /**
* @since 3.2.2
*/
public void addMappers(String packageName) {
addMappers(packageName, Object.class);
} }
我们清楚的看到注册Mapper和获取Mapper都是在一个Map对象中存取Mapper的代理对象MapperProxyFactory。
经过这篇文章,我们清楚的知道了MapperRegistry的功能就是注册和获取Mapper对象的代理。