在上一篇文章我们已经得到了DefaultSqlSession，接下来我们对sqlSession.getMapper(DemoMapper.class)这种语句进行分析
@Override
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
在这里又调用了如下方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
getMapper方法代码如下
@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);
}
}
在SqlSessionFactoryBuilder创建SqlSessionFactory的过程中已经将mapper接口到了knownMappers中，找不到的话这里会抛错。
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
首先实例化一个MapperProxy对象，我们对Mapper接口的调用实际上使用的MapperProxy对象，mapperInterface在build factory的过程中已经设置过值了，methodCache后面会提，暂时不提。
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
从这里可以得知我们返回的mapper对象实际是个代理对象。
由debug可清晰的看到返回的mapper对象是由jdk代理生成。
至此SqlSession的getMapper方法分析结束。
