MyBatis源码剖析之Mapper代理方式详解

//前三步都相同
InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
//这⾥不再调⽤SqlSession的api,⽽是获得了接⼝对象，调⽤接⼝中的⽅法。
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
//模拟ids的数据
List<Integer> ids = new ArrayList<Integer>();
ids.add(1);
ids.add(2);
List<User> userList = mapper.findByIds(ids);

public class Configuration {  
    protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
}

public class MapperRegistry {
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
}

<mappers>
  <mapper class="com.zjq.mapper.UserMapper"/>
  <package name="com.zjq.mapper"/>
</mappers>

public interface SqlSession extends Closeable {
      //调用configuration中的getMapper
      <T> T getMapper(Class<T> type);
}

public class DefaultSqlSession implements SqlSession {
  @Override
  public <T> T getMapper(Class<T> type) {
    //调用configuration 中的getMapper
    return configuration.<T>getMapper(type, this);
  }
}

public class Configuration {
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //调用MapperRegistry 中的getMapper
    return mapperRegistry.getMapper(type, sqlSession);
  }
}

public class MapperRegistry {
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //从 MapperRegistry 中的 HashMap 中拿 MapperProxyFactory
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type "   type   " is not known to the MapperRegistry.");
    }
    try {
      //调用MapperProxyFactory的newInstance通过动态代理⼯⼚⽣成实例
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: "   e, e);
    }
  }
}


public class MapperProxyFactory<T> {
  public T newInstance(SqlSession sqlSession) {
    //创建了 JDK动态代理的Handler类
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    //调⽤了重载⽅法
    return newInstance(mapperProxy);
  }
   
  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
}

//MapperProxy 类，实现了 InvocationHandler 接⼝
public class MapperProxy<T> implements InvocationHandler, Serializable {
  //省略部分源码
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;
    
  //构造，传⼊了 SqlSession，说明每个session中的代理对象的不同的！  
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }
  
   //省略部分源码
}

public class MapperProxy<T> implements InvocationHandler, Serializable { 
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      //如果是Object定义的⽅法，直接调⽤
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    // 获得 MapperMethod 对象
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //重点在这：最终调⽤了MapperMethod执⾏的⽅法
    return mapperMethod.execute(sqlSession, args);
  }
}

public class MapperMethod { 
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //判断mapper中的⽅法类型，最终调⽤的还是SqlSession中的⽅法 switch(command.getType()) 
    switch (command.getType()) {
      case INSERT: {
        //转换参数
        Object param = method.convertArgsToSqlCommandParam(args);
        //执⾏INSERT操作
        //转换rowCount
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        //转换参数
        Object param = method.convertArgsToSqlCommandParam(args);
        // 转换 rowCount
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        //转换参数
        Object param = method.convertArgsToSqlCommandParam(args);
        // 转换 rowCount
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        //⽆返回，并且有ResultHandler⽅法参数，则将查询的结果，提交给 ResultHandler 进⾏处理
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        //执⾏查询，返回列表
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        //执⾏查询，返回Map
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        //执⾏查询，返回Cursor
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        //执⾏查询，返回单个对象
        } else {
          //转换参数
          Object param = method.convertArgsToSqlCommandParam(args);
          //查询单条
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: "   command.getName());
    }
    //返回结果为null，并且返回类型为基本类型，则抛出BindingException异常
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '"   command.getName() 
            " attempted to return null from a method with a primitive return type ("   method.getReturnType()   ").");
    }
    //返回结果
    return result;
  }
}