MyBatis源码剖析之Mapper代理方式详解

目录

• 源码剖析-getmapper()
• 源码剖析-invoke()

具体代码如下：

//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);

思考⼀个问题，通常的Mapper接⼝我们都没有实现的⽅法却可以使⽤，是为什么呢？ 答案很简单：动态代理

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

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

开始之前介绍⼀下MyBatis初始化时对接⼝的处理：MapperRegistry是Configuration中的⼀个属性，它内部维护⼀个HashMap⽤于存放mapper接⼝的⼯⼚类，每个接⼝对应⼀个⼯⼚类。mappers中可以配置接⼝的包路径，或者某个具体的接⼝类。

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

当解析mappers标签时，它会判断解析到的是mapper配置⽂件时，会再将对应配置⽂件中的增删改查标签 封装成MappedStatement对象，存⼊mappedStatements中。(上⽂介绍了)当判断解析到接⼝时，会建此接⼝对应的MapperProxyFactory对象，存⼊HashMap中，key =接⼝的字节码对象，value =此接⼝对应MapperProxyFactory对象。

源码剖析-getmapper()

进⼊ sqlSession.getMapper(UserMapper.class )中

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
}

源码剖析-invoke()

在动态代理返回了示例后，我们就可以直接调⽤mapper类中的⽅法了，但代理对象调⽤⽅法，执⾏是在MapperProxy中的invoke⽅法中。

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);
  }
}

进⼊execute⽅法：

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;
  }
}

到此这篇关于MyBatis源码剖析之Mapper代理方式详解的文章就介绍到这了,更多相关MyBatis Mapper代理方式内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！