mybatis源码阅读(四)：mapper(dao)实例化

我就是我 2023-07-02 06:23 4阅读 0赞

转载自   [mybatis源码阅读(四)：mapper(dao)实例化][mybatis_mapper_dao]

在开始分析之前，先来了解一下这个模块中的核心组件之间的关系，如图：

![format_png][]

## 1.MapperRegistry&MapperProxyFactory ##

MapperRegistry是Mapper接口及其对应的代理对象工程的注册中心，Configuration是Mybatis全局性的配置对象，在初始化的过程中，所有配置信息会被解析成相应的对象并记录到Configuration对象中，这在之前也详细介绍了。Configuration.mapperRegistry字段记录当前使用的MapperRegistry对象，

public class MapperRegistry {
    
      // 全局唯一的配置对象，其中包含了所有的配置信息
      private final Configuration config;
      // 记录Mapper接口与对应MapperProxyFactory之间的关系
      private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
    }
    
    
    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)) {
            // 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);
            configuration.addMapper(boundType);
          }
        }
      }
    }
    public <T> void addMapper(Class<T> type) {
      mapperRegistry.addMapper(type);
    }
    public <T> void addMapper(Class<T> type) {
      if (type.isInterface()) {//是否为接口
        if (hasMapper(type)) {//是否已经加载过
          throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
        }
        boolean loadCompleted = false;
        try {
          knownMappers.put(type, new MapperProxyFactory<T>(type));
          // 注解处理
          MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
          parser.parse();
          loadCompleted = true;
        } finally {
          if (!loadCompleted) {
            knownMappers.remove(type);
          }
        }
      }
    }

在需要执行SQL语句时，会先获取mapper借口的代理对象，例如：

@Test
    public void findUserById() {
        SqlSession sqlSession = getSessionFactory().openSession();
        UserDao userMapper = sqlSession.getMapper(UserDao.class);
        User user = userMapper.findUserById(1);
        Assert.assertNotNull("没找到数据", user);
    }

DefaultSqlSession类中方法如下，实际上是通过JDK动态代理生成的代理对象

public <T> T getMapper(Class<T> type) {
        return this.configuration.getMapper(type, this);
    }

Configuration类方法如下：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
      return mapperRegistry.getMapper(type, sqlSession);
    }

MapperRegistry类中方法如下：

@SuppressWarnings("unchecked")
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
      //查找指定type对象的MapperProxyFactory对象
        final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
      if (mapperProxyFactory == null) {//如果为空抛出异常
        throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
      }
      try {
          // 创建实现了type接口的代理对象
        return mapperProxyFactory.newInstance(sqlSession);
      } catch (Exception e) {
        throw new BindingException("Error getting mapper instance. Cause: " + e, e);
      }
    }

MapperProxyFactory主要负责创建代理对象

@SuppressWarnings("unchecked")
    protected T newInstance(MapperProxy<T> mapperProxy) {
      return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
    }
    
    public T newInstance(SqlSession sqlSession) {
      final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
      return newInstance(mapperProxy);
    }

## 2.MapperProxy ##

MapperProxy实现了InvocationHandler接口，对动态代理的可以先去了解这篇文章[https://my.oschina.net/u/3737136/blog/1786175][https_my.oschina.net_u_3737136_blog_1786175]

public class MapperProxy<T> implements InvocationHandler, Serializable {
    
      private static final long serialVersionUID = -6424540398559729838L;
      // 记录关联的SQLSession对象
      private final SqlSession sqlSession;
      // mapper接口对应的class对象
      private final Class<T> mapperInterface;
      private final Map<Method, MapperMethod> methodCache;
    
      public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
        this.methodCache = methodCache;
      }
    
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
          // 如果目标方法是Object类继承来的，直接调用目标方法
          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);
        }
        // 从缓存中获取MapperMethod 对象,如果没有就创建新的并添加
        final MapperMethod mapperMethod = cachedMapperMethod(method);
        // 执行sql 语句
        return mapperMethod.execute(sqlSession, args);
        }
     }

## 3.MapperMethod ##

MapperMethod中封装了Mapper接口中对应方法的信息，以及对应SQL语句的信息，

public class MapperMethod {
    
      // 记录SQL语句的名称和类型
      private final SqlCommand command;
      // mapper接口中对应方法的相关信息
      private final MethodSignature method;
    
      public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
        this.command = new SqlCommand(config, mapperInterface, method);
        this.method = new MethodSignature(config, mapperInterface, method);
      }
    
      public Object execute(SqlSession sqlSession, Object[] args) {
        Object result;
        switch (command.getType()) {
          case INSERT: {
          Object param = method.convertArgsToSqlCommandParam(args);
            result = rowCountResult(sqlSession.insert(command.getName(), param));
            break;
          }
          case UPDATE: {
            Object param = method.convertArgsToSqlCommandParam(args);
            result = rowCountResult(sqlSession.update(command.getName(), param));
            break;
          }
          case DELETE: {
            Object param = method.convertArgsToSqlCommandParam(args);
            result = rowCountResult(sqlSession.delete(command.getName(), param));
            break;
          }
          case SELECT:
            if (method.returnsVoid() && method.hasResultHandler()) {
              // 处理返回值为void ,ResultSet 通过ResultHand处理的方法
              executeWithResultHandler(sqlSession, args);
              result = null;
            } else if (method.returnsMany()) {
              // 处理返回值为集合或者数组的方法
              result = executeForMany(sqlSession, args);
            } else if (method.returnsMap()) {
              // 处理返回值为map的方法
              result = executeForMap(sqlSession, args);
            } else if (method.returnsCursor()) {
              // 处理返回值为cursor的方法
              result = executeForCursor(sqlSession, args);
            } else {
              // 处理返回值为单一对象的方法
              Object param = method.convertArgsToSqlCommandParam(args);
              result = sqlSession.selectOne(command.getName(), param);
            }
            break;
          case FLUSH:
            result = sqlSession.flushStatements();
            break;
          default:
            throw new BindingException("Unknown execution method for: " + command.getName());
        }
        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() + ").");
        }
        return result;
      }
    }

[mybatis_mapper_dao]: https://my.oschina.net/u/3737136/blog/1811632
[format_png]: https://imgconvert.csdnimg.cn/aHR0cHM6Ly9zdGF0aWMub3NjaGluYS5uZXQvdXBsb2Fkcy9zcGFjZS8yMDE4LzA1MTIvMTE1NTU5X2JMQWpfMzczNzEzNi5wbmc?x-oss-process=image/format,png
[https_my.oschina.net_u_3737136_blog_1786175]: https://my.oschina.net/u/3737136/blog/1786175