MyBatis原理（2）-执行流程 4 Mapper的执行

执行方式2:

		DeptMapper mapper = openSession.getMapper(DeptMapper.class);
		Dept dept2= mapper.select(d);

DefaultSqlSession#getMapper

 @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }
  //从MapperRegistry获取
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry#getMapper

 public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
   // 此处的mapperProxyFactory，已在XMLConfigBuilder构建的过程中扫描了mappers节点下的package，使用ResolveUtil扫描了接口，并初始化放入knownMappers
    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);
    }
  }

MapperProxyFactory#newInstance

 public T newInstance(SqlSession sqlSession) {
   //创建Mapper代理
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

//使用mapperProxy，构造mapperInterface的动态代理
protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

openSession.getMapper 当前接口的动态代理，动态代理被执行时返回

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      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);
    }
    //缓存当前调用的Method对应的MapperMethod，所以一个MapperProxy对应一个接口实现类，MapperMethod则对应实现类的每一个方法，不过MapperProxy是代理，真正委托执行的是MapperMethod，MapperMethod，则负责将方法签名转换为命名空间委托sqlSession执行
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

MapperMethod#execute
使用MethodSignature获取方法参数和SqlCommand组合，最后通过sqlsession执行，后面的流程就和使用命名空间直接执行一致了，这里不难看出来，只要指定mapper.xml中命名空间和接口方法签名一致，就可以在执行期间进行动态绑定来执行sqlsession，而整个使用sqlsession获取接口代理的执行方式只比原生方式多了，创建MapperProxy这一过程，MapperProxy会对调用的MapperMethod进行缓存。

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()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          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;
  }

SqlCommand作为MapperMethod的内部类，使用接口当前包名+方法名称从Configuration中获取MapperStatement的id和type

public static class SqlCommand {

    private final String name;
    private final SqlCommandType type;

    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
      final String methodName = method.getName();
      final Class<?> declaringClass = method.getDeclaringClass();
      MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
          configuration);
      if (ms == null) {
        if (method.getAnnotation(Flush.class) != null) {
          name = null;
          type = SqlCommandType.FLUSH;
        } else {
          throw new BindingException("Invalid bound statement (not found): "
              + mapperInterface.getName() + "." + methodName);
        }
      } else {
        name = ms.getId();
        type = ms.getSqlCommandType();
        if (type == SqlCommandType.UNKNOWN) {
          throw new BindingException("Unknown execution method for: " + name);
        }
      }
    }

    public String getName() {
      return name;
    }

    public SqlCommandType getType() {
      return type;
    }

    private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
        Class<?> declaringClass, Configuration configuration) {
      String statementId = mapperInterface.getName() + "." + methodName;
      if (configuration.hasStatement(statementId)) {
        return configuration.getMappedStatement(statementId);
      } else if (mapperInterface.equals(declaringClass)) {
        return null;
      }
      for (Class<?> superInterface : mapperInterface.getInterfaces()) {
        if (declaringClass.isAssignableFrom(superInterface)) {
          MappedStatement ms = resolveMappedStatement(superInterface, methodName,
              declaringClass, configuration);
          if (ms != null) {
            return ms;
          }
        }
      }
      return null;
    }
  }