mybatis部分源码解析

当执行INSERT,UPDATE,DELETE类型的sql语句时，其执行结果都需要经过MapperMethod.rowCountResult()方法处理。sqlSession中的inert(),update(),delete()等方法返回的是int值，rowCountResult()方法会将该int值转换为Mapper接口中对应方法的返回值，其具体实现如下：

 private Object rowCountResult(int rowCount) {
        Object result;
        if (this.method.returnsVoid()) {
            result = null;
        } else {
            Object result;
            if ((Integer.class.equals(this.method.getReturnType())) || (Integer.TYPE.equals(this.method.getReturnType()))) {
                result = Integer.valueOf(rowCount);
            } else {
                Object result;
                if ((Long.class.equals(this.method.getReturnType())) || (Long.TYPE.equals(this.method.getReturnType()))) {
                    result = Long.valueOf(rowCount);
                } else {
                    Object result;
                    if ((Boolean.class.equals(this.method.getReturnType())) || (Boolean.TYPE.equals(this.method.getReturnType())))
                        result = Boolean.valueOf(rowCount > 0);
                    else
                        throw new BindingException("Mapper method '" + this.command.getName() + "' has an unsupported return type: " + this.method.getReturnType());
                }
            }
        }
        Object result;
        return result;
    }

异常信息：Expected one result (or null) to be returned by selectOne(), but found:2，原因：返回数据类型为单个对象，但查询结果查到不止一个。

  if (SqlCommandType.SELECT == this.command.getType()) {
                        Object result;
                        if ((this.method.returnsVoid()) && (this.method.hasResultHandler())) {
                        //如果Mapper接口中方法准备使用ResultHandler处理查询结果集，则...（当使用ResultHandler处理结果集时必须指定ResultMap或ResultType）
                            executeWithResultHandler(sqlSession, args);
                            result = null;
                        } else {
                            Object result;
      //如果Mapper接口中对应方法的返回值为数组或Collection接口实现类，则通过MapperMethod.executeForMany()方法处理
                            if (this.method.returnsMany()) {
                                result = executeForMany(sqlSession, args);
                            } else {
                                Object result;
                                //如果Mapper接口中对应方法的返回值为Map类型，则..
                                if (this.method.returnsMap()) {
                                    result = executeForMap(sqlSession, args);
                                } else {
                                  Object param = this.method.convertArgsToSqlCommandParam(args);
                                    result = sqlSession.selectOne(this.command.getName(), param);
                                }
                            }
                        }
                    }


 public <T> T selectOne(String statement, Object parameter) {
        List list = selectList(statement, parameter);
        if (list.size() == 1)
            return list.get(0);
        if (list.size() > 1) {
            throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
        }
        return null;
    }

绑定Mapper接口

1. XMLMapperBuilder.bindMapperForNamespace()方法完成了映射配置文件与对应Mapper接口的绑定。
2. 每个配置文件的命名空间可以绑定一个Mapper接口，并注册到MapperRegistry中。

 private void bindMapperForNamespace() {
         //获取映射配置文件的命名空间
        String namespace = this.builderAssistant.getCurrentNamespace();
        if (namespace != null) {
            Class boundType = null;
            try {
            //解析命名空间对应的类型
                boundType = Resources.classForName(namespace);
            } catch (ClassNotFoundException e) {
            }
            //是否已经加载了boundType接口
            if ((boundType == null) || (this.configuration.hasMapper(boundType))) {
                return;
            }
            //追加namespace前缀，并添加到configuration.LoadedResource集合中保存
            this.configuration.addLoadedResource("namespace:" + namespace);
            //调用MapperRegistry.addMapper()方法，注册boundType接口
            this.configuration.addMapper(boundType);
        }
    }