定义了一个接口实现类UserServiceImpl，并在控制类中用UserService userService=new UserServiceImpl(); 创建实现类的对象,调用实现类的方法userService.queryById(id),执行到jpaQueryFactory这句时会报空指针错误。

代码如下
实现类：
import ...

@Service("userService")
public class UserServiceImpl implements UserService {

@Autowired
JPAQueryFactory jpaQueryFactory;

@Override
public List<UserEntity> queryById(Interger id){
List<UserEntity> list=null;
QUserEntity qUserEntity=QUserEntity.userEntity;

Predicate predicate1=qUserEntity.id.eq(id).and(qUserEntity.state.eq("10A")); list=jpaQueryFactory.selectFrom(qUserEntity) .where(predicate1) .fetch(); return list;

};
}

控制类：
import ...
import io.swagger.annotations.Api;
import io.swagger.annotations.ApiOperation;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

@Controller

@RequestMapping("api/user/")

public class UserController {

@ResponseBody@RequestMapping(value = "getTree" , method = RequestMethod.POST)public Map<String, Object> getTree(@RequestBody UserTreeDto dto){ UserService userService=new UserServiceImpl(); List<UserEntity> list = userService.queryById(id); resultMap.put("data",list); resultMap.put("result","suc"); return resultMap;}

}

解决办法：修改控制类，用注入的方式定义实现类，如下
@Controller

@RequestMapping("api/user/")

public class UserController {

@AutowiredUserService userService;@ResponseBody@RequestMapping(value = "getTree" , method = RequestMethod.POST)public Map<String, Object> getTree(@RequestBody UserTreeDto dto){ Interger id=dto.getId(); List<UserEntity> list = userService.queryById(id); resultMap.put("data",list); resultMap.put("result","suc"); return resultMap;}

}

原因说明：这是因为自己new的对象没被spring管理导致的,这种情况就相当于没用spring管理，所以它不会自动进行依赖注入，jpaQueryFactory自然就是null，当用到的时候就报空指针异常了，尽管jpaQueryFactory是用注入方式定义的也不管用。