java - 实体不能为空？

Question

我想从数据库中检索找到的对象或空值。

在我的 ProjectFollowerImpl 中，对于 useFollowingProject 方法，我返回与查询匹配的单个实例，如果查询未返回结果，则返回 null。

@Override
public ProjectFollower userFollowingProject(Integer userId, Integer projectId) {
    return (ProjectFollower) session.createCriteria(classType)
            .add(Restrictions.eq("user.id", userId))
            .add(Restrictions.eq("project.id", projectId))
            .uniqueResult();
}

然后在 Tapestry 页面中，我有以下方法:

public ProjectFollower getUserFollowingProject() {
    return projectFollowerDao.userFollowingProject(loggedInUser.getId(), project.getId());
}

@CommitAfter
void onActionFromFollowProject() {
    ProjectFollower pf = getUserFollowingProject();
    if (pf != null) {
        projectFollowerDao.delete(pf.getId());
    } else {
        pf = new ProjectFollower();
        pf.setProjectId(project);
        pf.setUserId(loggedInUser);
        projectFollowerDao.merge(pf);
    }
}

但是，tapestry 抛出空指针异常，堆栈跟踪:

com.rile.issuetracker.pages.Tracker getUserFollowingProject()   Tracker.java    75
com.rile.issuetracker.pages.Tracker advised$onActionFromFollowProject_6d8b9673baf() Tracker.java    80

那么为什么对象返回空值会出现问题呢？我做错了什么？

更新:

public class Tracker {

@Property
@SessionState
private User loggedInUser;

@Property
@Inject
private ProjectDao projectDao;
@Property
private Project projectP1, project;
@Property
private List<Project> projectList;
@Property
@Inject
private ProjectFollowerDao projectFollowerDao;

@Property
@Inject
private TicketDao ticketDao;
@Property
private List<Ticket> ticketList;
@Property
private Ticket ticketP1;
@Property
@Inject
private TicketFollowerDao ticketFollowerDao;

@Property
private Util util = new Util();

public boolean getLoggedIn() {
    return loggedInUser.getEmail() != null;
}    

@PageLoaded
void onPageLoad() {
    projectList = projectDao.loadAll();
    ticketList = ticketDao.loadAll();
}

void onActivate(Integer contextValue) {
    if (contextValue != null) {  
        project = projectDao.getByID(contextValue);
    }
    if (project != null) {
        List ticketListByProjectID = ticketDao.getTicketsByProjectID(project.getId());
        if (!ticketListByProjectID.isEmpty()) {
            ticketList = ticketListByProjectID;
        } else {
            ticketList = null;
        }
    }
}

public ProjectFollower getUserFollowingProject() {
        return projectFollowerDao.userFollowingProject(loggedInUser.getId(), project.getId());        

}

@CommitAfter
void onActionFromFollowProject() {
    ProjectFollower pf = getUserFollowingProject();
    if (pf != null) {
        projectFollowerDao.delete(pf.getId());
    } else {
        pf = new ProjectFollower();
        pf.setProjectId(project);
        pf.setUserId(loggedInUser);
        projectFollowerDao.merge(pf);
    }
}

public boolean getIsUserFollowingTicket() {
    return ticketFollowerDao.isUserFollowingTicket(loggedInUser.getId(), ticketP1.getId());
}

@CommitAfter
void onActionFromFollowTicket() {}

public String getActiveFor(String parameter) {
    if (parameter == null || parameter.isEmpty()) {
        return null;
    }
    switch (parameter) {
        case "userFollowingProject":
            return getUserFollowingProject() != null ? "active" : "null";
        case "userFollowingTicket":
            return getIsUserFollowingTicket() ? "anchor-active" : "anchor-inactive";
        default: 
            return null;
    }
}

}

Answer 1

从完整的代码来看，最好的选择是project为null，在检查完整的类后，我们发现它没有被注入(inject)，并且它的值仅在onActivate函数中设置，如果contextValue 不为 null，并且 projectDao.getByID(contextValue) 返回一个非 null 值。

能否请您在调用 onActionFromFollowProject 之前确认是否正确发生