在livejournal中显示消息树（java）

我正在使用SpringMVC+Hibernate编写一个论坛。Hibernate使用惰性初始化，为了使其工作，我使用OpenSessionInviewWinterCeptor，它可以工作。延迟初始化应该没有任何问题。 我正在尝试显示消息树，就像在livejournal回复中一样。 我只有id、parentId和文本列

mysql> select * from posts;
+----+----------+----------+----------+--------+------------+----------+
| id | threadId | authorId | parentId | text   | created    | modified |
+----+----------+----------+----------+--------+------------+----------+
|  1 |        5 |     NULL |     NULL | fda    | 2011-11-24 | NULL     |
|  2 |        5 |     NULL |     NULL | aff    | 2011-11-24 | NULL     |
|  3 |        5 |     NULL |     NULL | faee   | 2011-11-24 | NULL     |
| 13 |        6 |     NULL |     NULL | f52    | 2011-11-26 | NULL     |
| 14 |        6 |     NULL |       13 | c431   | 2011-11-26 | NULL     |
| 15 |        6 |     NULL |     NULL | c31c13 | 2011-11-26 | NULL     |
| 16 |        6 |     NULL |       15 | n754   | 2011-11-26 | NULL     |
| 23 |        4 |     NULL |     NULL | v52    | 2011-11-26 | NULL     |
| 24 |        4 |     NULL |       23 | v53    | 2011-11-26 | NULL     |
| 25 |        4 |     NULL |     NULL | v423   | 2011-11-26 | NULL     |
| 26 |        4 |     NULL |       24 | v523   | 2011-11-26 | NULL     |
| 27 |        4 |     NULL |       23 | v253   | 2011-11-26 | NULL     |
+----+----------+----------+----------+--------+------------+----------+

POJO类职位：

@Entity
@Table(name="posts")
public class Post{
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
private Integer id;
@ManyToOne(cascade=CascadeType.REFRESH,fetch=FetchType.LAZY)
@JoinColumn(name="threadId")
private Thread thread;
@Column(name="authorId")
private Integer authorId;
@ManyToOne(cascade=CascadeType.REFRESH,fetch=FetchType.LAZY)
@JoinColumn(name="parentId")
private Post parentPost;
@Column(name="text")
private String text;
@Column(name="created")
private Date created;
@Column(name="modified")
private Date modified;
....Many getters and setters....
}

我已经编写了一个JSP自定义标记：

<custom:tree postList="${posts}"/> 

public class MessageTree extends SimpleTagSupport{
private List<Post> postList;
private StringBuffer output = new StringBuffer("<ul>");
public void setPostList(List<Post> postList){
    this.postList = postList;
}
public void doTag()throws JspException,IOException{
    retrieveOutput(null);
    output.append("</ul>");
    getJspContext().getOut().print(output.toString());
}
private void retrieveOutput(Integer parentId){
    int j = 0;
    while(j<postList.size()){
        if(parentId==null && postList.get(j).getParentPost()==null){
            output.append("<li>Id: "+postList.get(j).getId());
            output.append("<ul>");
            //retrieveOutput(postList.get(j).getId());
            output.append("</ul></li>");
        }else{
            if(postList.get(j).getParentPost().getId().equals(parentId)){ // !!!Here it throws java.lang.NullPointerException!!!!
                output.append("<li>Id: "+postList.get(j).getId());
                output.append("<ul>");
                retrieveOutput(postList.get(j).getId());
                output.append("</ul></li>");
            }
        }
        j++;
    }
}
}

但是当我尝试时，例如

public void doTag()throws JspException,IOException{
    output.append(postList.get(1).getParentPost().getId());
    getJspContext().getOut().print(output.toString());
}

它工作，它检索23！ 可能是我做得完全不对吗？你有什么建议

耶，我发现了错误！我把它改写了一点。问题是我没有在else子句中检查对象是否具有parrent post

private void retrieveOutput(Integer parentId){
    int j = 0;
    while(j<postList.size()){
        if(parentId==null && postList.get(j).getParentPost()==null){
            output.append("<li>Id: "+postList.get(j).getId()+"<br/>ParentId: 0<br/>Text: "+postList.get(j).getText()+"<br/>Posted: "+postList.get(j).getCreated()+"<br/><a href=\"/forums/deletePost/"+postList.get(j).getThread().getId()+"\">Delete this shit</a>");
            output.append("<ul>");
            retrieveOutput(postList.get(j).getId());
            output.append("</ul></li>");
        }else{
            if(postList.get(j).getParentPost()!=null && postList.get(j).getParentPost().getId().equals(parentId)){
                output.append("<li>Id: "+postList.get(j).getId()+"<br/>ParentId: "+postList.get(j).getParentPost().getId()+"<br/>Text: "+postList.get(j).getText()+"<br/>Posted: "+postList.get(j).getCreated()+"<br/><a href=\"/forums/deletePost/"+postList.get(j).getThread().getId()+"\">Delete this shit</a>");
                output.append("<ul>");
                retrieveOutput(postList.get(j).getId());
                output.append("</ul></li>");
            }
        }
        j++;
    }
    }

private void retrieveOutput（整数parentId）{
int j=0；
虽然（j我认为您的问题在于延迟抓取，但您不能访问事务之外的延迟抓取属性。请检查此项

public void doTag()throws JspException,IOException{
    output.append(postList.get(1).getParentPost().getId());
    getJspContext().getOut().print(output.toString());
}

private void retrieveOutput(Integer parentId){
    int j = 0;
    while(j<postList.size()){
        if(parentId==null && postList.get(j).getParentPost()==null){
            output.append("<li>Id: "+postList.get(j).getId()+"<br/>ParentId: 0<br/>Text: "+postList.get(j).getText()+"<br/>Posted: "+postList.get(j).getCreated()+"<br/><a href=\"/forums/deletePost/"+postList.get(j).getThread().getId()+"\">Delete this shit</a>");
            output.append("<ul>");
            retrieveOutput(postList.get(j).getId());
            output.append("</ul></li>");
        }else{
            if(postList.get(j).getParentPost()!=null && postList.get(j).getParentPost().getId().equals(parentId)){
                output.append("<li>Id: "+postList.get(j).getId()+"<br/>ParentId: "+postList.get(j).getParentPost().getId()+"<br/>Text: "+postList.get(j).getText()+"<br/>Posted: "+postList.get(j).getCreated()+"<br/><a href=\"/forums/deletePost/"+postList.get(j).getThread().getId()+"\">Delete this shit</a>");
                output.append("<ul>");
                retrieveOutput(postList.get(j).getId());
                output.append("</ul></li>");
            }
        }
        j++;
    }
    }