java - Yabe 中的 NPE 空指针异常

Question

我正在学习 Yabe 教程并在测试 useTheCommentsRelation 中遇到了一些空指针异常

useTheCommentsRelation

A java.lang.NullPointerException has been caught, null
In /test/BasicTest.java, line 96 :

 bobPost.addComment("Jeff", "Nice post"); 
 Hide tracejava.lang.NullPointerException
at models.Post.addComment(Post.java:30)
at BasicTest.useTheCommentsRelation(BasicTest.java:96)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)

这是测试代码；

@Test
public void useTheCommentsRelation() {
    // Create a new user and save it
    User bob = new User("bob@gmail.com", "secret", "Bob").save();

    // Create a new post
    Post bobPost = new Post(bob, "My first post", "Hello world").save();

    // Post a first comment
    bobPost.addComment("Jeff", "Nice post");
    bobPost.addComment("Tom", "I knew that !");

    // Count things
    assertEquals(1, User.count());
    assertEquals(1, Post.count());
    assertEquals(0, Comment.count()); // 2 when correct not 0

    // Retrieve Bob's post
    bobPost = Post.find("byAuthor", bob).first();
    assertNotNull(bobPost);

    // Navigate to comments
    assertEquals(2, bobPost.comments.size()); 
    assertEquals("Jeff", bobPost.comments.get(0).author);

    // Delete the post
    bobPost.delete();

    // Check that all comments have been deleted
    assertEquals(1, User.count());
    assertEquals(0, Post.count());
    assertEquals(0, Comment.count());
}

这里是 Post 类，末尾有 addComment 方法。

package models;

import java.util.*;
import javax.persistence.*;

import play.db.jpa.*;

@Entity
public class Post extends Model{

public String title;
public Date postedAt;

@Lob
public String content;

@ManyToOne
public User author;

@OneToMany(mappedBy="post", cascade=CascadeType.ALL)
public List<Comment> comments;  
public Post(User author, String title, String content){
    this.author = author;
    this.title = title;
    this.content = content;
    this.postedAt = new Date();
}
public Post addComment(String author, String content) {
    Comment newComment = new Comment(this, author, content).save();
    this.comments.add(newComment);
    this.save();
    return this;
}
}

评论类

package models;

import java.util.*;
import javax.persistence.*;
import play.db.jpa.*;

@Entity
public class Comment extends Model{
public String author;
public Date postedAt;
@Lob
public String content;
@ManyToOne
public Post post;
public Comment(Post post, String author, String content){
    this.post = post;
    this.author = author;
    this.content = content;
    this.postedAt = new Date();
}
}

感谢所有帮助我走到这一步的人。

Answer 1

我没有看到 Post.comments 在任何地方被初始化，但是 addComment() 试图用 this.comments.add(...) 取消引用它。