XML 字符串到 Java 对象 nullpointerexception

Question

错误：java.lang.NullPointerException：无法调用“java.util.List.iterator()”，因为“列表”为空

主要class:

try {
        jaxbContext = JAXBContext.newInstance(Recipes.class);
        Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
        Recipes rec = (Recipes) jaxbUnmarshaller.unmarshal(new StringReader(xml));

        List <Recipe> list = rec.getRecipes();
        for (Recipe re: list){
            System.out.println(re.getId());
        }
    } catch (JAXBException e) {
        e.printStackTrace();
      }

食谱class：

@XmlRootElement(name = "results")
public class Recipe {
private int id;
private String title;
private Image image;

public Recipe(){}

public Recipe(int id, String title, Image image){
    this.id = id;
    this.title = title;
    this.image = image;
}

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public Image getImage() {
    return image;
}

public void setImage(Image image) {
    this.image = image;
}

食谱class：

@XmlRootElement
public class Recipes {
private List<Recipe> recipes;

public Recipes(){}

public Recipes(List<Recipe> recipes){
    this.recipes = recipes;
}
@XmlElement
public List<Recipe> getRecipes() {
    return recipes;
}

public void setRecipes(List<Recipe> recipes) {
    this.recipes = recipes;
}
}

我正在尝试管理一个 xml 字符串，它可以转换为对象数组。一个错误给我这个列表是空的消息，我想，但我不明白为什么，谢谢你的帮助。

Answer 1

看起来 unmarshal 和 jaxbUnmarshaller returns 对您稍后使用的列表来说是空的。你能不能用调试检查它 returns 是什么？

此外，如果您的 class 是使用 no-args 构造函数。您可能想在该空构造函数中键入以下代码：

recipes = new ArrayList<>();

XML 字符串到 Java 对象 nullpointerexception

XML String to Java Object nullpoiterexception

java

nullpointerexception