12

我正在尝试解析一些 JSON（可以在这个 Gist中看到 JSON 的完整示例）。我在下面展示了 JSON 的一般结构：

[
    {
        "title": "Principles of Compiler Design",
        "authors": [
            "Aho",
            "Ullman"
        ],
        "publisher": "Addison Wesley",
        "year": 1977
    },
    {
        "title": "Compilers: Principles Techniques and Tools",
        "authors": [
            "Aho",
            "Sethi",
            "Ullman"
        ],
        "publisher": "Addison Wesley",
        "year": 1985
    }
]

我正在尝试使用 Jackson 库解析 JSON，但在测试时出现以下错误：

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_ARRAY token
 at [Source: library.json; line: 2, column: 49] (through reference chain: com.acme.datatypes.User["authors"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:163)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:588)
    at com.fasterxml.jackson.databind.deser.std.JdkDeserializers$StringDeserializer.deserialize(JdkDeserializers.java:90)
    at com.fasterxml.jackson.databind.deser.std.JdkDeserializers$StringDeserializer.deserialize(JdkDeserializers.java:59)
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:336)
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:89)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:290)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:112)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2563)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1759)
    at com.acme.datatypes.UserTest.main(UserTest.java:20)

这是我的代码：

用户测试类：

public class UserTest {
    public static void main(String[] args) throws JsonParseException,
            JsonMappingException, IOException {
        File jsonFile = new File("library.json");

        User user = null;

        ObjectMapper mapper = new ObjectMapper();

        user = mapper.readValue(jsonFile, User.class);
        System.out.println(user.getTitle());

        user = mapper.readValue(jsonFile, User.class);
        System.out.println(user.getAuthors());

        user = mapper.readValue(jsonFile, User.class);
        System.out.println(user.getPublisher());

        user = mapper.readValue(jsonFile, User.class);
        System.out.println(user.getYear());
    }
}

用户等级：

public class User {

    private String authors;
    private String publisher;
    private String title;
    private Number year;

    public String getAuthors() {
        return this.authors;
    }

    public void setAuthors(String authors) {
        this.authors = authors;
    }

    public String getPublisher() {
        return this.publisher;
    }

    public void setPublisher(String publisher) {
        this.publisher = publisher;
    }

    public String getTitle() {
        return this.title;
    }

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

    public Number getYear() {
        return this.year;
    }

    public void setYear(Number year) {
        this.year = year;
    }
}

有谁知道问题可能是什么？谢谢。

4

2 回答 2

16

两件快速的事情：

  1. 您的 User 类将authors属性定义为字符串。但在 JSON 中它是一个数组，因此您需要在 Java 对象中使用集合或数组类型。就像是：

    private List<String> authors

  2. 您在测试类中反复解析 JSON 文件。您只需要解析一次，并且需要使用超类型标记，因为 JSON 中有一个项目列表（不仅仅是一个）。您还使用了错误的类型来反序列化（User.class）。而不是所有这些行：

    user = mapper.readValue(jsonFile, User.class); System.out.println(user.getTitle());

    user = mapper.readValue(jsonFile, User.class); // <-- unnecessary parsing System.out.println(user.getAuthors());

    user = mapper.readValue(jsonFile, User.class); // <-- unnecessary parsing System.out.println(user.getPublisher());

    user = mapper.readValue(jsonFile, User.class); // <-- unnecessary parsing System.out.println(user.getYear());

只需使用：

List<User> userList =
    mapper.readValue(jsonFile, new TypeReference<List<User>>() {});

一旦您获得了测试类中的用户列表，您就可以使用增强的 for 循环对其进行迭代。

for(User user : userList) {
    System.out.println(user.getTitle());
}
于 2013-03-06T18:18:50.797 回答
4

由于您使用的是数组，因此您需要将其转换为数组或列表

作为数组

MyClass[] myObjects = mapper.readValue(json, MyClass[].class);

作为列表

List<MyClass> myObjects = mapper.readValue(jsonInput, new TypeReference<List<MyClass>>(){});

用户

public class User {

    private List<String> authors;
    private String publisher;
    private String title;
    private Number year;

    public List<String> getAuthors() {
        return this.authors;
    }

    public void setAuthors(List<String> authors) {
        this.authors = authors;
    }

    public String getPublisher() {
        return this.publisher;
    }

    public void setPublisher(String publisher) {
        this.publisher = publisher;
    }

    public String getTitle() {
        return this.title;
    }

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

    public Number getYear() {
        return this.year;
    }

    public void setYear(Number year) {
        this.year = year;
    }
}

用法：

List<User> l = mapper.readValue(new File(""),new TypeReference<List<User>>() {});
于 2013-03-06T15:19:36.900 回答