Error using Jackson and Json

Question

I am trying to parse some JSON (full example of the JSON can be seen in this Gist). I show the general structure of the JSON below:

[
    {
        "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
    }
]

I am trying to parse the JSON with Jackson libraries, but I get the following error while testing:

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)

Here is my code:

User Test class:

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());
    }
}

User class:

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;
    }
}

Does anyone know what the problem might be? Thanks.

Answer 1

Two quick things:

1. Your User class is defining the authors property as a String. But in JSON its an array, so you need to use a collections or array type in your Java object. Something like:

   private List<String> authors

2. You repeatedly parse the JSON file in your test class. You only need to parse it once, and you need to use a supertype token, since there is a list of items in the JSON (not just one). You are also using the wrong type to deserialize (User.class). Instead of all these lines:

   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());

Just use:

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

Once you get a list of users in your test class you can iterate them using an enhanced for loop.

for(User user : userList) {
    System.out.println(user.getTitle());
}

Answer 2

Since you are working with an array, you need to convert it into an array or a list

As Array

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

As List

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

User

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;
    }
}

Usage:

List<User> l = mapper.readValue(new File(""),new TypeReference<List<User>>() {});