Jackson JSON如何从对象数组反序列化日期

Question

I would serialize/deserialize a Object-Array with mixed objects. Therefor I created a MyList class und fill the items-array with strings, int and date-objects. Serialization with jackson works well. The json-string looks like this:

{"objects":["string",123,1390859948022]}

But on deserialize the json-string date is parsed as an integer. So I created a own class MyDate to wrap the date as a json-object with fieldname "date". The new result was:

{"objects":["string",123,1390859948022,{"date":1390859948022}]}

Now the result is a LinkedHashMap.

Here are my code-samples:

Maven-Dependencies:

    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>2.3.0</version>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.3.0</version>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-annotations</artifactId>
        <version>2.3.0</version>
    </dependency>

MyList.java

public class MyList {
    private Object[] objects;

    public MyList() {
    }

    public MyList(Object[] objects) {
        this.objects = objects;
    }

    // getter and setter
}

MyDate.java:

public class MyDate {
    private Date date;

    public MyDate() {
    }   

    public MyDate(Date date) {
        this.date = date;
    }
    // Getter and Setter
}

Serialize-Example:

public static void serializeTest() throws JsonGenerationException, JsonMappingException, IOException{
    ObjectMapper mapper = new ObjectMapper();       
    Object[] objects = {"string", 123, new Date(), new MyDate(new Date())};
    MyList liste = new MyList(objects);
    mapper.writeValue(System.out, liste);
}

Deserialize-Example:

public static void deserializeTest() throws JsonParseException,
        JsonMappingException, IOException {
    String input = "{\"objects\":[\"string\",123,1390858928698,{\"date\":1390858928698}]}";

    ObjectMapper mapper = new ObjectMapper();
    MyList list = mapper.readValue(input, MyList.class);
    System.out.println("#list:" + list);
    System.out.println("#list.getObjects:" + list.getObjects());
    for (Object i : list.getObjects()) {
        System.out.println("\t value=" + i + ", type=" + i.getClass());
    }
}

I have search for a possible solution, but nothing worked. I hope someone can help me.

Thanks a lot for all your answers. With your help I could solve my problem

I had to add JsonTypeInfo-Annotation to my object-array:

public class MyList {
    @JsonTypeInfo(  
        use = JsonTypeInfo.Id.NAME,  
        include = JsonTypeInfo.As.WRAPPER_OBJECT)  
    @JsonSubTypes({  
        @Type(value = Date.class, name = "date")})  
    private Object[] objects;

To solve this problem also in other object-arrays I have extended my code with the following line:

mapper.enableDefaultTyping(DefaultTyping.JAVA_LANG_OBJECT, As.WRAPPER_OBJECT);

This works for me and my concrete problem.

Answer 1

Read up on Polymorphic Deserialization . Currently all you're telling Jackson is that you want it to deserialize an array of Objects. In order for this to work, you need to annotate your MyDate class (or use Annotation Mixins to annotate the built-in Date class) so Jackson will output some additional type info when serializing your date. When deserializing, Jackson will use the that info to know what kind of class the JSON object represents.

Answer 2

Your problem is that there is no way for Jackson to know that a JSON String or Number should map to java.util.Date , if all you say is "map to java.lang.Object".

But you can force use of type information to make this possible; annotation to use is @JsonTypeInfo , and you can add that next to field like so:

@JsonTypeInfo(...)
private Object[] objects;

Answer 3

Regarding date serialization - see http://wiki.fasterxml.com/JacksonFAQDateHandling

Regarding deserialization: your attribute is generic - Object[]. From your current JSON it is impossible to determine in what Java type should it be instantiated. That's why JSON should contain additional info about it's date element, kind of ... {@type: "date", "date":1390859948022} ... or like this: ... {"date": {@type: "date", "value": 1390859948022} ...

How you do that - see the previous answer from "dnault" and link to Jackson there.