jackson serializing Collections.unmodifiable*

Question

I have a problem using the Jackson serialization from json, how do I serialize from Collections.unmodifiableMap?

The error I get is:

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of java.util.Collections$UnmodifiableMap, problem: No default constructor found

I wanted to use the SimpleAbstractTypeResolver from http://wiki.fasterxml.com/SimpleAbstractTypeResolver however I cannot get the inner class type Collections$UnmodifiableMap

Map<Integer, String> emailMap = newHashMap();
Account testAccount = new Account();
ObjectMapper mapper = new ObjectMapper();
mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL, As.PROPERTY);
String marshalled ;
emailMap.put(Integer.valueOf(10), "[email protected]");
testAccount.setMemberEmails(emailMap);

marshalled = mapper.writeValueAsString(testAccount);
System.out.println(marshalled);
Account returnedAccount = mapper.readValue(marshalled, Account.class);
System.out.println(returnedAccount.containsValue("[email protected]"));


public class Account {
  private Map<Integer, String> memberEmails = Maps.newHashMap();

  public void setMemberEmails(Map<Integer, String> memberEmails) {
    this.memberEmails = memberEmails;
  }

  public Map<Integer, String> getMemberEmails() {
    return Collections.unmodifiableMap(memberEmails);
  }

Any ideas? Thanks in advance.

Answer 1

Okay, you've run into an edge-ish type case with Jackson. The problem really is that the library will happily use your getter method to retrieve collection and map properties, and only falls back to instantiating these collections/maps if those getter methods return null.

This can fixed by a combination of @JsonProperty/@JsonIgnore annotations, with the caveat that the @class property in your JSON output will change.

Code example:

public class Account {
    @JsonProperty("memberEmails")
    private Map<Integer, String> memberEmails = Maps.newHashMap();

    public Account() {
        super();
    }

    public void setMemberEmails(Map<Integer, String> memberEmails) {
        this.memberEmails = memberEmails;
    }

    @JsonIgnore
    public Map<Integer, String> getMemberEmails() {
        return Collections.unmodifiableMap(memberEmails);
    }
}

If you serialize this class with your test code you will get the following JSON:

{
    "@class": "misc.stack.pojo.Account",
    "memberEmails": {
        "10": "[email protected]",
        "@class": "java.util.HashMap"
    }
}

Which will deserialize correctly.