java - How to prevent null values inside a Map and null fields inside a bean from getting serialized through Jackson

Question

I have a a Map<String,Foo> foosMap that I want to serialize through Jackson . Now I want following two settings on the serialization process:

1. The Map can have have plenty of null values and null keys and I don't want nulls to be serialized.
2. For all those Foos that are getting serialized, I do not want to serialize null objects referenced inside Foo.

What is the best way to achieve this ? I am using jackson-core1.9 and jackson-mapper1.9 jars in my project.

Answer 1

If it's reasonable to alter the original Map data structure to be serialized to better represent the actual value wanted to be serialized, that's probably a decent approach, which would possibly reduce the amount of Jackson configuration necessary. For example, just remove the null key entries, if possible, before calling Jackson. That said...

---

To suppress serializing Map entries with null values:

Before Jackson 2.9

you can still make use of WRITE_NULL_MAP_VALUES, but note that it's moved to SerializationFeature:

mapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);

Since Jackson 2.9

The WRITE_NULL_MAP_VALUES is deprecated, you can use the below equivalent:

mapper.setDefaultPropertyInclusion(
   JsonInclude.Value.construct(Include.ALWAYS, Include.NON_NULL))

---

To suppress serializing properties with null values, you can configure the ObjectMapper directly, or make use of the @JsonInclude annotation:

mapper.setSerializationInclusion(Include.NON_NULL);

or:

@JsonInclude(Include.NON_NULL)
class Foo
{
  public String bar;

  Foo(String bar)
  {
    this.bar = bar;
  }
}

---

To handle null Map keys, some custom serialization is necessary, as best I understand.

A simple approach to serialize null keys as empty strings (including complete examples of the two previously mentioned configurations):

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.SerializerProvider;

public class JacksonFoo
{
  public static void main(String[] args) throws Exception
  {
    Map<String, Foo> foos = new HashMap<String, Foo>();
    foos.put("foo1", new Foo("foo1"));
    foos.put("foo2", new Foo(null));
    foos.put("foo3", null);
    foos.put(null, new Foo("foo4"));

    // System.out.println(new ObjectMapper().writeValueAsString(foos));
    // Exception: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)

    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);
    mapper.setSerializationInclusion(Include.NON_NULL);
    mapper.getSerializerProvider().setNullKeySerializer(new MyNullKeySerializer());
    System.out.println(mapper.writeValueAsString(foos));
    // output: 
    // {"":{"bar":"foo4"},"foo2":{},"foo1":{"bar":"foo1"}}
  }
}

class MyNullKeySerializer extends JsonSerializer<Object>
{
  @Override
  public void serialize(Object nullKey, JsonGenerator jsonGenerator, SerializerProvider unused) 
      throws IOException, JsonProcessingException
  {
    jsonGenerator.writeFieldName("");
  }
}

class Foo
{
  public String bar;

  Foo(String bar)
  {
    this.bar = bar;
  }
}

To suppress serializing Map entries with null keys, further custom serialization processing would be necessary.