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...

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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)) 

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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;   } } 

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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.

Answer 2

For Jackson versions < 2.0 use this annotation on the class being serialized:

@JsonSerialize(include=JsonSerialize.Inclusion.NON_NULL)