Map Serialization and Deserialization with Jackson

1. Overview

In this quick tutorial, we’ll look at the serialization and deserialization of Java maps using Jackson.

We'll illustrate how to serialize and deserialize Map<String, String>, Map<Object, String>, and Map<Object, Object> to and from JSON-formatted Strings.

2. Maven Configuration

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.13.3</version>
</dependency>

We can get the latest version of Jackson here.

3. Serialization

Serialization converts a Java object into a stream of bytes, which can be persisted or shared as needed. Java Maps are collections that map a key Object to a value Object, and are often the least intuitive objects to serialize.

3.1. Map<String, String> Serialization

For a simple case, let's create a Map<String, String> and serialize it to JSON:

Map<String, String> map = new HashMap<>();
map.put("key", "value");

ObjectMapper mapper = new ObjectMapper();
String jsonResult = mapper.writerWithDefaultPrettyPrinter()
  .writeValueAsString(map);

ObjectMapper is Jackson's serialization mapper. It allows us to serialize our map, and write it out as a pretty-printed JSON String using the toString() method in String:

{
  "key" : "value"
}

3.2. Map<Object, String> Serialization

With a few extra steps, we can also serialize a map containing a custom Java class. Let's create a MyPair class to represent a pair of related String objects.

Note: the getters/setters should be public, and we annotate toString() with @JsonValue to ensure Jackson uses this custom toString() when serializing:

public class MyPair {

    private String first;
    private String second;
    
    @Override
    @JsonValue
    public String toString() {
        return first + " and " + second;
    }
 
    // standard getter, setters, equals, hashCode, constructors
}

Then we'll tell Jackson how to serialize MyPair by extending Jackson's JsonSerializer:

public class MyPairSerializer extends JsonSerializer<MyPair> {

    private ObjectMapper mapper = new ObjectMapper();

    @Override
    public void serialize(MyPair value, 
      JsonGenerator gen,
      SerializerProvider serializers) 
      throws IOException, JsonProcessingException {
 
        StringWriter writer = new StringWriter();
        mapper.writeValue(writer, value);
        gen.writeFieldName(writer.toString());
    }
}

JsonSerializer, as the name suggests, serializes MyPair to JSON using MyPair‘s toString() method. Furthermore, Jackson provides many Serializer classes to fit our serialization requirements.

Next we apply MyPairSerializer to our Map<MyPair, String> with the @JsonSerialize annotation. Note that we've only told Jackson how to serialize MyPair because it already knows how to serialize String:

@JsonSerialize(keyUsing = MyPairSerializer.class) 
Map<MyPair, String> map;

Then let's test our map serialization:

map = new HashMap<>();
MyPair key = new MyPair("Abbott", "Costello");
map.put(key, "Comedy");

String jsonResult = mapper.writerWithDefaultPrettyPrinter()
  .writeValueAsString(map);

The serialized JSON output is:

{
  "Abbott and Costello" : "Comedy"
}

3.3. Map<Object, Object> Serialization

The most complex case is serializing a Map<Object, Object>, but most of the work is already done. Let's use Jackson's MapSerializer for our map, and MyPairSerializer, from the previous section, for the map's key and value types:

@JsonSerialize(keyUsing = MapSerializer.class)
Map<MyPair, MyPair> map;
	
@JsonSerialize(keyUsing = MyPairSerializer.class)
MyPair mapKey;

@JsonSerialize(keyUsing = MyPairSerializer.class)
MyPair mapValue;

Then let's test out serializing our Map<MyPair, MyPair>:

mapKey = new MyPair("Abbott", "Costello");
mapValue = new MyPair("Comedy", "1940s");
map.put(mapKey, mapValue);

String jsonResult = mapper.writerWithDefaultPrettyPrinter()
  .writeValueAsString(map);

The serialized JSON output using MyPair‘s toString() method is:

{
  "Abbott and Costello" : "Comedy and 1940s"
}

4. Deserialization

Deserialization converts a stream of bytes into a Java object that we can use in code. In this section, we'll deserialize JSON input into Maps of different signatures.

4.1. Map<String, String> Deserialization

For a simple case, let's take a JSON-formatted input string and convert it to a Map<String, String> Java collection:

String jsonInput = "{\"key\": \"value\"}";
TypeReference<HashMap<String, String>> typeRef 
  = new TypeReference<HashMap<String, String>>() {};
Map<String, String> map = mapper.readValue(jsonInput, typeRef);

We use Jackson's ObjectMapper, as we did for serialization, using readValue() to process the input. Also, note our use of Jackson's TypeReference, which we'll use in all of our deserialization examples to describe the type of our destination Map. Here is the toString() representation of our map:

{key=value}

4.2. Map<Object, String> Deserialization

Now let's change our input JSON and the TypeReference of our destination to Map<MyPair, String>:

String jsonInput = "{\"Abbott and Costello\" : \"Comedy\"}";

TypeReference<HashMap<MyPair, String>> typeRef 
  = new TypeReference<HashMap<MyPair, String>>() {};
Map<MyPair,String> map = mapper.readValue(jsonInput, typeRef);

We need to create a constructor for MyPair that takes a String with both elements and parses them to the MyPair elements:

public MyPair(String both) {
    String[] pairs = both.split("and");
    this.first = pairs[0].trim();
    this.second = pairs[1].trim();
}

The toString() of our Map<MyPair,String> object is:

{Abbott and Costello=Comedy}

There is another option when we deserialize into a Java class that contains a Map; we can use Jackson's KeyDeserializer class, one of the many Deserialization classes that Jackson offers. Let's annotate our ClassWithAMap with @JsonCreator, @JsonProperty, and @JsonDeserialize:

public class ClassWithAMap {

  @JsonProperty("map")
  @JsonDeserialize(keyUsing = MyPairDeserializer.class)
  private Map<MyPair, String> map;

  @JsonCreator
  public ClassWithAMap(Map<MyPair, String> map) {
    this.map = map;
  }
 
  // public getters/setters omitted
}

Here we're telling Jackson to deserialize the Map<MyPair, String> contained in ClassWithAMap, so we need to extend KeyDeserializer to describe how to deserialize the map's key, a MyPair object, from an input String:

public class MyPairDeserializer extends KeyDeserializer {

  @Override
  public MyPair deserializeKey(
    String key, 
    DeserializationContext ctxt) throws IOException, 
    JsonProcessingException {
      
      return new MyPair(key);
    }
}

Then we can test the deserialization using readValue:

String jsonInput = "{\"Abbott and Costello\":\"Comedy\"}";

ClassWithAMap classWithMap = mapper.readValue(jsonInput,
  ClassWithAMap.class);

Again, the toString() method of our ClassWithAMap's map gives us the output we expect:

{Abbott and Costello=Comedy}

4.3. Map<Object,Object> Deserialization

Finally, let's change our input JSON and the TypeReference of our destination to Map<MyPair, MyPair>:

String jsonInput = "{\"Abbott and Costello\" : \"Comedy and 1940s\"}";
TypeReference<HashMap<MyPair, MyPair>> typeRef 
  = new TypeReference<HashMap<MyPair, MyPair>>() {};
Map<MyPair,MyPair> map = mapper.readValue(jsonInput, typeRef);

The toString() of our Map<MyPair, MyPair> object is:

{Abbott and Costello=Comedy and 1940s}

5. Conclusion

In this brief article, we learned how to serialize and deserialize Java Maps to and from JSON-formatted Strings.

As always, the example provided in this article is available in the GitHub repository.

res – Jackson (eBook) (cat=Jackson)

Do JSON right with Jackson

Download the E-book