Map Serialization and Deserialization with Jackson

向右看齐 2022-06-13 11:26 133阅读 0赞

## 1. Overview ##

In this article, we'll look at **serialization and deserialization of Java maps using**  [**Jackson**][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>2.8.7</version>
    </dependency>

You 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 which map a key *Object* to a value *Object* and are often the least intuitive objects to serialize.

### **3.1.**  Map<String, String>  **Serialization** ###

For the 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, which 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** ###

You can serialize a map containing a custom Java class with a few extra steps. 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
    }

Now let's 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. Jackson provides many [Serializer classes][] to fit your serialization requirements.

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;

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;

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 the 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();
    }

And the *toString()* of our *Map<MyPair,String>* object is:

{Abbott and Costello=Comedy}

**There is another option for the case when we deserialize into a Java class that contains a *\**** Map **\*— we can use Jackson's *\**** KeyDeserializer **\* class** , one of many [Deserialization][] classes that Jackson offers. We 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
    }

We are 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);
        }
    }

We test the deserialization out 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** ###

Lastly, 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);

And the *toString()* of our *Map<MyPair, MyPair>* object is:

{
       Abbott and Costello=Comedy and 1940s}

## 5. Conclusion ##

In this quick tutorial, we've seen how to serialize and deserialize Java *Maps* to and from JSON-formatted Strings.

As always, you can check out the example provided in this article in the [Github repository][].

原文链接：[http://www.baeldung.com/jackson-map][http_www.baeldung.com_jackson-map]

[Jackson]: https://github.com/FasterXML/jackson
[here]: http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22com.fasterxml.jackson.core%22%20AND%20a%3A%22jackson-databind%22
[Serializer classes]: https://fasterxml.github.io/jackson-databind/javadoc/2.3.0/com/fasterxml/jackson/databind/ser/std/package-summary.html
[Deserialization]: https://fasterxml.github.io/jackson-databind/javadoc/2.3.0/com/fasterxml/jackson/databind/deser/package-summary.html
[Github repository]: https://github.com/eugenp/tutorials/tree/master/jackson
[http_www.baeldung.com_jackson-map]: http://www.baeldung.com/jackson-map