Jackson, JAXB, OpenAPI and Spring

Imagine that you have a service that enriches some data. The model for the data that needs to be enriched is generated from an XSD. The model for the enriching data (the wrapper) is generated from OpenAPI specs. Those two separate models need to be combined and sent out via Spring’s RestTemplate. The problem is that the generated models interfere with each other, and the combination of models doesn’t serialize correctly.

To solve the problem of interference, we’re going to transform the XSD-generated model to a generic JSON model (using the org.json library). That way we eliminate the XML annotations, and we can just send the OpenAPI model.

XML model to JSONObject

Our wrapper model accepts any Object as payload. We can set our XmlModel there, but we can also set a JSONObject. However, we need to transform our XML model to JSONObject. We can use Jackson for this, when we use the right AnnotationInspector. We’re not going to transform the model directly to JSONObject, but we use a String representation as an intermediate step. Once we have converted our XmlModel to JSONObject, we can set that as our payload.

private JSONObject toJSONObject(XmlModel xmlModel) {
  try {
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.setAnnotationIntrospector(
    new JaxbAnnotationIntrospector(TypeFactory.defaultInstance()));
    String content = objectMapper.writeValueAsString(xmlModel);
    return new JSONObject(content);
  } catch (JsonProcessingException e) {
    // Shouldn't happen.
    log.severe("Error translating to JSONObject");
    throw new IllegalStateException(
      "Error translating to JSONObject");
  }
}

Configure RestTemplate

Now we have a model that can serialize, but our RestTemplate can’t handle it yet. Specifically, The ObjectMapper that the RestTemplate uses can’t handle the JSONObject. But we can configure our custom ObjectMapper and tell the RestTemplate to use that one. To serialize the JSONObject, we need to add the JsonOrgModule. And while we’re at it, we’re going to add the JavaTimeModule so we can serialize dates and times. We can’t set the ObjectMapper directly, we need to set it on a MessageConverter. Then we need to explicitly set our MessageConverter as the first to be sure that it’s going to be used.

@Bean
public RestTemplate myRestTemplate() {
  // Create and configurae
  ObjectMapper objectMapper = new ObjectMapper();
  objectMapper.registerModule(new JsonOrgModule());
  objectMapper.registerModule(new JavaTimeModule());

  // Configure MessageConverter
  MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
  converter.setObjectMapper(objectMapper);

  // Configure RestTemplate
  RestTemplate restTemplate =
     // create the RestTemplate;
     restTemplate.getMessageConverters()
       .add(0, outputgeneratiebeheerMessageConverter);

  return restTemplate;
}

Maven dependencies

The artifact jackson-module-jaxb-annotations contains the JaxbAnnotationIntrospector that we used to serialize the XML model. The jackson-datatype-json-org contains the JsonOrgModule that we used to serialize the JsonObjects. These dependencies need to be added, in addition to jackson

<properties>
  <jackson.version>2.12.1</jackson.version>
</properties>

<dependencies>
  <dependency>
    <groupId>com.fasterxml.jackson.module</groupId>
    <artifactId>jackson-module-jaxb-annotations</artifactId>
    <version>${jackson.version}</version>
  </dependency>
  <dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-json-org</artifactId>
    <version>${jackson.version}</version>
  </dependency>
</dependencies>

Java XML Api

Who uses XML in 2021? We all use JSON these days, aren’t we? Well, it turns out XML is still being used. These code fragments could help get you up to speed when you’re new to the Java XML API.

Create an empty XML document

To start from scratch, you’ll need to create an empty document:


Document doc = DocumentBuilderFactory
                .newInstance()
                .newDocumentBuilder()
                .newDocument();

Create document from existing file

To load an XML file, use the following fragment.


File source = new File("/location/of.xml");
Document doc = DocumentBuilderFactory
                    .newInstance()
                    .newDocumentBuilder()
                    .parse(target);

Add a new element

Now that we have the document, it’s time to add some elements and attributes. Note that Document and Element are both Nodes.


final Element newNode = doc.createElement(xmlElement.getName());
newNode.setAttribute(attributeName, attributeValue);
	
node.appendChild(newNode);

Get nodes matching XPath expression

XPath is a powerful way to search your XML document. Every XPath expression can match multiple nodes, or it can match none. Here’s how to use it in Java:


final String xPathExpression = "//node";
final XPath xpath = XPathFactory.newInstance().newXPath();
NodeList nodeList = (NodeList) xpath.evaluate(xPathExpression, 
        doc, 
        XPathConstants.NODESET);

JSON to XML

JSON is simpler and much more in use these days. However, it has less features than XML. Namespaces and attributes are missing, for example. Usually these aren’t needed, but they can be useful. To convert JSON to XML, you could use the org.json:json dependency. This will create an XML structure similar to the input JSON.

<properties>
	<org-json.version>20201115</org-json.version>
</properties>

<dependencies>
	<dependency>
		<groupId>org.json</groupId>
		<artifactId>json</artifactId>
		<version>${org-json.version}</version>
	</dependency>
</dependencies>

JSONObject content = new JSONObject(json);
String xmlFragment = XML.toString(content);

Writing XML

When we’re done manipulating the DOM, it’s time to write the XML to file or to a String. The following fragment does the trick


private void writeToConsole(final Document doc) 
    throws TransformerException{
	final StringWriter writer = new StringWriter();
	writeToTarget(doc, new StreamResult(writer));
	System.out.println(writer.toString());
}

private void writeToFile(final Document doc, File target) 
    throws TransformerException, IOException{
	try (final FileWriter fileWriter = new FileWriter(target)) {
		final StreamResult streamResult = new StreamResult(fileWriter);
		writeToTarget(doc, streamResult);
	}
}

private void writeToTarget(final Document doc, final StreamResult target) 
    throws TransformerException {
	final Transformer xformer = TransformerFactory.newInstance()
              .newTransformer();
	xformer.transform(new DOMSource(doc), target);
}

Spring Boot, MongoDB and raw JSON

Sometimes you want to store and retrieve raw JSON in MongoDB. With Spring Boot storing the JSON isn’t very hard, but retrieving can be a bit more challenging.

Setting up

To start using MongoDB from Spring Boot, you add the dependency to spring-boot-starter-data-mongodb

	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-data-mongodb</artifactId>
	</dependency>

And then you inject MongoTemplate into your class

@Autowired
private MongoTemplate mongoTemplate;

Inserting into MongoDB

Inserting JSON is just a matter of converting the JSON into a Document, and inserting that document into the right collection

String json = getJson();
Document doc = Document.parse(json);
mongoTemplate.insert(doc, "CollectionName");

Retrieving JSON

Retrieving JSON is a bit more complicated. First you need to get a cursor for the collection. This allows you to iterate over all the documents within that collection. Then you’ll retrieve each document from the collection, and cast it to a BasicDBObject. Once you have that, you can retrieve the raw JSON.

DBCursor cursor = mongoTemplate.getCollection("CollectionName").find();
Iterator iterator = cursor.iterator();
while (iterator.hasNext()){
   BasicDBObject next = (BasicDBObject) iterator.next();
   String json = next.toJson();
   // do stuff with json
}

Transforming raw JSON to Object

With Jackson you can transform the retrieved JSON to an object. However, your object might miss a few fields, since MongoDB adds some to keep track of the stored documents. To get around this problem, you need to configure the ObjectMapper to ignore those extra fields.

ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
MyObject object = mapper.readValue(json, MyObject.class);