1. Overview
JSON-LD is a JSON-based RDF format for representing Linked Data. It enables extending existing JSON objects with hypermedia capabilities; in other words, the capability to contain links in a machine-readable way.
In this tutorial, we’ll look at a couple of Jackson-based options to serialize and deserialize the JSON-LD format directly into POJOs. We’ll also cover the basic concepts of JSON-LD that will enable us to understand the examples.
2. Basic Concepts
The first time we see a JSON-LD document, we notice that some member names start with the @ character. These are JSON-LD keywords, and their values help us to understand the rest of the document.
To navigate the world of JSON-LD and to understand this tutorial, we need to be aware of four keywords:
- _@context _is the description of the JSON object that contains a key-value map of everything needed for the interpretation of the document
- @vocab is a possible key in @context that introduces a default vocabulary to make the @context object much shorter
- _@id _is the keyword to identify links either as a resource property to represent the direct link to the resource itself or as a @type value to mark any field as a link
- @type is the keyword to identify resource types either on the resource level or in the @context; for example, to define the type of embedded resources
3. Serialization in Java
Before we continue, we should take a look at our previous tutorials to refresh our memory on the Jackson ObjectMapper, Jackson Annotations, and custom Jackson Serializers.
Being already familiar with Jackson, we might realize that we could easily serialize two custom fields in any POJO as @id and @type using the @JsonProperty annotation. However, writing the @context by hand could be a lot of work and also prone to error.
Therefore, to avoid this error-prone approach, let’s take a closer look at two libraries that we could use for @context generation. Unfortunately, neither one of them is capable of generating all features of JSON-LD, but we’ll take a look at their shortcomings later as well.
4. Serialization With Jackson-Jsonld
Jackson-Jsonld** is a Jackson module that enables the annotation of POJOs in a convenient way to generate JSON-LD documents.**
4.1. Maven Dependencies
First, let’s add jackson-jsonld as a dependency to the pom.xml:
<dependency> <groupId>com.io-informatics.oss</groupId> <artifactId>jackson-jsonld</artifactId> <version>0.1.1</version></dependency>
4.2. Example
Then, let’s create our example POJO and annotate it for @context generation:
@JsonldResource@JsonldNamespace(name = "s", uri = "http://schema.org/")@JsonldType("s:Person")@JsonldLink(rel = "s:knows", name = "knows", href = "http://example.com/person/2345")public class Person { @JsonldId private String id; @JsonldProperty("s:name") private String name; // constructor, getters, setters}
Let’s deconstruct the steps to understand what we’ve done:
- With @JsonldResource we marked the POJO for processing as a JSON-LD resource
- In the @JsonldNamespace we defined a shorthand for the vocabulary we want to use
- The parameter we specified in @JsonldType will become the @type of the resource
- We used the @JsonldLink annotation to add links to the resource. When processed, the name parameter will be used as a field name and also added as a key to the @context. href will be the field value and rel will be the mapped value in the @context
- The field we marked with @JsonldId will become the @id of the resource
- The parameter we specified in @JsonldProperty will become the value mapped to the field’s name in the @context
Next, let’s generate the JSON-LD document.
#data #jackson #json #data analysis
