Lecture 32, Tue 03/03

What is JSON?

JSON is used in a few places in the course, and widely throughout real-world software development.

Let’s talk a little about what JSON is, and how its used.

JSON in a Nutshell

• JSON stands for “JavaScript Object Notation” but it’s used far more widely than just in JavaScript
• JSON has six data types: string, number, boolean, null, object, array

This page explains the six types quite nicely: https://restfulapi.net/json-data-types/

A few tips

• JSON looks a lot like Python syntax for lists and dictionaries, but one difference is that in JSON you must use double quotes, not single quotes.
• If you load json in a browser, depending on browser settings and extensions, it might or might not “format” it.
• If you get a bunch of JSON and it looks awful and hard to read, there are solutions:
  • Online “pretty-printers” such as: https://jsonformatter.org/json-pretty-print
  • Load into an editor such as VSCode, load a JSON formatting extension, and hit reformat
    • Reformat: Windows Shift + Alt + F, Mac Shift + Option + F, Ubuntu Ctrl + Shift + I.

Where JSON is used in CS56 this quarter

• localhost.json and heroku.json for the secrets file in all the projects that use GitHub or Google OAuth secrets
  • It stores json data that we load into an environment variable SPRING_APPLICATION_JSON that can override values in the application.properties files
• lab07 and the individual project track for API access
  • USGS Earthquakes API, e.g. https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&minmagnitude=2&maxradiuskm=100&latitude=34.414000&longitude=-119.848900
  • Location Search API
• ucsb-courses-search for:
  • the UCSB Curriculums API

JSON in Java

Note that the following is general Java, NOT Spring Boot specific.

There are a variety of libraries for dealing with JSON in Java.

• To the best of my knowledge, are all third-party, and thus require you to have appropriate dependency elements in your Maven pom.xml (or the equivalent for whatever build system you are using; in this course, it’s always Maven for now.)

The library we are using is called Jackson. In the pom.xml you’ll find:

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.10.0</version>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.10.0</version>
        </dependency>

With these two libraries, we can write so-called serializer/deserializer code for JSON.

• Serializer code turns Java Objects in JSON
• Deserializer code turns JSON into Java Objects.

More generally: Serializer/deserializer code converts between:

• objects of any given programming language, and
• a “serial format” that can be stored in a disk file, or sent over a network connection.

Examples from our code bases:

From UCSB Courses Search: https://github.com/ucsb-cs56-w20/ucsb-courses-search/blob/8cb242874d34aec4e3f2bf62baf220b464655221/src/main/java/edu/ucsb/cs56/ucsbapi/academics/curriculums/v1/classes/CoursePage.java#L39

From lab07:

/**
     * Create a FeatureCollection object from json representation
     * 
     * @param json String of json returned by API endpoint {@code /classes/search}
     * @return a new FeatureCollection object
     * @see <a href=
     *      "https://tools.ietf.org/html/rfc7946">https://tools.ietf.org/html/rfc7946</a>
     */
    public static FeatureCollection fromJSON(String json) {
        try {
            ObjectMapper objectMapper = new ObjectMapper();
            objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

            FeatureCollection featureCollection = objectMapper.readValue(json, FeatureCollection.class);
            return featureCollection;
        } catch (JsonProcessingException jpe) {
            logger.error("JsonProcessingException:" + jpe);
            return null;
        } catch (Exception e) {
            logger.error("Exception:" + e);
            return null;
        }
    }