Managing books

Of course, there wouldn’t be much point in always responding with the same books: we want to be able to manage them. To do this, we will need to add endpoints in our BooksController for creating, updating, and deleting books:

• POST /books should accept a Book and add it to our collection.
• GET /books/id should return the Book with that id.
• PUT /books/id should accept a Book and update the title and author of the book with that id.
• DELETE /books/id should delete the Book with that id.

We will keep track of the books in a Java map, so they will be lost when we restart the application. We will cover how to store the books in a database in later practicals.

Adding books

The declaration of the controller method for POST /books would look like this:

@Post
public void createBook(@Body Book book) {}

Note how the method has a book parameter annotated with @Body: this indicates to Micronaut that the endpoint will deserialise a JSON-formattted Book object from the HTTP request body. All the possible ways in which you can take parts of the HTTP request and bind them to variables are listed in the official Micronaut documentation.

Using the Java Collections Framework, add a map from integers to Book objects to your BooksController. If you are unfamiliar with the JCF, consult the Java Collections Framework resources in the Java knowledge map of the VLE.

Implement the createBook method so it updates the map, and change the getBooks method so it returns the values of the map.

Try restarting the application, adding a few books through POST /books, and listing them through GET /books.

Getting a specific book

The next method will be for retrieving a specific book:

@Get("/{id}")
public Book getBook(@PathVariable int id) {}

@PathVariable is for binding a part of the path of the URL to a variable: specifically, the ID of the book that we want to fetch. You can implement this method by simply returning the Book with that id, or null if we do not have it.

Try restarting the application. Try this:

• Add a book with POST /books.
• Check that GET /books/{id} with the given id returns it, with an HTTP 200 OK response.
• Check that GET /books/{id} with an id that we do not have produces an HTTP 404 Not Found response. This is because if a controller method returns null instead of a DTO, Micronaut will map that to a 404 response.

Updating a book

You should be able to write the declaration of the method for PUT /books/{id} yourself, based on the above examples: just use @Put as the annotation for the method (with the appropriate string parameter).

Follow this approach:

• Get the Book with the given id in your map. If it does not exist, respond with HTTP 404: when your method does not simply return a DTO, you can instead throw a new HttpStatusException(HttpStatus.NOT_FOUND, message), where the message is up to you.
• Update the title of the book in your map.
• Update the author of the book in your map.

Note how this HTTP endpoint does not use the id inside the Book object sent in the request, but instead uses the id in the URL. In fact, for update endpoints like these we would normally use a dedicated BookUpdateDTO that would not list id as a valid field.

Try it out in the Swagger UI before moving on.

Deleting a book

You should be able to write this method entirely yourself from the above examples, using the @Delete annotation on the new controller method, together with the appropriate string parameter.