Java REST API Basics
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What is a REST API?
REST (Representational State Transfer) is an architectural style for networked applications that maps operations onto standard HTTP methods -- GET to read, POST to create, PUT to replace, DELETE to remove -- applied to named resources rather than custom action verbs. Following these conventions is what makes an API predictable to other developers without extra documentation.
Example: What is a REST API?
public class Main {
public static void main(String[] args) {
String[] methods = {"GET /users", "POST /users", "PUT /users/1", "DELETE /users/1"};
for (String m : methods) System.out.println(m); // standard HTTP methods on named resources
}
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Designing REST Endpoints
REST endpoints use clear, resource-oriented URL paths like /users or /products rather than verb-based paths like /getUsers. You map incoming requests to these paths inside your application's routing layer, which is what decides which handler code runs for a given method-and-path combination.
Example: Designing REST Endpoints
public class Main {
static String route(String path) {
return switch (path) {
case "/users" -> "UserListHandler"; // resource-oriented, not /getUsers
case "/products" -> "ProductListHandler";
default -> "NotFound";
};
}
public static void main(String[] args) {
System.out.println(route("/users"));
}
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Serving JSON Data
JSON is the de facto standard format REST APIs use to exchange data because it's compact, human-readable, and natively supported by JavaScript. In your controllers you should explicitly set the Content-Type response header to 'application/json' so clients know how to parse the body correctly.
Example: Serving JSON Data
public class Main {
public static void main(String[] args) {
String contentType = "application/json";
String json = "{\"id\":1,\"name\":\"Riya\"}";
System.out.println("Content-Type: " + contentType);
System.out.println(json);
}
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Query Parameters in REST
You can accept extra, optional parameters through the request URL using query parameters, which are appended after a '?' and separated by '&' (for example ?page=2&limit=10). These are the natural place for filtering, pagination, or sorting options that aren't part of the resource's identity.
Example: Query Parameters in REST
public class Main {
public static void main(String[] args) {
String url = "/products?page=2&limit=10"; // filtering/pagination via query params
String query = url.substring(url.indexOf('?') + 1);
System.out.println(query);
}
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Mock REST Controller
By combining an in-memory list as fake data, basic routing, and hand-built JSON strings, you can mock out a complete REST controller quickly to prototype and test an API's shape before wiring up a real database behind it.
Example: Mock REST Controller
import java.util.*;
public class Main {
public static void main(String[] args) {
List<String> users = new ArrayList<>(List.of("Riya", "Aman")); // in-memory fake data
String path = "/users";
if (path.equals("/users")) {
System.out.println("[\"" + String.join("\",\"", users) + "\"]");
}
}
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