[BAEL-1659] Create REST microservices with javalin (#4145)

* [BAEL-1615] Code to support article.

* Adding example of provides...with

* Adding uses to main module

* [BAEL-1659] Creat REST microservices with javalin

* [BAEL-1659] Update article code based on feedback

* Revert parent pom change

* Using Optional instead of null

libraries/pom.xml

@@ -680,6 +680,12 @@
             <version>${unirest.version}</version>
         </dependency>
 
+        <!-- javalin -->
+        <dependency>
+            <groupId>io.javalin</groupId>
+            <artifactId>javalin</artifactId>
+            <version>1.6.0</version>
+        </dependency>
         <!-- Atlassian Fugue -->
         <dependency>
             <groupId>io.atlassian.fugue</groupId>

libraries/src/main/java/com/baeldung/javalin/JavalinApp.java

@@ -0,0 +1,16 @@
+package com.baeldung.javalin;
+
+import com.baeldung.javalin.User.UserController;
+import io.javalin.Javalin;
+
+public class JavalinApp {
+    public static void main(String[] args) {
+        Javalin app = Javalin.create()
+                .port(7000)
+                .start();
+
+        app.get("/hello", ctx -> ctx.html("Hello, Javalin!"));
+        app.get("/users", UserController.fetchAllUsernames);
+        app.get("/users/:id", UserController.fetchById);
+    }
+}

libraries/src/main/java/com/baeldung/javalin/User/User.java

@@ -0,0 +1,11 @@
+package com.baeldung.javalin.User;
+
+public class User {
+    public final int id;
+    public final String name;
+
+    public User(int id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+}

libraries/src/main/java/com/baeldung/javalin/User/UserController.java

@@ -0,0 +1,24 @@
+package com.baeldung.javalin.User;
+
+import io.javalin.Handler;
+
+import java.util.Objects;
+
+public class UserController {
+    public static Handler fetchAllUsernames = ctx -> {
+        UserDao dao = UserDao.instance();
+        Iterable<String> allUsers = dao.getAllUsernames();
+        ctx.json(allUsers);
+    };
+
+    public static Handler fetchById = ctx -> {
+        int id = Integer.parseInt(Objects.requireNonNull(ctx.param("id")));
+        UserDao dao = UserDao.instance();
+        User user = dao.getUserById(id);
+        if (user == null) {
+            ctx.html("Not Found");
+        } else {
+            ctx.json(user);
+        }
+    };
+}

libraries/src/main/java/com/baeldung/javalin/User/UserDao.java

@@ -0,0 +1,33 @@
+package com.baeldung.javalin.User;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+
+class UserDao {
+
+    private final List<User> users = Arrays.asList(
+            new User(0, "Steve Rogers"),
+            new User(1, "Tony Stark"),
+            new User(2, "Carol Danvers")
+    );
+
+    private static UserDao userDao = null;
+
+    private UserDao() {
+    }
+
+    static UserDao instance() {
+        if (userDao == null) {
+            userDao = new UserDao();
+        }
+        return userDao;
+    }
+
+    Optional<User> getUserById(int id) { return users.stream().filter(u -> u.id == id).findFirst(); }
+
+    Iterable<String> getAllUsernames() {
+        return users.stream().map(user -> user.name).collect(Collectors.toList());
+    }
+}