Spring Boot Basics — DI, Controllers & Services

8. Practice Problems

1. Extend the hand-rolled DI demo with a Logger that both OrderService and OrderController depend on. Verify the log line prints at each layer.
   হাতে-লেখা DI demo-তে একটি Logger যোগ করুন যা Service ও Controller দুই জায়গায় dependency। প্রতিটি layer-এ log যায় কিনা যাচাই করুন।
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   Main.java

   class Logger { void log(String m){ System.out.println("[LOG] "+m); } }
   
   class OrderService {
       private final Logger log;
       OrderService(Logger log){ this.log = log; }
       String summary(){ log.log("service"); return "42"; }
   }
   
   class OrderController {
       private final OrderService svc; private final Logger log;
       OrderController(OrderService s, Logger l){ svc=s; log=l; }
       String handle(){ log.log("controller"); return svc.summary(); }
   }
   
   class Main {
       public static void main(String[] a) {
           Logger l = new Logger();
           System.out.println(new OrderController(new OrderService(l), l).handle());
       }
   }

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2. Why is constructor injection preferred over field injection (@Autowired on a field)?
   Constructor injection field injection-এর চেয়ে কেন ভালো?
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   Answer: (1) Dependencies become final and can never be forgotten — the compiler refuses to build an incomplete object. (2) The class is testable without Spring — you just call its constructor in a test. (3) Circular dependencies surface as clear compile/startup errors instead of mysterious runtime NPEs. (4) The class's required collaborators become visible at a glance from its signature.

3. Write what a Spring Boot @RestController method for GET /health returning {"status":"UP"} looks like.
   Spring Boot-এ GET /health route লিখুন যা {"status":"UP"} JSON ফেরাবে।
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   HealthController.java

   import org.springframework.web.bind.annotation.*;
   import java.util.Map;
   
   @RestController
   class HealthController {
       @GetMapping("/health")
       public Map<String,String> health() {
           return Map.of("status", "UP");
       }
   }

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4. Build a hand-rolled "Spring profile" switcher: a Config class reads an env variable APP_ENV (default dev) and prints a different greeting for dev vs prod.
   হাতে-লেখা Spring profile বানান — env variable APP_ENV পড়ে dev/prod-এ ভিন্ন greeting প্রিন্ট করুন।
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   Main.java

   class Main {
       public static void main(String[] args) {
           String env = System.getenv("APP_ENV");
           if (env == null) env = "dev";
           System.out.println(switch (env) {
               case "prod" -> "Running in PRODUCTION — be careful.";
               case "dev"  -> "DEV mode — debug logs ON.";
               default      -> "Unknown env: " + env;
           });
       }
   }

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5. Explain, in 3 sentences, what happens when you call SpringApplication.run(App.class, args).
   তিন বাক্যে বলুন — SpringApplication.run(App.class, args) চালালে কী ঘটে।
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   Answer: (1) Spring bootstraps the ApplicationContext and scans the classpath for @Component / @Service / @RestController beans, instantiating and wiring them in dependency order. (2) Auto-configuration kicks in — Spring detects that spring-boot-starter-web is on the classpath and starts the embedded Tomcat server on port 8080. (3) The app stays up, listens for HTTP traffic, and dispatches requests to your mapped handler methods until it is stopped.