Learn SPRING-BOOT with Real Code Examples

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;

@SpringBootApplication
public class CounterApplication {
    public static void main(String[] args) {
        SpringApplication.run(CounterApplication.class, args);
    }
}

@RestController
@RequestMapping("/counter")
class CounterController {
    private int count = 0;

    @GetMapping
    public int getCount() {
        return count;
    }

    @PostMapping("/increment")
    public int increment() {
        return ++count;
    }

    @PostMapping("/decrement")
    public int decrement() {
        return --count;
    }

    @PostMapping("/reset")
    public int reset() {
        count = 0;
        return count;
    }
}

Demonstrates a simple Spring Boot REST API with a counter using a Controller and in-memory state.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;

@SpringBootApplication
public class HelloApplication {
    public static void main(String[] args) {
        SpringApplication.run(HelloApplication.class, args);
    }
}

@RestController
class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello World";
    }
}

A minimal Spring Boot API returning 'Hello World'.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;
import java.util.Map;

@SpringBootApplication
public class EchoApplication {
    public static void main(String[] args) {
        SpringApplication.run(EchoApplication.class, args);
    }
}

@RestController
class EchoController {
    @PostMapping("/echo")
    public Map<String, Object> echo(@RequestBody Map<String, Object> payload) {
        return payload;
    }
}

Echoes back JSON sent in POST request.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;

@SpringBootApplication
public class GreetApplication {
    public static void main(String[] args) {
        SpringApplication.run(GreetApplication.class, args);
    }
}

@RestController
class GreetController {
    @GetMapping("/greet")
    public String greet(@RequestParam(defaultValue="Guest") String name) {
        return "Hello " + name;
    }
}

Greets user using query parameters.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;

@SpringBootApplication
public class UserApplication {
    public static void main(String[] args) {
        SpringApplication.run(UserApplication.class, args);
    }
}

@RestController
@RequestMapping("/users")
class UserController {
    @GetMapping("/{id}")
    public String getUser(@PathVariable int id) {
        return "User " + id;
    }
}

Returns info for a user based on path variable.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;
import java.util.concurrent.CompletableFuture;

@SpringBootApplication
public class AsyncApplication {
    public static void main(String[] args) {
        SpringApplication.run(AsyncApplication.class, args);
    }
}

@RestController
class AsyncController {
    @GetMapping("/async")
    public CompletableFuture<String> asyncHello() {
        return CompletableFuture.supplyAsync(() -> {
        try { Thread.sleep(1000); } catch (InterruptedException e) {}
        return "Async Hello";
        });
    }
}

Asynchronous endpoint returning a delayed response.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.servlet.NoHandlerFoundException;

@SpringBootApplication
public class ErrorApplication {
    public static void main(String[] args) {
        SpringApplication.run(ErrorApplication.class, args);
    }
}

@RestController
@ControllerAdvice
class ErrorController {
    @ExceptionHandler(NoHandlerFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    public String handle404() {
        return "Custom 404 Not Found";
    }
}

Custom error handling for 404 requests.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;
import java.util.Map;

@SpringBootApplication
public class CombinedApplication {
    public static void main(String[] args) {
        SpringApplication.run(CombinedApplication.class, args);
    }
}

@RestController
class CombinedController {
    private int count = 0;

    @GetMapping("/counter")
    public int getCount() { return count; }
    @PostMapping("/counter/increment")
    public int increment() { return ++count; }
    @PostMapping("/counter/decrement")
    public int decrement() { return --count; }
    @PostMapping("/counter/reset")
    public int reset() { return count = 0; }

    @PostMapping("/echo")
    public Map<String, Object> echo(@RequestBody Map<String, Object> payload) { return payload; }

    @GetMapping("/greet")
    public String greet(@RequestParam(defaultValue="Guest") String name) { return "Hello " + name; }
}

Multiple endpoints: counter, echo, and greeting combined.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;

@SpringBootApplication
public class HealthApplication {
    public static void main(String[] args) {
        SpringApplication.run(HealthApplication.class, args);
    }
}

@RestController
class HealthController {
    @GetMapping("/health")
    public String health() { return "OK"; }
}

Simple health check endpoint returning status.