9 Spring Boot API Performance Tricks to Squeeze Every Millisecond

Introduction

As a system grows, API calls become a major performance bottleneck. Each request may involve database queries, object mapping, serialization, network I/O, and thread scheduling, any of which can add milliseconds to response time. In high‑concurrency scenarios, shaving off even a few milliseconds can significantly increase overall throughput.

1. Asynchronous Processing

Offload long‑running tasks to a separate thread to avoid blocking the main request thread.

@Service
public class AsyncService {
    @Async
    public void longRunningTask() {
        System.out.println("Starting long-running task");
        try {
            Thread.sleep(5000); // simulate long task
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Long-running task completed");
    }
}

Controller example using DeferredResult:

@GetMapping("/deferred")
@ResponseBody
public DeferredResult<Map<String, Object>> deferred() {
    long start = System.currentTimeMillis();
    System.out.printf("%s - start: %d%n", Thread.currentThread().getName(), start);
    DeferredResult<Map<String, Object>> result = new DeferredResult<>();
    new Thread(() -> {
        try {
            TimeUnit.SECONDS.sleep(3);
            Map<String, Object> data = new HashMap<>();
            data.put("code", 1);
            data.put("data", "your business data");
            result.setResult(data);
        } catch (InterruptedException ignored) {}
    }).start();
    long end = System.currentTimeMillis();
    System.out.printf("%s - end: %d%n", Thread.currentThread().getName(), end);
    System.out.printf("Total time: %d ms%n", (end - start));
    return result;
}

2. Caching

Cache rarely‑changing data to avoid repeated database or remote calls.

@Service
@CacheConfig(cacheNames = {"userCache"})
public class UserService {
    private final Logger logger = LoggerFactory.getLogger(UserService.class);

    @Cacheable(key = "#id")
    public User queryById(Long id) {
        logger.info("Querying user from DB: {}", id);
        return new User(id, "Name-" + new Random().nextInt(1000), new Random().nextInt(100));
    }
}

3. Database Query Optimization

Limit selected fields and avoid complex joins.

@Service
public class DatabaseOptimizationService {
    @PersistenceContext
    private EntityManager entityManager;

    public List<Object> optimizedQuery() {
        Query query = entityManager.createQuery("SELECT field1, field2 FROM MyTable WHERE condition");
        return query.getResultList();
    }
}

Projection interface example:

public interface UserProjection {
    Long getId();
    String getUsername();
    String getPhone();
}

public interface UserRepository extends JpaRepository<User, Long> {
    @Query("SELECT u.id AS id, u.username AS username, u.phone AS phone FROM User u WHERE u.status = 1")
    List<UserProjection> findUserProjection();
}

4. Response Compression

Compress large JSON or XML payloads with GZIP and set the appropriate response header.

@RestController
public class CompressionController {
    @GetMapping("/compressedData")
    public void getCompressedData(HttpServletResponse response) throws IOException {
        String data = "This is a large amount of data that needs to be compressed...";
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        GZIPOutputStream gzip = new GZIPOutputStream(baos);
        gzip.write(data.getBytes());
        gzip.close();
        response.addHeader("Content-Encoding", "gzip");
        response.getOutputStream().write(baos.toByteArray());
    }
}

Enable compression globally:

server:
  compression:
    enabled: true
    min-response-size: 1024
    mime-types: text/html,text/xml,text/plain,text/css,text/javascript,application/javascript,application/json,application/xml

5. WebFlux Reactive Endpoints

Use Spring WebFlux to handle massive concurrent requests with low resource consumption.

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

@RestController
public class WebFluxController {
    @GetMapping("/reactiveData")
    public Mono<String> getReactiveData() {
        return Mono.just("Reactive programming data");
    }
}

6. Logging Optimization

Reduce unnecessary logging; check log level before emitting messages.

@RestController
public class LoggingController {
    private static final Logger logger = LoggerFactory.getLogger(LoggingController.class);

    @GetMapping("/efficientLogging")
    public String getEfficientLogging() {
        if (logger.isInfoEnabled()) {
            logger.info("Efficient logging");
        }
        return "Logging optimization example";
    }
}

7. Index Optimization

Create composite indexes that match query predicates.

CREATE INDEX idx_username_email_age ON User(username, email, age);

Repository method leveraging the index:

public interface UserRepository extends JpaRepository<User, Long> {
    User findByUsernameAndEmail(String username, String email);
}

8. Connection‑Pool Configuration

Configure HikariCP to reuse connections and reduce creation overhead.

common-datasource: &common-ds
  driver-class-name: com.mysql.cj.jdbc.Driver
  username: root
  password: '{cipher}...'
  type: com.zaxxer.hikari.HikariDataSource
spring:
  datasource:
    url: jdbc:mysql://localhost:3306/testjpa
    <<: *common-ds
    hikari:
      minimumIdle: 20
      maximumPoolSize: 50
      autoCommit: true

9. CDN for Static Assets

Serve images, CSS, and JavaScript from a CDN to reduce latency for end users.

<script src="https://unpkg.com/vue@3/dist/vue.global.js"></script>

Placing static resources on a CDN shortens load time, indirectly improving overall API response speed.