1. Understanding Singleton Scope in Spring

The singleton scope in Spring ensures that only one instance of a bean is created and shared across the entire application context. This single instance is reused by all requests that need access to the bean.

1.1 What is Singleton Scope?

Singleton scope is the default bean scope in Spring. When a bean is declared as a singleton, Spring creates one instance of the bean and maintains it within the application context for the entire lifecycle of the application. Any subsequent requests for that bean return the same instance.

1.2 Code Example of Singleton Bean

@Component
public class SingletonService {
    public void process() {
        System.out.println("Processing request by " + Thread.currentThread().getName());
    }
}

1.3 Demo Code to Illustrate Singleton Behavior

@SpringBootApplication
public class SingletonDemoApplication {

    @Autowired
    private SingletonService singletonService;

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

    @PostConstruct
    public void simulateParallelRequests() {
        Runnable task = () -> singletonService.process();
        for (int i = 0; i < 5; i++) {
            new Thread(task).start();
        }
    }
}

1.4 Demo Results

Running the demo code would yield output like:

Processing request by Thread-1
Processing request by Thread-2
Processing request by Thread-3
Processing request by Thread-4
Processing request by Thread-5

Each thread is using the same SingletonService instance to process requests.

2. Why Singleton Scope Can Handle Multiple Parallel Requests

Despite the single instance nature of a singleton bean, it can handle multiple parallel requests effectively. This is possible due to how the JVM handles multiple threads and the design of the singleton beans themselves.

2.1 Thread-Safety in Singleton Beans

Singleton beans can handle multiple threads accessing them concurrently if the bean’s methods are thread-safe. Thread-safety ensures that multiple threads can access shared resources without causing inconsistent results.

Stateless Singleton Beans

If the singleton bean is stateless (i.e., it does not maintain any state across requests), it is inherently thread-safe. This is because there’s no shared state between threads that could lead to race conditions or other concurrency issues.

Stateful Singleton Beans

Stateful singleton beans, which maintain state across method invocations, require special care to ensure thread-safety. Developers need to implement synchronization mechanisms or use thread-local variables to ensure that each thread has its isolated copy of the state.

2.2 Code Example of Thread-Safe Singleton Bean

@Component
public class ThreadSafeService {

    public synchronized void safeProcess() {
        System.out.println("Thread-safe processing by " + Thread.currentThread().getName());
    }
}

2.3 Demo Code for Thread-Safe Singleton Bean

@SpringBootApplication
public class ThreadSafeDemoApplication {

    @Autowired
    private ThreadSafeService threadSafeService;

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

    @PostConstruct
    public void simulateParallelRequests() {
        Runnable task = () -> threadSafeService.safeProcess();
        for (int i = 0; i < 5; i++) {
            new Thread(task).start();
        }
    }
}

2.4 Demo Results

Running this code ensures that each thread processes the request sequentially due to the synchronized keyword:

Thread-safe processing by Thread-1
Thread-safe processing by Thread-2
Thread-safe processing by Thread-3
Thread-safe processing by Thread-4
Thread-safe processing by Thread-5

3. Conclusion

Singleton beans in Spring are powerful and versatile, capable of handling multiple parallel requests efficiently. However, when designing your application, it’s crucial to ensure that your singleton beans are thread-safe, especially if they maintain state. Understanding these nuances will help you avoid potential pitfalls in multi-threaded environments.

If you have any questions or need further clarification, feel free to comment below!

Read posts more at : Why Singleton Bean Scope Can Handle Multiple Parallel Requests And Key Considerations