No, Singleton beans are not inherently thread-safe in Spring. Since a single instance of a Singleton bean is shared across multiple threads, thread-safety depends on how the bean is designed and whether it maintains mutable state.
Key Points: • Spring creates only one instance of a Singleton bean per application context. • Multiple requests can access the same bean instance concurrently. • Stateless Singleton beans are generally thread-safe because they do not store request-specific data. • Stateful Singleton beans can cause race conditions and data inconsistency when accessed by multiple threads. • Thread safety can be achieved using synchronization, thread-safe collections, or by avoiding shared mutable state.
Example: A UserService bean that only performs business logic without storing data in instance variables is typically thread-safe. However, a bean that stores user-specific information in instance variables can lead to concurrency issues.
Code Example:
@Service
public class CounterService {
private int count = 0;
public void increment() {count++; // Not thread-safe
}
}Interview Tip: A concise interview answer is:
"Singleton beans are not thread-safe by default. Since the same bean instance is shared among multiple threads, thread safety depends on the bean's implementation. Stateless Singleton beans are generally safe, while stateful beans require proper synchronization or thread-safe design."