Spring Boot: How to handle ConcurrentModificationException when using HashMap in a multi-threaded environment?
I'm trying to configure I'm attempting to set up This might be a silly question, but I'm encountering a `ConcurrentModificationException` when using a `HashMap` to store user sessions in my Spring Boot application. The application is designed to handle multiple requests concurrently, and I'm updating the map from different threads. Here's a simplified version of my code: ```java import org.springframework.stereotype.Service; import java.util.HashMap; import java.util.Map; @Service public class SessionService { private final Map<String, String> userSessions = new HashMap<>(); public void addUserSession(String userId, String sessionId) { userSessions.put(userId, sessionId); } public void removeUserSession(String userId) { userSessions.remove(userId); } public String getSession(String userId) { return userSessions.get(userId); } } ``` In my controller, I'm invoking these methods in a multi-threaded context with the following snippet: ```java @RestController public class SessionController { private final SessionService sessionService; public SessionController(SessionService sessionService) { this.sessionService = sessionService; } @PostMapping("/session") public ResponseEntity<String> createSession(@RequestParam String userId, @RequestParam String sessionId) { sessionService.addUserSession(userId, sessionId); return ResponseEntity.ok("Session created"); } @DeleteMapping("/session") public ResponseEntity<String> deleteSession(@RequestParam String userId) { sessionService.removeUserSession(userId); return ResponseEntity.ok("Session deleted"); } } ``` When I perform concurrent requests to create and delete sessions, occasionally I get the following stack trace: ``` java.util.ConcurrentModificationException at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1583) at java.base/java.util.HashMap$KeyIterator.next(HashMap.java:1603) at com.myapp.SessionService.getSession(SessionService.java:20) ... ``` I have tried synchronizing the methods in `SessionService`, but that leads to performance bottlenecks and defeats the purpose of using a concurrent design. I also considered using `Collections.synchronizedMap()`, but I heard that it might still not be safe for concurrent access under heavy load. What are the best practices to handle concurrent modifications in a scenario like this? Should I switch to a different data structure, or would implementing proper locking mechanisms suffice? Any insights would be greatly appreciated! I'd really appreciate any guidance on this. This issue appeared after updating to Java latest.