Concurrent access occurs when multiple threads attempt to access shared resources simultaneously in a servlet environment.

Servlet containers handle multiple HTTP requests concurrently by creating separate threads for each request. This multi-threaded nature introduces potential race conditions when accessing shared data stored in SESSION and APPLICATION scopes.

• The REQUEST scope is inherently safe from concurrent access issues.
  Each HTTP request has its own dedicated instance of the REQUEST scope, and it is processed by a single thread throughout its lifecycle. This isolation ensures that one request cannot interfere with another request's data, making REQUEST scope the safest option for storing temporary data.
  
  
• The SESSION scope is not safe from concurrent access and requires careful handling.
  Multiple HTTP requests belonging to the same web session can access the SESSION scope instance simultaneously, especially in scenarios involving AJAX requests, multiple browser tabs, or frames. This concurrent access can lead to data corruption, inconsistent state, or race conditions when multiple threads modify session attributes simultaneously. The probability of concurrent access increases with applications that heavily use asynchronous requests or allow users to open multiple tabs.
  
  
• The APPLICATION scope presents the highest risk for concurrent access issues.
  Multiple HTTP requests from different users and sessions can access the APPLICATION scope instance simultaneously across the entire web application. Since APPLICATION scope data is shared among all users, improper handling can result in data corruption affecting all application users. This scope requires the most careful consideration regarding thread safety due to its global nature and high contention potential.
  

There are several approaches to handle concurrent access in servlet applications.

For SESSION Scope:

• Initialize the SESSION scope attributes when the web session is created, and use them only as read-only. This eliminates the need for synchronization since no modifications occur after initialization.
  
  
• Synchronize the block of code that reads from and modifies the SESSION scope attributes using synchronized blocks or methods. This ensures atomic operations but may impact performance.
  
  
• Use thread-safe data structures such as ConcurrentHashMap or Collections.synchronizedMap() for storing complex data in session attributes.
  

For APPLICATION Scope:

• Initialize the APPLICATION scope attributes when the web application is loaded by the web container using ServletContextListener, and use them only as read-only. This is the most efficient approach for configuration data or cached resources.
  
  
• Synchronize the block of code that reads from and modifies the APPLICATION scope attributes. Use fine-grained locking when possible to minimize performance impact.
  
  
• Consider using concurrent collections from java.util.concurrent package, such as ConcurrentHashMap, which provide better performance under high concurrency.
  
  
• Implement proper locking strategies, such as read-write locks, when you have frequent reads and infrequent writes to APPLICATION scope data.