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How do you ensure thread-safety and performance of Servlet Listeners?

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Servlet listeners are components that can monitor and react to events in the servlet container, such as application startup, session creation, or attribute changes. They can be useful for initializing resources, logging activities, or performing cleanup tasks. However, they also pose some challenges for thread-safety and performance, as they may be accessed by multiple threads concurrently or frequently. In this article, you will learn how to ensure thread-safety and performance of servlet listeners by following some best practices and avoiding common pitfalls.

本文章的要点总结

Embrace thread-safe collections:

Opt for data structures like ConcurrentHashMap or CopyOnWriteArrayList. These ensure your servlet listeners handle concurrent access smoothly, avoiding data corruption and boosting reliability.### *Delegate blocking tasks:Use ExecutorService to offload long-running operations from servlet listeners. This keeps your main threads free, improving overall application performance and responsiveness.

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Gokul Radhakrishnan
Sachin Gorade

Senior Software Engineer at Yelp

1 Use thread-safe data structures

One of the main sources of thread-safety issues in servlet listeners is the use of non-thread-safe data structures, such as ArrayList, HashMap, or StringBuilder. These classes are not designed to handle concurrent access and may result in inconsistent or corrupted data. To avoid this, you should use thread-safe data structures, such as CopyOnWriteArrayList, ConcurrentHashMap, or StringBuffer. These classes provide synchronization mechanisms to ensure atomicity and visibility of operations. Alternatively, you can use collections from the java.util.concurrent package, such as ConcurrentLinkedQueue, ConcurrentSkipListSet, or BlockingQueue. These classes offer more advanced features, such as lock-free algorithms, ordering guarantees, or blocking operations.

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2 Avoid blocking or long-running operations

Another factor that can affect the performance of servlet listeners is the execution of blocking or long-running operations, such as network calls, database queries, or file I/O. These operations can consume valuable resources, such as threads, memory, or CPU, and cause delays or timeouts for other requests. To avoid this, you should minimize the use of blocking or long-running operations in servlet listeners and delegate them to other components, such as asynchronous tasks, background threads, or schedulers. For example, you can use the java.util.concurrent.ExecutorService interface to submit tasks to a thread pool and execute them asynchronously. You can also use the javax.servlet.AsyncContext interface to create asynchronous requests and responses and free up the original thread.

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Gokul Radhakrishnan
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One thing i found useful was to prioritize the thread and use concurrency frameworks like Executor, ThreadPoolExecutor, or ForkJoinPool to manage your threads. I always advise engineers to define ThreadLogger using ThreadMXBean. Splunking this makes it easy in large systems.

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3 Use appropriate scopes and attributes

Another aspect that can influence the thread-safety and performance of servlet listeners is the choice of scopes and attributes. Servlet listeners can access and modify different scopes, such as application, session, or request, and different attributes, such as servlet context, session, or request attributes. However, not all scopes and attributes are suitable for all scenarios and may introduce concurrency problems or memory leaks. To avoid this, you should use appropriate scopes and attributes for your needs and follow some guidelines. For example, you should use application scope for global or shared data, such as configuration parameters, caches, or connection pools. You should use session scope for user-specific or stateful data, such as authentication information, shopping carts, or preferences. You should use request scope for short-lived or transient data, such as request parameters, headers, or cookies. You should also avoid storing large or complex objects as attributes, as they may consume too much memory or cause serialization issues.

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4 Implement the javax.servlet.ServletContextListener interface

One of the best practices for ensuring thread-safety and performance of servlet listeners is to implement the javax.servlet.ServletContextListener interface. This interface defines two methods: contextInitialized and contextDestroyed. These methods are invoked by the servlet container when the application is started and stopped, respectively. By implementing this interface, you can perform initialization and cleanup tasks for your servlet listeners in a controlled and consistent manner. For example, you can create and destroy thread pools, caches, or connections in these methods and avoid creating or destroying them in other methods. You can also register and unregister other servlet listeners in these methods and avoid duplicating or missing them in other places.

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5 Test and monitor your servlet listeners

The final tip for ensuring thread-safety and performance of servlet listeners is to test and monitor them regularly. Testing and monitoring can help you detect and fix any errors, bugs, or bottlenecks in your servlet listeners and improve their quality and reliability. For testing, you can use tools such as JUnit, TestNG, or Mockito to create unit tests, integration tests, or mock objects for your servlet listeners and verify their functionality and behavior. For monitoring, you can use tools such as JMeter, Gatling, or VisualVM to measure and analyze the performance, throughput, or memory usage of your servlet listeners and identify any issues or opportunities for optimization.

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Sachin Gorade

Senior Software Engineer at Yelp
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I agree to the point of using tools such as JMeter, Gatling or VisualVM to measure and analyze the performance, throughput, or memory usage of your servlet listeners but as mentioned in the other points, it's very important to separate the work that you are going to do in the listeners into other executor services to make sure logic is not blocking the requests. So Unit testing should be done for the separated code and listeners should contain the logic to submit the tasks to executor pool.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you ensure thread-safety and performance of Servlet Listeners?

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1 Use thread-safe data structures

2 Avoid blocking or long-running operations

3 Use appropriate scopes and attributes

4 Implement the javax.servlet.ServletContextListener interface

5 Test and monitor your servlet listeners

6 Here’s what else to consider

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