{"id":998,"date":"2022-07-03T12:11:13","date_gmt":"2022-07-03T10:11:13","guid":{"rendered":"https:\/\/ondro.inginea.eu\/?p=998"},"modified":"2023-03-18T12:01:01","modified_gmt":"2023-03-18T11:01:01","slug":"introduction-to-concurrency-and-threads-in-java-web-apps","status":"publish","type":"post","link":"https:\/\/ondro.inginea.eu\/index.php\/introduction-to-concurrency-and-threads-in-java-web-apps\/","title":{"rendered":"Introduction to concurrency and threads in Java web apps"},"content":{"rendered":"\n

Threads, concurrency, or synchronization are not very easy to understand concepts. When some concurrency is involved in our applications it’s pretty hard to avoid making mistakes. Although Java provides mechanisms to deal with parallel programming, sometimes there are just too many options. And often some essential options are missing. For web applications, Jakarta EE provides a simplified programming model to deal with parallel tasks. But in order to use it effectively and avoid mistakes, you need to understand the basic concepts which I’d like to explain here.<\/p>\n\n\n\n\n\n\n\n

Java provides a lot of mechanisms to help working with threads and concurrent tasks. As Java evolved, some new mechanisms were added while old mechanisms stayed. And it’s often not clear which of them are better and recommended to use in new applications. Jakarta EE builds on these features and makes them easier to understand and use. The standard Jakarta EE API intentionally specifies only interfaces and essential conceptual behavior. A lot of the complexity is abstracted away and provided by Jakarta EE runtimes to keep things simple. As a result, developers have a concise set of features. Easy to learn and understand but enough to build applications.<\/p>\n\n\n\n

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What is a thread pool<\/h2>\n\n\n\n

The basic concept of threading and parallelism in Jakarta EE runtimes is a thread pool<\/strong>. This is connected with the request processing model, where most of the tasks originate as a request from an external caller<\/strong>, they are then processed sequentially in a single thread<\/strong>, and produce some response<\/strong> that is usually sent back to the caller, persisted into a database or sent as a message to another system. Many separate tasks can run in parallel, each using its own separate thread. So there’s usually a simple mapping – one request needs one thread. After a task is finished, a thread doesn’t have to be destroyed. It can be reused to run another task, in order to avoid creating and destroying threads too often.<\/p>\n\n\n\n

Incoming tasks are not tied with any specific thread. A thread pool always makes sure there’s a thread available for a task. This is called “scheduling” and is often referred to as “thread scheduling”. Tasks are scheduled and processed either:<\/p>\n\n\n\n

by an existing thread that is finished with its previous task<\/p>\n\n\n\n

by a new thread if no free thread is available<\/p>\n\n\n\n

or they are queued and wait until a thread is available if all threads are busy<\/p>\n\n\n\n

Vice versa, threads aren’t tied to tasks either. They just take a task from a queue and process it. When threads are done with their tasks, they start processing another new task from a queue or wait for a task if the queue is empty. A group of such threads, together with the logic how they are managed and scheduled, is called a thread pool.<\/p>\n\n\n\n

Why thread pools?<\/h2>\n\n\n\n

Thread pools are used because it’s time-consuming to dispose a thread and create a new thread later. A new thread is created only when there’s no idle thread already available. And they are disposed only when there are already too many idle threads it’s likely not going to be needed for some time.<\/p>\n\n\n\n

Another aspect of thread pools is that their size is limited and adapts to the load. This is mainly for 3 reasons:<\/p>\n\n\n\n