How does the singleton Bean serve the concurrent request?

Question

I have a question regarding how singleton beans serve concurrent requests in detail.

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Answer 1

This question is 5+ years old by now (2019) and i hope you have found what you are looking for. but i would still post an answer. this may not only cover your question but also described multi-threading behavior in brief.

first of all. the singleton is a design pattern used in programming, which is used to create only one single instance for the entire application (only one for the JVM. as i hope there is only one JVM in your app). multi threading is a processing mechanism. it executes tasks concurrently. i think you are confused because a you've already know that a thread is a logical processing location. and an object is a memory instance. but you did not understand how multi-threading actually works under the hood. as for your question, i will explain this by with spring framework.

so when a user send a request to the server, the server dedicate a separate thread for each request. and in spring, the beans are singleton by default. so the first request begins to execute a method of your singleton bean, and before it is finished, an another request comes and it executes the same method by using another thread.

so what's happening here is that the second thread will not wait the first one to be finished executing the entire method. they executes concurrently, it means the first request will run the first line of the method and then the second thread begins to run the first line. and maybe second line also. Do note that while the first thread executes the first line, the second thread cannot execute the same line and while the second executes the first line and second line, the first cannot execute the second line until the second thread is finished the second line.

even though we call this concurrent execution, it does not execute concurrently at all. (one line is executed by only one thread at same time) according to your question, you are defined two methods in the bean, so they are separate methods. so two threads requesting the two methods at the same time will execute them at the same time. so what i have described will not apply for that scenario and if your beans are created newly for each request then also this will not happen and they execute concurrently.