What are Java thread groups, thread pools, and thread queues? What's the difference?

Hello, I can explain it to you in detail:

A thread group represents a collection of threads. In addition, thread groups can also contain other thread groups. Thread groups form a tree in which each thread group, except the initial thread group, has a parent thread group.

A thread is allowed to access information about its own thread group, but it is not allowed to access information about its thread group's parent thread group or any other thread group.

Thread pool: We can pass concurrently executed tasks to a thread pool instead of starting a new thread for each concurrently executed task. As long as there are idle threads in the pool, the task will be assigned to a thread for execution. Inside the thread pool, tasks are inserted into a blocking queue (Blocking Queue), and threads in the thread pool will fetch tasks from this queue. When a new task is inserted into the queue, an idle thread will successfully remove the task from the queue and execute it.

Thread pools are often used on multi-threaded servers. Each connection reaching the server over the network is wrapped into a task and passed to the thread pool. Threads in the thread pool will concurrently process requests on the connection. We will delve into the details of implementing a multi-threaded server in Java later.

Thread queue: refers to the scheduling queue formed when threads are congested

There are three general strategies for queuing:

Direct submission. The default option for work queues is SynchronousQueue, which submits tasks directly to threads without holding them. Here, if there is no thread available to run the task immediately, the attempt to enqueue the task will fail, so a new thread will be constructed. This strategy avoids locks when processing sets of requests that may have internal dependencies. Direct submission typically requires unbounded maximumPoolSizes to avoid rejecting newly submitted tasks. This policy allows unbounded threads the possibility to grow when commands arrive in succession at an average number that exceeds what the queue can handle.

Unbounded queue. Using an unbounded queue (for example, a LinkedBlockingQueue without a predefined capacity) will result in new tasks waiting in the queue while all corePoolSize threads are busy. This way, no more threads will be created than corePoolSize. (Therefore, the value of maximumPoolSize is invalid.) Unbounded queues are suitable for use when each task is completely independent of other tasks, that is, task execution does not affect each other; for example, in a Web page server. This kind of queuing can be used to handle transient bursts of requests. This strategy allows unbounded threads to have the possibility to grow when commands arrive consecutively at an average number that exceeds what the queue can handle.

Bounded queue. Bounded queues (such as ArrayBlockingQueue) help prevent resource exhaustion when using limited maximumPoolSizes, but can be difficult to tune and control. Queue size and maximum pool size may require trade-offs: using large queues and small pools can minimize CPU usage, operating system resources, and context switching overhead, but may result in artificially reduced throughput. If tasks block frequently (for example, if they are I/O bounds), the system may schedule more threads than you allow. Using small queues typically requires larger pool sizes and higher CPU usage, but may encounter unacceptable scheduling overhead, which can also reduce throughput.