Почему и/или когда на уровне web-приложения (веб-сайта, сайта) возникает необходимость выполнять работу одновременно или периодически?

E: «For example, a Servlet or JSP may need to pull data from several back end data sources or services before rendering HTML.»
Р: «Например, сервлету или web-странице (JSP) может потребоваться извлечь данные из нескольких внутренних источников данных или служб перед отображением HTML.»

Нуу, так себе причина, явно не первоочередная функциональность, которая нужна в каждом сайте. Простые сайты могут последовательно и синхронно извлечь, будет помедленнее, зато просто и без головоломок в коде.

Пример:
Пришел поисковый запрос от пользователя, искать в файлах будем по запросу, файлов много.
(Можно было бы заранее сделать индекс, зачем искать в момент запроса?)
Ну, тогда, будем формировать фрагменты ответов для каждого из файлов, подсвечивая слова в поисковом запросе.
Каждый фрагмент одновременно и параллельно.

Не получается скачать документы из-за санкций:

Надо попробовать в Jakarta EE посмотреть, вдруг есть там?

https://jakarta.ee/specifications/concurrency/
https://jakarta.ee/specifications/concurrency/3.2/
Target Environments
Java SE 21 and Java SE 25

https://projects.eclipse.org/projects/ee4j.cu

https://jakarta.ee/specifications/concurrency/2.0/
2020-10-08, Release (for Jakarta EE 9)
https://jakarta.ee/specifications/concu … y-spec-2.0
https://jakarta.ee/specifications/concu … ec-2.0.pdf

Tasks are units of work that represent a computation or some business logic.
Tasks are concrete implementations of the Java SE java.util.concurrent.Callable and java.lang.Runnable interfaces (see the Javadoc for java.util.concurrent.ExecutorService).
Task classes can optionally implement the jakarta.enterprise.concurrent.ManagedTask interface to provide execution properties and to register a jakarta.enterprise.concurrent.ManagedTaskListener instance to receive lifecycle events notifications.

java.util.concurrent.Callable
    call()

java.lang.Runnable
    run()

jakarta.enterprise.concurrent.ManagedTaskListener
    taskAborted()
    taskSubmitted()
    taskStarting()

jakarta.enterprise.concurrent.Trigger
    getNextRunTime()
    skipRun()

These methods can be made contextual through the ContextService, which can make any Java object contextual.
ContextService – The interface for creating contextual objects.

A contextual object is any Java object instance that has a particular application component’s thread context associated with it.
When a task instance is submitted to a managed instance of an ExecutorService, the task becomes a contextual task.
When the contextual task runs, the task behaves as if it were still running in the container it was submitted with.

Contextual Objects and Tasks referred here is not the same as the Context object as defined in the Jakarta Contexts and Dependency Injection specification.

ManagedExecutorService –The interface for submitting asynchronous tasks from a container.
ManagedScheduledExecutorService – The interface for scheduling tasks to run after a given delay or execute periodically.

ManagedThreadFactory – The interface for creating managed threads
А зачем нужны нити, если уже есть возможность запускать задачи через сервис ManagedExecutorService?
Не надо запускать, это просто имя, которое мы указываем в настройках пула задач
«ThreadFactory: A reference to a ManagedThreadFactory instance.
The managed ThreadFactory instance can create threads with different attributes (such as priority)»

jakarta.enterprise.concurrent.ManagedExecutorService
«is an interface that extends the java.util.concurrent.ExecutorService interface»

Instances for these objects are retrieved using the Java Naming and Directory Interface (JNDI) Naming Context (EE.5)
or through injection of resource environment references (EE.5.8.1.1).
(надо бы примерчик, в будущем/пото́м, как же в коде web-приложения получить объект типа ManagedExecutorService с интерфейсом ExecutorService, и как через него создать задачу)

Task instances are submitted to a ManagedExecutorService instance using any of the defined submit(), execute(), invokeAll(), or invokeAny() methods. Task instances will run as an extension of the Jakarta EE container instance that submitted the task and may interact with Jakarta EE resources

resource environment references are declared in the deployment descriptor

ManagedExecutorService references should be bound in the java:comp/env/concurrent subcontext

The Jakarta EE Product Provider must provide a preconfigured, default ManagedExecutorService for use by application components under the JNDI name java:comp/DefaultManagedExecutorService.

configure a ManagedExecutorService and the possible options that such a service could provide.

Core Size: The number of threads to keep in the pool, even if they are idle.
Maximum Size: The maximum number of threads to allow in the pool (could be unbounded).
Keep Alive: The time to allow threads to remain idle when the number of threads is greater than the core size.
Work Queue Capacity: The number of tasks that can be stored in the input bounded buffer (could be unbounded).
Hung Task Threshold: The amount of time in milliseconds that a task can execute before it is considered hung.

https://jakarta.ee/specifications/concu … extservice

jakarta.enterprise.concurrent.ContextService
«allows applications to create contextual objects without using a managed executor»

Instances for these objects are retrieved using the Java Naming and Directory Interface (JNDI) Naming Context (EE.5) or through injection of resource environment references (EE.5.8.1.1).

all ContextService references should be declared in the java:comp/env/concurrent subcontext.

The Jakarta EE Product Provider must provide a preconfigured, default ContextService for use by application components under the JNDI name java:comp/DefaultContextService.

whenever a method on the contextual object is invoked, the method executes with the thread context of the associated application component instance.

The ContextService uses the dynamic proxy capabilities found in the java.lang.reflect package to associate the application component container context with an object instance.
Contextual object proxy instances are created with a ContextService instance using the createContextualProxy() method.

task logs the request in a database, which requires the local namespace in order to locate the correct Datasource.