Package org.apache.commons.lang3.concurrent

Class MultiBackgroundInitializer

  • All Implemented Interfaces:
    ConcurrentInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults>
    public class MultiBackgroundInitializer
extends BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults>

    A specialized BackgroundInitializer implementation that can deal with multiple background initialization tasks.

    This class has a similar purpose as BackgroundInitializer. However, it is not limited to a single background initialization task. Rather it manages an arbitrary number of BackgroundInitializer objects, executes them, and waits until they are completely initialized. This is useful for applications that have to perform multiple initialization tasks that can run in parallel (i.e. that do not depend on each other). This class takes care about the management of an ExecutorService and shares it with the BackgroundInitializer objects it is responsible for; so the using application need not bother with these details.

    The typical usage scenario for MultiBackgroundInitializer is as follows:

    • Create a new instance of the class. Optionally pass in a pre-configured ExecutorService. Alternatively MultiBackgroundInitializer can create a temporary ExecutorService and delete it after initialization is complete.
    • Create specialized BackgroundInitializer objects for the initialization tasks to be performed and add them to the MultiBackgroundInitializer using the addInitializer(String, BackgroundInitializer) method.
    • After all initializers have been added, call the BackgroundInitializer.start() method.
    • When access to the result objects produced by the BackgroundInitializer objects is needed call the BackgroundInitializer.get() method. The object returned here provides access to all result objects created during initialization. It also stores information about exceptions that have occurred.

    MultiBackgroundInitializer starts a special controller task that starts all BackgroundInitializer objects added to the instance. Before the an initializer is started it is checked whether this initializer already has an ExecutorService set. If this is the case, this ExecutorService is used for running the background task. Otherwise the current ExecutorService of this MultiBackgroundInitializer is shared with the initializer.

    The easiest way of using this class is to let it deal with the management of an ExecutorService itself: If no external ExecutorService is provided, the class creates a temporary ExecutorService (that is capable of executing all background tasks in parallel) and destroys it at the end of background processing.

    Alternatively an external ExecutorService can be provided - either at construction time or later by calling the BackgroundInitializer.setExternalExecutor(ExecutorService) method. In this case all background tasks are scheduled at this external ExecutorService. Important note: When using an external ExecutorService be sure that the number of threads managed by the service is large enough. Otherwise a deadlock can happen! This is the case in the following scenario: MultiBackgroundInitializer starts a task that starts all registered BackgroundInitializer objects and waits for their completion. If for instance a single threaded ExecutorService is used, none of the background tasks can be executed, and the task created by MultiBackgroundInitializer waits forever.

    Since:
    3.0

    • Constructor Detail

      • MultiBackgroundInitializer

        public MultiBackgroundInitializer()
        Creates a new instance of MultiBackgroundInitializer.

      • MultiBackgroundInitializer

        public MultiBackgroundInitializer​(java.util.concurrent.ExecutorService exec)
        Creates a new instance of MultiBackgroundInitializer and initializes it with the given external ExecutorService.
        Parameters:
        exec - the ExecutorService for executing the background tasks

    • Method Detail

      • addInitializer

        public void addInitializer​(java.lang.String name,
                           BackgroundInitializer<?> backgroundInitializer)
        Adds a new BackgroundInitializer to this object. When this MultiBackgroundInitializer is started, the given initializer will be processed. This method must not be called after BackgroundInitializer.start() has been invoked.
        Parameters:
        name - the name of the initializer (must not be null)
        backgroundInitializer - the BackgroundInitializer to add (must not be null)
        Throws:
        java.lang.IllegalArgumentException - if a required parameter is missing
        java.lang.IllegalStateException - if start() has already been called

      • getTaskCount

        protected int getTaskCount()
        Returns the number of tasks needed for executing all child BackgroundInitializer objects in parallel. This implementation sums up the required tasks for all child initializers (which is necessary if one of the child initializers is itself a MultiBackgroundInitializer ). Then it adds 1 for the control task that waits for the completion of the children.
        Overrides:
        getTaskCount in class BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults>
        Returns:
        the number of tasks required for background processing