Class MultiBackgroundInitializer

  extended by org.apache.commons.lang3.concurrent.BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults>
      extended by org.apache.commons.lang3.concurrent.MultiBackgroundInitializer
All Implemented Interfaces:

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:

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.

$Id: 1082301 2011-03-16 21:02:15Z oheger $

Nested Class Summary
static class MultiBackgroundInitializer.MultiBackgroundInitializerResults
          A data class for storing the results of the background initialization performed by MultiBackgroundInitializer.
Constructor Summary
          Creates a new instance of MultiBackgroundInitializer.
MultiBackgroundInitializer(ExecutorService exec)
          Creates a new instance of MultiBackgroundInitializer and initializes it with the given external ExecutorService.
Method Summary
 void addInitializer(String name, BackgroundInitializer<?> init)
          Adds a new BackgroundInitializer to this object.
protected  int getTaskCount()
          Returns the number of tasks needed for executing all child BackgroundInitializer objects in parallel.
protected  MultiBackgroundInitializer.MultiBackgroundInitializerResults initialize()
          Creates the results object.
Methods inherited from class org.apache.commons.lang3.concurrent.BackgroundInitializer
get, getActiveExecutor, getExternalExecutor, getFuture, isStarted, setExternalExecutor, start
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public MultiBackgroundInitializer()
Creates a new instance of MultiBackgroundInitializer.


public MultiBackgroundInitializer(ExecutorService exec)
Creates a new instance of MultiBackgroundInitializer and initializes it with the given external ExecutorService.

exec - the ExecutorService for executing the background tasks
Method Detail


public void addInitializer(String name,
                           BackgroundInitializer<?> init)
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.

name - the name of the initializer (must not be null)
init - the BackgroundInitializer to add (must not be null)
IllegalArgumentException - if a required parameter is missing
IllegalStateException - if start() has already been called


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.

getTaskCount in class BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults>
the number of tasks required for background processing


protected MultiBackgroundInitializer.MultiBackgroundInitializerResults initialize()
                                                                           throws Exception
Creates the results object. This implementation starts all child BackgroundInitializer objects. Then it collects their results and creates a MultiBackgroundInitializerResults object with this data. If a child initializer throws a checked exceptions, it is added to the results object. Unchecked exceptions are propagated.

Specified by:
initialize in class BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults>
the results object
Exception - if an error occurs

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