org.apache.commons.lang3.concurrent

## Class BackgroundInitializer<T>

- java.lang.Object
- - org.apache.commons.lang3.concurrent.BackgroundInitializer<T>

- Type Parameters:
    `T` - the type of the object managed by this initializer class
  All Implemented Interfaces:
    ConcurrentInitializer<T>
  Direct Known Subclasses:
    CallableBackgroundInitializer, MultiBackgroundInitializer

      public abstract class BackgroundInitializer<T>
      extends Object
      implements ConcurrentInitializer<T>

  A class that allows complex initialization operations in a background task.

  Applications often have to do some expensive initialization steps when they are started, e.g. constructing a connection to a database, reading a configuration file, etc. Doing these things in parallel can enhance performance as the CPU load can be improved. However, when access to the resources initialized in a background thread is actually required, synchronization has to be performed to ensure that their initialization is complete.

  This abstract base class provides support for this use case. A concrete subclass must implement the `initialize()` method. Here an arbitrary initialization can be implemented, and a result object can be returned. With this method in place the basic usage of this class is as follows (where `MyBackgroundInitializer` is a concrete subclass):

       MyBackgroundInitializer initializer = new MyBackgroundInitializer();
       initializer.start();
       // Now do some other things. Initialization runs in a parallel thread
       ...
       // Wait for the end of initialization and access the result object
       Object result = initializer.get();

  After the construction of a `BackgroundInitializer` object its `start()` method has to be called. This starts the background processing. The application can now continue to do other things. When it needs access to the object produced by the `BackgroundInitializer` it calls its `get()` method. If initialization is already complete, `get()` returns the result object immediately. Otherwise it blocks until the result object is fully constructed.

  `BackgroundInitializer` is a thin wrapper around a `Future` object and uses an `ExecutorService` for running the background initialization task. It is possible to pass in an `ExecutorService` at construction time or set one using `setExternalExecutor()` before `start()` was called. Then this object is used to spawn the background task. If no `ExecutorService` has been provided, `BackgroundInitializer` creates a temporary `ExecutorService` and destroys it when initialization is complete.

  The methods provided by `BackgroundInitializer` provide for minimal interaction with the wrapped `Future` object. It is also possible to obtain the `Future` object directly. Then the enhanced functionality offered by `Future` can be used, e.g. to check whether the background operation is complete or to cancel the operation.

  Since:
    3.0
  Version:
    $Id: BackgroundInitializer.java 1583482 2014-03-31 22:54:57Z niallp $

- - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `BackgroundInitializer()` Creates a new instance of `BackgroundInitializer`.
    `protected` | `BackgroundInitializer(ExecutorService exec)` Creates a new instance of `BackgroundInitializer` and initializes it with the given `ExecutorService`.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `T` | `get()` Returns the result of the background initialization.
    `protected ExecutorService` | `getActiveExecutor()` Returns the `ExecutorService` that is actually used for executing the background task.
    `ExecutorService` | `getExternalExecutor()` Returns the external `ExecutorService` to be used by this class.
    `Future<T>` | `getFuture()` Returns the `Future` object that was created when `start()` was called.
    `protected int` | `getTaskCount()` Returns the number of background tasks to be created for this initializer.
    `protected abstract T` | `initialize()` Performs the initialization.
    `boolean` | `isStarted()` Returns a flag whether this `BackgroundInitializer` has already been started.
    `void` | `setExternalExecutor(ExecutorService externalExecutor)` Sets an `ExecutorService` to be used by this class.
    `boolean` | `start()` Starts the background initialization.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### BackgroundInitializer

          protected BackgroundInitializer()

      Creates a new instance of `BackgroundInitializer`. No external `ExecutorService` is used.

    - #### BackgroundInitializer

          protected BackgroundInitializer(ExecutorService exec)

      Creates a new instance of `BackgroundInitializer` and initializes it with the given `ExecutorService`. If the `ExecutorService` is not null, the background task for initializing this object will be scheduled at this service. Otherwise a new temporary `ExecutorService` is created.
      Parameters:
        `exec` - an external `ExecutorService` to be used for task execution

  - ### Method Detail

    - #### getExternalExecutor

          public final ExecutorService getExternalExecutor()

      Returns the external `ExecutorService` to be used by this class.
      Returns:
        the `ExecutorService`

    - #### isStarted

          public boolean isStarted()

      Returns a flag whether this `BackgroundInitializer` has already been started.
      Returns:
        a flag whether the `start()` method has already been called

    - #### setExternalExecutor

          public final void setExternalExecutor(ExecutorService externalExecutor)

      Sets an `ExecutorService` to be used by this class. The `ExecutorService` passed to this method is used for executing the background task. Thus it is possible to re-use an already existing `ExecutorService` or to use a specially configured one. If no `ExecutorService` is set, this instance creates a temporary one and destroys it after background initialization is complete. Note that this method must be called before `start()`; otherwise an exception is thrown.
      Parameters:
        `externalExecutor` - the `ExecutorService` to be used
      Throws:
        `IllegalStateException` - if this initializer has already been started

    - #### start

          public boolean start()

      Starts the background initialization. With this method the initializer becomes active and invokes the `initialize()` method in a background task. A `BackgroundInitializer` can be started exactly once. The return value of this method determines whether the start was successful: only the first invocation of this method returns true, following invocations will return false.
      Returns:
        a flag whether the initializer could be started successfully

    - #### get

          public T get()
                throws ConcurrentException

      Returns the result of the background initialization. This method blocks until initialization is complete. If the background processing caused a runtime exception, it is directly thrown by this method. Checked exceptions, including `InterruptedException` are wrapped in a `ConcurrentException`. Calling this method before `start()` was called causes an `IllegalStateException` exception to be thrown.
      Specified by:
        `get` in interface `ConcurrentInitializer<T>`
      Returns:
        the object produced by this initializer
      Throws:
        `ConcurrentException` - if a checked exception occurred during background processing
        `IllegalStateException` - if `start()` has not been called

    - #### getFuture

          public Future<T> getFuture()

      Returns the `Future` object that was created when `start()` was called. Therefore this method can only be called after `start()`.
      Returns:
        the `Future` object wrapped by this initializer
      Throws:
        `IllegalStateException` - if `start()` has not been called

    - #### getActiveExecutor

          protected final ExecutorService getActiveExecutor()

      Returns the `ExecutorService` that is actually used for executing the background task. This method can be called after `start()` (before `start()` it returns null). If an external executor was set, this is also the active executor. Otherwise this method returns the temporary executor that was created by this object.
      Returns:
        the `ExecutorService` for executing the background task

    - #### getTaskCount

          protected int getTaskCount()

      Returns the number of background tasks to be created for this initializer. This information is evaluated when a temporary `ExecutorService` is created. This base implementation returns 1. Derived classes that do more complex background processing can override it. This method is called from a synchronized block by the `start()` method. Therefore overriding methods should be careful with obtaining other locks and return as fast as possible.
      Returns:
        the number of background tasks required by this initializer

    - #### initialize

          protected abstract T initialize()
                                   throws Exception

      Performs the initialization. This method is called in a background task when this `BackgroundInitializer` is started. It must be implemented by a concrete subclass. An implementation is free to perform arbitrary initialization. The object returned by this method can be queried using the `get()` method.
      Returns:
        a result object
      Throws:
        `Exception` - if an error occurs

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