BackgroundInitializer

public class org.apache.commons.lang3.concurrent.BackgroundInitializer<T> extends AbstractConcurrentInitializer<T, Exception>

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.

Parameters:

<T> – The type of the object managed by this initializer class

Since:

3.0

Constructors

BackgroundInitializer()	Creates a new instance of BackgroundInitializer.
BackgroundInitializer(ExecutorService)	Creates a new instance of BackgroundInitializer and initializes it with the given ExecutorService.

protected BackgroundInitializer()

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

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

Methods

builder()	Creates a new builder.
get()	Gets the result of the background initialization.
getActiveExecutor()	Gets the ExecutorService that is actually used for executing the background task.
getExternalExecutor()	Gets the external ExecutorService to be used by this class.
getFuture()	Gets the Future object that was created when #start() was called.
getTaskCount()	Gets the number of background tasks to be created for this initializer.
getTypedException(Exception)	Gets an Exception with a type of E as defined by a concrete subclass of this class.
isInitialized()	Tests whether this instance is initialized.
isStarted()	Tests whether this BackgroundInitializer has already been started.
setExternalExecutor(ExecutorService)	Sets an ExecutorService to be used by this class.
start()	Starts the background initialization.

public static <T> BackgroundInitializer.Builder<BackgroundInitializer<T>, T> builder()

Creates a new builder.

Type parameters:

<T> – The type of object to build.

Returns:

A new builder.

Since:

3.14.0

@Override public T get() throws ConcurrentException

Gets 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.

Returns:

The object produced by this initializer.

Throws:

• org.apache.commons.lang3.concurrent.ConcurrentException – Thrown if a checked exception occurred during background processing.

• java.lang.IllegalStateException – Thrown if #start() has not been called.

protected final synchronized ExecutorService getActiveExecutor()

Gets 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.

public final synchronized ExecutorService getExternalExecutor()

Gets the external ExecutorService to be used by this class.

Returns:

The ExecutorService.

public synchronized Future<T> getFuture()

Gets 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:

java.lang.IllegalStateException – Thrown if #start() has not been called.

protected int getTaskCount()

Gets 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.

@Override protected Exception getTypedException(Exception e)

Gets an Exception with a type of E as defined by a concrete subclass of this class.

Parameters:

e – The actual exception that was thrown.

Returns:

A new exception with the actual type of E, that wraps e.

@Override public synchronized boolean isInitialized()

Tests whether this instance is initialized. Once initialized, always returns true. If initialization failed then the failure will be cached and this will never return true.

Returns:

true if initialization completed successfully, otherwise false.

Since:

3.14.0

public synchronized boolean isStarted()

Tests whether this BackgroundInitializer has already been started.

Returns:

A flag whether the #start() method has already been called.

public final synchronized 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:

java.lang.IllegalStateException – Thrown if this initializer has already been started.

public synchronized 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.