AtomicInitializer
Date: 2017-02-08

org.apache.commons.lang3.concurrent

## Class AtomicInitializer<T>

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

- Type Parameters:
    `T` - the type of the object managed by this initializer class
  All Implemented Interfaces:
    ConcurrentInitializer<T>

      public abstract class AtomicInitializer<T>
      extends java.lang.Object
      implements ConcurrentInitializer<T>

  A specialized implementation of the `ConcurrentInitializer` interface based on an `AtomicReference` variable.

  This class maintains a member field of type `AtomicReference`. It implements the following algorithm to create and initialize an object in its `get()` method:

  - First it is checked whether the `AtomicReference` variable contains already a value. If this is the case, the value is directly returned.
  - Otherwise the `initialize()` method is called. This method must be defined in concrete subclasses to actually create the managed object.
  - After the object was created by `initialize()` it is checked whether the `AtomicReference` variable is still undefined. This has to be done because in the meantime another thread may have initialized the object. If the reference is still empty, the newly created object is stored in it and returned by this method.
  - Otherwise the value stored in the `AtomicReference` is returned.

  Because atomic variables are used this class does not need any synchronization. So there is no danger of deadlock, and access to the managed object is efficient. However, if multiple threads access the `AtomicInitializer` object before it has been initialized almost at the same time, it can happen that `initialize()` is called multiple times. The algorithm outlined above guarantees that `get()` always returns the same object though.

  Compared with the `LazyInitializer` class, this class can be more efficient because it does not need synchronization. The drawback is that the `initialize()` method can be called multiple times which may be problematic if the creation of the managed object is expensive. As a rule of thumb this initializer implementation is preferable if there are not too many threads involved and the probability that multiple threads access an uninitialized object is small. If there is high parallelism, `LazyInitializer` is more appropriate.

  Since:
    3.0

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `AtomicInitializer()`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `T` | `get()` Returns the object managed by this initializer.
    `protected abstract T` | `initialize()` Creates and initializes the object managed by this `AtomicInitializer`.

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

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

- - ### Constructor Detail

    - #### AtomicInitializer

          public AtomicInitializer()

  - ### Method Detail

    - #### get

          public T get()
                throws ConcurrentException

      Returns the object managed by this initializer. The object is created if it is not available yet and stored internally. This method always returns the same object.
      Specified by:
        `get` in interface `ConcurrentInitializer<T>`
      Returns:
        the object created by this `AtomicInitializer`
      Throws:
        `ConcurrentException` - if an error occurred during initialization of the object

    - #### initialize

          protected abstract T initialize()
                                   throws ConcurrentException

      Creates and initializes the object managed by this `AtomicInitializer`. This method is called by `get()` when the managed object is not available yet. An implementation can focus on the creation of the object. No synchronization is needed, as this is already handled by `get()`. As stated by the class comment, it is possible that this method is called multiple times.
      Returns:
        the managed data object
      Throws:
        `ConcurrentException` - if an error occurs during object creation