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package org.threadly.concurrent.future;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * This future task has a special ability to start execution in the thread requesting the 
 * {@link #get()} IF it has not already started.  It is still expected that this future will be 
 * provided to an executor, but if that executor has been unable to start execution by the time 
 * {@link #get()} is requested, execution will start on the {@link #get()} thread.
 * 
 * This can make sense if for example you need to have the result of a given future before you can 
 * go forward with additional work.  That way if the thread pool is too busy with other things, we 
 * can try to get the result for this work unit as fast as possible (once we are ready for it).  
 * If execution starts on the {@link #get()} call, once the task is started in the thread pool, it 
 * will return immediately (WITHOUT invoking the task contained in the future).
 * 
 * This does come with some details to be aware of.  Execution may only occur on the 
 * {@link #get()} request.  Requesting {@link #get(long, java.util.concurrent.TimeUnit)} will NOT 
 * cause execution to start (as we can not ensure a timeout will be respected).  In addition this 
 * can not work with recurring tasks.
 * 
 * @since 2.4.0
 * @param  The result object type returned by this future
 */
public class ExecuteOnGetFutureTask extends ListenableFutureTask {
  private final AtomicBoolean executionStarted = new AtomicBoolean(false);
  
  /**
   * Constructs a runnable future with a runnable work unit.
   * 
   * @param task runnable to be run
   */
  public ExecuteOnGetFutureTask(Runnable task) {
    super(task);
  }
  
  /**
   * Constructs a runnable future with a runnable work unit.
   * 
   * @param task runnable to be run
   * @param result result to be provide after run has completed
   */
  public ExecuteOnGetFutureTask(Runnable task, T result) {
    super(task, result);
  }

  /**
   * Constructs a runnable future with a callable work unit.
   * 
   * @param task callable to be run
   */
  public ExecuteOnGetFutureTask(Callable task) {
    super(task);
  }
  
  /**
   * Starts execution of task if it has not already started.  This is thread safe, and will ensure 
   * that execution will only occur once.
   */
  protected void executeIfNotStarted() {
    if (! executionStarted.getPlain() && executionStarted.compareAndSet(false, true)) {
      super.run();
    }
  }
  
  @Override
  public void run() {
    executeIfNotStarted();
  }
  
  @Override
  public T get() throws InterruptedException, ExecutionException {
    executeIfNotStarted();
    
    return super.get();
  }
  
  @Override
  public Throwable getFailure() throws InterruptedException {
    executeIfNotStarted();
    
    return super.getFailure();
  }
}