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package com.cedarsoft.test.utils;

/**
 * @author Johannes Schneider ([email protected])
 */

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.awt.EventQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.InvocationTargetException;

import static org.assertj.core.api.Assertions.assertThat;


/**
 * A simple utility class that can verify that an object has been successfully garbage collected.
 */
public class MemoryLeakVerifier {
  private static final int MAX_GC_ITERATIONS = 50;
  private static final int GC_SLEEP_TIME = 100;

  @Nonnull
  private final WeakReference reference;

  public MemoryLeakVerifier( @Nonnull T object ) {
    this.reference = new WeakReference( object );
  }

  @Nonnull
  public WeakReference getReference() {
    return reference;
  }

  @Nullable
  public Object getObject() {
    return reference.get();
  }

  /**
   * Attempts to perform a full garbage collection so that all weak references will be removed. Usually only
   * a single GC is required, but there have been situations where some unused memory is not cleared up on the
   * first pass. This method performs a full garbage collection and then validates that the weak reference
   * now has been cleared. If it hasn't then the thread will sleep for 50 milliseconds and then retry up to
   * 10 more times. If after this the object still has not been collected then the assertion will fail.
   * 
   * Based upon the method described in: http://www.javaworld.com/javaworld/javatips/jw-javatip130.html
   */
  public void assertGarbageCollected() {
    Runtime runtime = Runtime.getRuntime();
    for ( int i = 0; i < MAX_GC_ITERATIONS; i++ ) {
      runtime.runFinalization();
      //noinspection CallToSystemGC
      runtime.gc();
      if ( getObject() == null ) {
        break;
      }

      // Pause for a while and then go back around the loop to try again...
      try {
        EventQueue.invokeAndWait( new NoOp() ); // Wait for the AWT event queue to have completed processing
        //noinspection BusyWait
        Thread.sleep( GC_SLEEP_TIME );
      } catch ( InterruptedException ignore ) {
        // Ignore any interrupts and just try again...
      } catch ( InvocationTargetException ignore ) {
        // Ignore any interrupts and just try again...
      }
    }

    assertThat( getObject() ).describedAs( "object should not exist after " + MAX_GC_ITERATIONS + " collections" ).isNull();
  }

  private static class NoOp implements Runnable {
    @Override
    public void run() {
    }
  }
}