詳解commons-pool2池化技術

/**
*向對象池中增加對象實例
*/
void addObject() throws Exception, IllegalStateException,
      UnsupportedOperationException;
/**
* 從對象池中獲取對象
*/
T borrowObject() throws Exception, NoSuchElementException,
      IllegalStateException;
/**
* 失效非法的對象
*/
void invalidateObject(T obj) throws Exception;
/**
* 釋放對象至對象池
*/
void returnObject(T obj) throws Exception;

public GenericObjectPool(final PooledObjectFactory<T> factory) {
      this(factory, new GenericObjectPoolConfig<T>());
  }
  
  public GenericObjectPool(final PooledObjectFactory<T> factory,
                            final GenericObjectPoolConfig<T> config) {
​
      super(config, ONAME_BASE, config.getJmxNamePrefix());
​
      if (factory == null) {
          jmxUnregister(); // tidy up
          throw new IllegalArgumentException("factory may not be null");
      }
      this.factory = factory;
​
      idleObjects = new LinkedBlockingDeque<>(config.getFairness());
      setConfig(config);
  }
​
  public GenericObjectPool(final PooledObjectFactory<T> factory,
                            final GenericObjectPoolConfig<T> config, final AbandonedConfig abandonedConfig) {
      this(factory, config);
      setAbandonedConfig(abandonedConfig);
  }

/**
* 向對象池中增加對象，一般在預加載的時候會使用該功能
*/
@Override
public void addObject() throws Exception {
  assertOpen();
  if (factory == null) {
      throw new IllegalStateException(
              "Cannot add objects without a factory.");
  }
  final PooledObject<T> p = create();
  addIdleObject(p);
}

final PooledObject<T> p;
try {
  p = factory.makeObject();
  if (getTestOnCreate() && !factory.validateObject(p)) {
      createCount.decrementAndGet();
      return null;
  }
} catch (final Throwable e) {
  createCount.decrementAndGet();
  throw e;
} finally {
  synchronized (makeObjectCountLock) {
      makeObjectCount--;
      makeObjectCountLock.notifyAll();
  }
}

/**
* 第一個節點
* Invariant: (first == null && last == null) ||
*           (first.prev == null && first.item != null)
*/
private transient Node<E> first; // @GuardedBy("lock")
​
/**
* 最後一個節點
* Invariant: (first == null && last == null) ||
*           (last.next == null && last.item != null)
*/
private transient Node<E> last; // @GuardedBy("lock")
​
/** 當前隊列長度 */
private transient int count; // @GuardedBy("lock")
​
/** 隊列最大容量 */
private final int capacity;
​
/** 主鎖 */
private final InterruptibleReentrantLock lock;
​
/** 隊列是否為空狀態鎖 */
private final Condition notEmpty;
​
/** 隊列是否滿狀態鎖 */
private final Condition notFull;

public boolean offerFirst(final E e, final long timeout, final TimeUnit unit)
  throws InterruptedException {
  Objects.requireNonNull(e, "e");
  long nanos = unit.toNanos(timeout);
  lock.lockInterruptibly();
  try {
      while (!linkFirst(e)) {
          if (nanos <= 0) {
              return false;
          }
          nanos = notFull.awaitNanos(nanos);
      }
      return true;
  } finally {
      lock.unlock();
  }
}

public E takeFirst() throws InterruptedException {
  lock.lock();
  try {
      E x;
      while ( (x = unlinkFirst()) == null) {
          notEmpty.await();
      }
      return x;
  } finally {
      lock.unlock();
  }
}

private boolean linkLast(final E e) {
  // assert lock.isHeldByCurrentThread();
  if (count >= capacity) {
      return false;
  }
  final Node<E> l = last;
  final Node<E> x = new Node<>(e, l, null);
  last = x;
  if (first == null) {
      first = x;
  } else {
      l.next = x;
  }
  ++count;
  notEmpty.signal();
  return true;
}

//1、嘗試從雙端隊列中獲取對象，pollFirst方法是非阻塞方法
p = idleObjects.pollFirst();
if (p == null) {
    p = create();
    if (p != null) {
        create = true;
    }
}
if (blockWhenExhausted) {
    if (p == null) {
        if (borrowMaxWaitMillis < 0) {
            //2、沒有設置最大阻塞等待時間，則無限等待
            p = idleObjects.takeFirst();
        } else {
            //3、設置最大等待時間瞭，則阻塞等待指定的時間
            p = idleObjects.pollFirst(borrowMaxWaitMillis,
                    TimeUnit.MILLISECONDS);
        }
    }
}

//修改對象狀態
if (!p.allocate()) {
    p = null;
}
if (p != null) {
    try {
        //初始化對象
        factory.activateObject(p);
    } catch (final Exception e) {
        try {
            destroy(p, DestroyMode.NORMAL);
        } catch (final Exception e1) {
        }
 
}
    if (p != null && getTestOnBorrow()) {
        boolean validate = false;
        Throwable validationThrowable = null;
        try {
            //驗證對象的可用性狀態
            validate = factory.validateObject(p);
        } catch (final Throwable t) {
            PoolUtils.checkRethrow(t);
            validationThrowable = t;
        }
        //對象不可用，驗證失敗，則進行destroy
        if (!validate) {
            try {
                destroy(p, DestroyMode.NORMAL);
               destroyedByBorrowValidationCount.incrementAndGet();
            } catch (final Exception e) {
                // Ignore - validation failure is more important
            }
 
        }
    }
}

public  class PoolState {
 
protected final List<PooledObject> idleObjects = new ArrayList<>();
protected final List<PooledObject> activeObjects = new ArrayList<>();
 
 
//...
 
}

//根據配置確定是否要為標簽刪除調用removeAbandoned方法
final AbandonedConfig ac = this.abandonedConfig;
if (ac != null && ac.getRemoveAbandonedOnBorrow() && (getNumIdle() < 2) && (getNumActive() > getMaxTotal() - 3) ) {
    removeAbandoned(ac);
}

public final void setTimeBetweenEvictionRunsMillis(
      final long timeBetweenEvictionRunsMillis) {
  this.timeBetweenEvictionRunsMillis = timeBetweenEvictionRunsMillis;
  startEvictor(timeBetweenEvictionRunsMillis);
}

final void startEvictor(final long delay) {
  synchronized (evictionLock) {
      EvictionTimer.cancel(evictor, evictorShutdownTimeoutMillis, TimeUnit.MILLISECONDS);
      evictor = null;
      evictionIterator = null;
      //如果delay<=0則不會開啟定時清理任務
      if (delay > 0) {
          evictor = new Evictor();
          EvictionTimer.schedule(evictor, delay, delay);
      }
  }
}

boolean evict;
try {
  evict = evictionPolicy.evict(evictionConfig, underTest,
  idleObjects.size());
} catch (final Throwable t) {
  // Slightly convoluted as SwallowedExceptionListener
  // uses Exception rather than Throwable
    PoolUtils.checkRethrow(t);
    swallowException(new Exception(t));
    // Don't evict on error conditions
    evict = false;
}
if (evict) {
    // 如果可以被回收則直接調用destroy進行回收
    destroy(underTest);
    destroyedByEvictorCount.incrementAndGet();
}

boolean active = false;
try {
  // 調用activateObject激活該空閑對象，本質上不是為瞭激活，
  // 而是通過這個方法可以判定是否還存活，這一步裡面可能會有一些資源的開辟行為。
  factory.activateObject(underTest);
  active = true;
} catch (final Exception e) {
  // 如果激活的時候，發生瞭異常，就說明該空閑對象已經失聯瞭。
  // 調用destroy方法銷毀underTest
  destroy(underTest);
  destroyedByEvictorCount.incrementAndGet();
}
if (active) {
  // 再通過進行validateObject校驗有效性
  if (!factory.validateObject(underTest)) {
      // 如果校驗失敗，說明對象已經不可用瞭
      destroy(underTest);
      destroyedByEvictorCount.incrementAndGet();
  } else {
      try {
          /*
            *因為校驗還激活瞭空閑對象，分配瞭額外的資源，那麼就通過passivateObject把在activateObject中開辟的資源釋放掉。
          */
          factory.passivateObject(underTest);
      } catch (final Exception e) {
          // 如果passivateObject失敗，也可以說明underTest這個空閑對象不可用瞭
          destroy(underTest);
          destroyedByEvictorCount.incrementAndGet();
      }
  }
}