开源项目剖析之apache-common-pool

前沿

该工程提供了对象池解决方案，该方案主要用于提高像文件句柄，数据库连接，socket通信这类大对象的调用效率。简单的说就是一种对象一次创建多次使用的技术。

整体结构

整个项目有三个包分别是org.apache.commons.pool2，org.apache.commons.pool2.impl和org.apache.commons.pool2.proxy。org.apache.commons.pool2主要定义整个项目要实现的接口；org.apache.commons.pool2.impl主要定义对接口的一般实现；org.apache.commons.pool2.proxy主要定义对接口的代理实现。

接下来我们看看整体的代码关系，如下类图

整个方案从ObjectPool，PooledObjectFactory和PooledObject三个接口展开，其中ObjectPool定义了对象池要实现的功能【比如怎么存取，怎么过期】；PooledObjectFactory定义了被池化的对象的创建，初始化，激活，钝化以及销毁功能；PooledObject定了一被池化对象的一些附加信息【创建时间，池中状态】；大概流程就是由PooledObjectFactory创建的对象经过PooledObject的包装然后放到ObjectPool里面来。

ObjectPool

//从池中获取对象
T borrowObject() throws Exception, NoSuchElementException,IllegalStateException;
//将对象放回池中
void returnObject(T obj) throws Exception;
//废弃对象
void invalidateObject(T obj) throws Exception;
//添加对象
void addObject() throws Exception, IllegalStateException,UnsupportedOperationException;
//获取对象个数
int getNumIdle();
//获取活跃对象个数
int getNumActive();
//清除池，池可用
void clear() throws Exception, UnsupportedOperationException;
//关闭池，池不可用
void close();

PooledObjectFactory

//创建一个新对象;当对象池中的对象个数不足时,将会使用此方法来"输出"一个新的"对象",并交付给对象池管理
PooledObject<T> makeObject() throws Exception;
//销毁对象,如果对象池中检测到某个"对象"idle的时间超时,或者操作者向对象池"归还对象"时检测到"对象"已经无效,那么此时将会导致"对象销毁";"销毁对象"的操作设计相差甚远,但是必须明确:当调用此方法时,"对象"的生命周期必须结束.如果object是线程,那么此时线程必须退出;如果object是socket操作,那么此时socket必须关闭;如果object是文件流操作,那么此时"数据flush"且正常关闭.
void destroyObject(PooledObject<T> p) throws Exception;
//检测对象是否"有效";Pool中不能保存无效的"对象",因此"后台检测线程"会周期性的检测Pool中"对象"的有效性,如果对象无效则会导致此对象从Pool中移除,并destroy;此外在调用者从Pool获取一个"对象"时,也会检测"对象"的有效性,确保不能讲"无效"的对象输出给调用者;当调用者使用完毕将"对象归还"到Pool时,仍然会检测对象的有效性.所谓有效性,就是此"对象"的状态是否符合预期,是否可以对调用者直接使用;如果对象是Socket,那么它的有效性就是socket的通道是否畅通/阻塞是否超时等.
boolean validateObject(PooledObject<T> p);
// "激活"对象,当Pool中决定移除一个对象交付给调用者时额外的"激活"操作,比如可以在activateObject方法中"重置"参数列表让调用者使用时感觉像一个"新创建"的对象一样;如果object是一个线程,可以在"激活"操作中重置"线程中断标记",或者让线程从阻塞中唤醒等;如果object是一个socket,那么可以在"激活操作"中刷新通道,或者对socket进行链接重建(假如socket意外关闭)等.
void activateObject(PooledObject<T> p) throws Exception;
//"钝化"对象,当调用者"归还对象"时,Pool将会"钝化对象";钝化的言外之意,就是此"对象"暂且需要"休息"一下.如果object是一个socket,那么可以passivateObject中清除buffer,将socket阻塞;如果object是一个线程,可以在"钝化"操作中将线程sleep或者将线程中的某个对象wait.需要注意的时,activateObject和passivateObject两个方法需要对应,避免死锁或者"对象"状态的混乱.
void passivateObject(PooledObject<T> p) throws Exception;

PooledObject

T getObject();
long getCreateTime();
long getActiveTimeMillis();
long getIdleTimeMillis();
long getLastBorrowTime();
long getLastReturnTime();
long getLastUsedTime();
int compareTo(PooledObject<T> other);
boolean equals(Object obj);
int hashCode();
String toString();
//后台清理线程
boolean startEvictionTest();
boolean endEvictionTest(Deque<PooledObject<T>> idleQueue);
boolean allocate();
boolean deallocate();
void invalidate()
void setLogAbandoned(boolean logAbandoned);
void use();
void printStackTrace(PrintWriter writer);
PooledObjectState getState();
//自动补偿功能
void markAbandoned();
void markReturning();

方案提供了三种类型的pool，分别是GenericKeyedObjectPool，SoftReferenceObjectPool和GenericObjectPool。其中GenericObjectPool是一般意义上的对象池；SoftReferenceObjectPool是弱引用的对象池；GenericKeyedObjectPool是具备分组的对象池。

以下是各个类的详细分析

SoftReferenceObjectPool

//可用对象列表
private final LinkedBlockingDeque<PooledSoftReference<T>> idleReferences = new LinkedBlockingDeque<PooledSoftReference<T>>();
//所有对象列表
private final ArrayList<PooledSoftReference<T>> allReferences = new ArrayList<PooledSoftReference<T>>();

borrowObject

//可用对象列表
private final LinkedBlockingDeque<PooledSoftReference<T>> idleReferences = new LinkedBlockingDeque<PooledSoftReference<T>>();
//所有对象列表
private final ArrayList<PooledSoftReference<T>> allReferences = new ArrayList<PooledSoftReference<T>>();

    public synchronized T borrowObject() throws Exception {
        assertOpen();//确定池打开
        T obj = null;
        boolean newlyCreated = false;
        PooledSoftReference<T> ref = null;
        while (null == obj) {
            if (idleReferences.isEmpty()) {
                if (null == factory) {
                    throw new NoSuchElementException();
                } else {
                    //如果可用列表为空则创建新的对象
                    newlyCreated = true;
                    obj = factory.makeObject().getObject();
                    //累加计数器
                    createCount++;
                    // Do not register with the queue
                    //关联
                    ref = new PooledSoftReference<T>(new SoftReference<T>(obj));
                    //添加进所有列表
                    allReferences.add(ref);
                }
            } else {
                //从可用队列获取对象
                ref = idleReferences.pollFirst();
                obj = ref.getObject();
                // Clear the reference so it will not be queued, but replace with a
                // a new, non-registered reference so we can still track this object
                // in allReferences
                //重建关联
                ref.getReference().clear();
                ref.setReference(new SoftReference<T>(obj));
            }
            if (null != factory && null != obj) {
                try {
                    //激活对象
                    factory.activateObject(ref);
                    if (!factory.validateObject(ref)) {
                        throw new Exception("ValidateObject failed");
                    }
                } catch (Throwable t) {
                    PoolUtils.checkRethrow(t);
                    try {
                        destroy(ref);
                    } catch (Throwable t2) {
                        PoolUtils.checkRethrow(t2);
                        // Swallowed
                    } finally {
                        obj = null;
                    }
                    if (newlyCreated) {
                        throw new NoSuchElementException(
                                "Could not create a validated object, cause: " +
                                        t.getMessage());
                    }
                }
            }
        }

        numActive++;
        //锁定
        ref.allocate();
        return obj;
    }

returnObject

    public synchronized void returnObject(T obj) throws Exception {
        boolean success = !isClosed();
        //判断对象来自于池【通过对象的equals方法】
        final PooledSoftReference<T> ref = findReference(obj);
        if (ref == null) {
            throw new IllegalStateException(
                "Returned object not currently part of this pool");
        }
        if (factory != null) {
            //判断对象合格
            if (!factory.validateObject(ref)) {
                success = false;
            } else {
                try {
                    //钝化对象
                    factory.passivateObject(ref);
                } catch (Exception e) {
                    success = false;
                }
            }
        }

        boolean shouldDestroy = !success;
        numActive--;
        if (success) {
            //如果对象合格并且钝化成功则解除锁定并添加到可用列表中
            // Deallocate and add to the idle instance pool
            ref.deallocate();
            idleReferences.add(ref);
        }
        notifyAll(); // numActive has changed

        if (shouldDestroy && factory != null) {
            try {
                destroy(ref);
            } catch (Exception e) {
                // ignored
            }
        }
    }

弱引用对象池最简单，没有后台清理线程只有当内存不够的情况下由虚拟机清除。

GenericObjectPool

//默认出队方式
private volatile boolean lifo = BaseObjectPoolConfig.DEFAULT_LIFO;
//后台清理逻辑
class Evictor extends TimerTask
//所有对象列表
private final Map<T, PooledObject<T>> allObjects = new ConcurrentHashMap<T, PooledObject<T>>();
//可用对象列表【双向链表】
private final LinkedBlockingDeque<PooledObject<T>> idleObjects = new LinkedBlockingDeque<PooledObject<T>>();

borrowObject

    public T borrowObject(long borrowMaxWaitMillis) throws Exception {
        assertOpen();
//是否在获取对象的时候检查对象，开启的话则检查【主要是检查过期】
        AbandonedConfig ac = this.abandonedConfig;
        if (ac != null && ac.getRemoveAbandonedOnBorrow() &&
                (getNumIdle() < 2) &&
                (getNumActive() > getMaxTotal() - 3) ) {
            removeAbandoned(ac);
        }

        PooledObject<T> p = null;

        // Get local copy of current config so it is consistent for entire
        // method execution
//当池耗尽的时候是否block，如果block的话则会idleObjects.pollFirst(borrowMaxWaitMillis,TimeUnit.MILLISECONDS);否则直接throw new NoSuchElementException("Pool exhausted");
        boolean blockWhenExhausted = getBlockWhenExhausted();

        boolean create;
        long waitTime = 0;

        while (p == null) {
            create = false;
            if (blockWhenExhausted) {
                p = idleObjects.pollFirst();
                if (p == null) {
                    create = true;
                    p = create();
                }
                if (p == null) {
                    if (borrowMaxWaitMillis < 0) {
                        p = idleObjects.takeFirst();
                    } else {
                        waitTime = System.currentTimeMillis();
                        p = idleObjects.pollFirst(borrowMaxWaitMillis,
                                TimeUnit.MILLISECONDS);
                        waitTime = System.currentTimeMillis() - waitTime;
                    }
                }
                if (p == null) {
                    throw new NoSuchElementException(
                            "Timeout waiting for idle object");
                }
                if (!p.allocate()) {
                    p = null;
                }
            } else {
                p = idleObjects.pollFirst();
                if (p == null) {
                    create = true;
                    p = create();
                }
                if (p == null) {
                    throw new NoSuchElementException("Pool exhausted");
                }
                if (!p.allocate()) {
                    p = null;
                }
            }

            if (p != null) {
                try {
//激活对象
                    factory.activateObject(p);
                } catch (Exception e) {
                    try {
                        destroy(p);
                    } catch (Exception e1) {
                        // Ignore - activation failure is more important
                    }
                    p = null;
                    if (create) {
                        NoSuchElementException nsee = new NoSuchElementException(
                                "Unable to activate object");
                        nsee.initCause(e);
                        throw nsee;
                    }
                }
//如果获取对象是检查则validateObject
                if (p != null && getTestOnBorrow()) {
                    boolean validate = false;
                    Throwable validationThrowable = null;
                    try {
                        validate = factory.validateObject(p);
                    } catch (Throwable t) {
                        PoolUtils.checkRethrow(t);
                        validationThrowable = t;
                    }
//检查不通过则destroy
                    if (!validate) {
                        try {
                            destroy(p);
                            destroyedByBorrowValidationCount.incrementAndGet();
                        } catch (Exception e) {
                            // Ignore - validation failure is more important
                        }
                        p = null;
                        if (create) {
                            NoSuchElementException nsee = new NoSuchElementException(
                                    "Unable to validate object");
                            nsee.initCause(validationThrowable);
                            throw nsee;
                        }
                    }
                }
            }
        }

        updateStatsBorrow(p, waitTime);

        return p.getObject();
    }

returnObject

    public void returnObject(T obj) {
        PooledObject<T> p = allObjects.get(obj);

        if (!isAbandonedConfig()) {
            if (p == null) {
                throw new IllegalStateException(
                        "Returned object not currently part of this pool");
            }
        } else {
            if (p == null) {
                return;  // Object was abandoned and removed
            } else {
                // Make sure object is not being reclaimed
                synchronized(p) {
                    final PooledObjectState state = p.getState();
                    if (state == PooledObjectState.ABANDONED ||
                            state == PooledObjectState.INVALID) {
                        return;
                    } else {
                        p.markReturning(); // Keep from being marked abandoned
                    }
                }
            }
        }

        long activeTime = p.getActiveTimeMillis();
//验证合格
        if (getTestOnReturn()) {
            if (!factory.validateObject(p)) {
                try {
                    destroy(p);
                } catch (Exception e) {
                    swallowException(e);
                }
                updateStatsReturn(activeTime);
                return;
            }
        }
//钝化
        try {
            factory.passivateObject(p);
        } catch (Exception e1) {
            swallowException(e1);
            try {
                destroy(p);
            } catch (Exception e) {
                swallowException(e);
            }
            updateStatsReturn(activeTime);
            return;
        }

        if (!p.deallocate()) {
            throw new IllegalStateException(
                    "Object has already been retured to this pool or is invalid");
        }
//池大小
        int maxIdleSave = getMaxIdle();
        if (isClosed() || maxIdleSave > -1 && maxIdleSave <= idleObjects.size()) {
            try {
                destroy(p);
            } catch (Exception e) {
                swallowException(e);
            }
        } else {
            if (getLifo()) {
                idleObjects.addFirst(p);
            } else {
                idleObjects.addLast(p);
            }
        }
        updateStatsReturn(activeTime);
    }

GenericKeyedObjectPool和GenericObjectPool类似。这里不再累述，下面主要说说怎么调用