开源项目剖析之apache-common-pool

285572001

　　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 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 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 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 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 p) throws Exception;

　　PooledObject



T getObject();
long getCreateTime();
long getActiveTimeMillis();
long getIdleTimeMillis();
long getLastBorrowTime();
long getLastReturnTime();
long getLastUsedTime();
int compareTo(PooledObject other);
boolean equals(Object obj);
int hashCode();
String toString();
//后台清理线程
boolean startEvictionTest();
boolean endEvictionTest(Deque 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 idleReferences = new LinkedBlockingDeque();
//所有对象列表
private final ArrayList allReferences = new ArrayList();

　　borrowObject



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

public synchronized T borrowObject() throws Exception {
assertOpen();//确定池打开
T obj = null;
boolean newlyCreated = false;
PooledSoftReference 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(new SoftReference(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(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 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 allObjects = new ConcurrentHashMap();
//可用对象列表【双向链表】
private final LinkedBlockingDeque idleObjects = new LinkedBlockingDeque();

　　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 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 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