commons-pool比较重要的几个接口：
- PooledObject
- ObjectPool
- PooledObjectFactory
- KeyedObjectPool
- KeyedPooledObjectFactory
PooledObjectState
池对象状态枚举，有如下状态：
- IDLE
- ALLOCATED
- EVICTION
- EVICTION_RETURN_TO_HEAD
- VALIDATION
- VALIDATION_PREALLOCATED
- VALIDATION_RETURN_TO_HEAD
- INVALID
- ABANDONED
- RETURNING
PooledObject
所有放入对象池中的对象最终都会被包装为PoolObject，这个包装类提供了一些辅助信息（如对象创建时间、Idle时间、Active时间、上次借出时间、上次归还时间等），便于我们对对象生命周期的管理。
接口方法：public interface PooledObject<T> {
T getObject();
PooledObjectState getState();
long getCreateTime();
long getActiveTimeMillis();
long getIdleTimeMillis();
long getLastBorrowTime();
long getLastReturnTime();
long getLastUsedTime();
boolean startEvictionTest();
boolean endEvictionTest(Deque<PooledObject<T>> idleQueue);
boolean allocate();
boolean deallocate();
void invalidate();
void setLogAbandoned(boolean logAbandoned);
void use();
void markAbandoned();
void markReturning();
}
PooledOjbect | DefaultPooledObject | PooledSoftReference
ObjectPool
ObjectPool有三种实现：
- GenericObjectPool
- SoftReferenceObjectPool
- ProxiedObjectPool
ObjectPool接口方法：public interface ObjectPool<T> {
T borrowObject();
void returnObject(T obj);
void invalidateObject(T obj);
void addObject();
int getNumIdle();
int getNumActive();
void clear();
void close();
}