1.資源池的動態伸縮
1.為了提升資源池的效能，需要設定最小閒置資源數量，在資源池初始化時完成初始化；而當使用的資源超過最小閒置資源數，消費者釋放回池中超過一定時間後要收縮到最小閒置資源數。
2.為了避免無限申請資源導致超出負載，需要設定最大資源數，池中資源不能超出最大資源數。
2.動態伸縮相關類結構
職責說明：
| 類 | 職責 |
|---|---|
| HouseKeeper | 實現了Runnable介面，負責池資源的動態伸縮。 |
| ScheduledExecutorService | 負責排程HouseKeeper。 |
| ScheduledFuture | 由ScheduledExecutorService排程返回，可以終止HouseKeeper的執行。 |
| HikariPool | 負責在構造器中初始化和呼叫以上類。 |
3.原始碼
3.1.HikariPool
// 初始化ScheduledExecutorService
this.houseKeepingExecutorService = initializeHouseKeepingExecutorService();
// 排程HouseKeeper， 延遲並週期性排程
this.houseKeeperTask = houseKeepingExecutorService.scheduleWithFixedDelay(new HouseKeeper(), 100L, housekeepingPeriodMs, MILLISECONDS);
複製程式碼3.2.HouseKeeper
private final class HouseKeeper implements Runnable
{
private volatile long previous = plusMillis(currentTime(), -housekeepingPeriodMs);
@Override
public void run()
{
try {
// refresh values in case they changed via MBean
connectionTimeout = config.getConnectionTimeout();
validationTimeout = config.getValidationTimeout();
leakTaskFactory.updateLeakDetectionThreshold(config.getLeakDetectionThreshold());
catalog = (config.getCatalog() != null && !config.getCatalog().equals(catalog)) ? config.getCatalog() : catalog;
final long idleTimeout = config.getIdleTimeout();
final long now = currentTime();
// Detect retrograde time, allowing +128ms as per NTP spec.
if (plusMillis(now, 128) < plusMillis(previous, housekeepingPeriodMs)) {
logger.warn("{} - Retrograde clock change detected (housekeeper delta={}), soft-evicting connections from pool.",
poolName, elapsedDisplayString(previous, now));
previous = now;
softEvictConnections();
return;
}
else if (now > plusMillis(previous, (3 * housekeepingPeriodMs) / 2)) {
// No point evicting for forward clock motion, this merely accelerates connection retirement anyway
logger.warn("{} - Thread starvation or clock leap detected (housekeeper delta={}).", poolName, elapsedDisplayString(previous, now));
}
previous = now;
String afterPrefix = "Pool ";
if (idleTimeout > 0L && config.getMinimumIdle() < config.getMaximumPoolSize()) {
logPoolState("Before cleanup ");
afterPrefix = "After cleanup ";
// 獲取未使用的資源
final List<PoolEntry> notInUse = connectionBag.values(STATE_NOT_IN_USE);
// 未使用資源大於配置的最小閒置資源則關閉多餘資源
int toRemove = notInUse.size() - config.getMinimumIdle();
for (PoolEntry entry : notInUse) {
if (toRemove > 0 && elapsedMillis(entry.lastAccessed, now) > idleTimeout && connectionBag.reserve(entry)) {
closeConnection(entry, "(connection has passed idleTimeout)");
toRemove--;
}
}
}
logPoolState(afterPrefix);
// 除了動態減少資源，這裡動態擴容資源
fillPool(); // Try to maintain minimum connections
}
catch (Exception e) {
logger.error("Unexpected exception in housekeeping task", e);
}
}
}
複製程式碼動態擴容資源：
//HikariPool.java
private synchronized void fillPool()
{
final int connectionsToAdd = Math.min(config.getMaximumPoolSize() - getTotalConnections(), config.getMinimumIdle() - getIdleConnections())
- addConnectionQueue.size();
for (int i = 0; i < connectionsToAdd; i++) {
addConnectionExecutor.submit((i < connectionsToAdd - 1) ? poolEntryCreator : postFillPoolEntryCreator);
}
}
複製程式碼3.3.ConcurrentBag
在ConcurrentBag中釋放資源時之會修改資源的狀態，不會去改變資源池可用資源數量。
public void requite(final T bagEntry)
{
// 這裡只是修改資源狀態，並不減少資源池中可用資源數量。
bagEntry.setState(STATE_NOT_IN_USE);
for (int i = 0; waiters.get() > 0; i++) {
if (bagEntry.getState() != STATE_NOT_IN_USE || handoffQueue.offer(bagEntry)) {
return;
}
else if ((i & 0xff) == 0xff) { // 0xff 是255， 每隔256進去一次
parkNanos(MICROSECONDS.toNanos(10));
}
else {
yield();
}
}
final List<Object> threadLocalList = threadList.get();
if (threadLocalList.size() < 50) {
threadLocalList.add(weakThreadLocals ? new WeakReference<>(bagEntry) : bagEntry);
}
}
複製程式碼4.總結
- 資源池在初始化時初始化最小資源數。
- 資源的動態伸縮可以通過JUC工具ScheduledExecutorService排程執行緒完成，而不需要額外使用第三方定時器。
- 消費者釋放資源時並不會立即減少資源池可用資源數量，因為很可能其他的消費者又會申請資源，為了避免減少無謂的建立資源操作，釋放的資源應該在超過一定時間後才真正關閉。
end.
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相關閱讀：
HikariPool原始碼（一）初識
HikariPool原始碼（二）設計思想借鑑
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