3、ConcurrentBag介紹
本文以v2.7.2源碼為主進行分析
HikariCP連接配接池是基于自主實作的ConcurrentBag完成的資料連接配接的多線程共享互動,是HikariCP連接配接管理快速的其中一個關鍵點。
ConcurrentBag是一個專門的并發包裹,在連接配接池(多線程資料互動)的實作上具有比LinkedBlockingQueue和LinkedTransferQueue更優越的性能。
ConcurrentBag通過拆分
CopyOnWriteArrayList、ThreadLocal和SynchronousQueue
進行并發資料互動。
- CopyOnWriteArrayList:負責存放ConcurrentBag中全部用于出借的資源
- ThreadLocal:用于加速線程本地化資源通路
- SynchronousQueue:用于存在資源等待線程時的第一手資源交接
private final CopyOnWriteArrayList<T> sharedList;
private final ThreadLocal<List<Object>> threadList;
private final SynchronousQueue<T> handoffQueue;
ConcurrentBag中全部的資源均隻能通過add方法進行添加,隻能通過remove方法進行移出。
public void add(final T bagEntry)
{
if (closed) {
LOGGER.info("ConcurrentBag has been closed, ignoring add()");
throw new IllegalStateException("ConcurrentBag has been closed, ignoring add()");
}
sharedList.add(bagEntry); //新添加的資源優先放入CopyOnWriteArrayList
// 當有等待資源的線程時,将資源交到某個等待線程後才傳回(SynchronousQueue)
while (waiters.get() > && !handoffQueue.offer(bagEntry)) {
yield();
}
}
public boolean remove(final T bagEntry)
{
// 如果資源正在使用且無法進行狀态切換,則傳回失敗
if (!bagEntry.compareAndSet(STATE_IN_USE, STATE_REMOVED) && !bagEntry.compareAndSet(STATE_RESERVED, STATE_REMOVED) && !closed) {
LOGGER.warn("Attempt to remove an object from the bag that was not borrowed or reserved: {}", bagEntry);
return false;
}
final boolean removed = sharedList.remove(bagEntry); // 從CopyOnWriteArrayList中移出
if (!removed && !closed) {
LOGGER.warn("Attempt to remove an object from the bag that does not exist: {}", bagEntry);
}
return removed;
}
ConcurrentBag中通過borrow方法進行資料資源借用,通過requite方法進行資源回收,注意其中borrow方法隻提供對象引用,不移除對象,是以使用時通過borrow取出的對象必須通過requite方法進行放回,否則容易導緻記憶體洩露!
public T borrow(long timeout, final TimeUnit timeUnit) throws InterruptedException
{
// 優先檢視有沒有可用的本地化的資源
final List<Object> list = threadList.get();
for (int i = list.size() - ; i >= ; i--) {
final Object entry = list.remove(i);
@SuppressWarnings("unchecked")
final T bagEntry = weakThreadLocals ? ((WeakReference<T>) entry).get() : (T) entry;
if (bagEntry != null && bagEntry.compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
return bagEntry;
}
}
final int waiting = waiters.incrementAndGet();
try {
// 當無可用本地化資源時,周遊全部資源,檢視是否存在可用資源
// 是以被一個線程本地化的資源也可能被另一個線程“搶走”
for (T bagEntry : sharedList) {
if (bagEntry.compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
if (waiting > ) {
// 因為可能“搶走”了其他線程的資源,是以提醒包裹進行資源添加
listener.addBagItem(waiting - );
}
return bagEntry;
}
}
listener.addBagItem(waiting);
timeout = timeUnit.toNanos(timeout);
do {
final long start = currentTime();
// 當現有全部資源全部在使用中,等待一個被釋放的資源或者一個新資源
final T bagEntry = handoffQueue.poll(timeout, NANOSECONDS);
if (bagEntry == null || bagEntry.compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
return bagEntry;
}
timeout -= elapsedNanos(start);
} while (timeout > _000);
return null;
}
finally {
waiters.decrementAndGet();
}
}
public void requite(final T bagEntry)
{
// 将狀态轉為未在使用
bagEntry.setState(STATE_NOT_IN_USE);
// 判斷是否存在等待線程,若存在,則直接轉手資源
for (int i = ; waiters.get() > ; i++) {
if (bagEntry.getState() != STATE_NOT_IN_USE || handoffQueue.offer(bagEntry)) {
return;
}
else if ((i & ) == ) {
parkNanos(MICROSECONDS.toNanos());
}
else {
yield();
}
}
// 否則,進行資源本地化
final List<Object> threadLocalList = threadList.get();
threadLocalList.add(weakThreadLocals ? new WeakReference<>(bagEntry) : bagEntry);
}
上述代碼中的
weakThreadLocals
是用來判斷是否使用弱引用,通過下述方法初始化:
private boolean useWeakThreadLocals()
{
try {
// 人工指定是否使用弱引用,但是官方不推薦進行自主設定。
if (System.getProperty("com.dareway.concurrent.useWeakReferences") != null) {
return Boolean.getBoolean("com.dareway.concurrent.useWeakReferences");
}
// 預設通過判斷初始化的ClassLoader是否是系統的ClassLoader來确定
return getClass().getClassLoader() != ClassLoader.getSystemClassLoader();
}
catch (SecurityException se) {
return true;
}
}