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/*
* Copyright (C) 2013, 2014 Brett Wooldridge
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.zaxxer.hikari.util;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_IN_USE;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_NOT_IN_USE;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_REMOVED;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_RESERVED;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.AbstractQueuedLongSynchronizer;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* This is a specialized concurrent bag that achieves superior performance
* to LinkedBlockingQueue and LinkedTransferQueue for the purposes of a
* connection pool. It uses ThreadLocal storage when possible to avoid
* locks, but resorts to scanning a common collection if there are no
* available items in the ThreadLocal list. Not-in-use items in the
* ThreadLocal lists can be "stolen" when the borrowing thread has none
* of its own. It is a "lock-less" implementation using a specialized
* AbstractQueuedLongSynchronizer to manage cross-thread signaling.
*
* Note that items that are "borrowed" from the bag are not actually
* removed from any collection, so garbage collection will not occur
* even if the reference is abandoned. Thus care must be taken to
* "requite" borrowed objects otherwise a memory leak will result. Only
* the "remove" method can completely remove an object from the bag.
*
* @author Brett Wooldridge
*
* @param the templated type to store in the bag
*/
public class ConcurrentBag
{
private static final Logger LOGGER = LoggerFactory.getLogger(ConcurrentBag.class);
protected final AbstractQueuedLongSynchronizer synchronizer;
protected final CopyOnWriteArrayList sharedList;
protected final AtomicLong sequence;
private final ThreadLocal>> threadList;
private final IBagStateListener listener;
private volatile boolean closed;
/**
* Construct a ConcurrentBag with the specified listener.
*
* @param listener the IBagStateListener to attach to this bag
*/
public ConcurrentBag(IBagStateListener listener)
{
this.sharedList = new CopyOnWriteArrayList();
this.synchronizer = createQueuedSynchronizer();
this.sequence = new AtomicLong(1);
this.listener = listener;
this.threadList = new ThreadLocal>>();
}
protected AbstractQueuedLongSynchronizer createQueuedSynchronizer()
{
throw new RuntimeException("createQueuedSynchronizer() method must be overridden");
}
/**
* The method will borrow a BagEntry from the bag, blocking for the
* specified timeout if none are available.
*
* @param timeout how long to wait before giving up, in units of unit
* @param timeUnit a TimeUnit determining how to interpret the timeout parameter
* @return a borrowed instance from the bag or null if a timeout occurs
* @throws InterruptedException if interrupted while waiting
*/
@SuppressWarnings("unchecked")
public T borrow(long timeout, final TimeUnit timeUnit) throws InterruptedException
{
if (!synchronizer.hasQueuedThreads()) {
// Try the thread-local list first
final ArrayList> list = threadList.get();
if (list == null) {
threadList.set(new ArrayList>(16));
}
else {
for (int i = list.size() - 1; i >= 0; i--) {
final IConcurrentBagEntry bagEntry = list.remove(i).get();
if (bagEntry != null && bagEntry.state().compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
return (T) bagEntry;
}
}
}
}
// Otherwise, scan the shared list ... for maximum of timeout
timeout = timeUnit.toNanos(timeout);
Future addItemFuture = null;
final long startScan = System.nanoTime();
final long originTimeout = timeout;
do {
long startSeq;
do {
startSeq = sequence.get();
for (final T bagEntry : sharedList) {
if (bagEntry.state().compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
return bagEntry;
}
}
} while (startSeq < sequence.get());
if (addItemFuture == null || addItemFuture.isDone()) {
addItemFuture = listener.addBagItem();
}
if (!synchronizer.tryAcquireSharedNanos(startSeq, timeout)) {
return null;
}
final long elapsed = (System.nanoTime() - startScan);
timeout = originTimeout - Math.max(elapsed, 100L); // don't trust the nanoTime() impl. not to go backwards due to NTP adjustments
}
while (timeout > 1000L); // 1000ns is the minimum resolution on many systems
return null;
}
/**
* This method will return a borrowed object to the bag. Objects
* that are borrowed from the bag but never "requited" will result
* in a memory leak.
*
* @param bagEntry the value to return to the bag
* @throws NullPointerException if value is null
* @throws IllegalStateException if the requited value was not borrowed from the bag
*/
public void requite(final T bagEntry)
{
if (bagEntry.state().compareAndSet(STATE_IN_USE, STATE_NOT_IN_USE)) {
final ArrayList> list = threadList.get();
if (list != null) {
list.add(new WeakReference(bagEntry));
}
synchronizer.releaseShared(sequence.incrementAndGet());
}
else {
LOGGER.warn("Attempt to remove an object from the bag that does not exist: {}", bagEntry.toString());
}
}
/**
* Add a new object to the bag for others to borrow.
*
* @param bagEntry an object to add to the bag
*/
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);
synchronizer.releaseShared(sequence.incrementAndGet());
}
/**
* Remove a value from the bag. This method should only be called
* with objects obtained by borrow(long, TimeUnit) or reserve(T)
*
* @param bagEntry the value to remove
* @return true if the entry was removed, false otherwise
* @throws IllegalStateException if an attempt is made to remove an object
* from the bag that was not borrowed or reserved first
*/
public boolean remove(final T bagEntry)
{
if (!bagEntry.state().compareAndSet(STATE_IN_USE, STATE_REMOVED) && !bagEntry.state().compareAndSet(STATE_RESERVED, STATE_REMOVED) && !closed) {
LOGGER.warn("Attempt to remove an object from the bag that was not borrowed or reserved: {}", bagEntry.toString());
return false;
}
final boolean removed = sharedList.remove(bagEntry);
if (!removed && !closed) {
LOGGER.warn("Attempt to remove an object from the bag that does not exist: {}", bagEntry.toString());
}
return removed;
}
/**
* Close the bag to further adds.
*/
public void close()
{
closed = true;
}
/**
* This method provides a "snaphot" in time of the BagEntry
* items in the bag in the specified state. It does not "lock"
* or reserve items in any way. Call reserve(T)
* on items in list before performing any action on them.
*
* @param state one of STATE_NOT_IN_USE or STATE_IN_USE
* @return a possibly empty list of objects having the state specified
*/
public List values(final int state)
{
final ArrayList list = new ArrayList(sharedList.size());
if (state == STATE_IN_USE || state == STATE_NOT_IN_USE) {
for (final T reference : sharedList) {
if (reference.state().get() == state) {
list.add(reference);
}
}
}
return list;
}
/**
* The method is used to make an item in the bag "unavailable" for
* borrowing. It is primarily used when wanting to operate on items
* returned by the values(int) method. Items that are
* reserved can be removed from the bag via remove(T)
* without the need to unreserve them. Items that are not removed
* from the bag can be make available for borrowing again by calling
* the unreserve(T) method.
*
* @param bagEntry the item to reserve
* @return true if the item was able to be reserved, false otherwise
*/
public boolean reserve(final T bagEntry)
{
return bagEntry.state().compareAndSet(STATE_NOT_IN_USE, STATE_RESERVED);
}
/**
* This method is used to make an item reserved via reserve(T)
* available again for borrowing.
*
* @param bagEntry the item to unreserve
*/
public void unreserve(final T bagEntry)
{
final long checkInSeq = sequence.incrementAndGet();
if (bagEntry.state().compareAndSet(STATE_RESERVED, STATE_NOT_IN_USE)) {
synchronizer.releaseShared(checkInSeq);
}
else {
LOGGER.warn("Attempt to relinquish an object to the bag that was not reserved: {}", bagEntry.toString());
}
}
/**
* Get the number of threads pending (waiting) for an item from the
* bag to become available.
*
* @return the number of threads waiting for items from the bag
*/
public int getPendingQueue()
{
return synchronizer.getQueueLength();
}
/**
* Get a count of the number of items in the specified state at the time of this call.
*
* @param state the state of the items to count
* @return a count of how many items in the bag are in the specified state
*/
public int getCount(final int state)
{
int count = 0;
for (final T reference : sharedList) {
if (reference.state().get() == state) {
count++;
}
}
return count;
}
/**
* Get the total number of items in the bag.
*
* @return the number of items in the bag
*/
public int size()
{
return sharedList.size();
}
public void dumpState()
{
for (T bagEntry : sharedList) {
LOGGER.info(bagEntry.toString());
}
}
}