netty是一个基础通信框架，管理的连接数较多，可能多至百万级，每一个连接都有或多或少有超时任务，比如发送数据超时，心跳检测等。如果为每一个连接都启动一个Timer，不仅效率低下，而且占用资源。基于论文Hashed and hierarchical timing wheels: data structures for the efficient implementation of a timer facility提出的定时轮，netty采用这种方式来管理和维护大量的定时任务，实现就在HashedWheelTimer这个类中，其原理如下：
定时轮其实就是一种环型的数据结构，可以把它想象成一个时钟，分成了许多格子，每个格子代表一定的时间，在这个格子上用一个链表来保存要执行的超时任务，同时有一个指针一格一格的走，走到哪个格子时就执行格子对应的超时任务，超时任务通过一定的规则放入到格子中，如下图所示：
以上图为例子，假设一格代表1s，上图能表示的时间段则为8s，假设当前指针指向3，有一个任务需要3s后执行，那么这个任务应该放在3+3=6的格子中，如果有一个任务需要6s后执行，那么这个任务应该放在(3+6)%8=1的格子中。接下来看下netty中的HashedWheelTimer类是具体如何实现这个算法的，构造函数如下：
public HashedWheelTimer(
ThreadFactory threadFactory,//用于创建worker线程
long tickDuration, //表示一格的时长，就是多久走一格
TimeUnit unit, //时间单位
int ticksPerWheel, //一圈有多少格
boolean leakDetection,//是否开启内存泄露检测
long maxPendingTimeouts
) {
if (threadFactory == null) {
throw new NullPointerException("threadFactory");
}
if (unit == null) {
throw new NullPointerException("unit");
}
if (tickDuration <= 0) {
throw new IllegalArgumentException("tickDuration must be greater than 0: " + tickDuration);
}
if (ticksPerWheel <= 0) {
throw new IllegalArgumentException("ticksPerWheel must be greater than 0: " + ticksPerWheel);
}
// Normalize ticksPerWheel to power of two and initialize the wheel.
wheel = createWheel(ticksPerWheel);//创建定时轮,格数为2的幂次方
mask = wheel.length - 1;//因为格子数为2的幂次方，此处用于代替%取余操作，可以提高效率
// Convert tickDuration to nanos.
this.tickDuration = unit.toNanos(tickDuration);//转换成纳秒
// Prevent overflow.
if (this.tickDuration >= Long.MAX_VALUE / wheel.length) {//校验是否存在溢出。即指针转动的时间间隔不能太长而导致tickDuration*wheel.length>Long.MAX_VALUE
throw new IllegalArgumentException(String.format(
"tickDuration: %d (expected: 0 < tickDuration in nanos < %d",
tickDuration, Long.MAX_VALUE / wheel.length));
}
workerThread = threadFactory.newThread(worker);
leak = leakDetection || !workerThread.isDaemon() ? leakDetector.track(this) : null;
this.maxPendingTimeouts = maxPendingTimeouts;
if (INSTANCE_COUNTER.incrementAndGet() > INSTANCE_COUNT_LIMIT &&
WARNED_TOO_MANY_INSTANCES.compareAndSet(false, true)) {
reportTooManyInstances();
}
}private static HashedWheelBucket[] createWheel(int ticksPerWheel) {
if (ticksPerWheel <= 0) {
throw new IllegalArgumentException(
"ticksPerWheel must be greater than 0: " + ticksPerWheel);
}
if (ticksPerWheel > 1073741824) {
throw new IllegalArgumentException(
"ticksPerWheel may not be greater than 2^30: " + ticksPerWheel);
}
ticksPerWheel = normalizeTicksPerWheel(ticksPerWheel);／／确保为2的米次方
HashedWheelBucket[] wheel = new HashedWheelBucket[ticksPerWheel];
for (int i = 0; i < wheel.length; i ++) {
wheel[i] = new HashedWheelBucket();
}
return wheel;
}private static final class HashedWheelBucket {
// Used for the linked-list datastructure
private HashedWheelTimeout head;
private HashedWheelTimeout tail;
.....
}public void start() {// 启动定时轮。这个方法其实在添加定时任务（newTimeout()方法）的时候会自动调用此方法，因为如果时间轮里根本没有定时任务，启动时间轮也是空耗资源
// 判断当前时间轮的状态，如果是初始化，则启动worker线程，启动整个定时轮；如果已经启动则略过；如果是已经停止，则报错。这里因为可能有多个线程争抢启动定时轮，所以采用了cas方式的无锁设计
switch (WORKER_STATE_UPDATER.get(this)) {
case WORKER_STATE_INIT:
if (WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_INIT, WORKER_STATE_STARTED)) {
workerThread.start();
}
break;
case WORKER_STATE_STARTED:
break;
case WORKER_STATE_SHUTDOWN:
throw new IllegalStateException("cannot be started once stopped");
default:
throw new Error("Invalid WorkerState");
}
// Wait until the startTime is initialized by the worker.
while (startTime == 0) {
try {
startTimeInitialized.await();//等待worker启动
} catch (InterruptedException ignore) {
// Ignore - it will be ready very soon.
}
}
}public Set<Timeout> stop() {
// worker线程不能停止定时轮，也就是加入的定时任务的线程不能调用这个方法。防止恶意的定时任务调用这个方法造成定时任务失效
if (Thread.currentThread() == workerThread) {
throw new IllegalStateException(
HashedWheelTimer.class.getSimpleName() +
".stop() cannot be called from " +
TimerTask.class.getSimpleName());
}
// 尝试CAS替换当前状态变为“停止：2”。如果失败，则当前时间轮的状态只能是“初始化：0”或者“停止：2”。直接将当前状态设置为“停止：2“
if (!WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_STARTED, WORKER_STATE_SHUTDOWN)) {
// workerState can be 0 or 2 at this moment - let it always be 2.
if (WORKER_STATE_UPDATER.getAndSet(this, WORKER_STATE_SHUTDOWN) != WORKER_STATE_SHUTDOWN) {
INSTANCE_COUNTER.decrementAndGet();
if (leak != null) {
boolean closed = leak.close(this);
assert closed;
}
}
return Collections.emptySet();
}
try {
boolean interrupted = false;
while (workerThread.isAlive()) {
workerThread.interrupt();//中断worker线程
try {
workerThread.join(100);
} catch (InterruptedException ignored) {
interrupted = true;
}
}
if (interrupted) {
Thread.currentThread().interrupt();
}
} finally {
INSTANCE_COUNTER.decrementAndGet();
if (leak != null) {
boolean closed = leak.close(this);
assert closed;
}
}
return worker.unprocessedTimeouts();//返回未处理的任务
}public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
if (task == null) {
throw new NullPointerException("task");
}
if (unit == null) {
throw new NullPointerException("unit");
}
long pendingTimeoutsCount = pendingTimeouts.incrementAndGet();
if (maxPendingTimeouts > 0 && pendingTimeoutsCount > maxPendingTimeouts) {
pendingTimeouts.decrementAndGet();
throw new RejectedExecutionException("Number of pending timeouts ("
+ pendingTimeoutsCount + ") is greater than or equal to maximum allowed pending "
+ "timeouts (" + maxPendingTimeouts + ")");
}
start();
// Add the timeout to the timeout queue which will be processed on the next tick.
// During processing all the queued HashedWheelTimeouts will be added to the correct HashedWheelBucket.
long deadline = System.nanoTime() + unit.toNanos(delay) - startTime;
// Guard against overflow.
if (delay > 0 && deadline < 0) {
deadline = Long.MAX_VALUE;
}
HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
timeouts.add(timeout);
return timeout;
}从阅读netty源码以及之前disruptor的源码后我们可以发现，无锁设计，juc并发包在框架中的应用非常普遍。