SynchronousQueue是一个没有数据缓冲的BlockingQueue，生产者线程对其的插入操作put必须等待消费者的移除操作take。
SynchronousQueue运用:
scss
public class SynchronousQueueDemo {
public static void main(String[] args) {
//新建一个SynchronousQueue同步队列
final BlockingQueue<Integer> synchronousQueue = new SynchronousQueue<>();
//启动一个生产者线程插入对象
SynchronousQueueProducer queueProducer = new SynchronousQueueProducer(synchronousQueue);
new Thread(queueProducer).start();
//启动两个消费者线程移除对象
SynchronousQueueConsumer queueConsumer1 = new SynchronousQueueConsumer(synchronousQueue);
new Thread(queueConsumer1).start();
SynchronousQueueConsumer queueConsumer2 = new SynchronousQueueConsumer(synchronousQueue);
new Thread(queueConsumer2).start();
}
}生产者:
csharp
public class SynchronousQueueProducer implements Runnable {
protected BlockingQueue<Integer> blockingQueue;
public SynchronousQueueProducer(BlockingQueue<Integer> queue) {
this.blockingQueue = queue;
}
@Override
public void run() {
int i = 0;
while (true) {
System.out.println(Thread.currentThread().getName() + " Put: " + ++i);
try {
blockingQueue.put(i);
Thread.sleep(1000); //每隔一秒生产一次
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}消费者:
java
public class SynchronousQueueConsumer implements Runnable {
protected BlockingQueue<Integer> blockingQueue;
public SynchronousQueueConsumer(BlockingQueue<Integer> queue) {
this.blockingQueue = queue;
}
@Override
public void run() {
while (true) {
try {
Integer data = blockingQueue.take();
System.out.println(Thread.currentThread().getName() + " take(): " + data);
Thread.sleep(5000); //每隔5秒消费一次
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}执行结果:
从结果我们可以看出,put一个元素,就会取出一个元素.
构造方法:
java
/**
* Creates a {@code SynchronousQueue} with nonfair access policy.
*/
public SynchronousQueue() {
this(false);
}
/**
* Creates a {@code SynchronousQueue} with the specified fairness policy.
*
* @param fair if true, waiting threads contend in FIFO order for
* access; otherwise the order is unspecified.
*/
public SynchronousQueue(boolean fair) {
transferer = fair ? new TransferQueue<E>() : new TransferStack<E>();
}默认无参的构造方法是非公平的.我们也可以指定参数来选择是否公平模式.
公平情况下是队列结构,先进先出.非公平是栈结构,后进先出.
数据结构:
TransferQueue:
构造方法:
ini
TransferQueue() {
QNode h = new QNode(null, false); // initialize to dummy node.
head = h;
tail = h;
}QNode方法:
arduino
/** Node class for TransferQueue. */
static final class QNode {
volatile QNode next; // next node in queue
volatile Object item; // CAS'ed to or from null
volatile Thread waiter; // to control park/unpark
final boolean isData;
QNode(Object item, boolean isData) {
this.item = item;
this.isData = isData;
}
boolean casNext(QNode cmp, QNode val) {
return next == cmp &&
UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
boolean casItem(Object cmp, Object val) {
return item == cmp &&
UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
}
/**
* Tries to cancel by CAS'ing ref to this as item.
*/
void tryCancel(Object cmp) {
UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
}
boolean isCancelled() {
return item == this;
}
/**
* Returns true if this node is known to be off the queue
* because its next pointer has been forgotten due to
* an advanceHead operation.
*/
boolean isOffList() {
return next == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long itemOffset;
private static final long nextOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = QNode.class;
itemOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("item"));
nextOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("next"));
} catch (Exception e) {
throw new Error(e);
}
}
}属性:
next:下一个节点.
item:元素的值.
waiter:执行的线程.
isData:执行的模式.(put添加操作还是take消费操作).
其余的方法不做过多解释了,获取偏移量,然后通过cas赋值.
SNode类:
java
/** Node class for TransferStacks. */
static final class SNode {
volatile SNode next; // next node in stack
volatile SNode match; // the node matched to this
volatile Thread waiter; // to control park/unpark
Object item; // data; or null for REQUESTs
int mode;
// Note: item and mode fields don't need to be volatile
// since they are always written before, and read after,
// other volatile/atomic operations.
SNode(Object item) {
this.item = item;
}
boolean casNext(SNode cmp, SNode val) {
return cmp == next &&
UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
/**
* Tries to match node s to this node, if so, waking up thread.
* Fulfillers call tryMatch to identify their waiters.
* Waiters block until they have been matched.
*
* @param s the node to match
* @return true if successfully matched to s
*/
boolean tryMatch(SNode s) {
if (match == null &&
UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
Thread w = waiter;
if (w != null) { // waiters need at most one unpark
waiter = null;
LockSupport.unpark(w);
}
return true;
}
return match == s;
}
/**
* Tries to cancel a wait by matching node to itself.
*/
void tryCancel() {
UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
}
boolean isCancelled() {
return match == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long matchOffset;
private static final long nextOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = SNode.class;
matchOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("match"));
nextOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("next"));
} catch (Exception e) {
throw new Error(e);
}
}
}属性:
next:下一个节点.
match:匹配的节点.
item:元素的值.
waiter:执行的线程.
mode:没研究这块相关的代码.
其余的方法不做过多解释了,获取偏移量,然后通过cas赋值.
队列方法:
put方法:
java
/**
* Inserts the specified element into this queue, waiting if necessary
* for space to become available.
*
* @param e the element to add
* @throws InterruptedException if interrupted while waiting
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
void put(E e) throws InterruptedException;
/**
* Adds the specified element to this queue, waiting if necessary for
* another thread to receive it.
*
* @throws InterruptedException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public void put(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
if (transferer.transfer(e, false, 0) == null) {
Thread.interrupted();
throw new InterruptedException();
}
}如果添加的元素为空会抛出空指针异常.所有的焦点都集中在了transferer(数据结构对象)的transfer方法.
transfer方法:
scss
/**
* Performs a put or take.
*
* @param e if non-null, the item to be handed to a consumer;
* if null, requests that transfer return an item
* offered by producer.
* @param timed if this operation should timeout
* @param nanos the timeout, in nanoseconds
* @return if non-null, the item provided or received; if null,
* the operation failed due to timeout or interrupt --
* the caller can distinguish which of these occurred
* by checking Thread.interrupted.
*/
abstract E transfer(E e, boolean timed, long nanos);
E transfer(E e, boolean timed, long nanos) {
QNode s = null; // constructed/reused as needed
boolean isData = (e != null);
for (;;) {
QNode t = tail;
QNode h = head;
if (t == null || h == null) // saw uninitialized value
continue; // spin
if (h == t || t.isData == isData) { // empty or same-mode
QNode tn = t.next;
if (t != tail) // inconsistent read
continue;
if (tn != null) { // lagging tail
advanceTail(t, tn);
continue;
}
if (timed && nanos <= 0) // can't wait
return null;
if (s == null)
s = new QNode(e, isData);
if (!t.casNext(null, s)) // failed to link in
continue;
advanceTail(t, s); // swing tail and wait
Object x = awaitFulfill(s, e, timed, nanos);
if (x == s) { // wait was cancelled
clean(t, s);
return null;
}
if (!s.isOffList()) { // not already unlinked
advanceHead(t, s); // unlink if head
if (x != null) // and forget fields
s.item = s;
s.waiter = null;
}
return (x != null) ? (E)x : e;
} else { // complementary-mode
QNode m = h.next; // node to fulfill
if (t != tail || m == null || h != head)
continue; // inconsistent read
Object x = m.item;
if (isData == (x != null) || // m already fulfilled
x == m || // m cancelled
!m.casItem(x, e)) { // lost CAS
advanceHead(h, m); // dequeue and retry
continue;
}
advanceHead(h, m); // successfully fulfilled
LockSupport.unpark(m.waiter);
return (x != null) ? (E)x : e;
}
}
}方法解释:
boolean isData = (e != null):如果为添加操作这个isData就为true,取出操作的话就为false.
注意:第一个判断条件进入和插入取出的顺序有关.具体理解可以通过断点来看.也可以找我具体了解.
advanceTail方法:
scss
/**
* Tries to cas nt as new tail.
*/
void advanceTail(QNode t, QNode nt) {
if (tail == t)
UNSAFE.compareAndSwapObject(this, tailOffset, t, nt);
}这个方法就是通过cas操作把tail节点换成新的.(因为这个条件逻辑处理有可能是处理取元素或者新增元素,所以看不懂就尝试这去理解另一种操作)
casNext方法:
kotlin
boolean casNext(QNode cmp, QNode val) {
return next == cmp &&
UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}awaitFulfill方法:
ini
/**
* Spins/blocks until node s is fulfilled.
*
* @param s the waiting node
* @param e the comparison value for checking match
* @param timed true if timed wait
* @param nanos timeout value
* @return matched item, or s if cancelled
*/
Object awaitFulfill(QNode s, E e, boolean timed, long nanos) {
/* Same idea as TransferStack.awaitFulfill */
final long deadline = timed ? System.nanoTime() + nanos : 0L;
Thread w = Thread.currentThread();
int spins = ((head.next == s) ?
(timed ? maxTimedSpins : maxUntimedSpins) : 0);
for (;;) {
if (w.isInterrupted())
s.tryCancel(e);
Object x = s.item;
if (x != e)
return x;
if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
s.tryCancel(e);
continue;
}
}
if (spins > 0)
--spins;
else if (s.waiter == null)
s.waiter = w;
else if (!timed)
LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanos);
}
}spins就是下面循环的次数.(很简单,不理解可以打个断点看一下就懂)
如果上面阻塞被唤醒了,就会继续循环执行.
返回这个值.
注意:这里是两个操作的逻辑.
casItem方法:
typescript
boolean casItem(Object cmp, Object val) {
return item == cmp &&
UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
}take方法:
java
/**
* Retrieves and removes the head of this queue, waiting if necessary
* until an element becomes available.
*
* @return the head of this queue
* @throws InterruptedException if interrupted while waiting
*/
E take() throws InterruptedException;
/**
* Retrieves and removes the head of this queue, waiting if necessary
* for another thread to insert it.
*
* @return the head of this queue
* @throws InterruptedException {@inheritDoc}
*/
public E take() throws InterruptedException {
E e = transferer.transfer(null, false, 0);
if (e != null)
return e;
Thread.interrupted();
throw new InterruptedException();
}transfer不做过多解释了,参考上面.
ini
/**
* Puts or takes an item.
*/
@SuppressWarnings("unchecked")
E transfer(E e, boolean timed, long nanos) {
QNode s = null; // constructed/reused as needed
boolean isData = (e != null);
for (;;) {
QNode t = tail;
QNode h = head;
if (t == null || h == null) // saw uninitialized value
continue; // spin
if (h == t || t.isData == isData) { // empty or same-mode
QNode tn = t.next;
if (t != tail) // inconsistent read
continue;
if (tn != null) { // lagging tail
advanceTail(t, tn);
continue;
}
if (timed && nanos <= 0) // can't wait
return null;
if (s == null)
s = new QNode(e, isData);
if (!t.casNext(null, s)) // failed to link in
continue;
advanceTail(t, s); // swing tail and wait
Object x = awaitFulfill(s, e, timed, nanos);
if (x == s) { // wait was cancelled
clean(t, s);
return null;
}
if (!s.isOffList()) { // not already unlinked
advanceHead(t, s); // unlink if head
if (x != null) // and forget fields
s.item = s;
s.waiter = null;
}
return (x != null) ? (E)x : e;
} else { // complementary-mode
QNode m = h.next; // node to fulfill
if (t != tail || m == null || h != head)
continue; // inconsistent read
Object x = m.item;
if (isData == (x != null) || // m already fulfilled
x == m || // m cancelled
!m.casItem(x, e)) { // lost CAS
advanceHead(h, m); // dequeue and retry
continue;
}
advanceHead(h, m); // successfully fulfilled
LockSupport.unpark(m.waiter);
return (x != null) ? (E)x : e;
}
}
}和上面方法一样.可以参考上面的.
非公平栈结构的put和take方法:
scss
/**
* Puts or takes an item.
*/
@SuppressWarnings("unchecked")
E transfer(E e, boolean timed, long nanos) {
SNode s = null; // constructed/reused as needed
int mode = (e == null) ? REQUEST : DATA;
for (;;) {
SNode h = head;
if (h == null || h.mode == mode) { // empty or same-mode
if (timed && nanos <= 0) { // can't wait
if (h != null && h.isCancelled())
casHead(h, h.next); // pop cancelled node
else
return null;
} else if (casHead(h, s = snode(s, e, h, mode))) {
SNode m = awaitFulfill(s, timed, nanos);
if (m == s) { // wait was cancelled
clean(s);
return null;
}
if ((h = head) != null && h.next == s)
casHead(h, s.next); // help s's fulfiller
return (E) ((mode == REQUEST) ? m.item : s.item);
}
} else if (!isFulfilling(h.mode)) { // try to fulfill
if (h.isCancelled()) // already cancelled
casHead(h, h.next); // pop and retry
else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
for (;;) { // loop until matched or waiters disappear
SNode m = s.next; // m is s's match
if (m == null) { // all waiters are gone
casHead(s, null); // pop fulfill node
s = null; // use new node next time
break; // restart main loop
}
SNode mn = m.next;
if (m.tryMatch(s)) {
casHead(s, mn); // pop both s and m
return (E) ((mode == REQUEST) ? m.item : s.item);
} else // lost match
s.casNext(m, mn); // help unlink
}
}
} else { // help a fulfiller
SNode m = h.next; // m is h's match
if (m == null) // waiter is gone
casHead(h, null); // pop fulfilling node
else {
SNode mn = m.next;
if (m.tryMatch(h)) // help match
casHead(h, mn); // pop both h and m
else // lost match
h.casNext(m, mn); // help unlink
}
}
}
}方法解釋：
mode的属性值是根据是插入操作还是取出操作来决定是什么模式.
snode方法:
vbnet
/**
* Creates or resets fields of a node. Called only from transfer
* where the node to push on stack is lazily created and
* reused when possible to help reduce intervals between reads
* and CASes of head and to avoid surges of garbage when CASes
* to push nodes fail due to contention.
*/
static SNode snode(SNode s, Object e, SNode next, int mode) {
if (s == null) s = new SNode(e);
s.mode = mode;
s.next = next;
return s;
}如果节点为空就新创建一个节点,如果不为空就进行指向和赋值模式.
casHead方法:
typescript
boolean casHead(SNode h, SNode nh) {
return h == head &&
UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
}就是把头节点通过cas操作换成新的节点.(通过队列模拟栈结构,后进先出.)
awaitFulfill方法:
ini
/**
* Spins/blocks until node s is matched by a fulfill operation.
*
* @param s the waiting node
* @param timed true if timed wait
* @param nanos timeout value
* @return matched node, or s if cancelled
*/
SNode awaitFulfill(SNode s, boolean timed, long nanos) {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
Thread w = Thread.currentThread();
int spins = (shouldSpin(s) ?
(timed ? maxTimedSpins : maxUntimedSpins) : 0);
for (;;) {
if (w.isInterrupted())
s.tryCancel();
SNode m = s.match;
if (m != null)
return m;
if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
s.tryCancel();
continue;
}
}
if (spins > 0)
spins = shouldSpin(s) ? (spins-1) : 0;
else if (s.waiter == null)
s.waiter = w; // establish waiter so can park next iter
else if (!timed)
LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanos);
}
}isFulfilling方法:
arduino
static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }一般情况下这里返回的都是false,所以会进入.如果头节点需要出队的话,就会返回true.进入else逻辑.
tryMatch方法:
java
/**
* Tries to match node s to this node, if so, waking up thread.
* Fulfillers call tryMatch to identify their waiters.
* Waiters block until they have been matched.
*
* @param s the node to match
* @return true if successfully matched to s
*/
boolean tryMatch(SNode s) {
if (match == null &&
UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
Thread w = waiter;
if (w != null) { // waiters need at most one unpark
waiter = null;
LockSupport.unpark(w);
}
return true;
}
return match == s;
}如果match不为空的话,证明不是取线程,所以需要把下一个线程替换为取线程节点.
这块理解可能会有偏差,不一定对,有懂的小伙伴们可以指点一二.
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