promise-framework是一个从netty抽取出来的通用promise异步框架，并简化了Listener架构。
github：https://github.com/zhaojigang/concurrent-framework
- 一、使用姿势
- 二、代码架构
- 三、代码分析
- 附、bug记录
一、使用姿势
1.1、回调方式（推荐 - 完全异步）
@Test
public void testListenerNotifyLater() {
int numListenersBefore = 2; // 设置结果前设置两个listener
int numListenersAfter = 3; // 设置结果后设置三个listener
CountDownLatch latch = new CountDownLatch(numListenersBefore + numListenersAfter);
DefaultPromise promise = new DefaultPromise<>();
for (int i = 0; i < numListenersBefore; i++) {
promise.addListener(new FutureListener() {
@Override
public void operationComplete(Future future) throws Exception {
latch.countDown();
}
});
}
new Thread(new Runnable() {
@Override
public void run() {
promise.setSuccess(null);
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < numListenersAfter; i++) {
promise.addListener(future -> {
latch.countDown();
});
}
}
}).start();
}
}).start();
try {
Assert.assertTrue(latch.await(100, TimeUnit.SECONDS), "expect notify " + (numListenersBefore + numListenersAfter) + " listeners");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
流程（非常重要）：
- 首先在main线程中为DefaultPromise实例添加了两个listener
- 之后启动另外一个线程A去设置值（此时就会回调已经加入到当前的DefaultPromise实例中的两个listener#operationComplete(Future
future)，然后删除这两个listener，也就是说一个listener只能被通知一遍） - 之后线程A又启动了另外的一条线程B为当前的DefaultPromise实例添加了3个listener，注意，此时每添加一个listener，就会立即回调其operationComplete方法，因为当前的DefaultPromise.isDone()==true了，就是说当前的DefaultPromise实例已经完成了。
1.2、阻塞get方式 - （不推荐 - 可能阻塞）
@Test
private void testFutureStyleWithWaitNotifyAll() throws ExecutionException, InterruptedException {
Promise promise = new DefaultPromise<>();
/**
* 一个线程在执行get()，进行wait()
*/
new Thread(new Runnable() {
@Override
public void run() {
try {
Object result = promise.get();// 等待条件
// 之后做相应的业务逻辑
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}).start();
// sleep 2s 使第一个线程先等待着
Thread.sleep(2000);
/**
* 另外一个线程在设置值，notifyAll唤醒wait()线程
*/
new Thread(new Runnable() {
@Override
public void run() {
promise.setSuccess(new Model(1L));
}
}).start();
}
步骤：
- 标准的future阻塞姿势：一个线程执行get()，进行wait()阻塞；另外一个线程设置值，执行notifyAll()
二、代码架构
说明：
- java.util.concurrent.Future：Java并发包提供的Future类。
- io.hulk.promise.framework.Future：继承java.util.concurrent.Future，增强功能。
- AbstractFuture：实现了java.util.concurrent.Future的get()和get(long timeout, TimeUnit unit)阻塞等待模式，使用模板模式搭建这两个方法的基本骨架。
- Promise：可写的Future，提供setSuccess()等接口方法。
- DefaultPromise：最终的实现类，该实现类实现了观察者模式。
三、代码分析
3.1 java.util.concurrent.Future
public interface Future {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
方法较为简陋，例如只有isDone()，而没有isSuccess()这样的方法，没有添加listener的接口，也没有设置是否可以取消的接口。所以使用io.hulk.promise.framework.Future增强java.util.concurrent.Future。
3.2 io.hulk.promise.framework.Future
/**
* from netty4.1
*/
public interface Future extends java.util.concurrent.Future {
/**
* Returns {@code true} if and only if the I/O operation was completed successfully.
*/
boolean isSuccess();
/**
* returns {@code true} if and only if the operation can be cancelled via {@link #cancel(boolean)}.
*/
boolean isCancellable();
/**
* Returns the cause of the failed I/O operation if the I/O operation has failed.
*
* @return the cause of the failure. {@code null} if succeeded or this future is not completed yet.
*/
Throwable cause();
/**
* Adds the specified listener to this future.
* The specified listener is notified when this future is {@linkplain #isDone() done}.
* If this future is already completed, the specified listener is notified immediately.
*/
Future addListener(FutureListener listener);
/**
* Adds the specified listeners to this future.
* The specified listeners is notified when this future is {@linkplain #isDone() done}.
* If this future is already completed, the specified listeners is notified immediately.
*/
Future addListeners(List> listeners);
/**
* Removes the first occurrence of the specified listener from this future.
* The specified listener is no longer notified when this future is {@linkplain #isDone() done}.
* If the specified listener is not associated with this future, this method does nothing and returns silently.
*/
Future removeListener(FutureListener listener);
/**
* Removes the first occurrence for each of the listeners from this future.
* The specified listeners is no longer notified when this future is {@linkplain #isDone() done}.
* If the specified listeners is not associated with this future, this method does nothing and returns silently.
*/
Future removeListeners(List> listeners);
/**
* Waits for this future until it is done, and rethrows the cause of the failure if this future failed.
*
* @throws InterruptedException
* if the current thread was interrupted
*/
Future sync() throws InterruptedException;
/**
* Waits for this future until it is done, and rethrows the cause of the failure if this future failed.
* This method catches an {@link InterruptedException} and discards it silently.
* 即不响应中断
*/
Future syncUninterruptibly();
/**
* Waits for this future to be completed.
*
* @throws InterruptedException
* if the current thread was interrupted
*/
Future await() throws InterruptedException;
/**
* Waits for this future to be completed without interruption.
* This method catches an {@link InterruptedException} and discards it silently.
*/
Future awaitUninterruptibly();
/**
* Waits for this future to be completed within the specified time limit.
*
* @return {@code true} if and only if the future was completed within the specified time limit
* @throws InterruptedException if the current thread was interrupted
*/
boolean await(long timeout, TimeUnit timeUnit) throws InterruptedException;
/**
* Waits for this future to be completed within the specified time limit.
*
* @return {@code true} if and only if the future was completed within the specified time limit without interruption.
* This method catches an {@link InterruptedException} and discards it silently.
*/
boolean awaitUninterruptibly(long timeout, TimeUnit timeUnit);
/**
* Waits for this future to be completed within the specified time limit.
*
* @return {@code true} if and only if the future was completed within the specified time limit
* @throws InterruptedException if the current thread was interrupted
*/
boolean await(long timeoutMillis) throws InterruptedException;
/**
* Waits for this future to be completed within the specified time limit.
*
* @return {@code true} if and only if the future was completed within the specified time limit without interruption.
* This method catches an {@link InterruptedException} and discards it silently.
*/
boolean awaitUninterruptibly(long timeoutMillis);
/**
* Return the result without blocking. If the future is not done yet this will return {@code null}.
*
* As it is possible that a {@code null} value is used to mark the future as successful you also need to check
* if the future is really done with {@link #isDone()} and not relay on the returned {@code null} value.
*/
V getNow();
/**
* {@inheritDoc}
*
* If the cancellation was successful it will fail the future with an {@link java.util.concurrent.CancellationException}.
*
*/
@Override
boolean cancel(boolean mayInterruptIfRunning); // {@inheritDoc} 用在一个@Override的方法上，表示为父类的方法添加详细的注释
}
3.3 FutureListener
/**
* Listens to the result of a {@link Future}.
* The result of the asynchronous operation is notified once
* this listener is added by calling {@link Future#addListener(FutureListener)}.
*
* @author zhaojigang
* @date 2018/7/16
*/
public interface FutureListener {
/**
* Invoked when the operation associated with the {@link Future} has been completed.
*
* @param future the source {@link Future} which called this callback
*/
void operationComplete(Future future) throws Exception;
}
注意：operationComplete(Future
3.4 AbstractFuture
/**
* from netty4.1
*/
public abstract class AbstractFuture implements Future{
@Override
public V get() throws InterruptedException, ExecutionException {
/**
* 阻塞等到await()调用完成，即失败或返回结果
*/
await();
/**
* 获取失败异常信息
*/
Throwable cause = cause();
/**
* 如果异常信息为null，直接获取响应结果
*/
if (cause == null) {
return getNow();
}
/**
* 如果返回结果result == CancellationException（即执行了cancel()），则抛出该异常
* 否则，抛出ExecutionException
*/
if (cause instanceof CancellationException) {
throw (CancellationException)cause;
}
throw new ExecutionException(cause);
}
@Override
public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
if(await(timeout, unit)) {
Throwable cause = cause();
if (cause == null) {
return getNow();
}
if (cause instanceof CancellationException) {
throw (CancellationException)cause;
}
throw new ExecutionException(cause);
}
/**
* 如果没有在指定的时间内await没有完成，抛出超时异常
*/
throw new TimeoutException();
}
}
使用模板模式，定义好get()和get(long timeout, TimeUnit unit)的基本框架，至于具体await()/await(timeout, unit)/cause()/getNow()等方法就由具体的类来实现了。
值得注意的是：
- 如果一个task被成功的cancel()了，会直接抛出CancellationException。
- get()/get(long timeout, TimeUnit unit)是阻塞获取结果的，所以netty不推荐使用这种方式。
3.5 Promise
/**
* from netty4.1
* Special {@link Future} which is writable.
*
* 添加设置操作
* 将Future中返回值为Future的全部override为Promise
* @author zhaojigang
* @date 2018/7/16
*/
public interface Promise extends Future {
/**
* Marks this future as a success and notifies all listeners.
* If it is success or failed already it will throw an {@link IllegalStateException}.
*/
Promise setSuccess(V result);
/**
* Marks this future as a success and notifies all listeners.
*
* @return {@code true} if and only if successfully marked this future as a success.
* Otherwise {@code false} because this future is already marked as either a success or a failure.
*/
boolean trySuccess(V result);
/**
* Marks this future as a failure and notifies all listeners.
* If it is success or failed already it will throw an {@link IllegalStateException}.
*/
Promise setFailure(Throwable cause);
/**
* Marks this future as a failure and notifies all listeners.
*
* @return {@code true} if and only if successfully marked this future as a failure.
* {@code false} because this future is already marked as either a success or a failure.
*/
boolean tryFailure(Throwable cause);
/**
* Make this future impossible to cancel.
*
* @return {@code true} if and only if successfully marked this future as uncancellable
* or it is already done without being cancelled.
* {@code false} if this future has been cancelled already.
*/
boolean setUncancellable();
@Override
Promise addListener(FutureListener listener);
@Override
Promise addListeners(List> listeners);
@Override
Promise removeListener(FutureListener listener);
@Override
Promise removeListeners(List> listeners);
@Override
Promise sync() throws InterruptedException;
@Override
Promise syncUninterruptibly();
@Override
Promise await() throws InterruptedException;
@Override
Promise awaitUninterruptibly();
}
Promise是一种可进行写的Future，具有设置成功结果，设置失败结果等功能，这样可以在成功或失败的时候回调注册到当前Promise实例的listeners了。就是一种完全异步的方式了，而AbstractFuture#get可能需要阻塞，所以netty推荐我们使用listener回调模式。
3.6 DefaultPromise
/**
* from netty4.1
*
* 一、DefaultPromise状态转换图：
* A {@link DefaultPromise} is either uncompleted or completed.
* When an I/O operation begins, a new future object is created.
* The new future is uncompleted initially - it is neither succeeded, failed, nor cancelled
* because the I/O operation is not finished yet.
* If the I/O operation is finished either successfully, with failure, or by cancellation,
* the future is marked as completed with more specific information, such as the cause of the
* failure.
* Please note that even failure and cancellation belong to the completed state.
*
* +---------------------------+
* | Completed successfully |
* +---------------------------+
* +----> isDone() = true |
* +--------------------------+ | | isSuccess() = true |
* | Uncompleted | | +===========================+
* +--------------------------+ | | Completed with failure |
* | isDone() = false | | +---------------------------+
* | isSuccess() = false |----+----> isDone() = true |
* | isCancelled() = false | | | cause() = non-null |
* | cause() = null | | +===========================+
* +--------------------------+ | | Completed by cancellation |
* | +---------------------------+
* +----> isDone() = true |
* | isCancelled() = true |
* +---------------------------+
*
*
*
* 二、DefaultPromise实现了两种执行机制：
* 1、future：wait/notify实现，可能要阻塞，使用方最终调用到DefaultPromise父类AbstractFuture#get或者DefaultPromise#syncXxx
* 2、listener：其实就是callback实现，不需要阻塞，当setSuccess/trySuccess/setFailure/tryFailure/cancel会直接调用listener（回调函数）当然如果有等待条件的其他线程，也会notifyAll
*
* 推荐使用第二种，完全异步的。
*/
public class DefaultPromise extends AbstractFuture implements Promise {
private static final Logger LOGGER = LoggerFactory.getLogger(DefaultPromise.class);
/**
* 返回结果result的原子更新器
*/
private static final AtomicReferenceFieldUpdater RESULT_UPDATER
= AtomicReferenceFieldUpdater.newUpdater(DefaultPromise.class, Object.class, "result");
/**
* 返回结果
*/
private volatile Object result;
/**
* 成功的返回标记
*/
private static final Object SUCCESS = new Object();
/**
* 不可取消的标记
*/
private static final Object UNCANCELLABLE = new Object();
/**
* wait线程的数量，注意该参数的修改要进行同步（恰好该参数的所有修改地方都在一个synchronized中）
*/
private short waiters;
/**
* cancel()时要将此项异常塞入result
*/
private static final Throwable CANCELLATION_CAUSE = new CancellationException(DefaultPromise.class.getName() + " invoked cancel()");
/**
* Threading - synchronized(this) 事件监听器列表
* If {@code empty}, it means either 1) no listeners were added yet or 2) all listeners were notified.
* 也就是说 一个listener通知过一次就会被删除，不会再通知第二次
*/
private List> listeners;
/**
* Threading - synchronized(this). We must prevent concurrent notification and FIFO listener notification.
*/
private boolean notifyingListeners;
/**
* future返回是否成功
*
* @return {@code true} 结果不为空 && 结果不是Throwable（失败）&& 结果不是UNCANCELLABLE（不能取消）
*/
@Override
public boolean isSuccess() {
/**
* 将成员变量result转换为局部变量进行操作的原因？
*
* 一、是因为 在下面的代码中会多次调用this.result，当外界的this.result引用发生变化时，由于this.result是被volatile修饰的，如果直接使用this.result将会导致多次获取的result不同，
* 但是this.result引用发生变化时，局部变量result不会发生变化（注意修改的是this.result引用的值，而不是this.result指向的地址的值，类似下边的程序）
*
* public static void main(String[] args) {
* DefaultPromiseTest test = new DefaultPromiseTest();
*
* Model m2 = test.m;
* System.out.println(m2);
*
* test.m = new Model(200L); // 注意：这里不是this.m.setId(300)，所以下面的m2不变
* System.out.println(m2);
* }
*
*
*
* 二、由于this.result是被volatile修饰的，每次获取都要强制从主存中获取，无法从工作线程直接获取，所以代价较大，而且将频繁操作的成员变量局部化更方便JIT优化
* https://blog.csdn.net/shaomingliang499/article/details/50549306
*/
Object result = this.result;
return result != null && !(result instanceof Throwable) && result != UNCANCELLABLE;
}
/**
* 等待线程是否可取消
*
* @return {@code true} 如果返回结果result为null，表示没有返回成功，也没有返回失败，也没有设置不可取消，此时可以取消
*/
@Override
public boolean isCancellable() {
return result == null;
}
/**
* 查询cause：如果result instanceof Throwable，那么表示返回结果出错了，否则 cause = null，表示一定没有错误
*
* @return
*/
@Override
public Throwable cause() {
Object result = this.result;
return result instanceof Throwable ? (Throwable) result : null;
}
@Override
public Promise setSuccess(V result) {
if (setSuccess0(result)) {
notifyListeners();
return this;
}
throw new IllegalStateException("complete already: " + this);
}
@Override
public boolean trySuccess(V result) {
if (setSuccess0(result)) {
notifyListeners();
return true;
}
return false;
}
@Override
public Promise setFailure(Throwable cause) {
if (setFailure0(cause)) {
notifyListeners();
return this;
}
throw new IllegalStateException("complete already: " + this);
}
@Override
public boolean tryFailure(Throwable cause) {
if (setFailure0(cause)) {
notifyListeners();
return true;
}
return false;
}
@Override
public boolean setUncancellable() {
Object result = this.result;
/**
* 从uncompleted设置为UNCANCELLABLE，如果设置成功，直接返回
*/
if (result == UNCANCELLABLE || RESULT_UPDATER.compareAndSet(this, null, UNCANCELLABLE)) {
return true;
}
/**
* 如果completed 而且又没被cancel()，此时返回true
*/
return isDone0(result) && !isCancelled();
}
@Override
public Promise addListener(FutureListener listener) {
ObjectUtil.checkNotNull(listener, "listener");
/**
* 防止多个线程同时操作listeners队列
*/
synchronized (this) {
if (listeners == null) {
listeners = new ArrayList<>();
}
listeners.add(listener);
}
/**
* 如果该listener是后加入的，则直接唤醒
*/
if (isDone()) {
notifyListeners();
}
return this;
}
@Override
public Promise addListeners(List> listeners) {
ObjectUtil.checkNotNull(listeners, "listeners");
synchronized (this) {
if (this.listeners == null) {
listeners = new ArrayList<>();
}
this.listeners.addAll(listeners);
}
if (isDone()) {
notifyListeners();
}
return this;
}
@Override
public Promise removeListener(FutureListener listener) {
ObjectUtil.checkNotNull(listeners, "listeners");
ObjectUtil.checkNotNull(listener, "listener");
synchronized (this) {
listeners.remove(listener);
}
return this;
}
@Override
public Promise removeListeners(List> listeners) {
ObjectUtil.checkNotNull(this.listeners, "listeners");
ObjectUtil.checkNotNull(listeners, "listeners");
synchronized (this) {
this.listeners.removeAll(listeners);
}
return this;
}
@Override
public Promise sync() throws InterruptedException {
/**
* 如果await()发生了异常，这里正好直接抛出
*/
await();
/**
* 如果await()返回了错误，也直接抛出
*/
rethrowIfFailed();
return this;
}
private void rethrowIfFailed() {
Throwable cause = cause();
if (cause == null) {
return;
}
throw (RuntimeException) cause;
}
@Override
public Promise syncUninterruptibly() {
awaitUninterruptibly();
rethrowIfFailed();
return this;
}
@Override
public Promise await() throws InterruptedException {
if (isDone()) {
return this;
}
if (Thread.interrupted()) {
throw new InterruptedException(toString());
}
/**
* wait()/notify()机制：
* 前提：每个对象都有一个锁 + 一个锁等待队列 + 一个条件等待队列。
* 线程协作：wait()/notify()通常都是由两个线程来协作的，一个wait()等待条件，另一个notify()唤醒等待线程
* 为什么加锁：wait()/notify()是必须加锁执行的（内部执行机制），否则会抛出异常IllegalMonitorStateException，锁对象是当前实例。
*
* wait内部执行机制：
* 1、把当前线程放入锁对象的条件等待队列，之后释放锁（注意：一定会释放锁，否则notify的线程将无法获取该对象锁），进入阻塞状态WAITING或TIMED_WAITING
* 2、当等待时间到了或者被其他线程notify/notifyAll了，则等待的当前线程从条件等待队列中移除出来，之后再尝试获取锁，如果没有获取锁，进入锁等待队列，线程状态改为BLOCKED；如果获取了锁，从wait调用中返回
*
* 为什么要写成:
*
* while (!isDone()) {
* wait();
* }
*
* 而不是
*
* if(!isDone()) {
* wait();
* }
*
*
* wait()表示阻塞等待，正常情况下while和if形式是等价的，但是为了防止wait()被意外唤醒，所以需要在wait()之后继续进行判断
*/
synchronized (this) {
while (!isDone()) {
/**
* 执行wait()之前：waiters加1
*/
incWaiters();
try {
wait();
} finally {
/**
* wait()结束之后，waiters减1
*/
decWaiters();
}
}
}
return this;
}
@Override
public Promise awaitUninterruptibly() {
if (isDone()) {
return this;
}
boolean interrupted = false;
synchronized (this) {
while (!isDone()) {
incWaiters();
try {
wait();
} catch (InterruptedException e) {
// Interrupted while waiting.
interrupted = true;
} finally {
decWaiters();
}
}
}
/**
* 捕获了中断异常，默默执行中断
*/
if (interrupted) {
Thread.currentThread().interrupt();
}
return this;
}
@Override
public boolean await(long timeout, TimeUnit timeUnit) throws InterruptedException {
return await0(timeUnit.toNanos(timeout), true);
}
@Override
public boolean awaitUninterruptibly(long timeout, TimeUnit timeUnit) {
try {
return await0(timeUnit.toNanos(timeout), false);
} catch (InterruptedException e) {
throw new InternalError();
}
}
@Override
public boolean await(long timeoutMillis) throws InterruptedException {
return await0(TimeUnit.MILLISECONDS.toNanos(timeoutMillis), true);
}
@Override
public boolean awaitUninterruptibly(long timeoutMillis) {
try {
return await0(TimeUnit.MILLISECONDS.toNanos(timeoutMillis), false);
} catch (InterruptedException e) {
throw new InternalError();
}
}
@Override
public V getNow() {
Object result = this.result;
if (result instanceof Throwable || result == SUCCESS || result == UNCANCELLABLE) {
return null;
}
return (V) result;
}
/**
* 查看java.util.concurrent.Future#cancel()的注释，
* This attempt will fail if the task has already completed（成功 || 失败 || 已被取消）, has already been cancelled,
* or could not be cancelled for some other reason
*
* @param mayInterruptIfRunning this value has no effect in this implementation.
* @return
*/
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
if (RESULT_UPDATER.compareAndSet(this, null, CANCELLATION_CAUSE)) {
checkNotifyWaiters();
notifyListeners();
return true;
}
return false;
}
@Override
public boolean isCancelled() {
return result instanceof CancellationException;
}
@Override
public boolean isDone() {
return isDone0(this.result);
}
/**
* 分析并发问题：
* 1、假设没有notifyingListeners：
* 当前线程A执行到while(true)的时候，假设另一条线程B也添加了FutureListener并进入了第一个同步块，此时线程B也进入了while(true)，
* B开始执行后来的这些FutureListeners，之后A才开始执行一开始的FutureListeners，这样就不能保证FIFO的执行FutureListener
*
* 2、加入notifyingListeners：
* 在线程A执行到第二个synchronized块中的if (this.listeners == null)中之前，线程B进入第一个同步块，由于notifyingListeners = true，则直接返回了，
* 而B后来添加的FutureListeners，A会在第二个同步快判断的时候发现当前的this.listeners.size>0，会继续赋值给本地变量继续第二轮循环.
*
* 这里有一个疑问：当外界的this.listeners发生变化时，temListeners是否变化，假设A执行到while(true)，B执行了addListener，则此时外界的this.listener改变了值，temListener是否发生变化
*/
private void notifyListeners() {
List> temListeners;
synchronized (this) {
if (notifyingListeners || this.listeners == null) {
return;
}
notifyingListeners = true;
temListeners = this.listeners;
this.listeners = null; // 通知完之后就置空，不再通知第二次
}
while (true) {
notifyListeners0(temListeners);
synchronized (this) {
if (this.listeners == null) {
notifyingListeners = false;
return;
}
temListeners = this.listeners;
this.listeners = null;
}
}
}
private void notifyListeners0(List> listeners) {
for (FutureListener listener : listeners) {
try {
listener.operationComplete(this);
} catch (Exception e) {
e.printStackTrace();
}
}
}
/**
* 设置成功标志
* result != null ? result : SUCCESS
*/
private boolean setSuccess0(V result) {
return setValue0(result != null ? result : SUCCESS);
}
/**
* 设置失败标志
* result == cause
*/
private boolean setFailure0(Throwable cause) {
return setValue0(ObjectUtil.checkNotNull(cause, "cause"));
}
private boolean setValue0(Object result) {
/**
* 更新result结果，唤醒所有阻塞线程
* 将result从null置为result 或者 从UNCANCELLABLE置为result（因为有可能是先将result置为UNCANCELLABLE的）
*/
if (RESULT_UPDATER.compareAndSet(this, null, result) ||
RESULT_UPDATER.compareAndSet(this, UNCANCELLABLE, result)) {
checkNotifyWaiters();
return true;
}
return false;
}
private void checkNotifyWaiters() {
synchronized (this) {
if (waiters > 0) {
notifyAll();
}
}
}
public boolean isDone0(Object result) {
return result != null && result != UNCANCELLABLE;
}
private void incWaiters() {
if (++waiters > Short.MAX_VALUE) {
throw new IllegalStateException("too many waiters :" + this);
}
}
private void decWaiters() {
--waiters;
}
/**
* 关于中断：
*
* 1、前提：线程有六种状态，{@link Thread#getState()}
* NEW：A thread that has not yet started is in this state.
* RUNNABLE：A thread executing in the Java virtual machine is in this state.
* it may be waiting for other resources from the operating system such as processor.
* BLOCKED：A thread that is blocked waiting for a monitor lock is in this state.
* A thread in the blocked state is waiting for a monitor lock
* to enter a synchronized block/method or
* reenter a synchronized block/method after calling {@link Object#wait() Object.wait}.
* WAITING：A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
* TIMED_WAITING：A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
* TERMINATED：A thread that has exited is in this state.
*
* 2、在不同阶段调用中断Thread.currentThread().interrupt()
* NEW/TERMINATED：interrupt()中断没有任何效果，中断位isInterrupted=false
* RUNNABLE: interrupt()中断没有效果，中断位isInterrupted=true，在run()方法中自己选择合适的点去处理
* BLOCKED：interrupt()中断位isInterrupted=true，不会使当前线程跳出锁等待队列，也就是说依然在等待锁
* WAITING/TIMED_WAITING: interrupt()抛出InterruptedException，设置isInterrupted=false，所以根据需要，需要自己去设置中断位
*
* @param timeoutNanos 纳秒级别的超时时间
* @param interruptable 是否可中断
* @return
* @throws InterruptedException
*/
private boolean await0(long timeoutNanos, boolean interruptable) throws InterruptedException {
/**
* 如果completed，直接返回
*/
if (isDone()) {
return true;
}
/**
* 如果传入的超时时间<=0，直接result
*/
if (timeoutNanos <= 0) {
return isDone();
}
/**
* 如果可中断 && 线程已被中断，抛出中断异常
*/
if (interruptable && Thread.interrupted()) {
throw new InterruptedException(toString());
}
boolean interrupted = false;
long startTimeNanos = System.nanoTime();
try {
while (true) {
synchronized (this) {
if (isDone()) {
return true;
}
incWaiters();
try {
wait(timeoutNanos / 1000000, (int) timeoutNanos % 1000000);
} catch (InterruptedException e) {
if (interruptable) {
throw e;
} else {
/**
* 对于中断来讲，抛出了中断异常时，Thread.currentThread().isInterrupted() == false，即不会设置中断标志位。
* 需要通过Thread.currentThread().interrupt()来设置中断标志位，来使外界自己根据中断位去做一些事
* Waits for this future to be completed without interruption. 所以在最后的finally才会中断
*/
interrupted = true;
}
} finally {
decWaiters();
}
}
if (isDone()) {
return true;
}
/**
* 防护性判断
*/
if (System.nanoTime() - startTimeNanos >= timeoutNanos) {
return isDone();
}
}
} finally {
if (interrupted) {
/**
* 此时线程处于RUNNABLE状态，执行interrupt()设置中断标志位
*/
Thread.currentThread().interrupt();
}
}
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder();
builder.append(this.getClass().getSimpleName()).append("@").append(Integer.toHexString(hashCode()));
Object result = this.result;
if (result == SUCCESS) {
builder.append("success");
} else if (result == UNCANCELLABLE) {
builder.append("uncancellable");
} else if (result instanceof Throwable) {
builder.append(result);
} else if (result != null) {
builder.append("success " + result);
} else {
builder.append("incompleted");
}
return builder.toString();
}
}
说明：
-
notifyListeners0(List方法中调用listener.operationComplete(this);而this就是当前的DefaultPromise实例。 - 通过使用notifyingListeners属性来实现监听器的先入先出。
- 实现机制：在业务逻辑执行前添加监听器
addListener(FutureListener在执行完业务逻辑之后，执行listener) setSuccess/trySuccess/setFailure/tryFailure等方法，此时会执行notifyAll()并回调添加进来的监听器。假设有线程阻塞在get()方法上时，在此时会做唤醒。
注意：
- DefaultPromise的状态机流转图：见类注释。
- DefaultPromise可以实现的两种使用机制：见类注释。再强调一点，建议使用回调方式。
- 学习使用AtomicReferenceFieldUpdater来实现属性的cas更新。
- 学习成员变量局部化的做法：不只是防止引用的并发修改，还是优化性能的一种方式。
- 学习wait/notify的实现机制：最佳实践见《Effective Java 中文版 第2版》的“第69条”
- 学习线程的六种状态与中断对各种状态的影响。
附、Bug记录
- netty的getNow有bug，我 这里 提了个issue，netty也将在4.1.28版本修复该bug。