Netty中的ChannelFuture和ChannelPromise
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2022-05-15 12:31:11
在Netty使用ChannelFuture和ChannelPromise进行异步操作的处理 这是官方给出的ChannelFutur描述 由图可以知道ChannelFutur有四种状态：Uncompleted、Completed successfully、Completed with failure、 ......
在netty使用channelfuture和channelpromise进行异步操作的处理
这是官方给出的channelfutur描述
1 * | completed successfully | 2 * +---------------------------+ 3 * +----> isdone() = true | 4 * +--------------------------+ | | issuccess() = true | 5 * | uncompleted | | +===========================+ 6 * +--------------------------+ | | completed with failure | 7 * | isdone() = false | | +---------------------------+ 8 * | issuccess() = false |----+----> isdone() = true | 9 * | iscancelled() = false | | | cause() = non-null | 10 * | cause() = null | | +===========================+ 11 * +--------------------------+ | | completed by cancellation | 12 * | +---------------------------+ 13 * +----> isdone() = true | 14 * | iscancelled() = true | 15 * +---------------------------+
由图可以知道channelfutur有四种状态：uncompleted、completed successfully、completed with failure、completed by cancellation，这几种状态是由isdone、issuccess、iscancelled、cause这四种方法的返回值决定的。
channelfutur接口的定义如下：
1 public interface channelfuture extends future<void> {
2 channel channel();
3
4 channelfuture addlistener(genericfuturelistener<? extends future<? super void>> var1);
5
6 channelfuture addlisteners(genericfuturelistener... var1);
7
8 channelfuture removelistener(genericfuturelistener<? extends future<? super void>> var1);
9
10 channelfuture removelisteners(genericfuturelistener... var1);
11
12 channelfuture sync() throws interruptedexception;
13
14 channelfuture syncuninterruptibly();
15
16 channelfuture await() throws interruptedexception;
17
18 channelfuture awaituninterruptibly();
19
20 boolean isvoid();
21 }
继承自netty的future：
1 public interface future<v> extends java.util.concurrent.future<v> {
2 boolean issuccess();
3
4 boolean iscancellable();
5
6 throwable cause();
7
8 future<v> addlistener(genericfuturelistener<? extends future<? super v>> var1);
9
10 future<v> addlisteners(genericfuturelistener... var1);
11
12 future<v> removelistener(genericfuturelistener<? extends future<? super v>> var1);
13
14 future<v> removelisteners(genericfuturelistener... var1);
15
16 future<v> sync() throws interruptedexception;
17
18 future<v> syncuninterruptibly();
19
20 future<v> await() throws interruptedexception;
21
22 future<v> awaituninterruptibly();
23
24 boolean await(long var1, timeunit var3) throws interruptedexception;
25
26 boolean await(long var1) throws interruptedexception;
27
28 boolean awaituninterruptibly(long var1, timeunit var3);
29
30 boolean awaituninterruptibly(long var1);
31
32 v getnow();
33
34 boolean cancel(boolean var1);
35 }
netty的future又继承自jdk的future：
1 public interface future<v> {
2
3 boolean cancel(boolean mayinterruptifrunning);
4
5 boolean iscancelled();
6
7 boolean isdone();
8
9 v get() throws interruptedexception, executionexception;
10
11 v get(long timeout, timeunit unit)
12 throws interruptedexception, executionexception, timeoutexception;
13 }
channelpromise继承了channelfuture：
1 public interface channelpromise extends channelfuture, promise<void> {
2 channel channel();
3
4 channelpromise setsuccess(void var1);
5
6 channelpromise setsuccess();
7
8 boolean trysuccess();
9
10 channelpromise setfailure(throwable var1);
11
12 channelpromise addlistener(genericfuturelistener<? extends future<? super void>> var1);
13
14 channelpromise addlisteners(genericfuturelistener... var1);
15
16 channelpromise removelistener(genericfuturelistener<? extends future<? super void>> var1);
17
18 channelpromise removelisteners(genericfuturelistener... var1);
19
20 channelpromise sync() throws interruptedexception;
21
22 channelpromise syncuninterruptibly();
23
24 channelpromise await() throws interruptedexception;
25
26 channelpromise awaituninterruptibly();
27
28 channelpromise unvoid();
29 }
其中promise接口定义如下：
1 public interface promise<v> extends future<v> {
2 promise<v> setsuccess(v var1);
3
4 boolean trysuccess(v var1);
5
6 promise<v> setfailure(throwable var1);
7
8 boolean tryfailure(throwable var1);
9
10 boolean setuncancellable();
11
12 promise<v> addlistener(genericfuturelistener<? extends future<? super v>> var1);
13
14 promise<v> addlisteners(genericfuturelistener... var1);
15
16 promise<v> removelistener(genericfuturelistener<? extends future<? super v>> var1);
17
18 promise<v> removelisteners(genericfuturelistener... var1);
19
20 promise<v> await() throws interruptedexception;
21
22 promise<v> awaituninterruptibly();
23
24 promise<v> sync() throws interruptedexception;
25
26 promise<v> syncuninterruptibly();
27 }
在netty中，无论是服务端还是客户端，在channel注册时都会为其绑定一个channelpromise，默认实现是defaultchannelpromise
defaultchannelpromise定义如下：
1 public class defaultchannelpromise extends defaultpromise<void> implements channelpromise, flushcheckpoint {
2
3 private final channel channel;
4 private long checkpoint;
5
6 public defaultchannelpromise(channel channel) {
7 this.channel = checknotnull(channel, "channel");
8 }
9
10 public defaultchannelpromise(channel channel, eventexecutor executor) {
11 super(executor);
12 this.channel = checknotnull(channel, "channel");
13 }
14
15 @override
16 protected eventexecutor executor() {
17 eventexecutor e = super.executor();
18 if (e == null) {
19 return channel().eventloop();
20 } else {
21 return e;
22 }
23 }
24
25 @override
26 public channel channel() {
27 return channel;
28 }
29
30 @override
31 public channelpromise setsuccess() {
32 return setsuccess(null);
33 }
34
35 @override
36 public channelpromise setsuccess(void result) {
37 super.setsuccess(result);
38 return this;
39 }
40
41 @override
42 public boolean trysuccess() {
43 return trysuccess(null);
44 }
45
46 @override
47 public channelpromise setfailure(throwable cause) {
48 super.setfailure(cause);
49 return this;
50 }
51
52 @override
53 public channelpromise addlistener(genericfuturelistener<? extends future<? super void>> listener) {
54 super.addlistener(listener);
55 return this;
56 }
57
58 @override
59 public channelpromise addlisteners(genericfuturelistener<? extends future<? super void>>... listeners) {
60 super.addlisteners(listeners);
61 return this;
62 }
63
64 @override
65 public channelpromise removelistener(genericfuturelistener<? extends future<? super void>> listener) {
66 super.removelistener(listener);
67 return this;
68 }
69
70 @override
71 public channelpromise removelisteners(genericfuturelistener<? extends future<? super void>>... listeners) {
72 super.removelisteners(listeners);
73 return this;
74 }
75
76 @override
77 public channelpromise sync() throws interruptedexception {
78 super.sync();
79 return this;
80 }
81
82 @override
83 public channelpromise syncuninterruptibly() {
84 super.syncuninterruptibly();
85 return this;
86 }
87
88 @override
89 public channelpromise await() throws interruptedexception {
90 super.await();
91 return this;
92 }
93
94 @override
95 public channelpromise awaituninterruptibly() {
96 super.awaituninterruptibly();
97 return this;
98 }
99
100 @override
101 public long flushcheckpoint() {
102 return checkpoint;
103 }
104
105 @override
106 public void flushcheckpoint(long checkpoint) {
107 this.checkpoint = checkpoint;
108 }
109
110 @override
111 public channelpromise promise() {
112 return this;
113 }
114
115 @override
116 protected void checkdeadlock() {
117 if (channel().isregistered()) {
118 super.checkdeadlock();
119 }
120 }
121
122 @override
123 public channelpromise unvoid() {
124 return this;
125 }
126
127 @override
128 public boolean isvoid() {
129 return false;
130 }
131 }
可以看到这个defaultchannelpromise仅仅是将channel封装了，而且其基本上所有方法的实现都依赖于父类defaultpromise
defaultpromise中的实现是整个channelfuture和channelpromise的核心所在：
defaultpromise中有如下几个状态量：
1 private static final int max_listener_stack_depth = math.min(8,
2 systempropertyutil.getint("io.netty.defaultpromise.maxlistenerstackdepth", 8));
3 private static final object success = new object();
4 private static final object uncancellable = new object();
5 private static final causeholder cancellation_cause_holder = new causeholder(throwableutil.unknownstacktrace(
6 new cancellationexception(), defaultpromise.class, "cancel(...)"));
7 private static final atomicreferencefieldupdater<defaultpromise, object> result_updater =
8 atomicreferencefieldupdater.newupdater(defaultpromise.class, object.class, "result");
max_listener_stack_depth： 表示最多可执行listeners的数量，默认是8
success ：表示异步操作正常完成
uncancellable：表示异步操作不可取消，并且尚未完成
cancellation_cause_holder：表示异步操作取消监听，用于cancel操作，
而causeholder 的实例对象是用来表示异步操作异常结束，同时保存异常信息：
1 private static final class causeholder {
2 final throwable cause;
3 causeholder(throwable cause) {
4 this.cause = cause;
5 }
6 }
result_updater：是一个原子更新器，通过cas操作，原子化更新 defaultpromise对象的名为result的成员，这个result成员是其异步操作判断的关键所在
defaultpromise的成员及构造方法定义：
1 public class defaultpromise<v> extends abstractfuture<v> implements promise<v> {
2 private volatile object result;
3 private final eventexecutor executor;
4 private object listeners;
5 private short waiters;
6 private boolean notifyinglisteners;
7
8 public defaultpromise(eventexecutor executor) {
9 this.executor = checknotnull(executor, "executor");
10 }
11 }
result：就是前面说的，判断异步操作状态的关键
result的取值有：success 、uncancellable、causeholder以及null (其实还可以是泛型v类型的任意对象，这里暂不考虑)
executor：就是channel绑定的nioeventloop，在我之前的博客说过，channel的异步操作都是在nioeventloop的线程中完成的（[netty中nioeventloopgroup的创建源码分析](https://blog.csdn.net/z_chenchen/article/details/90567863)）
listeners：通过一个object保存所有对异步操作的监听，用于异步操作的回调
waiters：记录阻塞中的listeners的数量
notifyinglisteners：是否需要唤醒的标志
首先来看isdone方法，通过之前的图可以知道，
isdone为false对应了uncompleted状态，即异步操作尚未完成；
isdone为true则代表了异步操作完成，但是还是有三种完成情况，需要结合别的判断方法才能具体知道是哪种情况；
isdone方法：
1 @override
2 public boolean isdone() {
3 return isdone0(result);
4 }
调用isdone0：
1 private static boolean isdone0(object result) {
2 return result != null && result != uncancellable;
3 }
有如下几种情况：
result等于null，result没有赋值，表示异步操作尚未完成（从这里就能想到异步操作完成，需要调用某个set方法来改变result的状态）
result是uncancellable状态，表示执行中的异步操作不可取消，当然也就是异步操作尚未完成
result不等于null，且不等于uncancellable，就表示异步操作完成（包括正常完成，以及异常结束，需要由cause方法进一步判断）
issuccess方法：
1 @override
2 public boolean issuccess() {
3 object result = this.result;
4 return result != null && result != uncancellable && !(result instanceof causeholder);
5 }
由这里可以知道当且仅当result 为success状态时，才返回true（其余除uncancellable和null的值其实也可以，这里暂不考虑）
iscancelled方法：
1 @override
2 public boolean iscancelled() {
3 return iscancelled0(result);
4 }
调用iscancelled0方法：
1 private static boolean iscancelled0(object result) {
2 return result instanceof causeholder && ((causeholder) result).cause instanceof cancellationexception;
3 }
只有当result是cancellationexception的实例时，表示取消异步操作
接着来看cause方法：
1 @override
2 public throwable cause() {
3 object result = this.result;
4 return (result instanceof causeholder) ? ((causeholder) result).cause : null;
5 }
和上面同理，通过判别resul是否是causeholder的实现类，若是，将causeholder保存的异常返回。
几种状态的判别说完了，下面看一下如何设置这几种状态的：
setsuccess方法：
1 @override
2 public promise<v> setsuccess(v result) {
3 if (setsuccess0(result)) {
4 notifylisteners();
5 return this;
6 }
7 throw new illegalstateexception("complete already: " + this);
8 }
首先调用setsuccess0方法，其中result的泛型通过defaultchannelpromise可知是void，在defaultchannelpromise中所有的set和try操作参数都是null，这里的result也就不去考虑：
1 private boolean setsuccess0(v result) {
2 return setvalue0(result == null ? success : result);
3 }
继续调用setvalue0方法：
1 private boolean setvalue0(object objresult) {
2 if (result_updater.compareandset(this, null, objresult) ||
3 result_updater.compareandset(this, uncancellable, objresult)) {
4 checknotifywaiters();
5 return true;
6 }
7 return false;
8 }
通过cas操作，将result状态变为success
其中checknotifywaiters方法：
1 private synchronized void checknotifywaiters() {
2 if (waiters > 0) {
3 notifyall();
4 }
5 }
检查waiters的个数，唤醒所有阻塞中的this，sync方法会引起阻塞
回到setsuccess方法中，setsuccess0通过cas操作，将result状态更新为success成功后，调用
notifylisteners方法，唤醒所有listener完成对异步操作的回调
listeners是通过addlistener方法添加的，用来对异步操作进行侦听：
看到addlistener方法：
1 @override
2 public promise<v> addlistener(genericfuturelistener<? extends future<? super v>> listener) {
3 checknotnull(listener, "listener");
4
5 synchronized (this) {
6 addlistener0(listener);
7 }
8
9 if (isdone()) {
10 notifylisteners();
11 }
12
13 return this;
14 }
15
16 @override
17 public promise<v> addlisteners(genericfuturelistener<? extends future<? super v>>... listeners) {
18 checknotnull(listeners, "listeners");
19
20 synchronized (this) {
21 for (genericfuturelistener<? extends future<? super v>> listener : listeners) {
22 if (listener == null) {
23 break;
24 }
25 addlistener0(listener);
26 }
27 }
28
29 if (isdone()) {
30 notifylisteners();
31 }
32
33 return this;
34 }
其中genericfuturelistener接口定义如下：
1 public interface genericfuturelistener<f extends future<?>> extends eventlistener {
2 /**
3 * invoked when the operation associated with the {@link future} has been completed.
4 *
5 * @param future the source {@link future} which called this callback
6 */
7 void operationcomplete(f future) throws exception;
8 }
可以看到listener其实就是通过operationcomplete方法，来完成回调，对future对象进行处理，由注释可知operationcomplete方法是在future操作完成时调用
addlisteners方法的实现比较简单，实现核心是在addlistener0中：
1 private void addlistener0(genericfuturelistener<? extends future<? super v>> listener) {
2 if (listeners == null) {
3 listeners = listener;
4 } else if (listeners instanceof defaultfuturelisteners) {
5 ((defaultfuturelisteners) listeners).add(listener);
6 } else {
7 listeners = new defaultfuturelisteners((genericfuturelistener<?>) listeners, listener);
8 }
9 }
其中defaultfuturelisteners是将genericfuturelistener对象封装的一个数组：
1 final class defaultfuturelisteners {
2
3 private genericfuturelistener<? extends future<?>>[] listeners;
4 private int size;
5 private int progressivesize;
6
7 @suppresswarnings("unchecked")
8 defaultfuturelisteners(
9 genericfuturelistener<? extends future<?>> first, genericfuturelistener<? extends future<?>> second) {
10 listeners = new genericfuturelistener[2];
11 listeners[0] = first;
12 listeners[1] = second;
13 size = 2;
14 if (first instanceof genericprogressivefuturelistener) {
15 progressivesize ++;
16 }
17 if (second instanceof genericprogressivefuturelistener) {
18 progressivesize ++;
19 }
20 }
21
22 public void add(genericfuturelistener<? extends future<?>> l) {
23 genericfuturelistener<? extends future<?>>[] listeners = this.listeners;
24 final int size = this.size;
25 if (size == listeners.length) {
26 this.listeners = listeners = arrays.copyof(listeners, size << 1);
27 }
28 listeners[size] = l;
29 this.size = size + 1;
30
31 if (l instanceof genericprogressivefuturelistener) {
32 progressivesize ++;
33 }
34 }
35
36 public void remove(genericfuturelistener<? extends future<?>> l) {
37 final genericfuturelistener<? extends future<?>>[] listeners = this.listeners;
38 int size = this.size;
39 for (int i = 0; i < size; i ++) {
40 if (listeners[i] == l) {
41 int listenerstomove = size - i - 1;
42 if (listenerstomove > 0) {
43 system.arraycopy(listeners, i + 1, listeners, i, listenerstomove);
44 }
45 listeners[-- size] = null;
46 this.size = size;
47
48 if (l instanceof genericprogressivefuturelistener) {
49 progressivesize --;
50 }
51 return;
52 }
53 }
54 }
55
56 public genericfuturelistener<? extends future<?>>[] listeners() {
57 return listeners;
58 }
59
60 public int size() {
61 return size;
62 }
63
64 public int progressivesize() {
65 return progressivesize;
66 }
67 }
size：记录listeners的个数
progressivesize：记录genericprogressivefuturelistener类型的listeners的个数
defaultfuturelisteners 中对数组的操作比较简单，
add方法，当size达到数组长度时，进行二倍扩容，
其中genericprogressivefuturelistener继承自genericfuturelistener：
1 public interface genericprogressivefuturelistener<f extends progressivefuture<?>> extends genericfuturelistener<f> {
2 /**
3 * invoked when the operation has progressed.
4 *
5 * @param progress the progress of the operation so far (cumulative)
6 * @param total the number that signifies the end of the operation when {@code progress} reaches at it.
7 * {@code -1} if the end of operation is unknown.
8 */
9 void operationprogressed(f future, long progress, long total) throws exception;
10 }
由注释可知operationprogressed是在future操作进行时调用，这里不对genericprogressivefuturelistener过多讨论
回到addlistener0方法，由defaultfuturelisteners就可以知道，实际上通过defaultfuturelisteners管理的一维数组来保存listeners
在addlistener方法完成对listener的添加后，还会调用isdone方法检查当前异步操作是否完成，若是完成需要调用notifylisteners，直接唤醒所有listeners完后对异步操作的回调
有add就有remove，removelistener方法：
1 @override
2 public promise<v> removelistener(final genericfuturelistener<? extends future<? super v>> listener) {
3 checknotnull(listener, "listener");
4
5 synchronized (this) {
6 removelistener0(listener);
7 }
8
9 return this;
10 }
11
12 @override
13 public promise<v> removelisteners(final genericfuturelistener<? extends future<? super v>>... listeners) {
14 checknotnull(listeners, "listeners");
15
16 synchronized (this) {
17 for (genericfuturelistener<? extends future<? super v>> listener : listeners) {
18 if (listener == null) {
19 break;
20 }
21 removelistener0(listener);
22 }
23 }
24
25 return this;
26 }
还是由removelistener0来实现：
1 private void removelistener0(genericfuturelistener<? extends future<? super v>> listener) {
2 if (listeners instanceof defaultfuturelisteners) {
3 ((defaultfuturelisteners) listeners).remove(listener);
4 } else if (listeners == listener) {
5 listeners = null;
6 }
7 }
看过之前的内容在看这里就比较简单了，通过defaultfuturelisteners去删除
notifylisteners方法：
1 private void notifylisteners() {
2 eventexecutor executor = executor();
3 if (executor.ineventloop()) {
4 final internalthreadlocalmap threadlocals = internalthreadlocalmap.get();
5 final int stackdepth = threadlocals.futurelistenerstackdepth();
6 if (stackdepth < max_listener_stack_depth) {
7 threadlocals.setfuturelistenerstackdepth(stackdepth + 1);
8 try {
9 notifylistenersnow();
10 } finally {
11 threadlocals.setfuturelistenerstackdepth(stackdepth);
12 }
13 return;
14 }
15 }
16
17 safeexecute(executor, new runnable() {
18 @override
19 public void run() {
20 notifylistenersnow();
21 }
22 });
23 }
其中executor方法：
1 protected eventexecutor executor() {
2 return executor;
3 }
用来获取executor轮询线程对象
判断executor是否处于轮询，否则需要通过safeexecute方法处理listeners的侦听，
safeexecute方法：
1 private static void safeexecute(eventexecutor executor, runnable task) {
2 try {
3 executor.execute(task);
4 } catch (throwable t) {
5 rejectedexecutionlogger.error("failed to submit a listener notification task. event loop shut down?", t);
6 }
7 }
这里保证了listeners的侦听回调是异步执行
internalthreadlocalmap在我之前的博客中说过，是netty使用的threadlocal （netty中fastthreadlocal源码分析）
去线程本地变量中找futurelistenerstackdepth（默认为0），判断stackdepth是否小于max_listener_stack_depth，否则也要通过safeexecute方法处理listeners的侦听
核心都是调用notifylistenersnow方法：
1 private void notifylistenersnow() {
2 object listeners;
3 synchronized (this) {
4 // only proceed if there are listeners to notify and we are not already notifying listeners.
5 if (notifyinglisteners || this.listeners == null) {
6 return;
7 }
8 notifyinglisteners = true;
9 listeners = this.listeners;
10 this.listeners = null;
11 }
12 for (;;) {
13 if (listeners instanceof defaultfuturelisteners) {
14 notifylisteners0((defaultfuturelisteners) listeners);
15 } else {
16 notifylistener0(this, (genericfuturelistener<?>) listeners);
17 }
18 synchronized (this) {
19 if (this.listeners == null) {
20 // nothing can throw from within this method, so setting notifyinglisteners back to false does not
21 // need to be in a finally block.
22 notifyinglisteners = false;
23 return;
24 }
25 listeners = this.listeners;
26 this.listeners = null;
27 }
28 }
29 }
先检查是否需要监听，满足条件后，判断listeners是否是defaultfuturelisteners，即包装后的数组
notifylisteners0方法：
1 private void notifylisteners0(defaultfuturelisteners listeners) {
2 genericfuturelistener<?>[] a = listeners.listeners();
3 int size = listeners.size();
4 for (int i = 0; i < size; i ++) {
5 notifylistener0(this, a[i]);
6 }
7 }
遍历这个数组，实则调用notifylistener0方法：
1 private static void notifylistener0(future future, genericfuturelistener l) {
2 try {
3 l.operationcomplete(future);
4 } catch (throwable t) {
5 if (logger.iswarnenabled()) {
6 logger.warn("an exception was thrown by " + l.getclass().getname() + ".operationcomplete()", t);
7 }
8 }
9 }
这里就可以看到，完成了对operationcomplete的回调，处理future
setsuccess结束，再来看trysuccess方法：
1 @override
2 public boolean trysuccess(v result) {
3 if (setsuccess0(result)) {
4 notifylisteners();
5 return true;
6 }
7 return false;
8 }
对比setsuccess来看，只有返回值不一样
setfailure方法：
1 @override
2 public promise<v> setfailure(throwable cause) {
3 if (setfailure0(cause)) {
4 notifylisteners();
5 return this;
6 }
7 throw new illegalstateexception("complete already: " + this, cause);
8 }
9
10 private boolean setfailure0(throwable cause) {
11 return setvalue0(new causeholder(checknotnull(cause, "cause")));
12 }
13
14 private boolean setvalue0(object objresult) {
15 if (result_updater.compareandset(this, null, objresult) ||
16 result_updater.compareandset(this, uncancellable, objresult)) {
17 checknotifywaiters();
18 return true;
19 }
20 return false;
21 }
和setsuccess逻辑一样，只不过cas操作将状态变为了causeholder对象，成功后唤醒listeners对异步操作的回调
tryfailure方法：
1 @override
2 public boolean tryfailure(throwable cause) {
3 if (setfailure0(cause)) {
4 notifylisteners();
5 return true;
6 }
7 return false;
8 }
也都是一个逻辑
还有一个setuncancellable方法：
1 @override
2 public boolean setuncancellable() {
3 if (result_updater.compareandset(this, null, uncancellable)) {
4 return true;
5 }
6 object result = this.result;
7 return !isdone0(result) || !iscancelled0(result);
8 }
若是result状态为null，异步操作尚未结束，直接通过cas操作将状态变为uncancellable
否则若是根据状态来判断
下来看到cancel方法：
1 /**
2 * {@inheritdoc}
3 *
4 * @param mayinterruptifrunning this value has no effect in this implementation.
5 */
6 @override
7 public boolean cancel(boolean mayinterruptifrunning) {
8 if (result_updater.compareandset(this, null, cancellation_cause_holder)) {
9 checknotifywaiters();
10 notifylisteners();
11 return true;
12 }
13 return false;
14 }
mayinterruptifrunning正如注释中所说，在这里没有什么作用
还是通过cas操作，将状态变为cancellation_cause_holder，调用checknotifywaiters唤醒因sync阻塞的线程，notifylisteners方法回调listeners的侦听
最后看到sync方法：
1 @override
2 public promise<v> sync() throws interruptedexception {
3 await();
4 rethrowiffailed();
5 return this;
6 }
先调用await方法：
1 @override
2 public promise<v> await() throws interruptedexception {
3 if (isdone()) {
4 return this;
5 }
6
7 if (thread.interrupted()) {
8 throw new interruptedexception(tostring());
9 }
10
11 checkdeadlock();
12
13 synchronized (this) {
14 while (!isdone()) {
15 incwaiters();
16 try {
17 wait();
18 } finally {
19 decwaiters();
20 }
21 }
22 }
23 return this;
24 }
先判断能否执行（异步操作尚未结束，当前线程没有被中断），然后调用checkdeadlock方法：
1 protected void checkdeadlock() {
2 eventexecutor e = executor();
3 if (e != null && e.ineventloop()) {
4 throw new blockingoperationexception(tostring());
5 }
6 }
检查轮询线程是否在工作
在synchronized块中以自身为锁，自旋等待异步操作的完成，若是没完成，调用incwaiters方法：
1 private void incwaiters() {
2 if (waiters == short.max_value) {
3 throw new illegalstateexception("too many waiters: " + this);
4 }
5 ++waiters;
6 }
在小于short.max_value的情况下，对waiters自增，
然后使用wait将自身阻塞，等待被唤醒
所以在之前setvalue0时，checknotifywaiters操作会notifyall，
由此可以知道sync方法的作用：在某一线程中调用sync方法会使得当前线程被阻塞，只有当异步操作执完毕，通过上面的set方法改变状态后，才会调用checknotifywaiters方法唤醒当前线程。
当从阻塞中被唤醒后调用decwaiters方法：
1 private void decwaiters() {
2 --waiters;
3 }
使得waiters自减
通过这样一种自旋方式，一直等到isdone成立，结束自旋，进而结束await方法，然后调用rethrowiffailed方法：
1 private void rethrowiffailed() {
2 throwable cause = cause();
3 if (cause == null) {
4 return;
5 }
6
7 platformdependent.throwexception(cause);
8 }
根据异步操作是否有异常，进而使用platformdependent抛出异常。
至此netty中的channelfuture和channelpromise分析到此全部结束。
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