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netty（九）初识Netty-Future & Promise

2021-11-10  本文已影响0人  我犟不过你

一、简介

在netty当中，我们需要进行异步处理的时候，经常会调用以下的两个方法：

Future & Promise

其实在我们使用JDK的时候，就知道有一个Future接口，用于异步时接收任务结果。

在Netty当中，基于JDK当中的Future接口，进行了扩展；后面又在Netty的Future基础之上，增加了Promise接口。

关于三者的关系请看以下的类图：

image.png

二、扩展了哪些主要能力？

在这一章节，主要分析前面提到的netty新增的两个接口都新增了哪些功能。

功能/名称 jdk Future netty Future Promise
cancel 取消任务 - -
isCanceled 任务是否取消 - -
isDone 任务是否完成，不能区分成功失败 - -
get 获取任务结果，阻塞等待 - -
getNow - 获取任务结果，非阻塞，还未产生结果时返回 null -
await - 等待任务结束，如果任务失败，不会抛异常，而是通过 isSuccess 判断 -
sync - 等待任务结束，如果任务失败，抛出异常 -
isSuccess - 判断任务是否成功 -
cause - 获取失败信息，非阻塞，如果没有失败，返回null -
addLinstener - 添加回调，异步接收结果 -
setSuccess - - 设置成功结果
setFailure - - 设置失败结果

三、使用示例

下面针对Promise我们重点学习，看看针对不同场景下的使用。

例1 同步处理任务成功

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        DefaultEventLoop eventExecutors = new DefaultEventLoop();
        DefaultPromise<Integer> promise = new DefaultPromise<>(eventExecutors);

        eventExecutors.execute(()->{
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +", set success: " + 10);
            promise.setSuccess(10);
        });

        System.out.println(Thread.currentThread().getName() +", start...");
        //未产生结果，不阻塞，返回null
        System.out.println(Thread.currentThread().getName() +", " + promise.getNow());
        //阻塞等待结果
        System.out.println(Thread.currentThread().getName() +", " + promise.get());
    }

结果：

main, start...
main, null
defaultEventLoop-1-1, set success: 10
main, 10

例2 异步处理任务成功

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        DefaultEventLoop eventExecutors = new DefaultEventLoop();
        DefaultPromise<Integer> promise = new DefaultPromise<>(eventExecutors);

        eventExecutors.execute(()->{
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +", set success: " + 10);
            promise.setSuccess(10);
        });

        System.out.println(Thread.currentThread().getName() +", start...");

        //添加异步监听，当有结果时调用getNow获取结果
        promise.addListener(future->{
            System.out.println(Thread.currentThread().getName() +", " + future.getNow());
        });
    }

结果：

main, start...
defaultEventLoop-1-1, set success: 10
defaultEventLoop-1-1, 10

例3 同步处理任务失败 sync & get

sync或者get，区别是get会对异常信息再包一层

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        DefaultEventLoop eventExecutors = new DefaultEventLoop();
        DefaultPromise<Integer> promise = new DefaultPromise<>(eventExecutors);

        eventExecutors.execute(()->{
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +", set failure");
            promise.setFailure(new RuntimeException());
        });

        System.out.println(Thread.currentThread().getName() +", start...");
        //未产生结果，不阻塞，返回null
        System.out.println(Thread.currentThread().getName() +", " + promise.getNow());
        //阻塞等待结果
        System.out.println(Thread.currentThread().getName() +", " + promise.get());
        //System.out.println(Thread.currentThread().getName() +", " + promise.get());
    }

结果：

main, start...
main, null
defaultEventLoop-1-1, set failure
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.RuntimeException
    at io.netty.util.concurrent.DefaultPromise.get(DefaultPromise.java:349)
    at com.cloud.bssp.netty.promise.Test3.main(Test3.java:34)
Caused by: java.lang.RuntimeException
    at com.cloud.bssp.netty.promise.Test3.lambda$main$0(Test3.java:27)
    at io.netty.channel.DefaultEventLoop.run(DefaultEventLoop.java:54)
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    at java.lang.Thread.run(Thread.java:748)

例4 同步处理任务失败 await

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        DefaultEventLoop eventExecutors = new DefaultEventLoop();
        DefaultPromise<Integer> promise = new DefaultPromise<>(eventExecutors);

        eventExecutors.execute(()->{
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +", set failure");
            promise.setFailure(new RuntimeException());
        });

        System.out.println(Thread.currentThread().getName() +", start...");
        //未产生结果，不阻塞，返回null
        System.out.println(Thread.currentThread().getName() +", " + promise.getNow());
        //阻塞等待结果
        promise.await();
        //isSuccess判断任务是否成功
        System.out.println(Thread.currentThread().getName() +", " + promise.isSuccess());
    }

结果：

main, start...
main, null
defaultEventLoop-1-1, set failure
main, false

例5 异步处理任务失败

    public static void main(String[] args) {
        DefaultEventLoop eventExecutors = new DefaultEventLoop();
        DefaultPromise<Integer> promise = new DefaultPromise<>(eventExecutors);

        eventExecutors.execute(() -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + ", set failure");
            promise.setFailure(new RuntimeException());
        });

        System.out.println(Thread.currentThread().getName() + ", start...");

        //添加异步监听，当有结果时调用getNow获取结果
        promise.addListener(future -> {
            if (!promise.isSuccess()) {
                //失败查看结果
                System.out.println(Thread.currentThread().getName() + ", " + promise.cause());
            }
            System.out.println(Thread.currentThread().getName() + ", " + future.getNow());
        });
    }

结果：

main, start...
defaultEventLoop-1-1, set failure
defaultEventLoop-1-1, java.lang.RuntimeException
defaultEventLoop-1-1, null

例6 await 死锁检查

    public static void main(String[] args) {
        DefaultEventLoop eventExecutors = new DefaultEventLoop();
        DefaultPromise<Integer> promise = new DefaultPromise<>(eventExecutors);

        eventExecutors.submit(()->{
            System.out.println("1");
            try {
                promise.await();
                // 注意不能仅捕获 InterruptedException 异常
                // 否则 死锁检查抛出的 BlockingOperationException 会继续向上传播
                // 而提交的任务会被包装为 PromiseTask，它的 run 方法中会 catch 所有异常然后设置为 Promise 的失败结果而不会抛出
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println("2");
        });
        eventExecutors.submit(()->{
            System.out.println("3");
            try {
                promise.await();
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println("4");
        });
    }

结果：

1
2
3
4
io.netty.util.concurrent.BlockingOperationException: DefaultPromise@6a77ffc2(incomplete)
    at io.netty.util.concurrent.DefaultPromise.checkDeadLock(DefaultPromise.java:461)
    at io.netty.util.concurrent.DefaultPromise.await(DefaultPromise.java:246)
    at com.cloud.bssp.netty.promise.Test6.lambda$main$0(Test6.java:21)
    at io.netty.util.concurrent.PromiseTask.runTask(PromiseTask.java:98)
    at io.netty.util.concurrent.PromiseTask.run(PromiseTask.java:106)
    at io.netty.channel.DefaultEventLoop.run(DefaultEventLoop.java:54)
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    at java.lang.Thread.run(Thread.java:748)
io.netty.util.concurrent.BlockingOperationException: DefaultPromise@6a77ffc2(incomplete)
    at io.netty.util.concurrent.DefaultPromise.checkDeadLock(DefaultPromise.java:461)
    at io.netty.util.concurrent.DefaultPromise.await(DefaultPromise.java:246)
    at com.cloud.bssp.netty.promise.Test6.lambda$main$1(Test6.java:33)
    at io.netty.util.concurrent.PromiseTask.runTask(PromiseTask.java:98)
    at io.netty.util.concurrent.PromiseTask.run(PromiseTask.java:106)
    at io.netty.channel.DefaultEventLoop.run(DefaultEventLoop.java:54)
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    at java.lang.Thread.run(Thread.java:748)
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