一、簡介
在netty當中,我們需要進行異步處理的時候,經常會調用以下的兩個方法:
Future & Promise
其實在我們使用JDK的時候,就知道有一個Future接口,用於異步時接收任務結果。
在Netty當中,基於JDK當中的Future接口,進行了擴展;後面又在Netty的Future基礎之上,增加了Promise接口。
關於三者的關係請看以下的類圖:
- JDK Future:只能同步等待任務結束(無論成功還是失敗)才能得到結果。
- Netty Future:可以同步等待任務結束得到結果,也可以異步方式得到結果,前提是任務必須要結束。
- Netty Promise:不僅有 netty Future 的功能,而且脫離了任務獨立存在,只作爲兩個線程間傳遞結果的容器。
二、擴展了哪些主要能力?
在這一章節,主要分析前面提到的netty新增的兩個接口都新增了哪些功能。
功能/名稱 | jdk Future | netty Future | Promise |
---|---|---|---|
cancel | 取消任務 | - | - |
isCanceled | 任務是否取消 | - | - |
isDone | 任務是否完成,不能區分成功失敗 | - | - |
get | 獲取任務結果,阻塞等待 | - | - |
getNow | - | 獲取任務結果,非阻塞,還未產生結果時返回 null | - |
await | - | 等待任務結束,如果任務失敗,不會拋異常,而是通過 isSuccess 判斷 | - |
sync | - | 等待任務結束,如果任務失敗,拋出異常 | - |
isSuccess | - | 判斷任務是否成功 | - |
cause | - | 獲取失敗信息,非阻塞,如果沒有失敗,返回null | - |
addLinstener | - | 添加回調,異步接收結果 | - |
setSuccess | - | - | 設置成功結果 |
setFailure | - | - | 設置失敗結果 |
三、使用示例
下面針對Promise我們重點學習,看看針對不同場景下的使用。
例1 同步處理任務成功
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...");
//未產生結果,不阻塞,返回null
System.out.println(Thread.currentThread().getName() +", " + promise.getNow());
//阻塞等待結果
System.out.println(Thread.currentThread().getName() +", " + promise.get());
}
結果:
main, start...
main, null
defaultEventLoop-1-1, set success: 10
main, 10
例2 異步處理任務成功
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...");
//添加異步監聽,當有結果時調用getNow獲取結果
promise.addListener(future->{
System.out.println(Thread.currentThread().getName() +", " + future.getNow());
});
}
結果:
main, start...
defaultEventLoop-1-1, set success: 10
defaultEventLoop-1-1, 10
例3 同步處理任務失敗 sync & get
sync或者get,區別是get會對異常信息再包一層
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...");
//未產生結果,不阻塞,返回null
System.out.println(Thread.currentThread().getName() +", " + promise.getNow());
//阻塞等待結果
System.out.println(Thread.currentThread().getName() +", " + promise.get());
//System.out.println(Thread.currentThread().getName() +", " + promise.get());
}
結果:
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 同步處理任務失敗 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...");
//未產生結果,不阻塞,返回null
System.out.println(Thread.currentThread().getName() +", " + promise.getNow());
//阻塞等待結果
promise.await();
//isSuccess判斷任務是否成功
System.out.println(Thread.currentThread().getName() +", " + promise.isSuccess());
}
結果:
main, start...
main, null
defaultEventLoop-1-1, set failure
main, false
例5 異步處理任務失敗
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...");
//添加異步監聽,當有結果時調用getNow獲取結果
promise.addListener(future -> {
if (!promise.isSuccess()) {
//失敗查看結果
System.out.println(Thread.currentThread().getName() + ", " + promise.cause());
}
System.out.println(Thread.currentThread().getName() + ", " + future.getNow());
});
}
結果:
main, start...
defaultEventLoop-1-1, set failure
defaultEventLoop-1-1, java.lang.RuntimeException
defaultEventLoop-1-1, null
例6 await 死鎖檢查
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();
// 注意不能僅捕獲 InterruptedException 異常
// 否則 死鎖檢查拋出的 BlockingOperationException 會繼續向上傳播
// 而提交的任務會被包裝爲 PromiseTask,它的 run 方法中會 catch 所有異常然後設置爲 Promise 的失敗結果而不會拋出
} 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");
});
}
結果:
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)