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                # 5.1-google Guava包的ListenableFuture解析 # 5.1-google Guava包的ListenableFuture解析 [原文地址](http://code.google.com/p/guava-libraries/wiki/ListenableFutureExplained) 譯者:**羅立樹** 校對:**方騰飛** 并發編程是一個難題,但是一個強大而簡單的抽象可以顯著的簡化并發的編寫。出于這樣的考慮,Guava 定義了 [ListenableFuture](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/ListenableFuture.html)接口并繼承了JDK concurrent包下的`Future` 接口。 **我們強烈地建議你在代碼中多使用`ListenableFuture來代替JDK的``Future`**, 因為: - `大多數Futures` 方法中需要它。 - 轉到`ListenableFuture` 編程比較容易。 - Guava提供的通用公共類封裝了公共的操作方方法,不需要提供Future和`ListenableFuture的擴展方法。` ## 接口 傳統JDK中的Future通過異步的方式計算返回結果:在多線程運算中可能或者可能在沒有結束返回結果,Future是運行中的多線程的一個引用句柄,確保在服務執行返回一個Result。 ListenableFuture可以允許你注冊回調方法(callbacks),在運算(多線程執行)完成的時候進行調用, 或者在運算(多線程執行)完成后立即執行。這樣簡單的改進,使得可以明顯的支持更多的操作,這樣的功能在JDK concurrent中的Future是不支持的。 `ListenableFuture` 中的基礎方法是[`addListener(Runnable, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6ommon/util/concurrent/ListenableFuture.html#addListener%28java.lang.Runnable,%20java.util.concurrent.Executor%29), 該方法會在多線程運算完的時候,指定的Runnable參數傳入的對象會被指定的Executor執行。 ## 添加回調(Callbacks) 多數用戶喜歡使用 [Futures.addCallback(ListenableFuture<V>, FutureCallback<V>, Executor)](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#addCallback%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.util.concurrent.FutureCallback,%20java.util.concurrent.Executor%29)的方式, 或者 另外一個版本[version](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#addCallback%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.util.concurrent.FutureCallback%29)(譯者注:[addCallback](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/src-html/com/google/common/util/concurrent/Futures.html#line.1106)([ListenableFuture](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/ListenableFuture.html "interface in com.google.common.util.concurrent")<V> future,[FutureCallback](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/FutureCallback.html "interface in com.google.common.util.concurrent")<? super V> callback)),默認是采用 `MoreExecutors.sameThreadExecutor()線程池`, 為了簡化使用,Callback采用輕量級的設計. [`FutureCallback&lt;V&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/FutureCallback.html) 中實現了兩個方法: - [`onSuccess(V)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/FutureCallback.html#onSuccess%28V%29),在Future成功的時候執行,根據Future結果來判斷。 - [`onFailure(Throwable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6e/common/util/concurrent/FutureCallback.html#onFailure%28java.lang.Throwable%29), 在Future失敗的時候執行,根據Future結果來判斷。 ## ListenableFuture的創建 對應JDK中的 [`ExecutorService.submit(Callable)`](http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html#submit%28java.util.concurrent.Callable%29) 提交多線程異步運算的方式,Guava 提供了[`ListeningExecutorService`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/ListeningExecutorService.html) 接口, 該接口返回 `ListenableFuture` 而相應的 `ExecutorService` 返回普通的 `Future`。將 `ExecutorService` 轉為 `ListeningExecutorService,`可以使用[MoreExecutors.listeningDecorator(ExecutorService)](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6MoreExecutors.html#listeningDecorator%28java.util.concurrent.ExecutorService%29)進行裝飾。 ``` <pre class="calibre11">``` ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10)); ListenableFuture explosion = service.submit(new Callable() { public Explosion call() { return pushBigRedButton(); } }); Futures.addCallback(explosion, new FutureCallback() { // we want this handler to run immediately after we push the big red button! public void onSuccess(Explosion explosion) { walkAwayFrom(explosion); } public void onFailure(Throwable thrown) { battleArchNemesis(); // escaped the explosion! } }); ``` ``` 另外, 假如你是從 [FutureTask](http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/FutureTask.html)轉換而來的, Guava 提供[`ListenableFutureTask.create(Callable&lt;V&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/concurrent/ListenableFutureTask.html#create%28java.util.concurrent.Callable%29) 和[`ListenableFutureTask.create(Runnable, V)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6common/util/concurrent/ListenableFutureTask.html#create%28java.lang.Runnable,%20V%29). 和 JDK不同的是, `ListenableFutureTask` 不能隨意被繼承(譯者注:ListenableFutureTask中的done方法實現了調用listener的操作)。 假如你喜歡抽象的方式來設置future的值,而不是想實現接口中的方法,可以考慮繼承抽象類[`AbstractFuture&lt;V&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/AbstractFuture.html) 或者直接使用 [`SettableFuture`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/SettableFuture.html) 。 假如你必須將其他API提供的Future轉換成 `ListenableFuture`,你沒有別的方法只能采用硬編碼的方式[`JdkFutureAdapters.listenInPoolThread(Future)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/JdkFutureAdapters.html) 來將 `Future` 轉換成 `ListenableFuture`。盡可能地采用修改原生的代碼返回 `ListenableFuture`會更好一些。 ## Application 使用`ListenableFuture` 最重要的理由是它可以進行一系列的復雜鏈式的異步操作。 ``` <pre class="calibre11">``` ListenableFuture rowKeyFuture = indexService.lookUp(query); AsyncFunction<RowKey, QueryResult> queryFunction = new AsyncFunction<RowKey, QueryResult>() { public ListenableFuture apply(RowKey rowKey) { return dataService.read(rowKey); } }; ListenableFuture queryFuture = Futures.transform(rowKeyFuture, queryFunction, queryExecutor); ``` ``` 其他更多的操作可以更加有效的支持而JDK中的Future是沒法支持的. 不同的操作可以在不同的Executors中執行,單獨的`ListenableFuture` 可以有多個操作等待。 當一個操作開始的時候其他的一些操作也會盡快開始執行–“fan-out”–`ListenableFuture` 能夠滿足這樣的場景:促發所有的回調(callbacks)。反之更簡單的工作是,同樣可以滿足“fan-in”場景,促發`ListenableFuture` 獲取(get)計算結果,同時其它的Futures也會盡快執行:可以參考 [the implementation of `Futures.allAsList`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/src-html/com/google/common/util/concurrent/Futures.html#line.1276) 。(譯者注:fan-in和fan-out是軟件設計的一個術語,可以參考這里: <http://baike.baidu.com/view/388892.htm#1> 或者看這里的解析 [Design Principles: Fan-In vs Fan-Out](http://it.toolbox.com/blogs/enterprise-solutions/design-principles-fanin-vs-fanout-16088 "Design Principles: Fan-In vs Fan-Out") ,這里fan-out的實現就是封裝的ListenableFuture通過回調,調用其它代碼片段。fan-in的意義是可以調用其它Future) 方法描述參考[`transform(ListenableFuture&lt;A&gt;, AsyncFunction&lt;A, B&gt;, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.util.concurrent.AsyncFunction,%20java.util.concurrent.Executor%29)`*``返回一個新的ListenableFuture` ,該`ListenableFuture` 返回的result是由傳入的`AsyncFunction` 參數指派到傳入的 `ListenableFuture中`.[`transform(ListenableFuture&lt;A&gt;, AsyncFunction&lt;A, B&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.util.concurrent.AsyncFunction%29)[`transform(ListenableFuture&lt;A&gt;, Function&lt;A, B&gt;, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function,%20java.util.concurrent.Executor%29)`返回一個新的ListenableFuture` ,該`ListenableFuture` 返回的result是由傳入的`Function` 參數指派到傳入的 `ListenableFuture中`.[`transform(ListenableFuture&lt;A&gt;, Function&lt;A, B&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)[`allAsList(Iterable&lt;ListenableFuture&lt;V&gt;&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/Futures.html#allAsList%28java.lang.Iterable%29)`返回一個ListenableFuture` ,該`ListenableFuture` 返回的result是一個List,List中的值是每個ListenableFuture的返回值,假如傳入的其中之一fails或者cancel,這個Future fails 或者canceled[`allAsList(ListenableFuture&lt;V&gt;...)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#allAsList%28com.google.common.util.concurrent.ListenableFuture...%29)[`successfulAsList(Iterable&lt;ListenableFuture&lt;V&gt;&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6le/common/util/concurrent/Futures.html#successfulAsList%28java.lang.Iterable%29)`返回一個ListenableFuture` ,該Future的結果包含所有成功的Future,按照原來的順序,當其中之一Failed或者cancel,則用null替代[`successfulAsList(ListenableFuture&lt;V&gt;...)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6s.html#successfulAsList%28com.google.common.util.concurrent.ListenableFuture...%29)[`AsyncFunction&lt;A, B&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/AsyncFunction.html) 中提供一個方法`ListenableFuture&lt;B&gt; apply(A input),`它可以被用于異步變換值。 ``` <pre class="calibre11">``` List<ListenableFuture> queries; // The queries go to all different data centers, but we want to wait until they're all done or failed. ListenableFuture<List> successfulQueries = Futures.successfulAsList(queries); Futures.addCallback(successfulQueries, callbackOnSuccessfulQueries); ``` ``` ## CheckedFuture Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。 Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。 Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。 Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。 Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。 Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。 Guava也提供了 [`CheckedFuture&lt;V, X extends Exception&gt;`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/CheckedFuture.html) 接口。`CheckedFuture` 是一個`ListenableFuture` ,其中包含了多個版本的get 方法,方法聲明拋出檢查異常.這樣使得創建一個在執行邏輯中可以拋出異常的Future更加容易 。將 `ListenableFuture` 轉換成`CheckedFuture`,可以使用 [`Futures.makeChecked(ListenableFuture&lt;V&gt;, Function&lt;Exception, X&gt;)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)。
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