5.1-google Guava包的ListenableFuture解析 · Google Guava 中文教程

# 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/…ommon/util/concurrent/ListenableFuture.html#addListener%28java.lang.Runnable, java.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/…utures.html#addCallback%28com.google.common.util.concurrent.ListenableFuture, com.google.common.util.concurrent.FutureCallback, java.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/…utures.html#addCallback%28com.google.common.util.concurrent.ListenableFuture, com.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<V>`](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/…e/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/…MoreExecutors.html#listeningDecorator%28java.util.concurrent.ExecutorService%29)進行裝飾。 ``` 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<V>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/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/…common/util/concurrent/ListenableFutureTask.html#create%28java.lang.Runnable, V%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<V>`](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。 ``` 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<A>, AsyncFunction<A, B>, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.util.concurrent.AsyncFunction, java.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<A>, AsyncFunction<A, B>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.util.concurrent.AsyncFunction%29) | | [`transform(ListenableFuture<A>, Function<A, B>, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function, java.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<A>, Function<A, B>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29) | | [`allAsList(Iterable<ListenableFuture<V>>)`](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<V>...)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#allAsList%28com.google.common.util.concurrent.ListenableFuture...%29) | | [`successfulAsList(Iterable<ListenableFuture<V>>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…le/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<V>...)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…s.html#successfulAsList%28com.google.common.util.concurrent.ListenableFuture...%29) | [`AsyncFunction<A, B>`](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<B> apply(A input)，`它可以被用于異步變換值。 ``` 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<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。 Guava也提供了?[`CheckedFuture<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。 Guava也提供了?[`CheckedFuture<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。 Guava也提供了?[`CheckedFuture<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。 Guava也提供了?[`CheckedFuture<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。 Guava也提供了?[`CheckedFuture<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。 Guava也提供了?[`CheckedFuture<V, X extends Exception>`](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<V>, Function<Exception, X>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…utures.html#makeChecked%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29)。