Reactor詳解之:異常處理

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"簡介"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不管是在響應式編程還是普通的程序設計中，異常處理都是一個非常重要的方面。今天將會給大家介紹Reactor中異常的處理流程。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"Reactor的異常一般處理方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"先舉一個例子，我們創建一個Flux，在這個Flux中，我們產生一個異常，看看是什麼情況："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Flux flux2= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i));\n flux2.subscribe(System.out::println);\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們會得到一個異常ErrorCallbackNotImplemented："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"100 / 1 = 100\n100 / 2 = 50\n\nreactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.ArithmeticException: / by zero\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那怎麼處理這個異常呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有兩種方式，第一種方式就是我們之前文章講過的，在subscribe的時候指定onError方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Flux flux2= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i));\n\n flux2.subscribe(System.out::println,\n error -> System.err.println(\"Error: \" + error));\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"還是剛纔的代碼，但是這次我們在subscribe的時候，添加了onError處理器，看下運行結果："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Divided by zero :(\n100 / 1 = 100\n100 / 2 = 50\nError: java.lang.ArithmeticException: / by zero\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到異常已經被我們捕獲了，並且進行了合適的處理。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了在subscribe中進行處理，我們還可以在publish的時候，就指定異常的處理模式，這就是我們要介紹的第二種方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" Flux flux= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .onErrorReturn(\"Divided by zero :(\");\n flux.subscribe(System.out::println);\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的例子中，在創建Flux的時候，手動指定了其onErrorReturn方法，我們看下輸出結果："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"100 / 1 = 100\n100 / 2 = 50\nDivided by zero :(\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"注意，對於Flux或者Mono來說，所有的異常都是一個終止的操作，即使你使用了異常處理，原生成序列也不會繼續。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"但是如果你對異常進行了處理，那麼它會將oneError信號轉換成爲新的序列的開始，並將替換掉之前上游產生的序列。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"各種異常處理方式詳解"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在一般的程序中，我們的異常應該怎麼處理呢？大家很容易想到的是try catch。而Reactor中subscribe的onError方法，就是try catch的一個具體應用："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Flux flux2= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i));\n\n flux2.subscribe(System.out::println,\n error -> System.err.println(\"Error: \" + error));\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"還是上的例子，我們在onError方法中，對異常進行了處理。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果轉換成爲常規代碼，應該是下面的樣子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void normalErrorHandle(){\n try{\n Arrays.asList(1,2,0).stream().map(i -> \"100 / \" + i + \" = \" + (100 / i)).forEach(System.out::println);\n }catch (Exception e){\n System.err.println(\"Error: \" + e);\n }\n }\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了這種最基本的異常處理方法之外，Reactor還提供了很多種不同的異常處理方法，下面我們來一一介紹一下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Static Fallback Value"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Static Fallback Value的意思是，在遇到異常的時候會fallback到一個靜態的默認值。比如我們之前講到的onErrorReturn。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Flux flux= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .onErrorReturn(\"Divided by zero :(\");\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當然onErrorReturn還支持一個Predicate參數，用來判斷要falback的異常是否滿足條件。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"public final Flux onErrorReturn(Predicate super Throwable> predicate, T fallbackValue) \n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Fallback Method"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了fallback Value之外，還支持Fallback Method。也就是說如果你想在捕獲異常之後調用其他的方法，就可以使用Fallback Method。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這裏Fallback Method是用onErrorResume來表示的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void useFallbackMethod(){\n Flux flux= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .onErrorResume(e -> System.out::println);\n flux.subscribe(System.out::println);\n }\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Dynamic Fallback Value"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所謂的動態Fallback Value就是根據你拋出的異常進行判斷，通過定位不同的Error從而fallback到不同的值："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void useDynamicFallback(){\n Flux flux= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .onErrorResume(error -> Mono.just(\n MyWrapper.fromError(error)));\n }\n\n public static class MyWrapper{\n public static String fromError(Throwable error){\n return \"That is a new Error\";\n }\n }\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Catch and Rethrow"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"同樣的，我們可以在捕獲異常之後進行rethrow:"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Flux flux= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .onErrorResume(error -> Flux.error(\n new RuntimeException(\"oops, ArithmeticException!\", error)));\n\n Flux flux2= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .onErrorMap(error -> new RuntimeException(\"oops, ArithmeticException!\", error));\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有兩種方式，第一種就是在onErrorResume中使用Flux.error構建一個新的Flux，另外一種就是直接在onErrorMap中進行處理。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Log or React on the Side"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有時候你只是想記錄一下異常信息，並不想破壞原來的React結構，那麼可以試着使用doOnError。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void useDoOnError(){\n Flux flux= Flux.just(1, 2, 0)\n .map(i -> \"100 / \" + i + \" = \" + (100 / i))\n .doOnError(error -> System.out.println(\"we got the error: \"+ error));\n }\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Finally Block"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果我們在代碼中使用了某些資源，一般情況下我們需要在finally中對其進行關閉，或者使用JDK7中引入的 try-with-resource 。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"舉個例子，下面的是使用finally的方式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Stats stats = new Stats();\nstats.startTimer();\ntry {\n doSomethingDangerous();\n}\nfinally {\n stats.stopTimerAndRecordTiming();\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面是使用try-with-resource的方式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"try (SomeAutoCloseable disposableInstance = new SomeAutoCloseable()) {\n return disposableInstance.toString();\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那麼在Reactor中，我們也有兩種方式和其對應。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第一種就是doFinally方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Stats stats = new Stats();\nLongAdder statsCancel = new LongAdder();\n\nFlux flux =\nFlux.just(\"foo\", \"bar\")\n .doOnSubscribe(s -> stats.startTimer())\n .doFinally(type -> { \n stats.stopTimerAndRecordTiming();\n if (type == SignalType.CANCEL) \n statsCancel.increment();\n })\n .take(1); \n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的例子中，doFinally實際上做的就是finally block做的事情。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第二種是使用using，我們先看一個using的定義："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public static Flux using(Callable extends D> resourceSupplier, Function super D, ? extends\n Publisher extends T>> sourceSupplier, Consumer super D> resourceCleanup)\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到using支持三個參數，resourceSupplier是一個生成器，用來在subscribe的時候生成要發送的resource對象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"sourceSupplier是一個生成Publisher的工廠，接收resourceSupplier傳過來的resource，然後生成Publisher對象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"resourceCleanup用來對resource進行收尾操作。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那麼我們怎麼用呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"舉個例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void useUsing(){\n AtomicBoolean isDisposed = new AtomicBoolean();\n Disposable disposableInstance = new Disposable() {\n @Override\n public void dispose() {\n isDisposed.set(true);\n }\n\n @Override\n public String toString() {\n return \"DISPOSABLE\";\n }\n };\n\n Flux flux =\n Flux.using(\n () -> disposableInstance,\n disposable -> Flux.just(disposable.toString()),\n Disposable::dispose);\n }\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的例子中，我們創建了一個Disposable對象，作爲resource，然後對這個resource進行加工，返回一個Flux對象，最後通過調用Disposable::dispose方法，對resource進行銷燬。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Retrying"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有時候我們遇到了異常，可能需要重試幾次，Reactor爲我們提供了retry方法，先看一個例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void testRetry(){\n Flux.interval(Duration.ofMillis(250))\n .map(input -> {\n if (input < 3){\n return \"tick \" + input;\n } \n throw new RuntimeException(\"boom\");\n })\n .retry(1)\n .elapsed()\n .subscribe(System.out::println, System.err::println);\n\n try {\n Thread.sleep(2100);\n } catch (InterruptedException e) {\n e.printStackTrace();\n }\n }\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"看下輸出結果："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"[264,tick 0]\n[255,tick 1]\n[241,tick 2]\n[506,tick 0]\n[252,tick 1]\n[253,tick 2]\njava.lang.RuntimeException: boom\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"retry的作用就是當遇到異常的時候，重啓一個新的序列。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"elapsed是用來展示產生的value時間之間的duration。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從結果我們可以看到，retry之前是不會產生異常信息的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文的例子"},{"type":"link","attrs":{"href":"https://github.com/ddean2009/learn-reactive/tree/master/reactorIntroduction","title":null},"content":[{"type":"text","text":"learn-reactive"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"本文作者：flydean程序那些事"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"本文鏈接："},{"type":"link","attrs":{"href":"http://www.flydean.com/reactor-handle-errors/","title":null},"content":[{"type":"text","text":"http://www.flydean.com/reactor-handle-errors/"}],"marks":[{"type":"italic"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"本文來源：flydean的博客"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"歡迎關注我的公衆號:「程序那些事」最通俗的解讀，最深刻的乾貨，最簡潔的教程，衆多你不知道的小技巧等你來發現！"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}