Akka2使用探索6（Futures）——实现并发和异步

 

Future用来获取某个并发操作的结果，这个结果可以同步（阻塞）或异步（非阻塞）的方式访问。

 

执行上下文

Future 需要一个ExecutionContext, 它与java.util.concurrent.Executor 很相像. 如果你在作用域内有一个 ActorSystem , 它可以用system.dispatcher()作 ExecutionContext。你也可以用ExecutionContext 伴生对象提供的工厂方法来将 Executors 和 ExecutorServices 进行包裹, 或者甚至创建自己的实例.

   //执行上下文可以自己指定线程池类型
        ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool())

Future的创建方法
        Future<String> f1 = Futures.successful("f1", ec);

        Future<String> f2 = Futures.future(new Callable() {
            @Override
            Object call() {
                return "f2"
            }
        }, ec)
        Future<String> f3 = Futures.successful("f3", ActorSystem.create("test").dispatcher());

    //使用actor的ask方法发送消息是也能创建一个Future

    Futuref4 = akka.pattern.Patterns.ask(actor, "msg", 1000 * 60)

函数式 Future

Akka 的 Future 有一些与Scala集合所使用的非常相似的 monadic 方法. 这使你可以构造出结果可以传递的‘管道’ 或 ‘数据流’.

map（对未来返回的结果进行处理）

让Future以函数式风格工作的第一个方法是 map. 它需要一个函数来对Future的结果进行处理, 返回一个新的结果。map 方法的返回值是包含新结果的另一个 Future:

static void map() throws Exception {
        ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        Future<String> f1 = Futures.successful("fof1o", ec);
        //map的作用是：对Futrue：f1的返回结果进行处理，返回一个新的结果
        Future<Integer> f2 = f1.map(new Mapper<String, Integer>() {
            public Integer apply(String s) {
                return s.length();
            }
        });  //这里对未来f1返回的字符串计算其长度

    对Future完成结果的处理方法

    //System.out.println(Await.result(f2, Duration.create(5, "s")));     //阻塞式，当前线程在此等待

        //下面是非阻塞式，异步返回
        f2.onComplete(new OnComplete<Integer>() {
            @Override
            public void onComplete(Throwable failure, Integer success) {
                System.out.println("f2返回结果:" + success + ",failure:" + failure);
            }
        });

        f2.onSuccess(new OnSuccess<Integer>() {
            @Override
            public void onSuccess(Integer result) {
                System.out.println("f2返回结果:" + result);
            }
        });

        f2.onFailure(new OnFailure() {
            @Override
            public void onFailure(Throwable failure) {
                System.out.println("f2返回failure:" + failure);
            }
        });
    }

 

flatMap（对多个Future返回的结果进行处理）

static void flatMap() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        Future<String> f1 = Futures.successful("hello", ec);

        //
        Future<Integer> fr = f1.flatMap(new Mapper<String, Future<Integer>>() {
            public Future<Integer> apply(final String s) {
                return Futures.future(new Callable<Integer>() {
                    public Integer call() {
                        return s.length();
                    }
                }, ec);
            }
        });  //
        System.out.println(Await.result(fr, Duration.create(5, "s")));     //阻塞式
    }

    //对两个Future的结果处理
    static void flatMap_concat2() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        final Future<String> f1 = Futures.successful("hello", ec);
        final Future<String> f2 = Futures.successful("world", ec);

        //如果要对多个Future的结果进行处理，需要用flatMap
        //本例中对f1和f2返回的结果用空格连接成“hello world”
        Future<String> fr = f1.flatMap(new Mapper<String, Future<String>>() {
            public Future<String> apply(final String s) {
                return f2.map(new Mapper<String, String>() {
                    @Override
                    public String apply(String v) {
                        return s + " " + v;
                    }
                });
            }
        });
        System.out.println(Await.result(fr, Duration.create(5, "s")));     //阻塞式
    }

    //对三个Future的结果处理
    static void flatMap_concat3() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        final Future<String> f1 = Futures.successful("How", ec);
        final Future<String> f2 = Futures.successful("are", ec);
        final Future<String> f3 = Futures.successful("you", ec);

        //如果要对多个Future的结果进行处理，需要用flatMap
        //本例中对f1、f2、f3返回的结果用空格连接成“How are you”
        Future<String> fr = f1.flatMap(new Mapper<String, Future<String>>() {
            public Future<String> apply(final String s) {
                return f2.flatMap(new Mapper<String, Future<String>>() {
                    @Override
                    public Future<String> apply(final String s2) {
                        return f3.map(new Mapper<String, String>() {
                            @Override
                            public String apply(String s3) {
                                return s + " " + s2 + " " + s3;
                            }
                        });
                    }
                });
            }
        });  /*用java写比较繁琐，用scala的话就简单多了
        val future1 = for {
            a: String <- actor ? "How" // returns How
            b: String <- actor ? "are" // returns "are"
            c: String <- actor ? "you" // returns "you"
        } yield a + " " + b + "" + c*/
        System.out.println(Await.result(fr, Duration.create(5, "s")));     //阻塞式
    }

filter（对Future进行条件筛选）

static void filter() throws Exception {
        ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        Future<String> f1 = Futures.successful("fof1o", ec);
        Future<String> f2 = Futures.successful("fo", ec);
        //map的作用是：对Futrue：f1的返回结果进行处理，返回一个新的结果
        Future<String> fs = f1.filter(Filter.filterOf(new Function<String, Boolean>() {
            @Override
            public Boolean apply(String param) {
                return param.length() == 5;
            }
        }));
        System.out.println(Await.result(fs, Duration.create(5, "s")));

        Future<String> ff = f2.filter(Filter.filterOf(new Function<String, Boolean>() {
            @Override
            public Boolean apply(String param) {
                return param.length() == 5;
            }
        }));  //不匹配的话会抛scala.MatchError异常
        System.out.println(Await.result(ff, Duration.create(5, "s")));
    }

 

组合Futures

如果Future的数目较多，用flatMap组合的话代码就过于复杂。可以使用sequence和traverse。

sequence（将 T[Future[A]] 转换为 Future[T[A]]）

public static void sequence() throws Exception {
        //将 T[Future[A]] 转换为 Future[T[A]]
        //简化了用flatMap来组合
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        final Future<String> f1 = Futures.successful("How", ec);
        final Future<String> f2 = Futures.successful("are", ec);
        final Future<String> f3 = Futures.successful("you", ec);

        List<Future<String>> futureList = new ArrayList<Future<String>>();
        futureList.add(f1);
        futureList.add(f2);
        futureList.add(f3);

        //这里将List<Future<String>> 组合成一个Future：Future<Iterable<String>>
        Future<Iterable<String>> future = Futures.sequence(futureList, ec);

        Future<String> fr = future.map(new Mapper<Iterable<String>, String>() {
            @Override
            public String apply(Iterable<String> parameter) {
                String result = "";
                for (String s : parameter) {
                    result += s + " ";
                }
                return result;
            }
        });
        System.out.println(Await.result(fr, Duration.create(5, "s")));
    }

traverse（将 T[A] 转换为 Future[T[A]]）

public static void traverse() throws Exception {
        //将 T[A] 转换为 Future[T[A]]
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());


        Iterable<String> list = Arrays.asList("How", "are", "you");

        //这里将List<String> 组合成一个Future：Future<Iterable<String>> ，对list中的每个元素做加工处理
        Future<Iterable<String>> future = Futures.traverse(list, new Function<String, Future<String>>() {
            @Override
            public Future<String> apply(final String param) {
                return Futures.future(new Callable<String>() {
                    @Override
                    public String call() throws Exception {
                        return param.toUpperCase();
                    }
                }, ec);
            }
        }, ec);

        Future<String> fr = future.map(new Mapper<Iterable<String>, String>() {
            @Override
            public String apply(Iterable<String> parameter) {
                String result = "";
                for (String s : parameter) {
                    result += s + " ";
                }
                return result;
            }
        });

        System.out.println(Await.result(fr, Duration.create(5, "s")));
    }

fold（从一个初始值开始递归地对Future序列进行处理（它将sequence和map操作合并成一步了））

public static void fold() throws Exception {
        //fold从一个初始值开始递归地对Future序列进行处理（它将sequence和map操作合并成一步了）
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        final Future<String> f1 = Futures.successful("How", ec);
        final Future<String> f2 = Futures.successful("are", ec);
        final Future<String> f3 = Futures.successful("you", ec);

        List<Future<String>> futureList = new ArrayList<Future<String>>();
        futureList.add(f1);
        futureList.add(f2);
        futureList.add(f3);

        //本例从初始值“Init”开始，递归地对futureList的返回值用"_"连接，返回“Init_How_are_you”
        Future<String> fr = Futures.fold("Init", futureList, new Function2<String, String, String>() {
            @Override
            public String apply(String arg1, String arg2) {
                System.out.println("arg1----" + arg1);    //第一次为Init，第二次为Init_How ，第三次为Init_How_are
                System.out.println("arg2----" + arg2);    //第一次为How ，第二次为are        第三次为you
                return arg1 + "_" + arg2;
            }
        }, ec);

        //如果futureList为空列表，则返回初始值“Init”
        System.out.println(Await.result(fr, Duration.create(5, "s")));  //结果为Init_How_are_you
    }
reduce（如果不想从给定的初始值开始递归，而想从future序列的第一个开始，则用reduce（它将sequence和map合并成一步了））

public static void reduce() throws Exception {
        //如果不想从给定的初始值开始递归，而想从future序列的第一个开始，则用reduce（它将sequence和map合并成一步了）
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        final Future<String> f1 = Futures.successful("How", ec);
        final Future<String> f2 = Futures.successful("are", ec);
        final Future<String> f3 = Futures.successful("you", ec);

        List<Future<String>> futureList = new ArrayList<Future<String>>();
        futureList.add(f1);
        futureList.add(f2);
        futureList.add(f3);

        //本例从初始值“How”开始，递归地对futureList的返回值用"_"连接，返回“How_are_you”
        Future<String> fr = Futures.reduce(futureList, new Function2<String, String, String>() {
            @Override
            public String apply(String arg1, String arg2) {
                System.out.println("arg1----" + arg1);    //第一次为How ，第二次为How_are
                System.out.println("arg2----" + arg2);    //第一次为are ，第二次为you
                return arg1 + "_" + arg2;
            }
        }, ec);

        //如果futureList为空列表，则返回初始值“Init”
        System.out.println(Await.result(fr, Duration.create(5, "s")));  //结果为Init_How_are_you
    }

 

andThen（由于回调的执行是无序的，而且可能是并发执行的, 当你需要一组有序操作的时候需要一些技巧。）

public static void andThen() throws Exception {
        //如果要对Future的结果分多次依次处理，需要使用andThen
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());

        Future<String> future = Futures.successful("hello", ec).andThen(new OnComplete<String>() {
            @Override
            public void onComplete(Throwable failure, String success) {
                System.out.println("先收到：" + success);
            }
        }).andThen(new OnComplete<String>() {
            @Override
            public void onComplete(Throwable failure, String success) {
                System.out.println("又收到：" + success);
            }
        }).andThen(new OnSuccess<Either<Throwable, String>>() {
            @Override
            public void onSuccess(Either<Throwable, String> result) {
                System.out.println("收到onSuccess：" + result);
            }
        });
    }

fallbackTo（将两个 Futures 合并成一个新的 Future, 如果第一个Future失败了，它将持有第二个 Future 的成功值）

public static void fallbackTo() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        Future<String> f1 = Futures.failed(new RuntimeException("ex1"), ec);
        Future<String> f2 = Futures.failed(new RuntimeException("ex2"), ec);
        Future<String> f3 = Futures.successful("ok", ec);
        //fallbackTo 将两个 Futures 合并成一个新的 Future, 如果第一个Future失败了，它将持有第二个 Future 的成功值
        Future<String> fr = f1.fallbackTo(f2).fallbackTo(f3);

        System.out.println(Await.result(fr, Duration.create(5, "s")));
    }

 

zip（操作将两个 Futures 组合压缩成一个新的Future，返回的新的Future持hold一个tuple实例，它包含二者成功的结果）

public static void zip() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        Future<String> f1 = Futures.future(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("f1---" + Thread.currentThread().getName());
                Thread.sleep(1000 * 10);
                return "hello";
            }
        }, ec);
        Future<String> f2 = Futures.future(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("f2---" + Thread.currentThread().getName());
                Thread.sleep(1000 * 5);
                return "world";
            }
        }, ec);
        //zip操作将两个 Futures 组合压缩成一个新的Future，返回的新的Future持hold一个tuple实例，它包含二者成功的结果
        Future<String> fr = f1.zip(f2).map(new Mapper<Tuple2<String, String>, String>() {
            @Override
            public String apply(Tuple2<String, String> ziped) {
                System.out.println("zip---" + Thread.currentThread().getName());
                return ziped._1() + " " + ziped._2();   //f1和f2的返回结果包含在zipped对象中
            }
        });
        System.out.println("主线程----" + Thread.currentThread().getName());
        System.out.println(Await.result(fr, Duration.create(15, "s")));
    }

    public static void zip2() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        Future<String> f1 = Futures.successful("hello", ec);
        Future<String> f2 = Futures.future(new Callable<String>() {
            @Override
            public String call() throws Exception {
                System.out.println("f2---" + Thread.currentThread().getName());
                Thread.sleep(1000 * 10);
                return (1 / 0) + "";
            }
        }, ec);
        //zip操作将两个 Futures 组合压缩成一个新的Future，返回的新的Future持hold一个tuple实例，它包含二者成功的结果
        Future<String> fr = f1.zip(f2).map(new Mapper<Tuple2<String, String>, String>() {
            @Override
            public String apply(Tuple2<String, String> ziped) {
                System.out.println("zip----" + Thread.currentThread().getName());
                return ziped._1() + " " + ziped._2();   //f1和f2的返回结果包含在zipped对象中
            }
        });

        System.out.println("主线程----" + Thread.currentThread().getName());
        System.out.println(Await.result(fr, Duration.create(15, "s")));
    }

 

recover（对Future的异常进行处理，相当于try..catch中对捕获异常后的处理）

public static void recover() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        //recover对Future的异常进行处理，相当于try..catch中对捕获异常后的处理
        Future<Integer> future = Futures.future(new Callable<Integer>() {
            public Integer call() {
                return 1 / 0;
            }
        }, ec).recover(new Recover<Integer>() {
            public Integer recover(Throwable problem) throws Throwable {
                System.out.println("捕获到异常：" + problem);
//                if (problem instanceof RuntimeException) {
//                    return 0;
//                } else {
//                    throw problem;
//                }
                return -2;    //这里捕获到异常后直接返回新值了，并没有再抛出异常，所以后面的recover不会再收到异常
            }
        }).recover(new Recover<Integer>() {
            public Integer recover(Throwable problem) throws Throwable {
                System.out.println("捕获到异常：" + problem);
                if (problem instanceof ArithmeticException) {  //捕获异常并处理，捕获到后，后面得到的result将会是-1
                    return -1;
                } else {
                    throw problem;
                }
            }
        });
        int result = Await.result(future, Duration.create(1, TimeUnit.SECONDS));
        System.out.println("result----" + result);
    }

 

recoverWith（和recover很类似，只是捕获到异常后返回Future，使其能够异步并发处理）

public static void recoverWith() throws Exception {
        final ExecutionContextExecutorService ec = ExecutionContexts.fromExecutorService(Executors.newCachedThreadPool());
        //recoverWith和recover很类似，只是捕获到异常后返回Future，使其能够异步并发处理
        Future<Integer> future = Futures.future(new Callable<Integer>() {
            public Integer call() {
                return 1 / 0;
            }
        }, ec).recoverWith(new Recover<Future<Integer>>() {
            @Override
            public Future<Integer> recover(Throwable failure) throws Throwable {
                if (failure instanceof ArithmeticException) {
                    return Futures.future(new Callable<Integer>() {
                        @Override
                        public Integer call() throws Exception {
                            return 0;
                        }
                    }, ec);
                } else throw failure;

            }
        });
        int result = Await.result(future, Duration.create(1, TimeUnit.SECONDS));
        System.out.println("result----" + result);
    }