精通多线程，却不会异步编程？

前言

以前需要异步执行一个任务时，一般是用Thread或者线程池Executor去创建。如果需要返回值，则是调用Executor.submit获取Future。但是多个线程存在依赖组合，我们又能怎么办？可使用同步组件CountDownLatch、CyclicBarrier等；其实有简单的方法，就是用CompletableFuture

• 线程任务的创建
• 线程任务的串行执行
• 线程任务的并行执行
• 处理任务结果和异常
• 多任务的简单组合
• 取消执行线程任务
• 任务结果的获取和完成与否判断

1、创建异步线程任务

根据supplier创建CompletableFuture任务

//使用内置线程ForkJoinPool.commonPool()，根据supplier构建执行任务
public static  CompletableFuture supplyAsync(Supplier supplier)
//指定自定义线程，根据supplier构建执行任务
public static  CompletableFuture supplyAsync(Supplier supplier, Executor executor)

根据runnable创建CompletableFuture任务

//使用内置线程ForkJoinPool.commonPool()，根据runnable构建执行任务
public static CompletableFuture runAsync(Runnable runnable)
//指定自定义线程，根据runnable构建执行任务
public static CompletableFuture runAsync(Runnable runnable, Executor executor)

• 使用示例

ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture rFuture = CompletableFuture
        .runAsync(() -> System.out.println("hello siting"), executor);
//supplyAsync的使用
CompletableFuture future = CompletableFuture
        .supplyAsync(() -> {
            System.out.print("hello ");
            return "siting";
        }, executor);

//阻塞等待，runAsync 的future 无返回值，输出null
System.out.println(rFuture.join());
//阻塞等待
String name = future.join();
System.out.println(name);
executor.shutdown(); // 线程池需要关闭
--------输出结果--------
hello siting
null
hello siting

常量值作为CompletableFuture返回

//有时候是需要构建一个常量的CompletableFuture
public static  CompletableFuture completedFuture(U value)

2 、线程串行执行

精通多线程，却不会异步编程？

任务完成则运行action，不关心上一个任务的结果，无返回值

public CompletableFuture thenRun(Runnable action)
public CompletableFuture thenRunAsync(Runnable action)
public CompletableFuture thenRunAsync(Runnable action, Executor executor)

• 使用示例

CompletableFuture future = CompletableFuture
        .supplyAsync(() -> "hello siting", executor)
        .thenRunAsync(() -> System.out.println("OK"), executor);
executor.shutdown();
--------输出结果--------
OK

任务完成则运行action，依赖上一个任务的结果，无返回值

public CompletableFuture thenAccept(Consumer action)
public CompletableFuture thenAcceptAsync(Consumer action)
public CompletableFuture thenAcceptAsync(Consumer action, Executor executor)

• 使用示例

ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        .supplyAsync(() -> "hello siting", executor)
        .thenAcceptAsync(System.out::println, executor);
executor.shutdown();
--------输出结果--------
hello siting

任务完成则运行fn，依赖上一个任务的结果，有返回值

public  CompletableFuture thenApply(Function fn)
public  CompletableFuture thenApplyAsync(Function fn)        
public  CompletableFuture thenApplyAsync(Function fn, Executor executor)

• 使用示例

ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        .supplyAsync(() -> "hello world", executor)
        .thenApplyAsync(data -> {
            System.out.println(data); return "OK";
        }, executor);
System.out.println(future.join());
executor.shutdown();
--------输出结果--------
hello world
OK

thenCompose - 任务完成则运行fn，依赖上一个任务的结果，有返回值

• 类似thenApply（区别是thenCompose的返回值是CompletionStage，thenApply则是返回 U），提供该方法为了和其他CompletableFuture任务更好地配套组合使用

public  CompletableFuture thenCompose(Function> fn) 
public  CompletableFuture thenComposeAsync(Function> fn)
public  CompletableFuture thenComposeAsync(Function> fn,
  Executor executor)        

• 使用示例

//第一个异步任务，常量任务
CompletableFuture f = CompletableFuture.completedFuture("OK");
//第二个异步任务
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        .supplyAsync(() -> "hello world", executor)
        .thenComposeAsync(data -> {
            System.out.println(data); return f; //使用第一个任务作为返回
        }, executor);
System.out.println(future.join());
executor.shutdown();
--------输出结果--------
hello world
OK

3 、线程并行执行

精通多线程，却不会异步编程？

两个CompletableFuture[并行]执行完，然后执行action，不依赖上两个任务的结果，无返回值

public CompletableFuture runAfterBoth(CompletionStage other, Runnable action)
public CompletableFuture runAfterBothAsync(CompletionStage other, Runnable action)
public CompletableFuture runAfterBothAsync(CompletionStage other, Runnable action, Executor executor)

• 使用示例

//第一个异步任务，常量任务
CompletableFuture first = CompletableFuture.completedFuture("hello world");
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // () -> System.out.println("OK") 是第三个任务
        .runAfterBothAsync(first, () -> System.out.println("OK"), executor);
executor.shutdown();
--------输出结果--------
OK

两个CompletableFuture[并行]执行完，然后执行action，依赖上两个任务的结果，无返回值

//第一个任务完成再运行other，fn再依赖消费两个任务的结果，无返回值
public  CompletableFuture thenAcceptBoth(CompletionStage other,
        BiConsumer action)
//两个任务异步完成，fn再依赖消费两个任务的结果，无返回值     
public  CompletableFuture thenAcceptBothAsync(CompletionStage other,
        BiConsumer action)  
//两个任务异步完成（第二个任务用指定线程池执行），fn再依赖消费两个任务的结果，无返回值                
public  CompletableFuture thenAcceptBothAsync(CompletionStage other,
        BiConsumer action, Executor executor) 

• 使用示例

//第一个异步任务，常量任务
CompletableFuture first = CompletableFuture.completedFuture("hello world");
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // (w, s) -> System.out.println(s) 是第三个任务
        .thenAcceptBothAsync(first, (s, w) -> System.out.println(s), executor);
executor.shutdown();
--------输出结果--------
hello siting

两个CompletableFuture[并行]执行完，然后执行action，依赖上两个任务的结果，有返回值

//第一个任务完成再运行other，fn再依赖消费两个任务的结果，有返回值
public  CompletableFuture thenCombine(CompletionStage other, 
  BiFunction fn)
//两个任务异步完成，fn再依赖消费两个任务的结果，有返回值
public  CompletableFuture thenCombineAsync(CompletionStage other,
        BiFunction fn)   
//两个任务异步完成（第二个任务用指定线程池执行），fn再依赖消费两个任务的结果，有返回值        
public  CompletableFuture thenCombineAsync(CompletionStage other,
        BiFunction fn, Executor executor)         

• 使用示例

//第一个异步任务，常量任务
CompletableFuture first = CompletableFuture.completedFuture("hello world");
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // (w, s) -> System.out.println(s) 是第三个任务
        .thenCombineAsync(first, (s, w) -> {
            System.out.println(s);
            return "OK";
        }, executor);
System.out.println(future.join());
executor.shutdown();
--------输出结果--------
hello siting
OK

4 、线程并行执行，谁先执行完则谁触发下一任务（二者选其最快）

精通多线程，却不会异步编程？

上一个任务或者other任务完成, 运行action，不依赖前一任务的结果，无返回值

public CompletableFuture runAfterEither(CompletionStage other, Runnable action)   
public CompletableFuture runAfterEitherAsync(CompletionStage other, Runnable action)
public CompletableFuture runAfterEitherAsync(CompletionStage other,
  Runnable action, Executor executor)

• 使用示例

//第一个异步任务，休眠1秒，保证最晚执行晚
CompletableFuture first = CompletableFuture.supplyAsync(()->{
    try{ Thread.sleep(1000); }catch (Exception e){}
    System.out.println("hello world");
    return "hello world";
});
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() ->{
            System.out.println("hello siting");
            return "hello siting";
        } , executor)
        //() ->  System.out.println("OK") 是第三个任务
        .runAfterEitherAsync(first, () ->  System.out.println("OK") , executor);
executor.shutdown();
--------输出结果--------
hello siting
OK

上一个任务或者other任务完成, 运行action，依赖最先完成任务的结果，无返回值

public CompletableFuture acceptEither(CompletionStage other,
  Consumer action)
public CompletableFuture acceptEitherAsync(CompletionStage other,
  Consumer action, Executor executor)       
public CompletableFuture acceptEitherAsync(CompletionStage other,
  Consumer action, Executor executor)     

• 使用示例

//第一个异步任务，休眠1秒，保证最晚执行晚
CompletableFuture first = CompletableFuture.supplyAsync(()->{
    try{ Thread.sleep(1000);  }catch (Exception e){}
    return "hello world";
});
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // data ->  System.out.println(data) 是第三个任务
        .acceptEitherAsync(first, data ->  System.out.println(data) , executor);
executor.shutdown();
--------输出结果--------
hello siting        

上一个任务或者other任务完成, 运行fn，依赖最先完成任务的结果，有返回值

public  CompletableFuture applyToEither(CompletionStage other,
  Function fn) 
public  CompletableFuture applyToEitherAsync(CompletionStage other,
  Function fn)         
public  CompletableFuture applyToEitherAsync(CompletionStage other,
  Function fn, Executor executor)         

• 使用示例

//第一个异步任务，休眠1秒，保证最晚执行晚
CompletableFuture first = CompletableFuture.supplyAsync(()->{
    try{ Thread.sleep(1000);  }catch (Exception e){}
    return "hello world";
});
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // data ->  System.out.println(data) 是第三个任务
        .applyToEitherAsync(first, data ->  {
            System.out.println(data);
            return "OK";
        } , executor);
System.out.println(future);
executor.shutdown();
--------输出结果--------
hello siting
OK

5 、处理任务结果或者异常

exceptionally-处理异常

精通多线程，却不会异步编程？

public CompletableFuture exceptionally(Function fn)

• 如果之前的处理环节有异常问题，则会触发exceptionally的调用相当于 try...catch
• 使用示例

CompletableFuture first = CompletableFuture
        .supplyAsync(() -> {
            if (true) {
                throw new RuntimeException("main error!");
            }
            return "hello world";
        })
        .thenApply(data -> 1)
        .exceptionally(e -> {
            e.printStackTrace(); // 异常捕捉处理，前面两个处理环节的日常都能捕获
            return 0;
        });

handle-任务完成或者异常时运行fn，返回值为fn的返回

• 相比exceptionally而言，即可处理上一环节的异常也可以处理其正常返回值

public  CompletableFuture handle(BiFunction fn) 
public  CompletableFuture handleAsync(BiFunction fn) 
public  CompletableFuture handleAsync(BiFunction fn, 
  Executor executor)        

• 使用示例

CompletableFuture first = CompletableFuture
        .supplyAsync(() -> {
            if (true) { throw new RuntimeException("main error!"); }
            return "hello world";
        })
        .thenApply(data -> 1)
        .handleAsync((data,e) -> {
            e.printStackTrace(); // 异常捕捉处理
            return data;
        });
System.out.println(first.join());
--------输出结果--------
java.util.concurrent.CompletionException: java.lang.RuntimeException: main error!
 ... 5 more
null

whenComplete-任务完成或者异常时运行action，有返回值

• whenComplete与handle的区别在于，它不参与返回结果的处理，把它当成监听器即可
• 即使异常被处理，在CompletableFuture外层，异常也会再次复现
• 使用whenCompleteAsync时，返回结果则需要考虑多线程操作问题，毕竟会出现两个线程同时操作一个结果

public CompletableFuture whenComplete(BiConsumer action) 
public CompletableFuture whenCompleteAsync(BiConsumer action) 
public CompletableFuture whenCompleteAsync(BiConsumer action,
  Executor executor)        

• 使用示例

CompletableFuture first = CompletableFuture
        .supplyAsync(() -> {
            if (true) {  throw new RuntimeException("main error!"); }
            return "hello world";
        })
        .thenApply(data -> new AtomicBoolean(false))
        .whenCompleteAsync((data,e) -> {
            //异常捕捉处理, 但是异常还是会在外层复现
            System.out.println(e.getMessage());
        });
first.join();
--------输出结果--------
java.lang.RuntimeException: main error!
Exception in thread "main" java.util.concurrent.CompletionException: java.lang.RuntimeException: main error!
 ... 5 more

6 、多个任务的简单组合

public static CompletableFuture allOf(CompletableFuture... cfs)
public static CompletableFuture