技术标签: spring spring boot java Java基础 多线程
在工作中,常常会调用多个服务或者方法去获取不同的数据,如果传统做法就是串行一个个获取,然后封装返回。我们可以尝试使用CompletableFuture,将多个操作交给异步线程执行,然后主线程等待最长任务完成,将所有结果一并返回即可。
当我们得到包含结果的Future时,我们可以使用get方法等待线程完成并获取返回值,但我们都知道future.get()是阻塞的方法,会一直等到线程执行完毕拿到返回值。我们可以看到FutureTask中的get方法,就是循环代码直到线程执行完成返回。
/**
* Awaits completion or aborts on interrupt or timeout.
*
* @param timed true if use timed waits
* @param nanos time to wait, if timed
* @return state upon completion
*/
private int awaitDone(boolean timed, long nanos) throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//循环 省略代码
...
}
}
我们考虑一种场景,如果我们执行完该异步任务1需要拿到返回值,然后使用该返回值执行其他异步调用2,那么我们就需要在主线程阻塞等待异步任务1完成,然后交由执行异步任务2,然后继续阻塞等待任务2的返回。这样不仅阻塞主线程,而且性能差。
什么是CompletableFuture:CompletableFuture结合了Future的优点,提供了非常强大的Future的扩展功能,可以帮助我们简化异步编程的复杂性,提供了函数式编程的能力,可以通过回调的方式处理计算结果,并且提供了转换和组合CompletableFuture的方法。
函数式编程:使用Functional Interface作为参数,可使用lambda表达式建议实现,编程便捷,之前有讲解过。
// 包括取消cancel、显示调用completeExceptionally、中断。
public boolean isCompletedExceptionally() {
Object r;
return ((r = result) instanceof AltResult) && r != NIL;
}
public boolean isCancelled() {
Object r;
// 判断异常是否是CancellationException
return ((r = result) instanceof AltResult) &&
(((AltResult)r).ex instanceof CancellationException);
}
public boolean isDone() {
return result != null;
}
public T get() throws InterruptedException, ExecutionException {
Object r;
return reportGet((r = result) == null ? waitingGet(true) : r);
}
public T join() {
Object r;
return reportJoin((r = result) == null ? waitingGet(false) : r);
}
join() 与get() 区别在于join() 返回计算的结果或者抛出一个unchecked异常(CompletionException),而get() 返回一个具体的异常.
CompletableFuture有无参数的构造方法,这个时候创建的是未完成的CompletableFuture。使用get会一直阻塞主线程。
所以我们一般使用静态方法来创建实例。
// 无入参有返回值
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier);
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor);
// 无入参无返回值,简单的执行
public static CompletableFuture<Void> runAsync(Runnable runnable);
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor);
我们注意到每种方法都有一个重构的方法。Executor参数可以手动指定线程池,否则默认ForkJoinPool.commonPool()系统级公共线程池。
【注意】:默认的commonPool的这些线程都是守护线程。我们在编程的时候需要谨慎使用守护线程,如果将我们普通的用户线程设置成守护线程,当我们的程序主线程结束,JVM中不存在其余用户线程,那么CompletableFuture的守护线程会直接退出,造成任务无法完成的问题!!
我们后续讲解的api一般会有三种相似的方法。我就只演示第三种。
- xxx(method);
- xxxAsync(method);
- xxxAsync(method, executor)
三种的区别就是第一种同步执行由主线程执行,第二种异步交由默认线程池,第三种异步交由创建的线程池。
现在就将CompletableFuture来试着使用。先从基本的构建开始。
/**
* 1.run + runAsync + supply + supplyAsync
*/
/** 1.异步运行交由默认线程池(forkjoinpool)无入参无返回值run
* 2.异步运行交由创建线程池(threadPoolExecutor)无入参无返回值run
*/
CompletableFuture<Void> run = CompletableFuture.runAsync(() ->
System.out.println("completablefuture runs asynchronously"));
CompletableFuture<Void> runCustomize = CompletableFuture.runAsync(() ->
System.out.println("completablefuture runs asynchronously in customize threadPool"
), threadPoolExecutor);
//================下面是supply===================
/** 1.异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
* 2.异步运行交由自己创建的线程池 无入参有返回值supply
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously");
return "success";
});
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success";
}, threadPoolExecutor);
result
completablefuture runs asynchronously
completablefuture runs asynchronously in customize threadPool
completablefuture supplys asynchronously
completablefuture supplys asynchronously in customize threadPool
前两种的CompletableFuture是没有值的,所以当后续链式调用想使用的时候入参是null的。
考虑当我们在CompletableFuture执行结束的时候,希望能够得到执行结果、或者异常,然后对结果或者异常做进一步处理。那么我们就需要使用到whenComplete。
/**
* 入参是BiConsumer,第一个参数是上一步结果、第二个是上一步执行的异常
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
int a = 1/0;
System.out.println("completablefuture supplys asynchronously");
return "success";
});
CompletableFuture<String> complete = supply.whenCompleteAsync((result, throwable) -> {
System.out.println("whenComplete: " + result + " throws " + throwable);
});
result:
whenComplete: null throws java.util.concurrent.CompletionException: java.lang.ArithmeticException: / by zero
handle和whenComplete入参是一样的,但是它可以在执行完成后返回执行结果,而whenComplete只能处理无法返回。
public <U> CompletableFuture<U> handle(BiFunction<? super T,Throwable,? extends U> fn)
public <U> CompletableFuture<U> handleAsync(BiFunction<? super T,Throwable,? extends U> fn)
public <U> CompletableFuture<U> handleAsync(BiFunction<? super T,Throwable,? extends U> fn, Executor executor)
example:
/**
* 入参是BiFunction 第一个参数是上一步结果、第二个是上一步执行的异常
* 返回值可以是任何类型
*/
CompletableFuture<String> handleAsyncCustomize = supply.handleAsync((a, throwable) -> {
System.out.println("handleAsyncCustomize: " + a + " throws " + throwable);
return "handleAsync success";
}, threadPoolExecutor);
System.out.println(handleAsyncCustomize.get());
result:
handleAsyncCustomize: success throws null
handleAsync success
thenApply又和handle很类似,有入参也有返回值的,但是他只有一个入参,无法处理上一异步任务异常的情况。如果发生异常get的时候会报错。
public <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn)
public <U> CompletableFuture<U> thenApplyAsync(Function<? super T,? extends U> fn)
public <U> CompletableFuture<U> thenApplyAsync(Function<? super T,? extends U> fn, Executor executor)
/**
* Function 入参是第一个异步任务执行结果、出参是返回值
* 若异步任务1异常 则2无法执行 get会报错
*/
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
int a = 1/0;
return "success";
}, threadPoolExecutor);
CompletableFuture<String> applyAsyncCustomize = supplyCustomize.thenApplyAsync(a -> {
System.out.println("applyAsyncCustomize " + a);
return "success";
}, threadPoolExecutor);
System.out.println(applyAsyncCustomize.get());
result:
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1908)
at CompletableFutureTask.main.main(main.java:48)
Caused by: java.lang.ArithmeticException: / by zero
at CompletableFutureTask.main.lambda$main$3(main.java:40)
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1604)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
thenAccept是有入参无返回值,如果继续链式调用那么下一个异步任务将会得到null值。
public CompletableFuture<Void> thenAccept(Consumer<? super T> action)
public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action)
public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action, Executor executor)
example:
/**
* Consumer第一个入参是上一步返回结果,若没返回结果则为null
* 无返回值
*/
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success";
}, threadPoolExecutor);
CompletableFuture<Void> applyAsyncCustomize = supplyCustomize.thenAcceptAsync(a ->{
System.out.println("accept " + a);
}, threadPoolExecutor);
System.out.println(applyAsyncCustomize.get());
result:
completablefuture supplys asynchronously in customize threadPool
accept success
null
和thenCombine有所不同,thenCombine是组合两个CompletableFuture返回结果进行异步处理,而thenCompose则是根据第一个返回结果,封装成新的CompletableFuture返回
example:
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously");
return "success";
});
CompletableFuture<String> future = supply.thenComposeAsync(a -> {
String name = a + " or fail";
return CompletableFuture.supplyAsync(() -> {
return name;
});
});
System.out.println(future.get());
result:
completablefuture supplys asynchronously in customize threadPool
success or fail
当发生异常时候的处理,注意异常后返回值类型需要和发生异常的CF返回值类型一致,相当于一种服务降级的思想。
example:
/**
* 发生异常时候,返回0
*/
CompletableFuture<Integer> supplyCustomize = CompletableFuture.supplyAsync(() -> {
int a = 1/0;
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return 2;
}, threadPoolExecutor).exceptionally(a->{
System.out.println(a);
return 0;
});
System.out.println(supplyCustomize.get());
result:
completablefuture runs asynchronously in customize threadPool
java.util.concurrent.CompletionException: java.lang.ArithmeticException: / by zero
0
thenCombine是组合式任务,上面的CompletableFuture使用是链式完成,当完成第一个时候,根据第一个执行结果进行下一步异步调用,而组合式异步,可以做到两个异步任务完全独立,只有当他们都完成时候才会继续执行。
example:
/**
* BiFunction入参,两个CF的返回结果作为入参,有返回值
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously");
return "success";
});
// 异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success";
}, threadPoolExecutor);
CompletableFuture<String> future = supply.thenCombine(supplyCustomize, (a, b) -> {
return "thenCombine result: " + a + "-" + b;
});
System.out.println(future.get());
result:
completablefuture supplys asynchronously
completablefuture supplys asynchronously in customize threadPool
thenCombine result: success-success
thenAcceptBoth和thenCombine区别就是没有返回值,将两个CF返回值进行处理,没有返回值
example:
/**
* 接受CF、BiConsumer,无返回值
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously");
return "success";
});
// 异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success";
}, threadPoolExecutor);
CompletableFuture<Void> bothAsync = supplyCustomize.thenAcceptBothAsync(supply, (a, b) -> {
System.out.println("thenAcceptBoth result " + a + "-" + b);
}, threadPoolExecutor);
result:
completablefuture supplys asynchronously
completablefuture supplys asynchronously in customize threadPool
thenAcceptBoth result success-success
null
无入参无返回值,只会在前面两者都运行完成后才会执行runAfterBoth的方法,我们可以在任一CF模拟长时间运行进行测试。
example:
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously");
return "success";
});
// 异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success";
}, threadPoolExecutor);
CompletableFuture<Void> bothAsync = supply.runAfterBothAsync(supplyCustomize, () -> {
System.out.println("run After Both Async");
});
result:
completablefuture supplys asynchronously
completablefuture supplys asynchronously in customize threadPool
run After Both Async
和thenAcceptBoth对应,两个CompletableFuture任一执行完成,就会继续下一步异步任务
example:
/**
* 任务一耗时长,所以当任务二完成就会执行acceptEitherAsync
* 参数 Consumer 入参为执行完成任务返回值
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("completablefuture supplys asynchronously");
return "success1";
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success2";
}, threadPoolExecutor);
CompletableFuture<Void> future = supply.acceptEitherAsync(supplyCustomize, a -> {
System.out.println("acceptEither result " + a);
});
result:
completablefuture supplys asynchronously in customize threadPool
acceptEither result success2
completablefuture supplys asynchronously
对应thenCombine,两个CompletableFuture任一执行完成,就会继续下一步异步任务
example:
/**
* 参数为Function,有入参也有返回值
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("completablefuture supplys asynchronously");
return "success1";
});
// 异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success2";
}, threadPoolExecutor);
CompletableFuture<String> future = supply.applyToEither(supplyCustomize, a -> {
System.out.println("acceptEither result " + a);
return "appleTo " + a;
});
对应runAfterBoth,两个CompletableFuture任一执行完成,就会继续下一步异步任务
/**
* runnable 无入参无返回值
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("completablefuture supplys asynchronously");
return "success1";
});
// 异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success2";
}, threadPoolExecutor);
CompletableFuture<Void> future = supply.runAfterEitherAsync(supplyCustomize, () -> {
System.out.println("runAfterEither result ");
});
所有完成get方法才能获得返回值,返回值是null,每个任务返回值需要调用每个CompletableFuture。
example:
/**
* 当所有任务完成后,返回null,具体每个任务返回值,需要调用具体
* 异步任务获取
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("completablefuture supplys asynchronously");
return "success1";
});
CompletableFuture<String> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return "success2";
}, threadPoolExecutor);
CompletableFuture<Void> allOf = CompletableFuture.allOf(supply, supplyCustomize);
allOf.get();
System.out.println(supplyCustomize.get() + "=>" + supply.get());
result:
completablefuture supplys asynchronously in customize threadPool
completablefuture supplys asynchronously
success2==success1
/**
* 任一先执行完毕的结果将会先返回
*/
CompletableFuture<String> supply = CompletableFuture.supplyAsync(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("completablefuture supplys asynchronously");
return "success1";
});
// 异步运行交由默认线程池(forkjoinpool)无入参有返回值supply
CompletableFuture<Integer> supplyCustomize = CompletableFuture.supplyAsync(() -> {
System.out.println("completablefuture supplys asynchronously in customize threadPool");
return 2;
}, threadPoolExecutor);
CompletableFuture<Object> anyOf = CompletableFuture.anyOf(supply, supplyCustomize);
System.out.println(anyOf.get());
result:先执行完所以先返回2
completablefuture supplys asynchronously in customize threadPool
2
completablefuture supplys asynchronously
CompletableFuture丰富的异步调用方法,可以帮助我们避免使用主线程来将多个异步任务连结,提高程序性能,加快响应速度。同时优秀的异常处理机制,在异步调用出错过程中也能完美解决。
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文章浏览阅读178次。项目运行环境配置:Jdk1.8 + Tomcat7.0 + Mysql + HBuilderX(Webstorm也行)+ Eclispe(IntelliJ IDEA,Eclispe,MyEclispe,Sts都支持)。项目技术:Springboot + mybatis + Maven +mysql5.7或8.0+html+css+js等等组成,B/S模式 + Maven管理等等。环境需要1.运行环境:最好是java jdk 1.8,我们在这个平台上运行的。其他版本理论上也可以。_基于java技术的停车场管理系统实现与设计
文章浏览阅读3.5k次。前言对于MediaPlayer播放器的源码分析内容相对来说比较多,会从Java-&amp;gt;Jni-&amp;gt;C/C++慢慢分析,后面会慢慢更新。另外,博客只作为自己学习记录的一种方式,对于其他的不过多的评论。MediaPlayerDemopublic class MainActivity extends AppCompatActivity implements SurfaceHolder.Cal..._android多媒体播放源码分析 时序图
文章浏览阅读2.4k次,点赞41次,收藏13次。java 数据结构与算法 ——快速排序法_快速排序法