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https://blog.youkuaiyun.com/weixin_43453386/article/details/88913809
java——》CompletableFuture
- 实现了CompletionStage 和 Future接口
- 简化了Java异步编程能力
- 任务执行完成之后执行“回调”(处理结果)
一、CompletableFuture使用实践
1、直接创建CompletableFuture
1)创建默认的CompletableFuture
默认的CompletableFuture是没有result的,
这时调用 future.get() 会一直 阻塞 ,直到有result或者出现异常
CompletableFuture<String> future = future = new CompletableFuture<>();
try {
future.get(1, TimeUnit.SECONDS);
} catch (Exception e) {
// todo
}
2)创建一个带result的CompletableFuture
CompletableFuture<String> future = CompletableFuture.completedFuture("result");
future.get();
3)给CompletableFuture填充一个result
CompletableFuture<String> future = future = new CompletableFuture<>();
future.complete("result");
4)给CompletableFuture填充一个异常
CompletableFuture<String> future = future = new CompletableFuture<>();
future.completeExceptionally(new RuntimeException("exception"));
try {
future.get();
} catch (Exception e) {
assert "exception".equals(e.getCause().getMessage());
}
2、通过方法runAsync创建CompletableFuture(没有返回值)
- 没有返回值
- Runnable任务
- 如果入参有executor,则使用executor来执行异步任务
- 如果入参没有executor,则默认使用ForkJoinPool.commonPool()作为执行异步任务的线程池
public static CompletableFuture<Void> runAsync(Runnable runnable)
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)
CompletableFuture.runAsync(() -> {
System.out.println("hello world");
}, executor);
3、通过方法supplyAsync创建CompletableFuture(有返回值)
- 有返回值
- 异步任务
- 如果入参有executor,则使用executor来执行异步任务
- 如果入参没有executor,则默认使用ForkJoinPool.commonPool()作为执行异步任务的线程池
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier)
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor)
CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
});
4、CompletableFuture的完成动作
1)whenComplete
- action是Action类型,既可以处理正常返回值也可以处理异常
- whenComplete:在任务执行完成后直接在当前线程内执行action动作
- whenCompleteAsync:交给其他线程执行action(如果是线程池,执行action的可能和之前执行异步任务的是同一个线程)
- 入参带executor的交给executor线程池来执行action动作
- 发生异常时,会在当前线程内执行exceptionally方法
public CompletableFuture<T> whenComplete(BiConsumer<? super T,? super Throwable> action)
public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T,? super Throwable> action)
public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T,? super Throwable> action, Executor executor)
public CompletableFuture<T> exceptionally(Function<Throwable,? extends T> fn)
CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
}).whenCompleteAsync((result, e) -> {
System.out.println(result + " " + e);
}).exceptionally((e) -> {
System.out.println("exception " + e);
return "exception";
});
2)handle
- 可以使用handle方法来执行CompletableFuture返回类型转换
- 处理正常返回值 和 异常(可以屏蔽异常,避免继续抛出)
- 返回新的CompletableFuture类型
- 对上一次CompletableFuture执行完的结果进行某些操作
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)
CompletableFuture<String> f1 = CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
});
CompletableFuture<Integer> f2 = f1.handle((r, e) -> {
System.out.println("handle");
return 1;
});
3)thenApply
- 可以使用thenApply方法来执行CompletableFuture返回类型转换
- 只有处理正常返回值,一旦有异常就会抛出
- 返回新的CompletableFuture类型
- 对上一次CompletableFuture执行完的结果进行某些操作
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)
CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
}).thenApply((r) -> {
System.out.println(r);
return "aaa";
}).thenApply((r) -> {
System.out.println(r);
return 1;
});
4)thenAccept
- 在CompletableFuture完成之后执行某些消费动作,而不返回新的CompletableFuture类型
- 只有处理正常返回值,一旦有异常就会抛出
- 对上一次CompletableFuture执行完的结果进行某些操作
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)
CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
}).thenAccept(r -> {
System.out.println(r);
}).thenAccept(r -> {
// 这里的r为Void(null)了
System.out.println(r);
})
5)thenAppeptBoth
- 同时对2个CompletableFuture执行结果执行某些操作
- 和handle/thenApply/thenAppep的流程是一样的
- 只不过thenAppeptBoth中包含了另一个CompletableFuture对象(注意,这里另一个CompletableFuture对象的执行可并不是上一个CompletableFuture执行结束才开始执行的)
- 无返回值
- thenAcceptBoth 和 runAfterBoth ,是2个CompletableFuture都计算完成
public <U> CompletableFuture<Void> thenAcceptBoth(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action)
public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action)
public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action, Executor executor)
public CompletableFuture<Void> runAfterBoth(CompletionStage<?> other, Runnable action)
CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
}).thenAcceptBoth(CompletableFuture.completedFuture("result2"), (r1, r2) -> {
System.out.println(r1 + "-" + r2);
});
6)thenCombine
- 同时对2个CompletableFuture执行结果执行某些操作
- 和handle/thenApply/thenAppep的流程是一样的
- 只不过thenAppeptBoth中包含了另一个CompletableFuture对象(注意,这里另一个CompletableFuture对象的执行可并不是上一个CompletableFuture执行结束才开始执行的)
- 有返回值
public <U,V> CompletableFuture<V> thenCombine(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn)
public <U,V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn)
public <U,V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn, Executor executor)
CompletableFuture.supplyAsync(() -> {
System.out.println("hello world");
return "result";
}).thenCombine(CompletableFuture.completedFuture("result2"), (r1, r2) -> {
System.out.println(r1 + "-" + r2);
return r1 + "-" + r2;
});
7)acceptEither
- 当任意一个CompletableFuture计算完成的时候就会执行
public CompletableFuture<Void> acceptEither(CompletionStage<? extends T> other, Consumer<? super T> action)
public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other, Consumer<? super T> action)
public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other, Consumer<? super T> action, Executor executor)
public <U> CompletableFuture<U> applyToEither(CompletionStage<? extends T> other, Function<? super T,U> fn)
public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T,U> fn)
public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T,U> fn, Executor executor)
- allOf :在多个CompletableFuture都计算完成后执行某个动作
- anyOf:在多个CompletableFuture中的一个计算完成后执行某个动作
public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs)
public static CompletableFuture<Object> anyOf(CompletableFuture<?>... cfs)
二、CompletableFuture实现机制
CompletableFuture实现流程:
- 执行任务
- 添加任务完成之后的动作(回调方法)
- 执行回调
CompletableFuture.supplyAsync(() -> {
// callable任务
System.out.println("hello world");
return "result";
}).thenApply(r -> {
// 任务完成之后的动作(回调方法),类似于ThreadPoolExecutor.afterExecute方法
System.out.println(r);
return r;
});
1、执行任务:AsyncSupply.run
public void run() {
CompletableFuture<T> d; Supplier<T> f;
// dep是当前CompletableFuture,fn是任务执行逻辑
if ((d = dep) != null && (f = fn) != null) {
dep = null; fn = null;
if (d.result == null) {
try {
// 1 任务执行 & result cas设置
d.completeValue(f.get());
} catch (Throwable ex) {
// 1.1 result cas异常设置
d.completeThrowable(ex);
}
}
// 2 任务完成,可能涉及到回调的执行
d.postComplete();
}
}
2、添加回调:thenApply
public <U> CompletableFuture<U> thenApply(
Function<? super T,? extends U> fn) {
return uniApplyStage(null, fn);
}
private <V> CompletableFuture<V> uniApplyStage(
Executor e, Function<? super T,? extends V> f) {
if (f == null) throw new NullPointerException();
CompletableFuture<V> d = new CompletableFuture<V>();
if (e != null || !d.uniApply(this, f, null)) {
// 当上一个CompletableFuture未完成时,将该CompletableFuture添加
// 到上一个CompletableFuture的statck中
UniApply<T,V> c = new UniApply<T,V>(e, d, this, f);
push(c);
c.tryFire(SYNC);
}
return d;
}
3、执行回调:CompletableFuture.postComplete
final void postComplete() {
/*
* On each step, variable f holds current dependents to pop
* and run. It is extended along only one path at a time,
* pushing others to avoid unbounded recursion.
*/
CompletableFuture<?> f = this; Completion h;
while ((h = f.stack) != null ||
(f != this && (h = (f = this).stack) != null)) {
CompletableFuture<?> d; Completion t;
// cas设置h.next到当前CompletableFuture.statck
if (f.casStack(h, t = h.next)) {
if (t != null) {
if (f != this) {
pushStack(h);
continue;
}
h.next = null; // detach
}
f = (d = h.tryFire(NESTED)) == null ? this : d;
}
}
}
// UniAccept
final CompletableFuture<Void> tryFire(int mode) {
CompletableFuture<Void> d; CompletableFuture<T> a;
if ((d = dep) == null ||
!d.uniAccept(a = src, fn, mode > 0 ? null : this)) // 执行回调
return null;
dep = null; src = null; fn = null;
// 返回当前CompletableFuture 或者 递归调用postComplete
return d.postFire(a, mode);
}