一 Future（jdk5引入）简介： Future接口是Java多线程Future模式的实现，可以来进行异步计算。
可以使用isDone方法检查计算是否完成，或者使用get阻塞住调用线程，直到计算完成返回结果，也可以使用cancel方法停止任务的执行。
简单测试 - 主题 ： Future模式可以理解成：我有一个任务，提交给了Future，Future替我完成这个任务，期间程序可以处理其他任务。
一段时间之后，主线程可以从Future那儿取出结果。
public class ThreadPoolTest {
private static Logger logger= LoggerFactory.getLogger(ThreadPoolTest.class);
public static void main(String[] args) {
ExecutorService threadPool = Executors.newSingleThreadExecutor();//创建单一线程
Future<String> future = threadPool.submit(new Callable<String>() {
@Override
public String call() throws Exception {
Thread.sleep(2000);
logger.info("out2-----");
return "Hello world";
}
});
try {
try {
BigDecimal bigDecimal=new BigDecimal(0);
logger.info("out1:"+bigDecimal.toString());
logger.info(future.get(3000,TimeUnit.MILLISECONDS));//Hello world
} catch (TimeoutException e) {
logger.error("timeout-exception",e);
}
} catch (InterruptedException e) {
logger.error("interrupted-exception",e);
} catch (ExecutionException e) {
logger.error("execution-exception",e);
}finally {
threadPool.shutdown();
}
}
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console输出：
11:17:17.558 [main] INFO tk.mybatis.springboot.util.thread.ThreadPoolTest - out1:0
Disconnected from the target VM, address: '127.0.0.1:54966', transport: 'socket'
11:17:19.556 [pool-1-thread-1] INFO tk.mybatis.springboot.util.thread.ThreadPoolTest - out2-----
11:17:19.557 [main] INFO tk.mybatis.springboot.util.thread.ThreadPoolTest - Hello world
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超时会报错：
SLF4J: Actual binding is of type [ch.qos.logback.classic.util.ContextSelectorStaticBinder]
10:44:07.413 [main] ERROR tk.mybatis.springboot.util.thread.ThreadPoolTest - timeout-exception
java.util.concurrent.TimeoutException: null
at java.util.concurrent.FutureTask.get(FutureTask.java:205)
at tk.mybatis.springboot.util.thread.ThreadPoolTest.main(ThreadPoolTest.java:30)
Process finished with exit code 0
总结：1 比起future.get()，推荐使用get (long timeout, TimeUnit unit) 方法，
设置了超时时间可以防止程序无限制的等待future的结果，可以进行异常处理逻辑。
2 虽然Future以及相关使用方法提供了异步执行任务的能力，但是对于结果的获取却是很不方便，
只能通过阻塞或者轮询的方式得到任务的结果。
阻塞的方式显然和我们的异步编程的初衷相违背，轮询的方式又会耗费无谓的CPU资源，而且也不能及时地得到计算结果，
为什么不能用观察者设计模式当计算结果完成及时通知监听者呢？
Future的接口很简单，只有五个方法。jdk8中源码如下：
package java.util.concurrent;
/**
* A {@code Future} represents the result of an asynchronous
* computation. Methods are provided to check if the computation is
* complete, to wait for its completion, and to retrieve the result of
* the computation. The result can only be retrieved using method
* {@code get} when the computation has completed, blocking if
* necessary until it is ready. Cancellation is performed by the
* {@code cancel} method. Additional methods are provided to
* determine if the task completed normally or was cancelled. Once a
* computation has completed, the computation cannot be cancelled.
* If you would like to use a {@code Future} for the sake
* of cancellability but not provide a usable result, you can
* declare types of the form {@code Future<?>} and
* return {@code null} as a result of the underlying task.
*
* <p>
* <b>Sample Usage</b> (Note that the following classes are all
* made-up.)
* <pre> {@code
* interface ArchiveSearcher { String search(String target); }
* class App {
* ExecutorService executor = ...
* ArchiveSearcher searcher = ...
* void showSearch(final String target)
* throws InterruptedException {
* Future<String> future
* = executor.submit(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* displayOtherThings(); // do other things while searching
* try {
* displayText(future.get()); // use future
* } catch (ExecutionException ex) { cleanup(); return; }
* }
* }}</pre>
*
* The {@link FutureTask} class is an implementation of {@code Future} that
* implements {@code Runnable}, and so may be executed by an {@code Executor}.
* For example, the above construction with {@code submit} could be replaced by:
* <pre> {@code
* FutureTask<String> future =
* new FutureTask<String>(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* executor.execute(future);}</pre>
*
* <p>Memory consistency effects: Actions taken by the asynchronous computation
* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
* actions following the corresponding {@code Future.get()} in another thread.
*
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
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看懂英文还是最直接的。
二. CompletableFuture介绍
2.1 Future模式的缺点
- Future虽然可以实现获取异步执行结果的需求，但是它没有提供通知的机制，我们无法得知Future什么时候完成。
- 要么使用阻塞，在future.get()的地方等待future返回的结果，这时又变成同步操作。要么使用isDone()轮询地判断Future是否完成，这样会耗费CPU的资源。
2.2 CompletableFuture介绍
public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
}
Netty、Guava分别扩展了Java 的 Future 接口，方便异步编程。
在Java 8中, 新增加了一个包含50个方法左右的类: CompletableFuture，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。
1 )主动完成计算
CompletableFuture类实现了CompletionStage和Future接口，所以你还是可以像以前一样通过阻塞或者轮询的方式获得结果，尽管这种方式不推荐使用。
valueIfAbsent
join返回计算的结果或者抛出一个unchecked异常(CompletionException)，它和get对抛出的异常的处理有些细微的区别
@Test
public void test(){
CompletableFuture<Integer> future = CompletableFuture.supplyAsync(()->{
int i=1/0;
return 100;
});
logger.info(String.format("join=%d", future.join()));
try {
logger.info(String.format("get=%s", future.get()));
} catch (InterruptedException e) {
logger.error("error=%s",e);
} catch (ExecutionException e) {
logger.error("error=%s",e);
}
}
尽管Future可以代表在另外的线程中执行的一段异步代码，但是你还是可以在本身线程中执行：
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class BasicMain {
private static Logger logger = LoggerFactory.getLogger(BasicMain.class);
public static CompletableFuture<Integer> compute() {
final CompletableFuture<Integer> future = new CompletableFuture<>();
return future;
}
public static void main(String[] args) throws Exception {
final CompletableFuture<Integer> f = compute();
class Client extends Thread {
CompletableFuture<Integer> f;
Client(String threadName, CompletableFuture<Integer> f) {
super(threadName);
this.f = f;
}
@Override
public void run() {
try {
logger.info(this.getName() + ": " + f.get());
} catch (InterruptedException e) {
logger.error(e.getMessage());
} catch (ExecutionException e) {
logger.error(e.getMessage());
}
}
}
new Client("Client1", f).start();
new Client("Client2", f).start();
logger.info("waiting....");
f.complete(100);
f.obtrudeValue(200);
//logger Client1: 200 Client2: 200 或者 Client1: 100 Client1: 100 或者 Client1: 200 Client2: 100 ，都有可能
// f.completeExceptionally(new Exception());}
}
说明：
可以看到我们并没有把f.complete(100) 放在另外的线程中去执行，但是在大部分情况下我们可能会用一个线程池去执行这些异步任 务。CompletableFuture.complete()、CompletableFuture.completeExceptionally只能被调用一次。
但是我们有两个后门方法可以重设这个值:obtrudeValue、obtrudeException，但是使用的时候要小心，
因为complete已经触发了客户端，有可能导致客户端会得到不期望的结果
2）创建CompletableFuture对象。
CompletableFuture的静态工厂方法
方法名 | 描述 |
runAsync(Runnable runnable) | 使用ForkJoinPool.commonPool()作为它的线程池执行异步代码。 |
runAsync(Runnable runnable, Executor executor) | 使用指定的thread pool执行异步代码。 |
supplyAsync(Supplier<U> supplier) | 使用ForkJoinPool.commonPool()作为它的线程池执行异步代码，异步操作有返回值 |
supplyAsync(Supplier<U> supplier, Executor executor) | 使用指定的thread pool执行异步代码，异步操作有返回值 |
eg： runAsync 和 supplyAsync 方法比较
区别是：
runAsync返回的CompletableFuture是没有返回值的
原因：
Runnable函数式接口类型为参数，所以CompletableFuture的计算结果为空
共同点：
因为方法的参数类型都是函数式接口，所以可以使用lambda表达式实现异步任务，比如：
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
return "Hello";
});
try {
logger.info("out = "+ future.get());//out = Hello
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
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CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
System.out.println("Hello");
});
try {
logger.info("out = "+ future.get());//out = null
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
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简单试验 - 主题：在两个线程里并行执行任务A和任务B，只要有一个任务完成了，就执行任务C
package tk.mybatis.springboot.util.thread;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.time.LocalTime;
import java.util.Random;
import java.util.concurrent.*;
/**
* 在两个线程里并行执行任务A和任务B，只要有一个任务完成了，就执行任务C
*
* 两种方法useFuture和useCompletableFuture相比：
*
* 首先，useCompletableFuture 比 useFuture 的代码简单。
* 在useFuture 里，既要自己照顾线程池的创建和销毁，还要负责对任务A和任务B的监控。
* 而useCompletableFuture，只需要用CompletableFuture的runAfterEither就完成了任务A、任务B和任务C之间的依赖关系的定义。
*
*/
public class CompletableFutureTest {
private static Logger logger = LoggerFactory.getLogger(CompletableFutureTest.class);
private static Random random = new Random();
/**
* useFuture test
* @throws InterruptedException
* @throws ExecutionException
*/
private static void useFuture() throws InterruptedException, ExecutionException {
logger.info("useFuture");
ExecutorService exector = Executors.newFixedThreadPool(3);
Future<Void> futureA = exector.submit(() -> work("A1"));
Future<Void> futureB = exector.submit(() -> work("B1"));
while (true) {
try {
futureA.get(1, TimeUnit.SECONDS);
break;
} catch (TimeoutException e) {
}
try {
futureB.get(1, TimeUnit.SECONDS);
break;
} catch (TimeoutException e) {
}
}
exector.submit(() -> work("C1")).get();
exector.shutdown();
}
private static void useCompletableFuture() throws InterruptedException, ExecutionException {
logger.info("useCompletableFuture");
CompletableFuture<Void> futureA = CompletableFuture.runAsync(() -> work("A2"));
CompletableFuture<Void> futureB = CompletableFuture.runAsync(() -> work("B2"));
logger.info("get="+futureA.runAfterEither(futureB, () -> work("C2")).get());
// 或者
// CompletableFuture.runAsync(() -> work("A2"))
// .runAfterEither(CompletableFuture.runAsync(() -> work("B2"))
// , () -> work("C2"))
// .get();
}
/**
* logger 输出
* @param name
* @return
*/
public static Void work(String name) {
logger.info(name + " starts at " + LocalTime.now());
try {
TimeUnit.SECONDS.sleep(random.nextInt(10));
} catch (InterruptedException e) {
}
logger.info(name + " ends at " + LocalTime.now());
return null;
}
/**
* 从useFuture的输出可以看出，
* 任务C1的开始并不是紧随着任务A1的完成，差了0.001秒，
* 原因是在方法1里，是对任务A1和任务B1都用get(1,TimeUnit.SECONDS)来询问他们的状态，
* 当其中一个任务先完成时，主线程可能正阻塞在另一个未完成任务的get上
*
* 而从useCompletableFuture完全不存在这样的问题，
* 任务C2的开始于任务A1的结束之间没有任何的时间差
*
* @param args
* @throws InterruptedException
* @throws ExecutionException
*/
public static void main(String[] args) throws InterruptedException, ExecutionException {
// useFuture();
//logger 输出
// B1 starts at 10:28:09.562
// A1 starts at 10:28:09.562
// B1 ends at 10:28:17.566
// C1 starts at 10:28:17.567
// A1 ends at 10:28:18.563
// C1 ends at 10:28:20.570
// TimeUnit.SECONDS.sleep(10);
useCompletableFuture();
//logger 输出
// A2 starts at 10:43:46.867
// B2 starts at 10:43:46.867
// B2 ends at 10:43:51.871
// A2 ends at 10:43:51.871
// C2 starts at 10:43:51.871
// C2 ends at 10:44:00.874
TimeUnit.SECONDS.sleep(10);//避免打印出的start - end 不全
}
}
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3）计算结果完成时的处理
当CompletableFuture的计算结果完成，或者抛出异常的时候，我们可以执行特定的Action。主要是下面的方法：
public CompletableFuture<T> whenComplete(BiConsumer<? super T, ? super Throwable> action) {
return uniWhenCompleteStage(null, action);
}
public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action) {
return uniWhenCompleteStage(asyncPool, action);
}
public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action, Executor executor) {
return uniWhenCompleteStage(screenExecutor(executor), action);
}
public CompletableFuture<T> exceptionally(Function<Throwable, ? extends T> fn) {
return uniExceptionallyStage(fn);
}
可以看到Action的类型是BiConsumer<? super T,? super Throwable>，它可以处理正常的计算结果，或者异常情况。
方法不以Async结尾，意味着Action使用相同的线程执行，
而Async可能会使用其它的线程去执行(如果使用相同的线程池，也可能会被同一个线程选中执行)。
注意这几个方法都会返回CompletableFuture，当Action执行完毕后它的结果返回原始的CompletableFuture的计算结果或者返回异常。
package tk.mybatis.springboot.util.thread;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Future;
public class MainTest {
private static Logger logger = LoggerFactory.getLogger(MainTest.class);
private static Random random = new Random();
private static long t = System.currentTimeMillis();
static Map<String,String> getMoreData() {
logger.info("start");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
logger.info("end ");
Map map=new HashMap();
map.put(random.nextInt(1000),random.nextInt(1000));
return map;
}
public static void main(String[] args) throws Exception {
CompletableFuture<Map<String,String>> future = CompletableFuture.supplyAsync(MainTest::getMoreData);
Future<Map<String,String>> f = future.whenComplete((v, e) -> {
logger.info("v="+v);
logger.info("e="+e);
});
logger.info("get="+f.get());
}
}
exceptionally方法返回一个新的CompletableFuture，
当原始的CompletableFuture抛出异常的时候，就会触发这个CompletableFuture的计算，调用function计算值，
？？）。
也就是这个exceptionally方法用来处理异常的情况。
CompletableFuture<Map<String,String>> futureException = future.exceptionally(new Function<Throwable, Map<String, String>>() {
@Override
public Map<String, String> apply(Throwable throwable) {
logger.error("error="+throwable.getMessage());
Map map=new HashMap();
map.put(random.nextInt(1000),random.nextInt(1000));
return map;
}
});
logger.info("get.exception="+futureException.get());
下面一组方法虽然也返回CompletableFuture对象，但是对象的值和原来的CompletableFuture计算的值不同。
当原先的CompletableFuture的值计算完成或者抛出异常的时候，会触发这个CompletableFuture对象的计算，结果由BiFunction参数计算而得。
因此这组方法兼有whenComplete和转换的两个功能。
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)
同样，不以Async结尾的方法由原来的线程计算，以Async结尾的方法由默认的线程池ForkJoinPool.commonPool()或者指定的线程池executor运行。
4）转换
CompletableFuture可以作为monad(单子)和functor。由于回调风格的实现，我们不必因为等待一个计算完成而阻塞着调用线程，
而是告诉CompletableFuture当计算完成的时候请执行某个function。而且我们还可以将这些操作串联起来，或者将CompletableFuture组合起来。
三 ThreadPoolTaskExecutor
包路径：org.springframework.scheduling.concurrent
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