CompletableFuture是java8引入的一个很实用的特性，可以视为Future的升级版本，以下几个示例可以说明其主要用法（注：示例来自《java8实战》一书第11章）
一、引子：化同步为异步
为了方便描述，假设"查询电商报价"的场景：有一个商家Shop类，对外提供价格查询的服务getPrice
import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Future;
public class Shop {
public String name;
private Random random = new Random();
public Shop(String name) {
this.name = name;
}
/**
* 计算价格
*
* @param product
* @return
*/
private double calculatePrice(String product) {
delay();
return random.nextDouble() * product.charAt(0) + product.charAt(1);
}
/**
* 模拟计算价格的耗时
*/
private static void delay() {
try {
Thread.sleep(1000L);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
/**
* 对外提供的报价服务方法
*
* @param product
* @return
*/
public double getPrice(String product) {
return calculatePrice(product);
}
}
平台可以调用getPrice方法获取某个商家的报价：
public static void main(String[] args) {
testSyncGetPrice();
}
private static void doSomethingElse() {
System.out.println("do something else");
}
public static void testSyncGetPrice() {
Shop shop = new Shop("BestShop");
long start = System.currentTimeMillis();
System.out.printf("Price is %.2f\n", shop.getPrice("my favorite product"));
doSomethingElse();
System.out.println("(SyncGetPrice) Invocation returned after : " + (System.currentTimeMillis() - start) + " ms\n");
}
显然，这是1个同步调用，在shop.getPrice()方法执行完前，后面的doSomethingElse()只能等着，输出结果如下：
Price is 222.01 do something else (SyncGetPrice) Invocation returned after : 1015 ms
为了消除同步阻塞，可以借用Future将同步的getPrice方法调用，转换成异步。
public Future<Double> getPriceAsync(String product) {
Future<Double> submit = Executors.newFixedThreadPool(1).submit(() -> calculatePrice(product));
return submit;
}
上面的submit方法，最终调用的是java.util.concurrent.AbstractExecutorService#submit(java.util.concurrent.Callable<T>)
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
如果继续追下去的话，execute方法，又是调用的java.util.concurrent.ThreadPoolExecutor#execute方法，创建一个线程来异步执行。将同步转换成异步后，doSomethingElse方法，在getPriceAsync执行期间，就能并发执行了。
public static void doSomethingElse() {
System.out.println("do something else");
}
public static void testAsyncGetPrice() {
Shop shop = new Shop("BestShop");
long start = System.currentTimeMillis();
Future<Double> futurePrice = shop.getPriceAsync("my favorite product");
doSomethingElse();
try {
Double price = futurePrice.get();
System.out.printf("Price is %.2f\n", price);
} catch (Exception e) {
throw new RuntimeException(e);
}
System.out.println("(AsyncGetPrice) Invocation returned after : " + (System.currentTimeMillis() - start) + " ms\n");
}
public static void main(String[] args) {
testAsyncGetPrice();
}
输出结果：
do something else Price is 201.69 (AsyncGetPrice) Invocation returned after : 1111 ms
二、同步转换成异步的其它方式
CompletableFuture出现后，"同步调用"转换成"异步调用"的方式，有了新的选择：
public Future<Double> getPriceAsync1(String product) {
CompletableFuture<Double> futurePrice = new CompletableFuture<Double>();
new Thread(() -> {
try {
double price = calculatePrice(product);
futurePrice.complete(price);
} catch (Exception e) {
futurePrice.completeExceptionally(e);
}
}).start();
return futurePrice;
}
public Future<Double> getPriceAsync2(String product) {
return CompletableFuture.supplyAsync(() -> calculatePrice(product));
}
上面这2种方法效果等价，显然第2种supplyAsync的写法更简洁。需要说明的是：CompletableFuture内部其实也是使用线程池来处理的，只不过这个线程池的类型默认是ForkJoinPool，这一点可以从java.util.concurrent.CompletableFuture#asyncPool源码看出来：
/**
* Default executor -- ForkJoinPool.commonPool() unless it cannot
* support parallelism.
*/
private static final Executor asyncPool = useCommonPool ?
ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();
三、CompletableFuture中使用自定义线程池
如果需要查询报价的商家有很多，比如：6个，逐一同步调用getPrice方法，时长大约就是6个商家的总时长累加
static List<Shop> shops = Arrays.asList(
new Shop("1-shop"),
new Shop("2-shop"),
new Shop("3-shop"),
new Shop("4-shop"),
new Shop("5-shop"),
new Shop("6-shop")
);
public static void main(String[] args) {
testFindPrices();
}
public static List<String> findPrices(String product) {
return shops.stream()
.map(shop -> String.format("%s price is %.2f", shop.name, shop.getPrice(product)))
.collect(Collectors.toList());
}
输出：
[1-shop price is 180.36, 2-shop price is 206.13, 3-shop price is 205.49, 4-shop price is 184.62, 5-shop price is 222.73, 6-shop price is 143.19] do something else (findPrices-Stream) Invocation returned after : 6102 ms
这显然太慢了，要知道现代计算机都是多核cpu体系，很容易想到把stream换成parallelStream，可以充分发挥多核优势：
public static List<String> findPricesParallel(String product) {
return shops.parallelStream()
.map(shop -> String.format("%s price is %.2f", shop.name, shop.getPrice(product)))
.collect(Collectors.toList());
}
还是刚才的测试场景，这时输出结果类似下面这样：（注：测试机器为mac 4核笔记本）
[1-shop price is 137.42, 2-shop price is 168.93, 3-shop price is 182.89, 4-shop price is 154.60, 5-shop price is 192.70, 6-shop price is 179.06] do something else (findPrices-parallelStream) Invocation returned after : 2102 ms
比刚才好多了，耗时从6s缩短到2s，但仔细想一想：6个商家的getPrice处理，分摊到4个核上，还是有2个核会出现阻塞(即：平均1个核并行处理1个task，6-4=2，仍然有2个task要排队)。
如果换成用CompletableFuture默认的ForkJoinPool呢，性能会不会好一些？
public static List<String> findPricesFuture() {
List<CompletableFuture<String>> priceFutures = shops.parallelStream()
.map(shop -> CompletableFuture.supplyAsync(() ->
String.format("%s price is %.2f", shop.name, shop.getPrice("myPhone27"))))
.collect(Collectors.toList());
return priceFutures.parallelStream().map(CompletableFuture::join).collect(Collectors.toList());
}
public static void testFindPricesCompletableFuture() {
long start = System.currentTimeMillis();
System.out.printf(findPricesFuture().toString() + "\n");
doSomethingElse();
System.out.println("(findPrices-CompletableFuture) Invocation returned after : " + (System.currentTimeMillis() - start) + " ms\n");
}
输出结果（注：上面代码中的parallelStream换成stream，下面的输出结果也差不多）
[1-shop price is 168.57, 2-shop price is 159.43, 3-shop price is 200.08, 4-shop price is 165.64, 5-shop price is 195.11, 6-shop price is 206.83] do something else (findPrices-CompletableFuture) Invocation returned after : 2092 ms
从结果上看，使用CompletableFuture与使用仅使用parallelStream的耗时差不多，并没有性能上的提升。原因在于默认的ForkJoinPool，其默认线程数也是跟CPU核数相关的。在这个场景中，我们至少要6个线程（即：shops.size()），才能让6个商家的getPrice并发处理。按这个思路，我们可以自定义一个线程池，然后传入supplyAsync方法中：
private static final Executor executor =
Executors.newFixedThreadPool(Math.min(shops.size(), 100),
new ThreadFactory() {
public Thread newThread(Runnable r) {
Thread t = new Thread(r);
t.setDaemon(true);
return t;
}
});
public static List<String> findPricesFutureWithExecutor() {
List<CompletableFuture<String>> priceFutures = shops.stream()
.map(shop -> CompletableFuture.supplyAsync(() ->
String.format("%s price is %.2f", shop.name, shop.getPrice("myPhone27")), executor))
.collect(Collectors.toList());
return priceFutures.stream().map(CompletableFuture::join).collect(Collectors.toList());
}
public static void testFindPricesExecutor() {
long start = System.currentTimeMillis();
System.out.printf(findPricesFutureWithExecutor().toString() + "\n");
doSomethingElse();
System.out.println("(findPrices-FutureWithExecutor) Invocation returned after : " + (System.currentTimeMillis() - start) + " ms\n");
}
public static void main(String[] args) {
testFindPricesExecutor();
}
输出结果如下：
[1-shop price is 177.26, 2-shop price is 227.09, 3-shop price is 179.98, 4-shop price is 127.19, 5-shop price is 208.93, 6-shop price is 229.91] do something else (findPrices-FutureWithExecutor) Invocation returned after : 1121 ms
从耗时上看，仅相当于单个商家getPrice的耗时，已经达到最佳效果。
四、多个异步操作组合
前面提到的商家报价场景，我们再加点料，引入“打折”功能。先把shop调整下：
package future;
import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Future;
public class Shop {
public String name;
private static Random random = new Random();
public Shop(String name) {
this.name = name;
}
private double calculatePrice(String product) {
randomDelay();
return random.nextDouble() * product.charAt(0) + product.charAt(1);
}
public static void randomDelay() {
int delay = 500 + random.nextInt(2000);
try {
Thread.sleep(delay);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
/**
* 查询“(原始)价格”及"对应的折扣"
*
* @param product
* @return
*/
public String getPriceWithDiscount(String product) {
double price = calculatePrice(product);
Discount.Code code = Discount.Code.values()[random.nextInt(Discount.Code.values().length)];
String result = String.format("%s:%.2f:%s", name, price, code);
System.out.println(result);
return result;
}
}
主要有2处改动：
1 是delay方法引入了随机数，模拟不同商家查询价格时，有着不同的处理时间，显得更真实。
2 是getPriceWithDiscount方法，返回的价格不再是1个double，而是类似下面这样的字符串
1-shop:212.78:NONE 2-shop:182.22:DIAMOND 3-shop:148.91:PLATINUM 4-shop:203.78:SILVER 5-shop:152.75:DIAMOND 6-shop:212.43:NONE
同时包括了原始的价格，以及打折等级（无折扣、白银等级、钻石等级...之类），这里有一个Discount类，代码如下：
package future;
import java.math.BigDecimal;
public class Discount {
/**
* 打折类型
*/
public enum Code {
NONE(0), SILVER(5), GOLD(10), PLATINUM(15), DIAMOND(20);
/**
* 折扣百分比
*/
private final int percentage;
Code(int percentage) {
this.percentage = percentage;
}
}
/**
* 计算折扣后的价格
* @param price
* @param code
* @return
*/
private static double apply(double price, Code code) {
Shop.randomDelay();
return format(price * (100 - code.percentage) / 100);
}
private static double format(double d) {
BigDecimal decimal = new BigDecimal(d);
return decimal.setScale(2, BigDecimal.ROUND_HALF_DOWN).doubleValue();
}
/**
* 应用折扣，输出最后的处理结果
*
* @param quote
* @return
*/
public static String applyDiscount(Quote quote) {
return quote.shopName + " price is " + apply(quote.price, quote.discountCode);
}
}
apply模拟了计算折扣价时，需要一定的耗时randomDelay()，而getPriceWithDiscount返回的字符串，还需要有1个Quota类专门解析其中的原始价格以及折扣等级
/**
* 带折扣的报价
*/
public class Quote {
public final String shopName;
public final double price;
public final Discount.Code discountCode;
public Quote(String shopName, double price, Discount.Code code) {
this.shopName = shopName;
this.price = price;
this.discountCode = code;
}
/**
* 解析价格结果
*
* @param s
* @return
*/
public static Quote parse(String s) {
String[] split = s.split(":");
String shopName = split[0];
double price = Double.parseDouble(split[1]);
Discount.Code discountCode = Discount.Code.valueOf(split[2]);
return new Quote(shopName, price, discountCode);
}
}
引入折扣功能后，原来的“查询商家价格”，可分解成3个步骤：
1. 先调用shop.getPriceWithDiscount 返回“原始价格及折扣等级”字符串
2. 解析1中返回的字符串，将price与discount信息提取出来，并最终封装成Quota对象
3. 调用Discount的applyDiscount，返回最终打折后的价格信息
而且，上面的步骤，3依赖2的完成，2依赖1的完成，用标准写出来的话，大致是下面这个样子：
public static List<String> findDiscountPrices() {
return shops.stream()
.map(shop -> shop.getPriceWithDiscount("myPhone27"))
.map(Quote::parse)
.map(Discount::applyDiscount)
.collect(Collectors.toList());
}
public static void testFindDiscountPrices() {
long start = System.currentTimeMillis();
System.out.printf(findDiscountPrices().toString() + "\n");
doSomethingElse();
System.out.println("(findDiscountPrices-stream) Invocation returned after : " + (System.currentTimeMillis() - start) + " ms\n");
}
public static void main(String[] args) {
testFindDiscountPrices();
}
这是同步的调用方式，可想而知，最终耗时会很大：
1-shop:157.77:DIAMOND 2-shop:138.03:DIAMOND 3-shop:204.60:DIAMOND 4-shop:202.52:NONE 5-shop:155.14:GOLD 6-shop:224.15:SILVER [1-shop price is 126.22, 2-shop price is 110.42, 3-shop price is 163.68, 4-shop price is 202.52, 5-shop price is 139.63, 6-shop price is 212.94] do something else (findDiscountPrices-stream) Invocation returned after : 16449 ms
使用CompletableFuture，可以把1-2-3 这3个步骤都转换成异步，且保证相互之间的依赖关系，代码如下：
public static List<String> findDiscountPricesFuture() {
List<CompletableFuture<String>> list = shops.stream()
.map(shop -> CompletableFuture.supplyAsync(() -> shop.getPriceWithDiscount("myPhone27"), executor))
.map(f -> f.thenApply(Quote::parse))
.map(f -> f.thenCompose(quote -> CompletableFuture.supplyAsync(() -> Discount.applyDiscount(quote), executor)))
.collect(Collectors.toList());
return list.stream().map(CompletableFuture::join).collect(Collectors.toList());
}
输出结果如下：
从结果上看，确实已经是异步了（1个线程处理1个商家的getPrice及Discount计算)，整体耗时也大幅下降。但是有一个细节问题，6个商家的最终结果（即：最后的[...]列表输出），是等所有异步操作都执行完，1次性输出的，这在实际应用中，意味着，最终买家能多快看到价格输出，取决于最慢的那个商家，这是不能接受的，理想情况下，应该是哪个商家的服务快，能先计算出结果 ，就应该第1时间展示这家店的价格。
修正后的代码如下：
public static void findDiscountPricesFuture() {
long start = System.currentTimeMillis();
CompletableFuture[] futureArray = shops.stream()
.map(shop -> CompletableFuture.supplyAsync(() -> shop.getPriceWithDiscount("myPhone27"), executor))
.map(f -> f.thenApply(Quote::parse))
.map(f -> f.thenCompose(quote -> CompletableFuture.supplyAsync(() -> Discount.applyDiscount(quote), executor)))
.map(f -> f.thenAccept(s -> System.out.println(s + " (done in " + (System.currentTimeMillis() - start) + " ms)")))
.toArray(size -> new CompletableFuture[size]);
CompletableFuture.allOf(futureArray).join();
}
解释：主要是利用了CompletableFuture.allOf()方法，该方法会把数组结果，按完成时间快慢，快的先返回。
从运行效果上看，最终的报价输出，不再是等6个商家全计算好才返回。
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