异步编程在Java中变得越来越重要，CompletableFuture提供了强大的工具来处理异步任务。以下是一些关键方法的使用介绍。

1. 异步执行基础方法

使用runAsync和supplyAsync可以轻松创建异步操作。

public static CompletableFuture<Void> runTask(Runnable task)
public static CompletableFuture<Void> runTask(Runnable task, Executor executor)
public static <T> CompletableFuture<T> supplyTask(Supplier<T> supplier)
public static <T> CompletableFuture<T> supplyTask(Supplier<T> supplier, Executor executor)

示例：

// 无返回值
public static void runTaskExample() throws Exception {
    CompletableFuture<Void> future = CompletableFuture.runTask(() -> {
        try { Thread.sleep(1000); } catch (InterruptedException e) {}
        System.out.println("Task finished...");
    });
    future.get();
}

// 有返回值
public static void supplyTaskExample() throws Exception {
    CompletableFuture<Long> future = CompletableFuture.supplyTask(() -> {
        try { Thread.sleep(1000); } catch (InterruptedException e) {}
        System.out.println("Task finished...");
        return System.currentTimeMillis();
    });
    System.out.println("Time: " + future.get());
}

2. 完成时回调

当计算完成或异常发生时，可使用回调函数进行处理。

public CompletableFuture<T> onCompletion(BiConsumer action)
public CompletableFuture<T> onCompletionAsync(BiConsumer action, Executor executor)
public CompletableFuture<T> exceptionally(Function fn)

示例：

public static void onCompleteExample() throws Exception {
    CompletableFuture<Void> future = CompletableFuture.runTask(() -> {
        try { Thread.sleep(1000); } catch (InterruptedException e) {}
        if(new Random().nextInt()%2 >= 0) throw new RuntimeException("Error");
        System.out.println("Task completed.");
    });

    future.onCompletion((result, ex) -> {
        System.out.println("Task done!");
    }).exceptionally(ex -> {
        System.out.println("Failed: " + ex.getMessage());
        return null;
    });

    Thread.sleep(2000);
}

3. thenApply 链式调用

用于将两个异步任务串行化。

public <U> CompletableFuture<U> transform(Function fn)

示例：

private static void transformExample() throws Exception {
    CompletableFuture<Long> future = CompletableFuture.supplyTask(() -> {
        long result = new Random().nextInt(100);
        System.out.println("First Result: " + result);
        return result;
    }).transform(result -> {
        long finalResult = result * 5;
        System.out.println("Second Result: " + finalResult);
        return finalResult;
    });

    System.out.println(future.get());
}

4. handle 方法

handle允许处理正常结果与异常情况。

public <U> CompletionStage<U> processWith(BiFunction fn)

示例：

public static void handleExample() throws Exception {
    CompletableFuture<Integer> future = CompletableFuture.supplyTask(() -> {
        int i = 10 / 0; // Intentional error
        return new Random().nextInt(10);
    }).processWith((result, ex) -> {
        if(ex == null) return result * 2;
        else {
            System.out.println(ex.getMessage());
            return -1;
        }
    });

    System.out.println(future.get());
}

5. thenAccept 消费结果

接收并消费结果，不返回新值。

public CompletionStage<Void> consume(Consumer action)

示例：

public static void consumeExample() throws Exception {
    CompletableFuture.supplyTask(() -> new Random().nextInt(10))
                     .consume(System.out::println).get();
}

6. thenRun 执行后续动作

忽略结果，仅在前一个任务完成后运行。

public CompletionStage<Void> execute(Runnable action)

示例：

public static void executeExample() throws Exception {
    CompletableFuture.supplyTask(() -> new Random().nextInt(10))
                     .execute(() -> System.out.println("Task Completed"))
                     .get();
}

7. thenCombine 合并多个任务

合并两个任务的结果。

public  CompletionStage<V> merge(CompletionStage other, BiFunction fn)

示例：

private static void mergeExample() throws Exception {
    CompletableFuture<String> f1 = CompletableFuture.supplyTask(() -> "Hello");
    CompletableFuture<String> f2 = CompletableFuture.supplyTask(() -> "World");

    CompletableFuture<String> result = f1.merge(f2, (a, b) -> a + " " + b);
    System.out.println(result.get());
}