关于CompletableFuture的兜底方法exceptionally的问题

CompletableFuture<String> stringFuture = CompletableFuture.supplyAsync(() -> {
            String uuid = UUID.randomUUID().toString();
            System.out.println("内部打印。。。uuid = " + uuid);
//            int a = 1 / 0;
            return uuid;
        }, MyThreadPool.executorService)
                .whenComplete((s, throwable) -> {
                    System.out.println("执行完毕后打印。。。CompleteUUID = " + s);
                    System.out.println(throwable.getMessage());
                })
                .exceptionally(throwable -> {
                    System.out.println(throwable.getMessage());

                    String uuid = UUID.randomUUID().toString();
                    System.out.println("exceptionally兜底打印。。。uuid = " + uuid);

                    return uuid;
                });
        String outUUID = stringFuture.get();

老师，我这边不理解，为什么

stringFuture 要链式调用whenComplete、exceptionally。
为什么stringFuture 分开调用
stringFuture.whenComplete((s, throwable) -> {
    System.out.println("执行完毕后打印。。。CompleteUUID = " + s);
    System.out.println(throwable.getMessage());
});
stringFuture.exceptionally(throwable -> {
    System.out.println(throwable.getMessage());

    String uuid = UUID.randomUUID().toString();
    System.out.println("exceptionally兜底打印。。。uuid = " + uuid);

    return uuid;
});
String outUUID = stringFuture.get();
这样主线程为异常，无法兜底