CompletableFuture异常处理全攻略

CompletableFuture future = CompletableFuture.supplyAsync(() -> {
    if (Math.random() > 0.5) throw new RuntimeException("模拟失败");
    return "success";
}).exceptionally(ex -> {
    log.error("任务执行失败", ex);
    return "fallback-result"; // 返回替代结果，类型需匹配
});

CompletableFuture result = CompletableFuture.supplyAsync(() -> 42)
    .thenApply(x -> x / 0) // 故意除零
    .handle((val, ex) -> {
        if (ex != null) {
            if (ex instanceof ArithmeticException) {
                return -1;
            } else if (ex instanceof TimeoutException) {
                return -2;
            }
            throw new RuntimeException("不可预期异常", ex);
        }
        return val;
    });

CompletableFuture load = fetchDataAsync()
    .whenComplete((data, ex) -> {
        if (ex != null) {
            metrics.counter("load.fail").increment();
            auditLogger.warn("加载失败，ID={}", id, ex);
        } else {
            metrics.timer("load.success").record(Duration.between(start, Instant.now()));
        }
    });

CompletableFuture userFuture = fetchRemoteUser(id)
    .exceptionally(ex -> {
        if (ex instanceof IOException) {
            return loadFromCache(id).join(); // 同步查缓存（谨慎！）
        }
        throw new ServiceException("获取用户失败", ex);
    })
    .thenApply(user -> {
        if (user == null) {
            throw new UserNotFoundException("用户不存在: " + id);
        }
        return enrichUser(user);
    })
    .exceptionally(ex -> {
        notifyAlert(ex); // 上报告警
        return buildDefaultUser(id); // 构造兜底用户
    });