重试 HTTP 请求（Java 11 - HttpClient）

Question

问题

使用HttpClientJava 11（JDK，而不是 Apache），如何重试请求？

假设我想重试一个请求最多10 次，如果它没有返回状态代码或200抛出异常。

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试图

目前，我正在循环中重新安排返回的未来，我想知道是否有更好或更优雅的方式。

CompletableFuture<HttpResponse<Foo>> task = client.sendAsync(request, bodyHandler);

for (int i = 0; i < 10; i++) {
    task = task.thenComposeAsync(response -> response.statusCode() == 200 ?
        CompletableFuture.completedFuture(response) :
        client.sendAsync(request, bodyHandler));
}

// Do something with 'task' ...

如果我们也为特殊情况添加重试，我最终会得到

CompletableFuture<HttpResponse<Foo>> task = client.sendAsync(request, bodyHandler);

for (int i = 0; i < 10; i++) {
    task = task.thenComposeAsync(response ->
            response.statusCode() == 200 ?
            CompletableFuture.completedFuture(response) :
            client.sendAsync(request, bodyHandler))
        .exceptionallyComposeAsync(e ->
            client.sendAsync(request, bodyHandler));
}

// Do something with 'task' ...

不幸的是，似乎没有任何composeAsync触发器可以同时触发常规完成和异常。有handleAsync但没有compose，lambda 需要返回U但不CompletionStage<U>存在。

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其他框架

为了进行质量检查，我也对展示如何使用其他框架实现这一目标的答案感兴趣，但我不会接受它们。

例如，我见过一个名为Failsafe的库，它可能为此提供一个优雅的解决方案（请参阅jodah.net/failsafe）。

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参考

作为参考，以下是一些相关的 JavaDoc 链接：

• Http客户端
• 完整的未来
• 完成阶段

Answer 1

我建议按照这些思路做一些事情（假设没有安全经理）：

public static int MAX_RESEND = 10;

public static void main(String[] args) {
    HttpClient client = HttpClient.newHttpClient();
    HttpResponse.BodyHandler<String> handler = HttpResponse.BodyHandlers.ofString();
    HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("http://example.com/")).build();
    var response = client.sendAsync(request, handler)
            .thenComposeAsync(r -> tryResend(client, request, handler, 1, r));
    // do something with response...
}

public static <T> CompletableFuture<HttpResponse<T>>
        tryResend(HttpClient client, HttpRequest request, BodyHandler<T> handler,
                 int count, HttpResponse<T> resp) {
    if (resp.statusCode() == 200 || count >= MAX_RESEND) {
        return CompletableFuture.completedFuture(resp);
    } else {
        return client.sendAsync(request, handler)
                .thenComposeAsync(r -> tryResend(client, request, handler, count+1, r));
    }
}

如果你想同时处理常规和特殊情况，你可以这样做：

public static int MAX_RESEND = 5;

public static void main(String[] args) {
    HttpClient client = HttpClient.newHttpClient();
    BodyHandler<String> handler = BodyHandlers.ofString();
    HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("http://example.com/")).build();
    CompletableFuture<HttpResponse<String>> response = 
        client.sendAsync(request, handler)
            .handleAsync((r, t) -> tryResend(client, request, handler, 1, r, t))
            .thenCompose(Function.identity());
    // do something with response ...
}

public static boolean shouldRetry(HttpResponse<?> r, Throwable t, int count) {
    if (r != null && r.statusCode() == 200 || count >= MAX_RESEND) return false;
    if (t instanceof ... ) return false;
    return true;
}

public static <T> CompletableFuture<HttpResponse<T>>
        tryResend(HttpClient client, HttpRequest request,
                  BodyHandler<T> handler, int count,
                  HttpResponse<T> resp, Throwable t) {
    if (shouldRetry(resp, t, count)) {
        return client.sendAsync(request, handler)
                .handleAsync((r, x) -> tryResend(client, request, handler, count + 1, r, x))
                .thenCompose(Function.identity());
    } else if (t != null) {
        return CompletableFuture.failedFuture(t);
    } else {
        return CompletableFuture.completedFuture(resp);
    }
}