使用Java 11 HttpClient反序列化JSON，使用JSON停止自定义BodyHandler，并且不会继续

Question

在直接使用带有自定义HttpClient::send的Java11 HttpResponse.BodyHandler将JSON反序列化为定制对象时，我遇到了问题。我在回答这个问题时遇到了这个问题。

我正在使用的版本：

• OpenJDK 11
• 杰克逊2.9.9.3

我创建了一个简单的泛型JsonBodyHandler类，它实现了HttpResponse.BodyHandler。

public class JsonBodyHandler<W> implements HttpResponse.BodyHandler<W> {

    private final Class<W> wClass;

    public JsonBodyHandler(Class<W> wClass) {
        this.wClass = wClass;
    }

    @Override
    public HttpResponse.BodySubscriber<W> apply(HttpResponse.ResponseInfo responseInfo) {
        return asJSON(wClass);
    }

}

asJSON方法定义为：

public static <W> HttpResponse.BodySubscriber<W> asJSON(Class<W> targetType) {
        HttpResponse.BodySubscriber<String> upstream = HttpResponse.BodySubscribers.ofString(StandardCharsets.UTF_8);

        return HttpResponse.BodySubscribers.mapping(
                upstream,
                (String body) -> {
                    try {
                        ObjectMapper objectMapper = new ObjectMapper();
                        return objectMapper.readValue(body, targetType);
                    } catch (IOException e) {
                        throw new UncheckedIOException(e);
                    }
            });
}

因此，它返回一个自定义HttpResponse.BodySubscriber，它将主体作为String，然后应用从JSON到给定的targetType的映射来测试它：

public static void main(String[] args) throws URISyntaxException, IOException, InterruptedException {

        HttpRequest request = HttpRequest.newBuilder(new URI("https://jsonplaceholder.typicode.com/todos/1"))
                .header("Accept", "application/json")
                .build();

        Model model = HttpClient.newHttpClient()
                .send(request, new JsonBodyHandler<>(Model.class))
                .body();

        System.out.println(model);

}

Model类：

public class Model {
        private String userId;
        private String id;
        private String title;
        private boolean completed;


    //getters setters constructors toString
}

产出与预期相符：

Model{userId='1', id='1', title='delectus aut autem', completed=false}

但是，当我首先将asJSON方法更改为读取InputStream而不是String时：

public static <W> HttpResponse.BodySubscriber<W> asJSON(Class<W> targetType) {
    HttpResponse.BodySubscriber<InputStream> upstream = HttpResponse.BodySubscribers.ofInputStream();

    return HttpResponse.BodySubscribers.mapping(
                upstream,
                (InputStream is) -> {
                    try (InputStream stream = is) {
                        ObjectMapper objectMapper = new ObjectMapper();
                        return objectMapper.readValue(stream, targetType);
                    } catch (IOException e) {
                        throw new UncheckedIOException(e);
                    }
            });
}

它在使用ObjectMapper调用该值后挂起，并且不继续(我已经检查它是否成功地从端点获得响应，状态代码为200)，但是它只是挂起。有人知道有什么问题吗？

Answer 1

我刚刚发现了这个问题，它有同样的问题，但与GZIPInputStream。事实证明，HttpResponse.BodySubscribers.mapping是有缺陷的，而且不像文档所描述的那样工作。以下是官方的链接 OpenJDK bug站点。它是为OpenJDK 13修正的。因此，一个解决办法是使用HttpResponse.BodySubscribers::ofString而不是HttpResponse.BodySubscribers::ofInputStream作为HttpResponse.BodySubscribers::mapping的上游--在我的问题中向您展示了如何做到这一点。

或者，正如@daniel在注释中指出的那样，更好的解决方案是返回一个Supplier，而不是模型类：

public static <W> HttpResponse.BodySubscriber<Supplier<W>> asJSON(Class<W> targetType) {
        HttpResponse.BodySubscriber<InputStream> upstream = HttpResponse.BodySubscribers.ofInputStream();

        return HttpResponse.BodySubscribers.mapping(
                upstream,
                inputStream -> toSupplierOfType(inputStream, targetType));
    }

    public static <W> Supplier<W> toSupplierOfType(InputStream inputStream, Class<W> targetType) {
        return () -> {
            try (InputStream stream = inputStream) {
                ObjectMapper objectMapper = new ObjectMapper();
                return objectMapper.readValue(stream, targetType);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        };
    }

JsonBodyHandler还使用Supplier

public class JsonBodyHandler<W> implements HttpResponse.BodyHandler<Supplier<W>> {

    private final Class<W> wClass;

    public JsonBodyHandler(Class<W> wClass) {
        this.wClass = wClass;
    }

    @Override
    public HttpResponse.BodySubscriber<Supplier<W>> apply(HttpResponse.ResponseInfo responseInfo) {
        return asJSON(wClass);
    }

}

然后我们可以这样称呼它：

public static void main(String[] args) throws URISyntaxException, IOException, InterruptedException {

    HttpRequest request = HttpRequest.newBuilder(new URI("https://jsonplaceholder.typicode.com/todos/1"))
            .header("Accept", "application/json")
            .build();

    Model model = HttpClient.newHttpClient()
            .send(request, new JsonBodyHandler<>(Model.class))
            .body()
            .get();

    System.out.println(model);

}

这甚至被推广到在OpenJDK 13 docs中描述的方法中。

映射函数使用客户端的执行器执行，因此可以用于映射任何响应体类型，包括阻塞InputStream。但是，在mapper函数中执行任何阻塞操作都有可能阻塞执行器线程的未知时间(至少在阻塞操作结束之前)，这最终可能会使执行者缺乏可用的线程。因此，在映射到所需类型可能会阻塞的情况下(例如，通过在InputStream上读取)，然后映射到所需类型的Supplier并将阻塞操作推迟到调用者的线程调用Supplier::get时应该是首选的。

Answer 2

通过使用杰克逊TypeReference，我能够单独使用泛型来实现它，而不需要冗余的Class<T>参数。

package com.company;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;
import java.io.InputStream;
import java.io.UncheckedIOException;
import java.net.http.HttpResponse;
import java.util.function.Supplier;

public class JsonBodyHandler<W> implements HttpResponse.BodyHandler<Supplier<W>> {

    public JsonBodyHandler() {
    }

    private static <W> Supplier<W> toSupplierOfType(InputStream inputStream) {
        return () -> {
            try (InputStream stream = inputStream) {
                return new ObjectMapper().readValue(stream, new TypeReference<W>() {
                });
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        };
    }

    private static <W> HttpResponse.BodySubscriber<Supplier<W>> asJSON() {
        return HttpResponse.BodySubscribers.mapping(
                HttpResponse.BodySubscribers.ofInputStream(),
                JsonBodyHandler::toSupplierOfType);
    }

    @Override
    public HttpResponse.BodySubscriber<Supplier<W>> apply(HttpResponse.ResponseInfo responseInfo) {
        return JsonBodyHandler.asJSON();
    }

}

并在使用中

package com.company;

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.time.Duration;
import java.util.List;


public class Main {

    private static final HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://myApi"))
            .timeout(Duration.ofMinutes(1))
            .header("Content-Type", "application/json")
            .build();

    private static final HttpClient client = HttpClient.newBuilder().build();

    public static void main(String[] args) throws InterruptedException {
        client.sendAsync(Main.request, new JsonBodyHandler<List<MyDto>>())
                .thenAccept(response -> {
                    List<MyDto> myDtos = response.body().get();
                    System.out.println(myDtos);
                }).join();
    }
}

Answer 3

在科特林：

    val objectMapper = ObjectMapper()

    fun jsonBodyHandler(): HttpResponse.BodyHandler<JsonNode> {
      val jsonNodeSubscriber = BodySubscribers.mapping(BodySubscribers.ofByteArray()) {
        objectMapper.readTree(it)
      }
      return HttpResponse.BodyHandler { jsonNodeSubscriber }
    }