我是 Java 11 HttpClient 的新手，想尝试一下。我有一个返回 JSON 的简单 GET 请求，我想将 JSON 响应映射到名为的 Java 类Questionnaire.

我知道我可以将响应直接转换为字符串或输入流，如下所示

HttpRequest request = HttpRequest.newBuilder(new URI(String.format("%s%s", this.baseURI, "/state")))
          .header(ACCEPT, APPLICATION_JSON)
          .PUT(noBody()).build();

HttpResponse<String> response = this.client.send(request, HttpResponse.BodyHandlers.ofString());

我怎样才能编写将 JSON 字符串转换为我的 Questionnaire 类的东西？

HttpResponse<Questionnaire> response = this.client.send(request, HttpResponse.BodyHandlers./* what can I do here? */);

我使用 Jackson 将 JSON 转换为 Java 类实例。 Jackson 支持新的 Java 标准 HttpClient 了吗？

UPDATE 1我的表达不够准确，抱歉。我正在寻找一个阻塞 get 的例子。我知道http://openjdk.java.net/groups/net/httpclient/recipes.html#jsonGet http://openjdk.java.net/groups/net/httpclient/recipes.html#jsonGet

Java 11 的解决方案HttpClient::sendAsync only

基于这个链接 https://openjdk.java.net/groups/net/httpclient/recipes.html#jsonGet你可以这样做：

public static void main(String[] args) throws IOException, URISyntaxException, ExecutionException, InterruptedException {
        UncheckedObjectMapper uncheckedObjectMapper = new UncheckedObjectMapper();

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

        Model model = HttpClient.newHttpClient()
                .sendAsync(request, HttpResponse.BodyHandlers.ofString())
                .thenApply(HttpResponse::body)
                .thenApply(uncheckedObjectMapper::readValue)
                .get();

        System.out.println(model);

}

class UncheckedObjectMapper extends com.fasterxml.jackson.databind.ObjectMapper {
        /**
         * Parses the given JSON string into a Map.
         */
        Model readValue(String content) {
            try {
                return this.readValue(content, new TypeReference<Model>() {
                });
            } catch (IOException ioe) {
                throw new CompletionException(ioe);
            }
        }

}

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


    //getters setters constructors toString
}

我使用了一些虚拟端点，它提供示例 JSON 输入和示例模型类，将响应直接映射到Model使用杰克逊的课程。

Java 11 的解决方案HttpClient::send and HttpClient::sendAsync

我通过定义自定义找到了一种方法HttpResponse.BodyHandler :

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

    private Class<W> wClass;

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

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

    public static <T> HttpResponse.BodySubscriber<T> asJSON(Class<T> 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);
                    }
                });
    }
}

然后我称之为：

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);

}

回应是：

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

JavaDoc 的HttpResponse.BodySubscribers::mapping对于解决这个问题特别有用。还可以进一步改进使用HttpResponse.BodySubscribers::ofInputStream代替HttpResponse.BodySubscribers.ofString(StandardCharsets.UTF_8)定义BodySubscriber为了JsonBodyHandler.