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Examples and Recipes

By Guest Blogger Chris Hegarty 

The following are a number of examples and recipes that can be followed to perform common tasks using the JDK HTTP Client. See here for an introduction to the JDK HTTP Client.

Synchronous Get

Response body as a String

public void get(String uri) throws Exception {
    HttpClient client = HttpClient.newHttpClient();
    HttpRequest request = HttpRequest.newBuilder()
          .uri(URI.create(uri))
          .build();

    HttpResponse<String> response =
          client.send(request, BodyHandler.asString());

    System.out.println(response.body());
}

The above example uses the asString BodyHandler to convert the response body bytes into a String. A BodyHandler must be supplied for each HttpRequest sent. The BodyHandler determines how to handle the response body, if any.

The BodyHandler is invoked once the response status code and headers are available, but before the response body bytes are received. The BodyHandler is responsible for creating the BodySubscriber which is a reactive-stream subscriber that receives streams of data with non-blocking back pressure. The BodySubscriber is responsible for, possibly, converting the response body bytes into a higher-level Java type.

The HttpResponse class provides a number of convenience static factory methods for creating a BodyHandler. A number of these accumulate the response bytes in memory until it is completely received, after which it is converted into the higher-level Java type, for example, asString, and asByteArray. Others stream the response data as it arrives; asFile, asByteArrayConsumer, and asInputStream. Alternatively, a custom written subscriber implementation can be provided.

Response body as a File

public void get(String uri) throws Exception {
    HttpClient client = HttpClient.newHttpClient();
    HttpRequest request = HttpRequest.newBuilder()
          .uri(URI.create(uri))
          .build();

    HttpResponse<Path> response =
          client.send(request, BodyHandler.asFile(Paths.get("body.txt")));

    System.out.println("Response in file:" + response.body());
}

Asynchronous Get

The asynchronous API returns immediately with a CompletableFuture that completes with the HttpResponse when it becomes available. CompletableFuture was added in Java 8 and supports composable asynchronous programming.

Response body as a String

public CompletableFuture<String> get(String uri) {
    HttpClient client = HttpClient.newHttpClient();
    HttpRequest request = HttpRequest.newBuilder()
          .uri(URI.create(uri))
          .build();

    return client.sendAsync(request, BodyHandler.asString())
          .thenApply(HttpResponse::body);
}

The CompletableFuture.thenApply(Function) method can be used to map the HttpResponse to its body type, status code, etc.

Response body as a File

public CompletableFuture<Path> get(String uri) {
    HttpClient client = HttpClient.newHttpClient();
    HttpRequest request = HttpRequest.newBuilder()
          .uri(URI.create(uri))
          .build();

    return client.sendAsync(request, BodyHandler.asFile(Paths.get("body.txt")))
          .thenApply(HttpResponse::body);
}

Post

A request body can be supplied by an HttpRequest.BodyPublisher.

public void post(String uri, String data) throws Exception {
    HttpClient client = HttpClient.newBuilder().build();
    HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create(uri))
            .POST(BodyPublisher.fromString(data))
            .build();

    HttpResponse<?> response = client.send(request, BodyHandler.discard(""));
    System.out.println(response.statusCode());
}

The above example uses the fromString BodyPublisher to convert the given String into request body bytes.

The BodyPublisher is a reactive-stream publisher that publishes streams of request body on-demand. HttpRequest.Builder has a number of methods that allow setting a BodyPublisher; Builder::POST, Builder::PUT, and Builder::method. The HttpRequest class has a number of convenience static factory methods that create a BodyPublisher for common types of data; fromString, fromByteArray, fromFile.

The discard BodyHandler can be used to receive and discard the response body when it is not of interest.

Concurrent Requests

It's easy to combine Java Streams and the CompletableFuture API to issue a number of requests and await their responses. The following example sends a GET request for each of the URIs in the list and stores all the responses as Strings.

public void getURIs(List<URI> uris) {
    HttpClient client = HttpClient.newHttpClient();
    List<HttpRequest> requests = uris.stream()
            .map(HttpRequest::newBuilder)
            .map(reqBuilder -> reqBuilder.build())
            .collect(toList());

    CompletableFuture.allOf(requests.stream()
            .map(request -> client.sendAsync(request, asString()))
            .toArray(CompletableFuture<?>[]::new))
            .join();
}

Get JSON

In many cases the response body will be in some higher-level format. The convenience response body handlers can be used, along with a third-party library to convert the response body into that format.

The following example demonstrates how to use the Jackson library, in combination with BodyHandler::asString to convert a JSON response into a Map of String key/value pairs.

public CompletableFuture<Map<String,String>> JSONBodyAsMap(URI uri) {
    UncheckedObjectMapper objectMapper = new UncheckedObjectMapper();

    HttpRequest request = HttpRequest.newBuilder(uri)
          .header("Accept", "application/json")
          .build();

    return HttpClient.newHttpClient()
          .sendAsync(request, asString())
          .thenApply(HttpResponse::body)
          .thenApply(objectMapper::readValue);
}

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

The above example uses asString which accumulates the response body bytes in memory. Alternatively, a streaming subscriber, like asInputStream could be used.

Post JSON

In many cases the request body will be in some higher-level format. The convenience request body handlers can be used, along with a third-party library to convert the request body into that format.

The following example demonstrates how to use the Jackson library, in combination with the BodyPublisher::fromString to convert a Map of String key/value pairs into JSON.

public CompletableFuture<Void> postJSON(URI uri,
                                        Map<String,String> map)
    throws IOException
{
    ObjectMapper objectMapper = new ObjectMapper();
    String requestBody = objectMapper
          .writerWithDefaultPrettyPrinter()
          .writeValueAsString(map);

    HttpRequest request = HttpRequest.newBuilder(uri)
          .header("Content-Type", "application/json")
          .POST(BodyPublisher.fromString(requestBody))
          .build();

    return HttpClient.newHttpClient()
          .sendAsync(request, BodyHandler.asString())
          .thenApply(HttpResponse::statusCode)
          .thenAccept(System.out::println);
}

Setting a Proxy

A ProxySelector can be configured on the HttpClient through the client's Builder::proxy method. The ProxySelector API returns a specific proxy for a given URI. In many cases a single static proxy is sufficient. The ProxySelector::of static factory method can be used to create such a selector.

Response body as a String with a specified proxy

public CompletableFuture<String> get(String uri) {
    HttpClient client = HttpClient.newBuilder()
          .proxy(ProxySelector.of(new InetSocketAddress("www-proxy.com", 8080)))
          .build();

    HttpRequest request = HttpRequest.newBuilder()
          .uri(URI.create(uri))
          .build();

    return client.sendAsync(request, asString())
          .thenApply(HttpResponse::body);
}

Alternatively, the system-wide default proxy selector can be used, which is the default on macOS.

HttpClient.newBuilder()
      .proxy(ProxySelector.getDefault())
      .build();

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