How to POST JSON request using Apache HttpClient?

I have something like the following:

final String url = "http://example.com";

final HttpClient httpClient = new HttpClient();
final PostMethod postMethod = new PostMethod(url);
postMethod.addRequestHeader("Content-Type", "application/json");
postMethod.addParameters(new NameValuePair[]{
        new NameValuePair("name", "value)
});
httpClient.executeMethod(httpMethod);
postMethod.getResponseBodyAsStream();
postMethod.releaseConnection();

It keeps coming back with a 500. The service provider says I need to send JSON. How is that done with Apache HttpClient 3.1+?

Apache HttpClient doesn't know anything about JSON, so you'll need to construct your JSON separately. To do so, I recommend checking out the simple JSON-java library from json.org. (If "JSON-java" doesn't suit you, json.org has a big list of libraries available in different languages.)

Once you've generated your JSON, you can use something like the code below to POST it

StringRequestEntity requestEntity = new StringRequestEntity(
    JSON_STRING,
    "application/json",
    "UTF-8");

PostMethod postMethod = new PostMethod("http://example.com/action");
postMethod.setRequestEntity(requestEntity);

int statusCode = httpClient.executeMethod(postMethod);

Edit

Note - The above answer, as asked for in the question, applies to Apache HttpClient 3.1. However, to help anyone looking for an implementation against the latest Apache client:

StringEntity requestEntity = new StringEntity(
    JSON_STRING,
    ContentType.APPLICATION_JSON);

HttpPost postMethod = new HttpPost("http://example.com/action");
postMethod.setEntity(requestEntity);

HttpResponse rawResponse = httpclient.execute(postMethod);

For Apache HttpClient 4.5 or newer version:

    CloseableHttpClient httpclient = HttpClients.createDefault();
	HttpPost httpPost = new HttpPost("http://targethost/login");
	String JSON_STRING="";
	HttpEntity stringEntity = new StringEntity(JSON_STRING,ContentType.APPLICATION_JSON);
	httpPost.setEntity(stringEntity);
	CloseableHttpResponse response2 = httpclient.execute(httpPost);
	

Note:

1 in order to make the code compile, both httpclient package and httpcore package should be imported.

2 try-catch block has been ommitted.

Reference: appache official guide > the Commons HttpClient project is now end of life, and is no longer > being developed. It has been replaced by the Apache HttpComponents > project in its HttpClient and HttpCore modules

As mentioned in the excellent answer by janoside, you need to construct the JSON string and set it as a StringEntity.

To construct the JSON string, you can use any library or method you are comfortable with. Jackson library is one easy example:

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ObjectNode;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.StringEntity;

ObjectMapper mapper = new ObjectMapper();
ObjectNode node = mapper.createObjectNode();
node.put("name", "value"); // repeat as needed
String JSON_STRING = node.toString();
postMethod.setEntity(new StringEntity(JSON_STRING, ContentType.APPLICATION_JSON));

I use JACKSON library to convert object to JSON and set the request body like below. Here is full example.

try (CloseableHttpClient httpclient = HttpClients.createDefault()) {

    HttpPost httpPost = new HttpPost("https://jsonplaceholder.typicode.com/posts");

    Post post = new Post("foo", "bar", 1);
    ObjectWriter ow = new ObjectMapper().writer();
    String strJson = ow.writeValueAsString(post);
    System.out.println(strJson);

    StringEntity strEntity = new StringEntity(strJson, ContentType.APPLICATION_JSON);
    httpPost.setEntity(strEntity);
    httpPost.setHeader("Content-type", "application/json");

    try (CloseableHttpResponse response = httpclient.execute(httpPost)) {
        System.out.println(response.getCode() + " " + response.getReasonPhrase());

        HttpEntity entity = response.getEntity();
        String result = EntityUtils.toString(entity);
        System.out.println(result);

        EntityUtils.consume(entity);
    } catch (ParseException e) {
        e.printStackTrace();
    }

} catch (IOException e) {
    System.out.println(e.getMessage());
}