How to POST JSON request using Apache HttpClient?

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 It has been replaced by the Apache HttpComponents project in its HttpClient and HttpCore modules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 neededString JSON_STRING = node.toString();postMethod.setEntity(new StringEntity(JSON_STRING, ContentType.APPLICATION_JSON));