HttpURLConnection can post raw text in any format, such as text, json, xml, and html. In fact, you do not necessary to specify the field key when sending a request. Just write the data directly, using getOutputStream().write(data).

Example

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Iterator;
import java.util.Map;

public class HttpPostRaw {
    // Set the connect timeout value in milliseconds
    private final int CONNECT_TIMEOUT = 15000;
    // Set the read timeout value in milliseconds
    private final int READ_TIMEOUT = 60000;

    private HttpURLConnection httpConn;
    private String postData;
    private String charset;

    /**
     * This constructor initializes a new HTTP POST request with content type
     * is set to multipart/form-data
     *
     * @param requestURL
     * @param charset
     * @param headers
     * @param postData
     * @throws IOException
     */
    public HttpPostRaw(String requestURL, String charset, Map<String, String> headers, String postData) throws IOException {
        this.charset = charset;
        this.postData = postData;
        URL url = new URL(requestURL);
        httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setConnectTimeout(CONNECT_TIMEOUT);
        httpConn.setReadTimeout(READ_TIMEOUT);
        httpConn.setUseCaches(false);
        httpConn.setDoOutput(true);    // indicates POST method
        httpConn.setDoInput(true);
        httpConn.setRequestProperty("Content-Type", "text/plain");
        if (headers != null && headers.size() > 0) {
            Iterator<String> it = headers.keySet().iterator();
            while (it.hasNext()) {
                String key = it.next();
                String value = headers.get(key);
                httpConn.setRequestProperty(key, value);
            }
        }
    }

    public HttpPostRaw(String requestURL, String charset, Map<String, String> headers) throws IOException {
        this(requestURL, charset, headers, null);
    }

    public HttpPostRaw(String requestURL, String charset) throws IOException {
        this(requestURL, charset, null, null);
    }

    /**
     * Adds a header to the request
     *
     * @param key
     * @param value
     */
    public void addHeader(String key, String value) {
        httpConn.setRequestProperty(key, value);
    }

    /**
     * Adds a form field to the request
     *
     * @param postData
     */
    public void setPostData(String postData) {
        this.postData = postData;
    }

    /**
     * Convert the request data to a byte array
     *
     * @return
     */
    private byte[] getParamsByte() {
        byte[] result = null;
        try {
            result = this.postData.toString().getBytes("UTF-8");
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        return result;
    }

    /**
     * Completes the request and receives response from the server.
     *
     * @return String as response in case the server returned
     * status OK, otherwise an exception is thrown.
     * @throws IOException
     */
    public String finish() throws IOException {
        String response = "";
        byte[] postDataBytes = this.getParamsByte();
        httpConn.getOutputStream().write(postDataBytes);
        // Check the http status
        int status = httpConn.getResponseCode();
        if (status == HttpURLConnection.HTTP_OK) {
            ByteArrayOutputStream result = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int length;
            while ((length = httpConn.getInputStream().read(buffer)) != -1) {
                result.write(buffer, 0, length);
            }
            response = result.toString(this.charset);
            httpConn.disconnect();
        } else {
            throw new IOException("Server returned non-OK status: " + status);
        }
        return response;
    }
}

Use it

try {
    HttpPostRaw post = new HttpPostRaw("http://o2o-test.jdk5.com", "utf-8");
    String json = "{\"id\":\"288285\",\"value\":\"test\"}";
    post.setPostData(json);
    post.addHeader("Content-Type", "application/json");
    String out = post.finish();
    System.out.println(out);
} catch (Exception e) {
    e.printStackTrace();
}

This http request content look like:

POST /json HTTP/1.1
Content-Type: application/json
Cache-Control: no-cache
Pragma: no-cache
User-Agent: Java/1.8.0_211
Host: http://localhost
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
Content-Length: 30
Connection: keep-alive

{"id":"288285","value":"test"}