Using Java/Android HttpUrlConnection to send post request withapplication/x-www-form-urlencodedbody. The post data will be encoded as key-value pairs separated by '&'. This type does not support binary data. If you want to send a file, refer to HttpURLConnection multipart/form-data example.

Example

By default, the browser sends theapplication/x-www-form-urlencoded content-type header.

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

public class HttpPostForm {
    private HttpURLConnection httpConn;
    private Map<String, Object> queryParams;
    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 queryParams
     * @throws IOException
     */
    public HttpPostForm(String requestURL, String charset, Map<String, String> headers, Map<String, Object> queryParams) throws IOException {
        this.charset = charset;
        if (queryParams == null) {
            this.queryParams = new HashMap<>();
        } else {
            this.queryParams = queryParams;
        }
        URL url = new URL(requestURL);
        httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setUseCaches(false);
        httpConn.setDoOutput(true);    // indicates POST method
        httpConn.setDoInput(true);
        httpConn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        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 HttpPostForm(String requestURL, String charset, Map<String, String> headers) throws IOException {
        this(requestURL, charset, headers, null);
    }

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

    /**
     * Adds a form field to the request
     *
     * @param name  field name
     * @param value field value
     */
    public void addFormField(String name, Object value) {
        queryParams.put(name, value);
    }

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

    /**
     * Convert the request fields to a byte array
     *
     * @param params
     * @return
     */
    private byte[] getParamsByte(Map<String, Object> params) {
        byte[] result = null;
        StringBuilder postData = new StringBuilder();
        for (Map.Entry<String, Object> param : params.entrySet()) {
            if (postData.length() != 0) {
                postData.append('&');
            }
            postData.append(this.encodeParam(param.getKey()));
            postData.append('=');
            postData.append(this.encodeParam(String.valueOf(param.getValue())));
        }
        try {
            result = postData.toString().getBytes("UTF-8");
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        return result;
    }

    /**
     * URL-encoding keys and values
     *
     * @param data
     * @return
     */
    private String encodeParam(String data) {
        String result = "";
        try {
            result = URLEncoder.encode(data, "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(queryParams);
        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 {
    // Headers
    Map<String, String> headers = new HashMap<>();
    headers.put("User-Agent", "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36");
    HttpPostForm httpPostForm = new HttpPostForm("http://localhost", "utf-8", headers);
    // Add form field
    httpPostForm.addFormField("username", "用户名");
    httpPostForm.addFormField("password", "test_psw");
    // Result
    String response = httpPostForm.finish();
    System.out.println(response);
} catch (Exception e) {
    e.printStackTrace();
}

Below is the request header information, you can see that the request parameters are concatenated by the '&' symbol.

POST / HTTP/1.1
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36
Host: localhost
Cache-Control: no-cache
Pragma: no-cache
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
Content-Length: 54
Connection: keep-alive

password=test_psw&username=%E7%94%A8%E6%88%B7%E5%90%8D