在新版本的Android中，你必须把所有的web I/O请求放到一个新的线程中。AsyncTask最适合小请求。

注(2020年10月):以下答案中使用的AsyncTask已在Android API级别30中弃用。有关更新的示例，请参考官方文档或本博客文章

更新(2017年6月)答案，适用于Android 6.0+。感谢@Rohit Suthar， @Tamis Bolvari和@sudhiskr的评论。

    public class CallAPI extends AsyncTask<String, String, String> {
    
        public CallAPI(){
            //set context variables if required
        }
    
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }
    
         @Override
         protected String doInBackground(String... params) {
            String urlString = params[0]; // URL to call
            String data = params[1]; //data to post
            OutputStream out = null;

            try {
                URL url = new URL(urlString);
                HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
                out = new BufferedOutputStream(urlConnection.getOutputStream());

                BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(out, "UTF-8"));
                writer.write(data);
                writer.flush();
                writer.close();
                out.close();

                urlConnection.connect();
            } catch (Exception e) {
                System.out.println(e.getMessage());
            }
        }
    }

引用:

https://developer.android.com/reference/java/net/HttpURLConnection.html 如何使用NameValuePair使用POST添加参数到HttpURLConnection

原答案(2010年5月)

注意:此解决方案已过时。它只能在5.1版本以下的安卓设备上运行。Android 6.0及以上版本不包括本回答中使用的Apache http客户端。

来自Apache Commons的Http Client是可行的。它已经包含在android中。下面是一个简单的例子，说明如何使用它来执行HTTP Post。

public void postData() {
    // Create a new HttpClient and Post Header
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost("http://www.yoursite.com/script.php");

    try {
        // Add your data
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
        nameValuePairs.add(new BasicNameValuePair("id", "12345"));
        nameValuePairs.add(new BasicNameValuePair("stringdata", "Hi"));
        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

        // Execute HTTP Post Request
        HttpResponse response = httpclient.execute(httppost);
        
    } catch (ClientProtocolException e) {
        // TODO Auto-generated catch block
    } catch (IOException e) {
        // TODO Auto-generated catch block
    }
} 

通过这种方式，我们可以用http post方法发送数据并获得结果

     public class MyHttpPostProjectActivity extends Activity implements OnClickListener {

    private EditText usernameEditText;
    private EditText passwordEditText;
    private Button sendPostReqButton;
    private Button clearButton;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.login);

        usernameEditText = (EditText) findViewById(R.id.login_username_editText);
        passwordEditText = (EditText) findViewById(R.id.login_password_editText);

        sendPostReqButton = (Button) findViewById(R.id.login_sendPostReq_button);
        sendPostReqButton.setOnClickListener(this);

        clearButton = (Button) findViewById(R.id.login_clear_button);
        clearButton.setOnClickListener(this);        
    }

    @Override
    public void onClick(View v) {

        if(v.getId() == R.id.login_clear_button){
            usernameEditText.setText("");
            passwordEditText.setText("");
            passwordEditText.setCursorVisible(false);
            passwordEditText.setFocusable(false);
            usernameEditText.setCursorVisible(true);
            passwordEditText.setFocusable(true);
        }else if(v.getId() == R.id.login_sendPostReq_button){
            String givenUsername = usernameEditText.getEditableText().toString();
            String givenPassword = passwordEditText.getEditableText().toString();

            System.out.println("Given username :" + givenUsername + " Given password :" + givenPassword);

            sendPostRequest(givenUsername, givenPassword);
        }   
    }

    private void sendPostRequest(String givenUsername, String givenPassword) {

        class SendPostReqAsyncTask extends AsyncTask<String, Void, String>{

            @Override
            protected String doInBackground(String... params) {

                String paramUsername = params[0];
                String paramPassword = params[1];

                System.out.println("*** doInBackground ** paramUsername " + paramUsername + " paramPassword :" + paramPassword);

                HttpClient httpClient = new DefaultHttpClient();

                // In a POST request, we don't pass the values in the URL.
                //Therefore we use only the web page URL as the parameter of the HttpPost argument
                HttpPost httpPost = new HttpPost("http://www.nirmana.lk/hec/android/postLogin.php");

                // Because we are not passing values over the URL, we should have a mechanism to pass the values that can be
                //uniquely separate by the other end.
                //To achieve that we use BasicNameValuePair             
                //Things we need to pass with the POST request
                BasicNameValuePair usernameBasicNameValuePair = new BasicNameValuePair("paramUsername", paramUsername);
                BasicNameValuePair passwordBasicNameValuePAir = new BasicNameValuePair("paramPassword", paramPassword);

                // We add the content that we want to pass with the POST request to as name-value pairs
                //Now we put those sending details to an ArrayList with type safe of NameValuePair
                List<NameValuePair> nameValuePairList = new ArrayList<NameValuePair>();
                nameValuePairList.add(usernameBasicNameValuePair);
                nameValuePairList.add(passwordBasicNameValuePAir);

                try {
                    // UrlEncodedFormEntity is an entity composed of a list of url-encoded pairs. 
                    //This is typically useful while sending an HTTP POST request. 
                    UrlEncodedFormEntity urlEncodedFormEntity = new UrlEncodedFormEntity(nameValuePairList);

                    // setEntity() hands the entity (here it is urlEncodedFormEntity) to the request.
                    httpPost.setEntity(urlEncodedFormEntity);

                    try {
                        // HttpResponse is an interface just like HttpPost.
                        //Therefore we can't initialize them
                        HttpResponse httpResponse = httpClient.execute(httpPost);

                        // According to the JAVA API, InputStream constructor do nothing. 
                        //So we can't initialize InputStream although it is not an interface
                        InputStream inputStream = httpResponse.getEntity().getContent();

                        InputStreamReader inputStreamReader = new InputStreamReader(inputStream);

                        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

                        StringBuilder stringBuilder = new StringBuilder();

                        String bufferedStrChunk = null;

                        while((bufferedStrChunk = bufferedReader.readLine()) != null){
                            stringBuilder.append(bufferedStrChunk);
                        }

                        return stringBuilder.toString();

                    } catch (ClientProtocolException cpe) {
                        System.out.println("First Exception caz of HttpResponese :" + cpe);
                        cpe.printStackTrace();
                    } catch (IOException ioe) {
                        System.out.println("Second Exception caz of HttpResponse :" + ioe);
                        ioe.printStackTrace();
                    }

                } catch (UnsupportedEncodingException uee) {
                    System.out.println("An Exception given because of UrlEncodedFormEntity argument :" + uee);
                    uee.printStackTrace();
                }

                return null;
            }

            @Override
            protected void onPostExecute(String result) {
                super.onPostExecute(result);

                if(result.equals("working")){
                    Toast.makeText(getApplicationContext(), "HTTP POST is working...", Toast.LENGTH_LONG).show();
                }else{
                    Toast.makeText(getApplicationContext(), "Invalid POST req...", Toast.LENGTH_LONG).show();
                }
            }           
        }

        SendPostReqAsyncTask sendPostReqAsyncTask = new SendPostReqAsyncTask();
        sendPostReqAsyncTask.execute(givenUsername, givenPassword);     
    }
}

使用来自Square的开源okHttp库。okHttp适用于Android 2.3及以上版本，并在GitHub上拥有Apache 2.0许可证。

发送POST数据就像在AsyncTask中添加以下内容一样简单:

OkHttpClient client = new OkHttpClient();
RequestBody formBody = new FormBody.Builder()
                      .add("email", emailString) // A sample POST field
                      .add("comment", commentString) // Another sample POST field
                      .build();
Request request = new Request.Builder()
                 .url("https://yourdomain.org/callback.php") // The URL to send the data to
                 .post(formBody)
                 .build();

okHttp在maven上也有一个命名空间，所以将它添加到Android Studio项目很简单。只需将编译'com.squareup.okhttp3:okhttp:3.11.0'添加到应用程序的build.gradle中。

完整的代码

在你的活动中添加以下内容:

public class CallAPI extends AsyncTask<String, String, String> {

    String emailString;
    String commentString;

    public CallAPI(String email, String commnt){
           emailString = email;
           commentString = commnt;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected String doInBackground(String... params) {
        OkHttpClient client = new OkHttpClient();
        RequestBody formBody = new FormBody.Builder()
                      .add("email", emailString) // A sample POST field
                      .add("comment", commentString) // Another sample POST field
                      .build();
        Request request = new Request.Builder()
                 .url("https://yourdomain.org/callback.php") // The URL to send the data to
                 .post(formBody)
                 .build();
        return "";
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);
    }
}

并将其称为using:

new CallAPI(emailString, commentString).execute();

这是一个如何在不使用外部Apache库的情况下POST多部分数据的例子:

byte[] buffer = getBuffer();

if(buffer.length > 0) {
   String lineEnd = "\r\n"; 
   String twoHyphens = "--"; 
   String boundary =  "RQdzAAihJq7Xp1kjraqf"; 

   ByteArrayOutputStream baos = new ByteArrayOutputStream();
   DataOutputStream dos = new DataOutputStream(baos);

   // Send parameter #1
   dos.writeBytes(twoHyphens + boundary + lineEnd); 
   dos.writeBytes("Content-Disposition: form-data; name=\"param1\"" + lineEnd);
   dos.writeBytes("Content-Type: text/plain; charset=US-ASCII" + lineEnd);
   dos.writeBytes("Content-Transfer-Encoding: 8bit" + lineEnd);
   dos.writeBytes(lineEnd);
   dos.writeBytes(myStringData + lineEnd);

   // Send parameter #2
   //dos.writeBytes(twoHyphens + boundary + lineEnd); 
   //dos.writeBytes("Content-Disposition: form-data; name=\"param2\"" + lineEnd + lineEnd);
   //dos.writeBytes("foo2" + lineEnd);

   // Send a binary file
   dos.writeBytes(twoHyphens + boundary + lineEnd); 
   dos.writeBytes("Content-Disposition: form-data; name=\"param3\";filename=\"test_file.dat\"" + lineEnd); 
   dos.writeBytes("Content-Type: application/octet-stream" + lineEnd);
   dos.writeBytes("Content-Transfer-Encoding: binary" + lineEnd);
   dos.writeBytes(lineEnd); 
   dos.write(buffer);
   dos.writeBytes(lineEnd); 
   dos.writeBytes(twoHyphens + boundary + twoHyphens + lineEnd); 
   dos.flush(); 
   dos.close();

   ByteArrayInputStream content = new ByteArrayInputStream(baos.toByteArray());
   BasicHttpEntity entity = new BasicHttpEntity();
   entity.setContent(content);

   HttpPost httpPost = new HttpPost(myURL);
   httpPost.addHeader("Connection", "Keep-Alive");
   httpPost.addHeader("Content-Type", "multipart/form-data; boundary="+boundary);

   //MultipartEntity entity = new MultipartEntity();
   //entity.addPart("param3", new ByteArrayBody(buffer, "test_file.dat"));
   //entity.addPart("param1", new StringBody(myStringData));

   httpPost.setEntity(entity);

   /*
   String httpData = "";
   ByteArrayOutputStream baos1 = new ByteArrayOutputStream();
   entity.writeTo(baos1);
   httpData = baos1.toString("UTF-8");
   */

   /*
   Header[] hdrs = httpPost.getAllHeaders();
   for(Header hdr: hdrs) {
     httpData += hdr.getName() + " | " + hdr.getValue() + " |_| ";
   }
   */

   //Log.e(TAG, "httpPost data: " + httpData);
   response = httpClient.execute(httpPost);
}

方法将数据作为HTTP请求发布，

public static InputStream callPostService(String Url,
        List<NameValuePair> data) {
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost(Url);
    try {
        httppost.setEntity(new UrlEncodedFormEntity(data));
        HttpResponse response = httpclient.execute(httppost);
        HttpEntity entity = response.getEntity();
        return entity.getContent();
    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}