废话就不多说了，，直接上代码：
服务器端：
①MySQL中建立一个数据库hello，建一张表tab_user，添加字段id，username,password。然后随便添加几条记录。
②新建Java Web工程HelloServer。
③在WEB-INF目录下的lib中引入mysql-connector-java-5.0.8-bin.jar与org.json.jar。
④新建Servlet类LoginServlet。
LoginServlet.java:
package cn.domain.hello.servlet;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStreamReader;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.json.JSONArray;import org.json.JSONObject;import cn.domain.hello.bean.UserBean;import cn.domain.hello.dao.UserDao;public class LoginServlet extends HttpServlet {/** * */private static final long serialVersionUID = 1L;/** * The doGet method of the servlet. <br> * * This method is called when a form has its tag value method equals to get. * * @param request *the request send by the client to the server * @param response *the response send by the server to the client * @throws ServletException *if an error occurred * @throws IOException *if an error occurred */public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doPost(request, response);}/** * The doPost method of the servlet. <br> * * This method is called when a form has its tag value method equals to * post. * * @param request *the request send by the client to the server * @param response *the response send by the server to the client * @throws ServletException *if an error occurred * @throws IOException *if an error occurred */public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8");response.setContentType("text/json;charset=UTF-8");String reqMessage, respMessage;JSONArray reqObject = null;JSONArray respObject = null;try {BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream(), "UTF-8"));StringBuffer sb = new StringBuffer("");String temp;while ((temp = br.readLine()) != null) {sb.append(temp);}br.close();reqMessage = sb.toString();System.out.println("请求报文:" + reqMessage);reqObject = new JSONArray(reqMessage);UserDao userDao = new UserDao();UserBean ub = userDao.getUserByName(reqObject.getJSONObject(0).getString("username"));if (ub.getPassword() != null&& ub.getPassword().equals(reqObject.getJSONObject(0).getString("password"))) {respObject = new JSONArray().put(new JSONObject().put("userId",ub.getId()));}} catch (Exception e) {e.printStackTrace();} finally {respMessage = respObject == null ? "" : respObject.toString();System.out.println("返回报文:" + respMessage);PrintWriter pw = response.getWriter();pw.write(respMessage);pw.flush();pw.close();}}}
简单抽象出了数据访问层，并实现数据库的操作，目录结构如下，由于文件多，详细请下载下面的源码查看：
好了，到这里服务器端就已经建立完成了。客户端访问这个servlet的URL为：:8080/HelloServer/servlet/LoginServlet。
记住，在android中localhost指的是模拟器本身，而这里的10.0.2.2指你的计算机服务器本地测试IP，可改成内网或外网IP。
Android客户端端：
①新建一个android application：Hello。
②引入org.json.jar。
③在activity_main.xml布局文件中加入两个EditText，来输入用户名和密码，添加按钮来提交json数据。
④先封装一下访问网络的HTTP操作：
WebUtil.java:
package cn.domain.hello.util;import org.apache.http.HttpResponse;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpPost;import org.apache.http.entity.StringEntity;import org.apache.http.impl.client.DefaultHttpClient;import org.apache.http.params.BasicHttpParams;import org.apache.http.params.HttpParams;import org.apache.http.util.EntityUtils;import org.json.JSONArray;import cn.domain.hello.config.Config;public class WebUtil {public static JSONArray getJSONArrayByWeb(String methodName,JSONArray params) {String returnValue = "";JSONArray result = null;HttpParams httpParams = new BasicHttpParams();httpParams.setParameter("charset", "UTF-8");HttpClient hc = new DefaultHttpClient(httpParams);HttpPost hp = new HttpPost(Config.SERVER_IP + "/HelloServer/servlet/"+ methodName);try {hp.setEntity(new StringEntity(params.toString(), "UTF-8"));HttpResponse hr = hc.execute(hp);if (hr.getStatusLine().getStatusCode() == 200) {returnValue = EntityUtils.toString(hr.getEntity(), "UTF-8");result = new JSONArray(returnValue);}} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();}if (hc != null) {hc.getConnectionManager().shutdown();}return result;}}⑤在MainActivity中实现登录操作：package cn.domain.hello.activity;import org.json.JSONArray;import org.json.JSONException;import org.json.JSONObject;import cn.domain.hello.R;import cn.domain.hello.config.Config;import cn.domain.hello.util.WebUtil;import android.app.Activity;import android.os.AsyncTask;import android.os.Bundle;import android.util.Log;import android.view.View;import android.view.ViewStub;import android.view.View.OnClickListener;import android.view.ViewGroup;import android.widget.Button;import android.widget.EditText;import android.widget.TextView;import android.widget.Toast;public class MainActivity extends Activity {private EditText etUsername;private EditText etPassword;private Button btnLogin;private ViewGroup vsProgress;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);this.etUsername = (EditText) this.findViewById(R.id.etUsername);this.etPassword = (EditText) this.findViewById(R.id.etPassword);this.btnLogin = (Button) this.findViewById(R.id.btnLogin);this.btnLogin.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {// TODO Auto-generated method stubString username = MainActivity.this.etUsername.getText().toString().trim();String password = MainActivity.this.etPassword.getText().toString().trim();if ("".equals(username)) {Toast.makeText(MainActivity.this, "请填写用户名",Toast.LENGTH_SHORT).show();return;}if ("".equals(password)) {Toast.makeText(MainActivity.this, "请填写密码",Toast.LENGTH_SHORT).show();return;}//如果已经填写了用户名和密码，执行登录操作executeLogin(username, password);}});}private void executeLogin(String username, String password) {new LoginTask().execute(username, password);}private void onLoginComplete(Integer userId) {if (userId == null || userId == 0) {//如果没有获取到用户ID，说明登录失败Toast.makeText(MainActivity.this, "用户名或密码错误", Toast.LENGTH_SHORT).show();if (vsProgress != null) {vsProgress.setVisibility(View.INVISIBLE);}return;}if (vsProgress != null) {vsProgress.setVisibility(View.INVISIBLE);}//如果成功获取到返回的用户ID，说明登录成功，跳转到HelloActivityToast.makeText(MainActivity.this, "登陆成功", Toast.LENGTH_SHORT).show();HelloActivity.actionStart(MainActivity.this, userId, etUsername.getText().toString());}private class LoginTask extends AsyncTask<String, Void, Integer> {@Overrideprotected void onPreExecute() {// TODO Auto-generated method stubsuper.onPreExecute();//进行登录验证时，显示登录进度条if (vsProgress == null) {ViewStub vs = (ViewStub) findViewById(R.id.vsProgress);vsProgress = (ViewGroup) vs.inflate();} else {vsProgress.setVisibility(View.VISIBLE);}}@Overrideprotected Integer doInBackground(String… params) {// TODO Auto-generated method stubInteger result = null;JSONArray reqValue;try {//将用户名和密码封装到JSONArray中，进行HTTP通信reqValue = new JSONArray().put(new JSONObject().put("username",params[0]).put("password", params[1]));JSONArray rec = WebUtil.getJSONArrayByWeb(Config.METHOD_LOGIN,reqValue);if (rec != null) {//如果成功获取用户IDresult = rec.getJSONObject(0).getInt("userId");}} catch (JSONException e) {// TODO Auto-generated catch blocke.printStackTrace();}return result;}@Overrideprotected void onPostExecute(Integer result) {// TODO Auto-generated method stubsuper.onPostExecute(result);//回调onLoginComplete(result);}}}即使爬到最高的山上，一次也只能脚踏实地地迈一步。
