public class Person
{
  private int id;
  private String name;
  private String address;

  public Person()
  {
  }

  public int getId()
  {
    return id;
  }

  public void setId(int id)
  {
    this.id = id;
  }

  public String getName()
  {
    return name;
  }

  public void setName(String name)
  {
    this.name = name;
  }

  public String getAddress()
  {
    return address;
  }

  public void setAddress(String address)
  {
    this.address = address;
  }

  public Person(int id, String name, String address)
  {
    super();
    this.id = id;
    this.name = name;
    this.address = address;
  }

  @Override
  public String toString()
  {
    return "Person [id=" + id + ", name=" + name + ", address=" + address
        + "]";
  }

}

public class JsonTools
{
  /**
   * 得到一个json类型的字符串对象
   * @param key
   * @param value
   * @return
   */
  public static String getJsonString(String key, Object value)
  {
    JSONObject jsonObject = new JSONObject();
    //put和element都是往JSONObject对象中放入 key/value 对
//    jsonObject.put(key, value);
    jsonObject.element(key, value);
    return jsonObject.toString();
  }
  
  /**
   * 得到一个json对象
   * @param key
   * @param value
   * @return
   */
  public static JSONObject getJsonObject(String key, Object value)
  {
    JSONObject jsonObject = new JSONObject();
    jsonObject.put(key, value);
    return jsonObject;
  }
  
}

　 Person person = new Person(1, "xiaoluo", "广州");
    //  将Person对象转换成一个json类型的字符串对象
    String personString = JsonTools.getJsonString("person", person);
    System.out.println(personString.toString());

　　　　 JSONObject jsonObject = JsonTools.getJsonObject("person", person);
    //  通过JSONObject的toBean方法可以将json对象转换成一个javabean
    JSONObject personObject = jsonObject.getJSONObject("person");
    Person person2 = (Person) JSONObject.toBean(personObject, Person.class);
    System.out.println(person2);

Person [id=1, name=xiaoluo, address=广州]

@Test
  public void testPersonsJson()
  {
    List<Person> persons = new ArrayList<Person>();
    Person person = new Person(1, "xiaoluo", "广州");
    Person person2 = new Person(2, "android", "上海");
    persons.add(person);
    persons.add(person2);
    String personsString = JsonTools.getJsonString("persons", persons);
    System.out.println(personsString);
    
    JSONObject jsonObject = JsonTools.getJsonObject("persons", persons);
    //  List<Person>相当于一个JSONArray对象
    JSONArray personsArray = (JSONArray)jsonObject.getJSONArray("persons");
    List<Person> persons2 = (List<Person>) personsArray.toCollection(personsArray, Person.class);
    System.out.println(persons2);
  }

{"persons":[{"address":"广州","id":1,"name":"xiaoluo"},{"address":"上海","id":2,"name":"android"}]}
[Person [id=1, name=xiaoluo, address=广州], Person [id=2, name=android, address=上海]]

@Test
  public void testMapJson()
  {
    List<Map<String, String>> list = new ArrayList<Map<String, String>>();
    Map<String, String> map1 = new HashMap<String, String>();
    map1.put("id", "001");
    map1.put("name", "xiaoluo");
    map1.put("age", "20");
    Map<String, String> map2 = new HashMap<String, String>();
    map2.put("id", "002");
    map2.put("name", "android");
    map2.put("age", "33");
    list.add(map1);
    list.add(map2);
    String listString = JsonTools.getJsonString("list", list);
    System.out.println(listString);
    
    JSONObject jsonObject = JsonTools.getJsonObject("list", list);
    JSONArray listArray = jsonObject.getJSONArray("list");
    List<Map<String, String>> list2 = (List<Map<String, String>>) listArray.toCollection(listArray, Map.class);
    System.out.println(list2);
  }

{"list":[{"id":"001","age":"20","name":"xiaoluo"},{"id":"002","age":"33","name":"android"}]}
[{id=001, name=xiaoluo, age=20}, {id=002, name=android, age=33}]

public class JsonService
{
  public Person getPerson()
  {
    Person person = new Person(1, "xiaoluo", "广州");
    return person;
  }
  
  public List<Person> getPersons()
  {
    List<Person> persons = new ArrayList<Person>();
    Person person = new Person(1, "xiaoluo", "广州");
    Person person2 = new Person(2, "android", "上海");
    persons.add(person);
    persons.add(person2);
    return persons;
  }
  
  public List<String> getString()
  {
    List<String> list = new ArrayList<String>();
    list.add("广州");
    list.add("上海");
    list.add("北京");
    return list;
  }
  
  public List<Map<String, String>> getMapList()
  {
    List<Map<String, String>> list = new ArrayList<Map<String, String>>();
    Map<String, String> map1 = new HashMap<String, String>();
    map1.put("id", "001");
    map1.put("name", "xiaoluo");
    map1.put("age", "20");
    Map<String, String> map2 = new HashMap<String, String>();
    map2.put("id", "002");
    map2.put("name", "android");
    map2.put("age", "33");
    list.add(map1);
    list.add(map2);
    return list;
  }
}

public static void main(String[] args)
  {
    Gson gson = new Gson();
    JsonService jsonService = new JsonService();
    Person person = jsonService.getPerson();
    System.out.println("person: " + gson.toJson(person));
    //  对于Object类型，使用 fromJson(String, Class)方法来将Json对象转换成Java对象
    Person person2 = gson.fromJson(gson.toJson(person), Person.class);
    System.out.println(person2);
    System.out.println("------------------------------------------------");
    
    List<Person> persons = jsonService.getPersons();
    System.out.println("persons: " + gson.toJson(persons));
    /*
     * 对于泛型对象，使用fromJson(String, Type)方法来将Json对象转换成对应的泛型对象
     * new TypeToken<>(){}.getType()方法
     */
    List<Person> persons2 = gson.fromJson(gson.toJson(persons), new TypeToken<List<Person>>(){}.getType());
    System.out.println(persons2);
    System.out.println("------------------------------------------------");
    
    List<String> list = jsonService.getString();
    System.out.println("String---->" + gson.toJson(list));
    List<String> list2 = gson.fromJson(gson.toJson(list), new TypeToken<List<String>>(){}.getType());
    System.out.println("list2---->" + list2);
    System.out.println("------------------------------------------------");
    
    List<Map<String, String>> listMap = jsonService.getMapList();
    System.out.println("Map---->" + gson.toJson(listMap));
    List<Map<String, String>> listMap2 = gson.fromJson(gson.toJson(listMap), new TypeToken<List<Map<String, String>>>(){}.getType());
    System.out.println("listMap2---->" + listMap2);
    System.out.println("------------------------------------------------");
  }

@WebServlet("/CityServlet")
public class CityServlet extends HttpServlet
{
  private static final long serialVersionUID = 1L;

  public CityServlet()
  {
    super();
  }

  protected void doGet(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException
  {
    this.doPost(request, response);
  }

  protected void doPost(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException
  {
    response.setContentType("text/html;charset=utf-8");
    request.setCharacterEncoding("utf-8");
    response.setCharacterEncoding("utf-8");
    PrintWriter writer = response.getWriter();
    
    String type = request.getParameter("type");
    if("json".equals(type))
    {
      List<String> cities = new ArrayList<String>();
      cities.add("广州");
      cities.add("上海");
      cities.add("北京");
      cities.add("湖南");
      Map<String, List<String>> map = new HashMap<String, List<String>>();
      map.put("cities", cities);
      String citiesString = JSON.toJSONString(map);
      writer.println(citiesString);
    }
    
    writer.flush();
    writer.close();
  }

}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent" >

  <TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:layout_marginLeft="64dp"
    android:layout_marginTop="64dp"
    android:textSize="20sp"
    android:text="城市" />
  
  <Spinner 
    android:id="@+id/spinner"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignTop="@id/textView1"
    android:layout_toRightOf="@id/textView1"/>

  <Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView1"
    android:layout_below="@+id/spinner"
    android:layout_marginLeft="22dp"
    android:layout_marginTop="130dp"
    android:text="加载数据" />

</RelativeLayout>

public class JsonUtils
{
  /**
   * @param citiesString  从服务器端得到的JSON字符串数据
   * @return  解析JSON字符串数据，放入List当中
   */
  public static List<String> parseCities(String citiesString)
  {
    List<String> cities = new ArrayList<String>();
    
    try
    {
      JSONObject jsonObject = new JSONObject(citiesString);
      JSONArray jsonArray = jsonObject.getJSONArray("cities");
      for(int i = 0; i < jsonArray.length(); i++)
      {
        cities.add(jsonArray.getString(i));
      }
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
    
    return cities;
  }
}

public class HttpUtils
{
  /**
   * @param path  请求的服务器URL地址
   * @param encode  编码格式
   * @return  将服务器端返回的数据转换成String
   */
  public static String sendPostMessage(String path, String encode)
  {
    String result = "";
    HttpClient httpClient = new DefaultHttpClient();
    try
    {
      HttpPost httpPost = new HttpPost(path);
      HttpResponse httpResponse = httpClient.execute(httpPost);
      if(httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK)
      {
        HttpEntity httpEntity = httpResponse.getEntity();
        if(httpEntity != null)
        {
          result = EntityUtils.toString(httpEntity, encode);
        }
      }
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
    finally
    {
      httpClient.getConnectionManager().shutdown();
    }
    
    return result;
  }

public class MainActivity extends Activity
{
  private Spinner spinner;
  private Button button;
  private ArrayAdapter<String> adapter;
  private ProgressDialog dialog;
  private final String CITY_PATH_JSON = "http://172.25.152.34:8080/httptest/CityServlet?type=json";
  @Override
  protected void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    
    spinner = (Spinner)findViewById(R.id.spinner);
    button = (Button)findViewById(R.id.button);
    dialog = new ProgressDialog(MainActivity.this);
    button.setOnClickListener(new OnClickListener()
    {
      @Override
      public void onClick(View v)
      {
        dialog.setTitle("提示信息");
        dialog.setMessage("loading......");
        dialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
        dialog.setCancelable(false);
        
        new MyAsyncTask().execute(CITY_PATH_JSON);
      }
    });
  }

  public class MyAsyncTask extends AsyncTask<String, Void, List<String>>
  {
    @Override
    protected void onPreExecute()
    {
      dialog.show();
    }
    @Override
    protected List<String> doInBackground(String... params)
    {
      List<String> cities = new ArrayList<String>();
      String citiesString = HttpUtils.sendPostMessage(params[0], "utf-8");
      //  解析服务器端的json数据
      cities = JsonUtils.parseCities(citiesString);return cities;
    }
    @Override
    protected void onPostExecute(List<String> result)
    {
      adapter = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_spinner_item, result);
      adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
      spinner.setAdapter(adapter);
      dialog.dismiss();
    }
  }
  
  @Override
  public boolean onCreateOptionsMenu(Menu menu)
  {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
  }

}

<uses-permission android:name="android.permission.INTERNET"/>