Android 简明教程

Android - JSON Parser

JSON 代表 JavaScript 对象表示法。它是一个独立数据交换格式，是 XML 的最佳替代方案。本章说明如何解析 JSON 文件，并从中提取所需的信息。

JSON stands for JavaScript Object Notation.It is an independent data exchange format and is the best alternative for XML. This chapter explains how to parse the JSON file and extract necessary information from it.

Android 提供了四个不同的类来操作 JSON 数据。这些类是 JSONArray,JSONObject,JSONStringer and JSONTokenizer.

Android provides four different classes to manipulate JSON data. These classes are JSONArray,JSONObject,JSONStringer and JSONTokenizer.

第一步是识别你感兴趣的 JSON 数据中的字段。例如，在下面给出的 JSON 中，我们仅有兴趣获取温度。

The first step is to identify the fields in the JSON data in which you are interested in. For example. In the JSON given below we interested in getting temperature only.

{
   "sys":
   {
      "country":"GB",
      "sunrise":1381107633,
      "sunset":1381149604
   },
   "weather":[
      {
         "id":711,
         "main":"Smoke",
         "description":"smoke",
         "icon":"50n"
      }
   ],

  "main":
   {
      "temp":304.15,
      "pressure":1009,
   }
}

JSON - Elements

一个 JSON 文件由许多组件组成。以下是定义 JSON 文件的组件及其说明的表格：

An JSON file consist of many components. Here is the table defining the components of an JSON file and their description −

Sr.No	Component & description
1	Array([) In a JSON file , square bracket ([) represents a JSON array
2	Objects({) In a JSON file, curly bracket ({) represents a JSON object
3	Key A JSON object contains a key that is just a string. Pairs of key/value make up a JSON object
4	Value Each key has a value that could be string , integer or double e.t.c

JSON - Parsing

为了分析 JSON 对象，我们将创建一个 JSONObject 类对象，并指定一个包含 JSON 数据的字符串给它。其语法如下 −

For parsing a JSON object, we will create an object of class JSONObject and specify a string containing JSON data to it. Its syntax is −

String in;
JSONObject reader = new JSONObject(in);

最后一步是分析 JSON。一个 JSON 文件由具有不同键/值对的不同对象组成，等等。因此，JSONObject 具有单独的函数来分析 JSON 文件的每个组件。其语法如下 −

The last step is to parse the JSON. A JSON file consist of different object with different key/value pair e.t.c. So JSONObject has a separate function for parsing each of the component of JSON file. Its syntax is given below −

JSONObject sys  = reader.getJSONObject("sys");
country = sys.getString("country");

JSONObject main  = reader.getJSONObject("main");
temperature = main.getString("temp");

方法 getJSONObject 返回 JSON 对象。方法 getString 返回指定键的字符串值。

The method getJSONObject returns the JSON object. The method getString returns the string value of the specified key.

除了这些方法，此类还提供了其他方法来更好地分析 JSON 文件。以下列出了这些方法 −

Apart from the these methods , there are other methods provided by this class for better parsing JSON files. These methods are listed below −

Sr.No	Method & description
1	get(String name) This method just Returns the value but in the form of Object type
2	getBoolean(String name) This method returns the boolean value specified by the key
3	getDouble(String name) This method returns the double value specified by the key
4	getInt(String name)This method returns the integer value specified by the key
5	getLong(String name) This method returns the long value specified by the key
6	length() This method returns the number of name/value mappings in this object..
7	names() This method returns an array containing the string names in this object.

Example

要使用此示例，你可以在实际设备或模拟器上运行此示例。

To experiment with this example , you can run this on an actual device or in an emulator.

Steps	Description
1	You will use Android studio to create an Android application.
2	Modify src/MainActivity.java file to add necessary code.
3	Modify the res/layout/activity_main to add respective XML components
4	Modify the res/values/string.xml to add necessary string components
5	Run the application and choose a running android device and install the application on it and verify the results

以下是修改的主活动文件 src/MainActivity.java 的内容。

Following is the content of the modified main activity file src/MainActivity.java.

package com.example.tutorialspoint7.myapplication;

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;

public class MainActivity extends AppCompatActivity {

   private String TAG = MainActivity.class.getSimpleName();
   private ListView lv;

   ArrayList<HashMap<String, String>> contactList;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);

      contactList = new ArrayList<>();
      lv = (ListView) findViewById(R.id.list);

      new GetContacts().execute();
   }

   private class GetContacts extends AsyncTask<Void, Void, Void> {
      @Override
      protected void onPreExecute() {
         super.onPreExecute();
         Toast.makeText(MainActivity.this,"Json Data is
            downloading",Toast.LENGTH_LONG).show();

      }

      @Override
      protected Void doInBackground(Void... arg0) {
         HttpHandler sh = new HttpHandler();
         // Making a request to url and getting response
         String url = "http://api.androidhive.info/contacts/";
         String jsonStr = sh.makeServiceCall(url);

         Log.e(TAG, "Response from url: " + jsonStr);
         if (jsonStr != null) {
            try {
               JSONObject jsonObj = new JSONObject(jsonStr);

               // Getting JSON Array node
               JSONArray contacts = jsonObj.getJSONArray("contacts");

               // looping through All Contacts
               for (int i = 0; i < contacts.length(); i++) {
                  JSONObject c = contacts.getJSONObject(i);
                  String id = c.getString("id");
                  String name = c.getString("name");
                  String email = c.getString("email");
                  String address = c.getString("address");
                  String gender = c.getString("gender");

                  // Phone node is JSON Object
                  JSONObject phone = c.getJSONObject("phone");
                  String mobile = phone.getString("mobile");
                  String home = phone.getString("home");
                  String office = phone.getString("office");

                  // tmp hash map for single contact
                  HashMap<String, String> contact = new HashMap<>();

                  // adding each child node to HashMap key => value
                  contact.put("id", id);
                  contact.put("name", name);
                  contact.put("email", email);
                  contact.put("mobile", mobile);

                  // adding contact to contact list
                  contactList.add(contact);
               }
            } catch (final JSONException e) {
               Log.e(TAG, "Json parsing error: " + e.getMessage());
               runOnUiThread(new Runnable() {
                  @Override
                  public void run() {
                     Toast.makeText(getApplicationContext(),
                     "Json parsing error: " + e.getMessage(),
                        Toast.LENGTH_LONG).show();
                  }
               });

            }

         } else {
            Log.e(TAG, "Couldn't get json from server.");
            runOnUiThread(new Runnable() {
               @Override
               public void run() {
                  Toast.makeText(getApplicationContext(),
                     "Couldn't get json from server. Check LogCat for possible errors!",
                     Toast.LENGTH_LONG).show();
               }
            });
         }

         return null;
      }

      @Override
      protected void onPostExecute(Void result) {
         super.onPostExecute(result);
         ListAdapter adapter = new SimpleAdapter(MainActivity.this, contactList,
            R.layout.list_item, new String[]{ "email","mobile"},
               new int[]{R.id.email, R.id.mobile});
         lv.setAdapter(adapter);
      }
   }
}

以下是 HttpHandler.java xml 的修改内容。

Following is the modified content of the xml HttpHandler.java.

package com.example.tutorialspoint7.myapplication;

import android.util.Log;

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;

public class HttpHandler {

   private static final String TAG = HttpHandler.class.getSimpleName();

   public HttpHandler() {
   }

   public String makeServiceCall(String reqUrl) {
      String response = null;
      try {
         URL url = new URL(reqUrl);
         HttpURLConnection conn = (HttpURLConnection) url.openConnection();
         conn.setRequestMethod("GET");
         // read the response
         InputStream in = new BufferedInputStream(conn.getInputStream());
         response = convertStreamToString(in);
      } catch (MalformedURLException e) {
         Log.e(TAG, "MalformedURLException: " + e.getMessage());
      } catch (ProtocolException e) {
         Log.e(TAG, "ProtocolException: " + e.getMessage());
      } catch (IOException e) {
         Log.e(TAG, "IOException: " + e.getMessage());
      } catch (Exception e) {
         Log.e(TAG, "Exception: " + e.getMessage());
      }
      return response;
   }

   private String convertStreamToString(InputStream is) {
      BufferedReader reader = new BufferedReader(new InputStreamReader(is));
      StringBuilder sb = new StringBuilder();

      String line;
      try {
         while ((line = reader.readLine()) != null) {
            sb.append(line).append('\n');
         }
      } catch (IOException e) {
         e.printStackTrace();
      } finally {
         try {
            is.close();
         } catch (IOException e) {
            e.printStackTrace();
         }
      }

      return sb.toString();
   }
}

以下是修改后的 xml res/layout/activity_main.xml 内容。

Following is the modified content of the xml res/layout/activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
<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"
    tools:context="com.example.tutorialspoint7.myapplication.MainActivity">

   <ListView
      android:id="@+id/list"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content" />
</RelativeLayout>

以下是 res/layout/list_item.xml xml 的修改内容。

Following is the modified content of the xml res/layout/list_item.xml.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:orientation="vertical"
   android:padding="@dimen/activity_horizontal_margin">
   <TextView
      android:id="@+id/email"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:paddingBottom="2dip"
      android:textColor="@color/colorAccent" />

   <TextView
      android:id="@+id/mobile"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:textColor="#5d5d5d"
      android:textStyle="bold" />
</LinearLayout>

下面是 AndroidManifest.xml 文件的内容。

Following is the content of AndroidManifest.xml file.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.tutorialspoint7.myapplication">

   <uses-permission android:name="android.permission.INTERNET"/>
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
         <activity android:name=".MainActivity">
            <intent-filter>
               <action android:name="android.intent.action.MAIN" />
               <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

让我们尝试运行我们刚刚修改的应用程序。我假设你在进行环境设置时已创建了 AVD 。要从 Android Studio 运行该应用程序，请打开一个项目的活动文件，并单击工具栏中的运行图标。Android Studio 会将该应用程序安装在你的 AVD 上并启动它，如果你的设置和应用程序一切正常，它将显示以下模拟器窗口−

Let’s try to run our application we just modified. I assume you had created your AVD while doing environment setup. To run the app from Android studio, open one of your project’s activity files and click Run icon from the toolbar. Android studio installs the app on your AVD and starts it and if everything is fine with your setup and application, it will display following Emulator window −

上面的示例显示了字符串 JSON 中的数据，这些数据包含雇员详细信息以及薪酬信息。

Above Example showing the data from string json,The data has contained employer details as well as salary information.