android执行网络操作

 

本篇我们会介绍连接到网络中涉及的基本任务，监测的网络连接（包括连接更改），并给予用户控制应用程序的网络使用情况。还介绍了如何解析和使用XML数据。

这个类包含一个示例应用程序来说明如何执行常见的网络操作。您可以下载示例（在右边），并用它作为自己的应用程序源代码的可重用代码。本章的重点有三：

1.连接到网络

2.管理网络的使用

3.解析XML数据

 

一、连接到网络

在mainfest中声明权限，代码如下：

 

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

 

选择http客户端

大多数联网的Android应用程序使用HTTP来发送和接收数据。Android包括两个HTTP客户:HttpURLConnection HttpClient和Apache。都支持HTTPS,流媒体上传和下载,可配置的超时,IPv6,和连接池。我们建议使用HttpURLConnection目标应用程序。

检查网络连接

在你的应用程序尝试连接到网络,它应该检查是否一个网络连接可用使用getActiveNetworkInfo()和一个()。记住,这个装置可能范围的一个网络,或用户可能已经禁用wi - fi和移动数据访问。

 

public void myClickHandler(View view) {
    ...
    ConnectivityManager connMgr = (ConnectivityManager) 
        getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
    if (networkInfo != null && networkInfo.isConnected()) {
        // fetch data
    } else {
        // display error
    }
    ...
}

 

在单独线程中执行网络操作

网络操作可以包括不可预测的延迟。为了防止这种导致一个糟糕的用户体验,总是执行网络操作在一个单独的线程。

 

public class HttpExampleActivity extends Activity {
    private static final String DEBUG_TAG = "HttpExample";
    private EditText urlText;
    private TextView textView;
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);   
        urlText = (EditText) findViewById(R.id.myUrl);
        textView = (TextView) findViewById(R.id.myText);
    }
// When user clicks button, calls AsyncTask.
    // Before attempting to fetch the URL, makes sure that there is a network connection.
    public void myClickHandler(View view) {
        // Gets the URL from the UI's text field.
        String stringUrl = urlText.getText().toString();
        ConnectivityManager connMgr = (ConnectivityManager) 
            getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
        if (networkInfo != null && networkInfo.isConnected()) {
            new DownloadWebpageText().execute(stringUrl);
        } else {
            textView.setText("No network connection available.");
        }
    }
// Uses AsyncTask to create a task away from the main UI thread. This task takes a 
     // URL string and uses it to create an HttpUrlConnection. Once the connection
     // has been established, the AsyncTask downloads the contents of the webpage as
     // an InputStream. Finally, the InputStream is converted into a string, which is
     // displayed in the UI by the AsyncTask's onPostExecute method.
     private class DownloadWebpageText extends AsyncTask {
        @Override
        protected String doInBackground(String... urls) {
              
            // params comes from the execute() call: params[0] is the url.
            try {
                return downloadUrl(urls[0]);
            } catch (IOException e) {
                return "Unable to retrieve web page. URL may be invalid.";
            }
        }
        // onPostExecute displays the results of the AsyncTask.
        @Override
        protected void onPostExecute(String result) {
            textView.setText(result);
       }
    }
    ...
}

 

连接和下载数据

在你的线程执行您的网络交易,你可以使用HttpURLConnection来执行一个GET和下载数据。在您调用connect(),你可以得到一个InputStream的数据通过调用getInputStream()。

 

// Given a URL, establishes an HttpUrlConnection and retrieves
// the web page content as a InputStream, which it returns as
// a string.
private String downloadUrl(String myurl) throws IOException {
    InputStream is = null;
    // Only display the first 500 characters of the retrieved
    // web page content.
    int len = 500;
        
    try {
        URL url = new URL(myurl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(10000 /* milliseconds */);
        conn.setConnectTimeout(15000 /* milliseconds */);
        conn.setRequestMethod("GET");
        conn.setDoInput(true);
        // Starts the query
        conn.connect();
        int response = conn.getResponseCode();
        Log.d(DEBUG_TAG, "The response is: " + response);
        is = conn.getInputStream();
// Convert the InputStream into a string
        String contentAsString = readIt(is, len);
        return contentAsString;
        
    // Makes sure that the InputStream is closed after the app is
    // finished using it.
    } finally {
        if (is != null) {
            is.close();
        } 
    }
}

 

getResponseCode()返回连接的状态码。这是一种有用的方式获得一些额外的信息的连接。一个200的状态代码表示成功。

转换InputStream到String

 

// Reads an InputStream and converts it to a String.
public String readIt(InputStream stream, int len) throws IOException, UnsupportedEncodingException {
    Reader reader = null;
    reader = new InputStreamReader(stream, "UTF-8");        
    char[] buffer = new char[len];
    reader.read(buffer);
    return new String(buffer);
}

 

 

 

二、管理网络

检查设备的网络连接

一个设备可以有各种类型的网络连接。这节课的重点是使用wi - fi或手机或网络连接，这个代码片段测试网络连接wi - fi和移动。它确定这些网络接口是可用的或连接的(即网络连接是否存在,如果可以建立套接字和传递数据)

 

private static final String DEBUG_TAG = "NetworkStatusExample";
...      
ConnectivityManager connMgr = (ConnectivityManager) 
        getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connMgr.getNetworkInfo(ConnectivityManager.TYPE_WIFI); 
boolean isWifiConn = networkInfo.isConnected();
networkInfo = connMgr.getNetworkInfo(ConnectivityManager.TYPE_MOBILE);
boolean isMobileConn = networkInfo.isConnected();
Log.d(DEBUG_TAG, "Wifi connected: " + isWifiConn);
Log.d(DEBUG_TAG, "Mobile connected: " + isMobileConn);

 

管理网络的使用

您可以实现一个首选项活动,让用户明确控制应用程序的使用网络资源。例如: 1.你可能允许用户上传的视频只有当设备被连接到wi - fi网络。2.你可能会同步(或没有)根据特定标准如网络可用性、时间间隔,等等。

编写一个应用程序,支持网络访问和管理网络的使用,你的清单必须有正确的权限和意图过滤器。

在样例应用程序中,这个规定了SettingsActivity,将显示一个UI让用户知道何时可以进行下载操作。

 

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.android.networkusage"
    ...>
<uses-sdk android:minSdkVersion="4" 
           android:targetSdkVersion="14" />
        
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
        ...>
        ...
        <activity android:label="SettingsActivity" android:name=".SettingsActivity">
             <intent-filter>
                <action android:name="android.intent.action.MANAGE_NETWORK_USAGE" />
                <category android:name="android.intent.category.DEFAULT" />
          </intent-filter>
        </activity>
    </application>
</manifest>

 

 

 

public class SettingsActivity extends PreferenceActivity implements OnSharedPreferenceChangeListener {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        // Loads the XML preferences file
        addPreferencesFromResource(R.xml.preferences);
    }
  
    @Override
    protected void onResume() {
        super.onResume();
// Registers a listener whenever a key changes            
        getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(this);
    }
  
    @Override
    protected void onPause() {
        super.onPause();
// Unregisters the listener set in onResume().
       // It's best practice to unregister listeners when your app isn't using them to cut down on 
       // unnecessary system overhead. You do this in onPause().            
       getPreferenceScreen().getSharedPreferences().unregisterOnSharedPreferenceChangeListener(this);    
    }
  
    // When the user changes the preferences selection, 
    // onSharedPreferenceChanged() restarts the main activity as a new
    // task. Sets the the refreshDisplay flag to "true" to indicate that 
    // the main activity should update its display.
    // The main activity queries the PreferenceManager to get the latest settings.
    
    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {    
        // Sets refreshDisplay to true so that when the user returns to the main
        // activity, the display refreshes to reflect the new settings.
        NetworkActivity.refreshDisplay = true;
    }
}

 

 

响应网络变动

如果有一个匹配发生在设置和设备的网络连接(例如,如果设置为“wi - fi”和设备有一个wi - fi连接)之间,应用程序下载提继续并刷新显示

 

public class NetworkActivity extends Activity {
    public static final String WIFI = "Wi-Fi";
    public static final String ANY = "Any";
    private static final String URL = "http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest";
   
    // Whether there is a Wi-Fi connection.
    private static boolean wifiConnected = false; 
    // Whether there is a mobile connection.
    private static boolean mobileConnected = false;
    // Whether the display should be refreshed.
    public static boolean refreshDisplay = true;
    
    // The user's current network preference setting.
    public static String sPref = null;
    
    // The BroadcastReceiver that tracks network connectivity changes.
    private NetworkReceiver receiver = new NetworkReceiver();
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        // Registers BroadcastReceiver to track network connection changes.
        IntentFilter filter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
        receiver = new NetworkReceiver();
        this.registerReceiver(receiver, filter);
    }
    
    @Override 
    public void onDestroy() {
        super.onDestroy();
        // Unregisters BroadcastReceiver when app is destroyed.
        if (receiver != null) {
            this.unregisterReceiver(receiver);
        }
    }
    
    // Refreshes the display if the network connection and the
    // pref settings allow it.
    
    @Override
    public void onStart () {
        super.onStart();  
        
        // Gets the user's network preference settings
        SharedPreferences sharedPrefs = PreferenceManager.getDefaultSharedPreferences(this);
        
        // Retrieves a string value for the preferences. The second parameter
        // is the default value to use if a preference value is not found.
        sPref = sharedPrefs.getString("listPref", "Wi-Fi");
updateConnectedFlags(); 
       
        if(refreshDisplay){
            loadPage();    
        }
    }
    
    // Checks the network connection and sets the wifiConnected and mobileConnected
    // variables accordingly. 
    public void updateConnectedFlags() {
        ConnectivityManager connMgr = (ConnectivityManager) 
                getSystemService(Context.CONNECTIVITY_SERVICE);
        
        NetworkInfo activeInfo = connMgr.getActiveNetworkInfo();
        if (activeInfo != null && activeInfo.isConnected()) {
            wifiConnected = activeInfo.getType() == ConnectivityManager.TYPE_WIFI;
            mobileConnected = activeInfo.getType() == ConnectivityManager.TYPE_MOBILE;
        } else {
            wifiConnected = false;
            mobileConnected = false;
        }  
    }
      
    // Uses AsyncTask subclass to download the XML feed from stackoverflow.com.
    public void loadPage() {
        if (((sPref.equals(ANY)) && (wifiConnected || mobileConnected))
                || ((sPref.equals(WIFI)) && (wifiConnected))) {
            // AsyncTask subclass
            new DownloadXmlTask().execute(URL);
        } else {
            showErrorPage();
        }
    }
...
    
}

 

检测网络连接变化

 

public class NetworkReceiver extends BroadcastReceiver {   
      
@Override
public void onReceive(Context context, Intent intent) {
    ConnectivityManager conn =  (ConnectivityManager)
        context.getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = conn.getActiveNetworkInfo();
       
    // Checks the user prefs and the network connection. Based on the result, decides whether
    // to refresh the display or keep the current display.
    // If the userpref is Wi-Fi only, checks to see if the device has a Wi-Fi connection.
    if (WIFI.equals(sPref) && networkInfo != null && networkInfo.getType() == ConnectivityManager.TYPE_WIFI) {
        // If device has its Wi-Fi connection, sets refreshDisplay
        // to true. This causes the display to be refreshed when the user
        // returns to the app.
        refreshDisplay = true;
        Toast.makeText(context, R.string.wifi_connected, Toast.LENGTH_SHORT).show();
// If the setting is ANY network and there is a network connection
    // (which by process of elimination would be mobile), sets refreshDisplay to true.
    } else if (ANY.equals(sPref) && networkInfo != null) {
        refreshDisplay = true;
                 
    // Otherwise, the app can't download content--either because there is no network
    // connection (mobile or Wi-Fi), or because the pref setting is WIFI, and there 
    // is no Wi-Fi connection.
    // Sets refreshDisplay to false.
    } else {
        refreshDisplay = false;
        Toast.makeText(context, R.string.lost_connection, Toast.LENGTH_SHORT).show();
    }
}

 

 

三、解析xml

上传和解析XML数据是很常见的任务,网络连接应用程序。这一课解释了如何解析XML文档并使用他们的数据

选择转换器

我们建议XmlPullParser,这是一种高效且可维护的方式来解析XML在Android里。

分析需求

 

<?xml version="1.0" encoding="utf-8"?> 
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:creativeCommons="http://backend.userland.com/creativeCommonsRssModule" ...">     
<title type="text">newest questions tagged android - Stack Overflow</title>
...
    <entry>
    ...
    </entry>
    <entry>
        <id>http://stackoverflow.com/q/9439999</id>
        <re:rank scheme="http://stackoverflow.com">0</re:rank>
        <title type="text">Where is my data file?</title>
        <category scheme="http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest/tags" term="android"/>
        <category scheme="http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest/tags" term="file"/>
        <author>
            <name>cliff2310</name>
            <uri>http://stackoverflow.com/users/1128925</uri>
        </author>
        <link rel="alternate" href="http://stackoverflow.com/questions/9439999/where-is-my-data-file" />
        <published>2012-02-25T00:30:54Z</published>
        <updated>2012-02-25T00:30:54Z</updated>
        <summary type="html">
            <p>I have an Application that requires a data file...</p>
</summary>
    </entry>
    <entry>
    ...
    </entry>
...
</feed>

 

实例化转化器

 

public class StackOverflowXmlParser {
    // We don't use namespaces
    private static final String ns = null;
   
    public List parse(InputStream in) throws XmlPullParserException, IOException {
        try {
            XmlPullParser parser = Xml.newPullParser();
            parser.setFeature(XmlPullParser.FEATURE_PROCESS_NAMESPACES, false);
            parser.setInput(in, null);
            parser.nextTag();
            return readFeed(parser);
        } finally {
            in.close();
        }
    }
 ... 
}
读取xml
private List readFeed(XmlPullParser parser) throws XmlPullParserException, IOException {
    List entries = new ArrayList();
parser.require(XmlPullParser.START_TAG, ns, "feed");
    while (parser.next() != XmlPullParser.END_TAG) {
        if (parser.getEventType() != XmlPullParser.START_TAG) {
            continue;
        }
        String name = parser.getName();
        // Starts by looking for the entry tag
        if (name.equals("entry")) {
            entries.add(readEntry(parser));
        } else {
            skip(parser);
        }
    }  
    return entries;
}

 

 

下面这个代码片段展示了如何解析器解析条目、标题、链接和总结：

 

public static class Entry {
    public final String title;
    public final String link;
    public final String summary;
private Entry(String title, String summary, String link) {
        this.title = title;
        this.summary = summary;
        this.link = link;
    }
}
  
// Parses the contents of an entry. If it encounters a title, summary, or link tag, hands them off
// to their respective "read" methods for processing. Otherwise, skips the tag.
private Entry readEntry(XmlPullParser parser) throws XmlPullParserException, IOException {
    parser.require(XmlPullParser.START_TAG, ns, "entry");
    String title = null;
    String summary = null;
    String link = null;
    while (parser.next() != XmlPullParser.END_TAG) {
        if (parser.getEventType() != XmlPullParser.START_TAG) {
            continue;
        }
        String name = parser.getName();
        if (name.equals("title")) {
            title = readTitle(parser);
        } else if (name.equals("summary")) {
            summary = readSummary(parser);
        } else if (name.equals("link")) {
            link = readLink(parser);
        } else {
            skip(parser);
        }
    }
    return new Entry(title, summary, link);
}
// Processes title tags in the feed.
private String readTitle(XmlPullParser parser) throws IOException, XmlPullParserException {
    parser.require(XmlPullParser.START_TAG, ns, "title");
    String title = readText(parser);
    parser.require(XmlPullParser.END_TAG, ns, "title");
    return title;
}
  
// Processes link tags in the feed.
private String readLink(XmlPullParser parser) throws IOException, XmlPullParserException {
    String link = "";
    parser.require(XmlPullParser.START_TAG, ns, "link");
    String tag = parser.getName();
    String relType = parser.getAttributeValue(null, "rel");  
    if (tag.equals("link")) {
        if (relType.equals("alternate")){
            link = parser.getAttributeValue(null, "href");
            parser.nextTag();
        } 
    }
    parser.require(XmlPullParser.END_TAG, ns, "link");
    return link;
}
// Processes summary tags in the feed.
private String readSummary(XmlPullParser parser) throws IOException, XmlPullParserException {
    parser.require(XmlPullParser.START_TAG, ns, "summary");
    String summary = readText(parser);
    parser.require(XmlPullParser.END_TAG, ns, "summary");
    return summary;
}
// For the tags title and summary, extracts their text values.
private String readText(XmlPullParser parser) throws IOException, XmlPullParserException {
    String result = "";
    if (parser.next() == XmlPullParser.TEXT) {
        result = parser.getText();
        parser.nextTag();
    }
    return result;
}
  ...
}

 

跳过无用的标签

 

private void skip(XmlPullParser parser) throws XmlPullParserException, IOException {
    if (parser.getEventType() != XmlPullParser.START_TAG) {
        throw new IllegalStateException();
    }
    int depth = 1;
    while (depth != 0) {
        switch (parser.next()) {
        case XmlPullParser.END_TAG:
            depth--;
            break;
        case XmlPullParser.START_TAG:
            depth++;
            break;
        }
    }
 }

 

使用xml数据

 

public class NetworkActivity extends Activity {
    public static final String WIFI = "Wi-Fi";
    public static final String ANY = "Any";
    private static final String URL = "http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest";
   
    // Whether there is a Wi-Fi connection.
    private static boolean wifiConnected = false; 
    // Whether there is a mobile connection.
    private static boolean mobileConnected = false;
    // Whether the display should be refreshed.
    public static boolean refreshDisplay = true; 
    public static String sPref = null;
...
      
    // Uses AsyncTask to download the XML feed from stackoverflow.com.
    public void loadPage() {  
      
        if((sPref.equals(ANY)) && (wifiConnected || mobileConnected)) {
            new DownloadXmlTask().execute(URL);
        }
        else if ((sPref.equals(WIFI)) && (wifiConnected)) {
            new DownloadXmlTask().execute(URL);
        } else {
            // show error
        }  
    }

 

动态下载xml

 

// Implementation of AsyncTask used to download XML feed from stackoverflow.com.
private class DownloadXmlTask extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... urls) {
        try {
            return loadXmlFromNetwork(urls[0]);
        } catch (IOException e) {
            return getResources().getString(R.string.connection_error);
        } catch (XmlPullParserException e) {
            return getResources().getString(R.string.xml_error);
        }
    }
@Override
    protected void onPostExecute(String result) {  
        setContentView(R.layout.main);
        // Displays the HTML string in the UI via a WebView
        WebView myWebView = (WebView) findViewById(R.id.webview);
        myWebView.loadData(result, "text/html", null);
    }
}

 

加载xml

 

// Uploads XML from stackoverflow.com, parses it, and combines it with
// HTML markup. Returns HTML string.
private String loadXmlFromNetwork(String urlString) throws XmlPullParserException, IOException {
    InputStream stream = null;
    // Instantiate the parser
    StackOverflowXmlParser stackOverflowXmlParser = new StackOverflowXmlParser();
    List<Entry> entries = null;
    String title = null;
    String url = null;
    String summary = null;
    Calendar rightNow = Calendar.getInstance(); 
    DateFormat formatter = new SimpleDateFormat("MMM dd h:mmaa");
        
    // Checks whether the user set the preference to include summary text
    SharedPreferences sharedPrefs = PreferenceManager.getDefaultSharedPreferences(this);
    boolean pref = sharedPrefs.getBoolean("summaryPref", false);
        
    StringBuilder htmlString = new StringBuilder();
    htmlString.append("<h3>" + getResources().getString(R.string.page_title) + "</h3>");
    htmlString.append("<em>" + getResources().getString(R.string.updated) + " " + 
            formatter.format(rightNow.getTime()) + "</em>");
        
    try {
        stream = downloadUrl(urlString);        
        entries = stackOverflowXmlParser.parse(stream);
    // Makes sure that the InputStream is closed after the app is
    // finished using it.
    } finally {
        if (stream != null) {
            stream.close();
        } 
     }
    
    // StackOverflowXmlParser returns a List (called "entries") of Entry objects.
    // Each Entry object represents a single post in the XML feed.
    // This section processes the entries list to combine each entry with HTML markup.
    // Each entry is displayed in the UI as a link that optionally includes
    // a text summary.
    for (Entry entry : entries) {       
        htmlString.append("<p><a href='");
        htmlString.append(entry.link);
        htmlString.append("'>" + entry.title + "</a></p>");
        // If the user set the preference to include summary text,
        // adds it to the display.
        if (pref) {
            htmlString.append(entry.summary);
        }
    }
    return htmlString.toString();
}
// Given a string representation of a URL, sets up a connection and gets
// an input stream.
private InputStream downloadUrl(String urlString) throws IOException {
    URL url = new URL(urlString);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setReadTimeout(10000 /* milliseconds */);
    conn.setConnectTimeout(15000 /* milliseconds */);
    conn.setRequestMethod("GET");
    conn.setDoInput(true);
    // Starts the query
    conn.connect();
    InputStream stream = conn.getInputStream();      
}

 

 

上面的片段代码为了帮助大家理解，这里还是老习惯，贴上项目的源代码，本来是有项目截图，运行效果图之类的图的，只是这篇文章太长了，再弄图片，就更占篇幅了，所以就只贴源码了，大家可以自己的运行起来看看，希望能从整体的架构和具体的代码细节上帮助到大家。