Android(安卓)Applications Tutorial 23. HTTP Internet Connection

Android Applications Tutorial
23. HTTP Internet Connection

23.0 HTTP Connection
23.1 HTTP Connection with Handler - Example 1
23.2 HTTP Connection with Handler - Example 2

23.0 HTTP Connection How do we set up internet connection from our Android activity? There is nothing new the way we make HTTP connection in Android platform compared with regular application written in core Java. However, in this tutorial, in additon to the Http Connection, we will see how the communication is done between threadand the Messageobject.

23.1 HTTP Connection with Handler - Example 1

In this example, we download an image and text from the web.

Let's preview the whole Java code first,HttpURLConncecitonA.java:

package com.bogotobogo.httpconnecta;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;import java.net.URLConnection;import android.app.Activity;import android.app.ProgressDialog;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.ImageView;import android.widget.TextView;public class HttpURLConnectionA extends Activity {    private ProgressDialog progressDialog;private Bitmap bitmap = null;private String text = null;/** Called when the activity is first created. */@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.main);        Button imageBtn = (Button)findViewById(R.id.Button01);Button textBtn = (Button)findViewById(R.id.Button02);        imageBtn.setOnClickListener( new OnClickListener() {@Overridepublic void onClick(View v) {downloadImage("http://www.bogotobogo.com/images/smalltiger.gif");}});        textBtn.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {downloadText("http://www.bogotobogo.com/android.html");}});}    private void downloadImage(String urlStr) {progressDialog = ProgressDialog.show(this, "", "Downloading Image from " + urlStr);final String url = urlStr;new Thread() {public void run() {InputStream in = null;Message msg = Message.obtain();msg.what = 1;try {    in = openHttpConnection(url);    bitmap = BitmapFactory.decodeStream(in);    Bundle b = new Bundle();    b.putParcelable("bitmap", bitmap);    msg.setData(b);    in.close();} catch (IOException e1) {    e1.printStackTrace();}messageHandler.sendMessage(msg);} }.start();}private void downloadText(String urlStr) {progressDialog = ProgressDialog.show(this, "", "Download Text from " + urlStr);final String url = urlStr;new Thread () {public void run() {int BUFFER_SIZE = 2000;        InputStream in = null;        Message msg = Message.obtain();        msg.what=2;        try {        in = openHttpConnection(url);                        InputStreamReader isr = new InputStreamReader(in);            int charRead;              text = "";              char[] inputBuffer = new char[BUFFER_SIZE];                  while ((charRead = isr.read(inputBuffer))>0)                  {                                          String readString =                           String.copyValueOf(inputBuffer, 0, charRead);                                          text += readString;                      inputBuffer = new char[BUFFER_SIZE];                  }                 Bundle b = new Bundle();    b.putString("text", text);    msg.setData(b);                  in.close();                  }catch (IOException e2) {                e2.printStackTrace();            }messageHandler.sendMessage(msg);}}.start();    }private InputStream openHttpConnection(String urlStr) {InputStream in = null;int resCode = -1;try {URL url = new URL(urlStr);URLConnection urlConn = url.openConnection();if (!(urlConn instanceof HttpURLConnection)) {throw new IOException ("URL is not an Http URL");}HttpURLConnection httpConn = (HttpURLConnection)urlConn;httpConn.setAllowUserInteraction(false);httpConn.setInstanceFollowRedirects(true);            httpConn.setRequestMethod("GET");            httpConn.connect();             resCode = httpConn.getResponseCode();                             if (resCode == HttpURLConnection.HTTP_OK) {                in = httpConn.getInputStream();                                             }         } catch (MalformedURLException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}return in;}private Handler messageHandler = new Handler() {public void handleMessage(Message msg) {super.handleMessage(msg);switch (msg.what) {case 1:ImageView img = (ImageView) findViewById(R.id.imageview01);img.setImageBitmap((Bitmap)(msg.getData().getParcelable("bitmap")));break;case 2:TextView text = (TextView) findViewById(R.id.textview01);text.setText(msg.getData().getString("text"));break;}progressDialog.dismiss();}};}

Since retrieving data from the internet can be a time consuming, we use a separate thread to get the image file. We should not use UI/main thread doing this kind of task as we discussed in the22. Threads.

How do we get a connection when we press the Download button?
At the click of a button, Download Image, we first open an HTTP connection to the server and then request for the image. Here is the our method,openHttpConnection(String urlStr), just for that task. Basically it is our utility code for retrieving image/text from the internet.

private InputStream openHttpConnection(String urlStr) {InputStream in = null;int resCode = -1;try {URL url = new URL(urlStr);URLConnection urlConn = url.openConnection();if (!(urlConn instanceof HttpURLConnection)) {throw new IOException ("URL is not an Http URL");}HttpURLConnection httpConn = (HttpURLConnection)urlConn;httpConn.setAllowUserInteraction(false);httpConn.setInstanceFollowRedirects(true);            httpConn.setRequestMethod("GET");            httpConn.connect();             resCode = httpConn.getResponseCode();                             if (resCode == HttpURLConnection.HTTP_OK) {                in = httpConn.getInputStream();                                             }         } catch (MalformedURLException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}return in;}

Here, we open aURLConnection

URLConnection urlConn = url.openConnection();

Then, checks if it is an instance ofHttpURLConnection

if (!(urlConn instanceof HttpURLConnection)) {throw new IOException ("URL is not an Http URL");}

then, sets the parameters required

HttpURLConnection httpConn = (HttpURLConnection)urlConn;httpConn.setAllowUserInteraction(false);httpConn.setInstanceFollowRedirects(true);httpConn.setRequestMethod("GET");

and made the connection by calling theconnect()method.

httpConn.connect();

Then, we check if the response code is OK and I get a handle to the input stream.

resCode = httpConn.getResponseCode();                 if (resCode == HttpURLConnection.HTTP_OK) {in = httpConn.getInputStream();                                 }

Now, we know how we are connected to the internet.
And we get input stream through the connection using a hard coded URL of our image.
Let's step back a little bit, and check how we feed that URL at the click of Download Image button.

Button imageBtn = (Button)findViewById(R.id.Button01);...imageBtn.setOnClickListener( new OnClickListener() {@Overridepublic void onClick(View v) {downloadImage("http://www.bogotobogo.com/images/smalltiger.gif");}});

So, actually downloading an image is done through our method,downloadImage(String url).

private void downloadImage(String urlStr) {progressDialog = ProgressDialog.show(this, "", "Downloading Image from " + urlStr);final String url = urlStr;new Thread() {public void run() {InputStream in = null;Message msg = Message.obtain();msg.what = 1;try {    in = openHttpConnection(url);    bitmap = BitmapFactory.decodeStream(in);    Bundle b = new Bundle();    b.putParcelable("bitmap", bitmap);    msg.setData(b);    in.close();} catch (IOException e1) {    e1.printStackTrace();}messageHandler.sendMessage(msg);} }.start();}

If we are not getting the image in a separate thread, we only need the three lines of code.

in = openHttpConnection(url);bitmap = BitmapFactory.decodeStream(in); ......in.close();

Open a connection, get the bitmap and close the connection. However, since this is a typical task with unpredictable response time, it should be done in a separate thread of its own.

So, before we start a new thread, we let the UI thread to show aProgressDialogfor our impatient user.

progressDialog = ProgressDialog.show(this, "", "Downloading Image from " + urlStr);

Then, we start a new thread. we make the URL string accessible within the new thread by making it a final variable. Since message is an object that we can use for communication between threads, we create aMessageobject in the thread. Then, we set the message number to 1, so that we can use it later.

Message msg = Message.obtain();msg.what = 1;

Then, we bundle our bitmap already put into aBundleobject that can be sent back in the Message object.

Bundle b = new Bundle();b.putParcelable("bitmap", bitmap);msg.setData(b);

Once we close the input stream as in:

in.close();

the thread has completed the job. So, we notify the main / UI thread through this method and also pass on the Message object:

messageHandler.sendMessage(msg);

Now we got the image through aMessagein a separate thread.

But, how do we retrieve the image from the Message object in the main thread?
As soon as the child thread notifies, the method called back in the main thread is thehandleMessage(msg)method.
It is inhandleMessage(msg)method that we retrieve the bitmap and set it to theImageViewin the UI.

private Handler messageHandler = new Handler() {public void handleMessage(Message msg) {super.handleMessage(msg);switch (msg.what) {case 1:ImageView img = (ImageView) findViewById(R.id.imageview01);img.setImageBitmap((Bitmap)(msg.getData().getParcelable("bitmap")));break;case 2:TextView text = (TextView) findViewById(R.id.textview01);text.setText(msg.getData().getString("text"));break;}progressDialog.dismiss();}};

Within this method, first we check themsg.whatvariable to see what the type of message we're expecting is. If it is 1, which we had set indownloadImage(..)method, then, we do the required things to get a handle to the ImageView object and then give the bitmap to it.

How do we get the data from the msg object?
ThroughgetData(). Then we use the keybitmapto retrieve the bitmap and cast it to Bitmap before setting it to the ImageView.
Finally, we dismiss the progress dialog.

Our layout file,/res/layout/main.xml:

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:orientation="vertical"    android:layout_width="fill_parent"    android:layout_height="fill_parent"    android:scrollbars="vertical">    <Button android:id="@+id/Button01" android:text="@string/button01" android:layout_height="wrap_content" android:layout_width="wrap_content"></Button><Button android:id="@+id/Button02" android:layout_height="wrap_content" android:text="@string/button02" android:layout_width="wrap_content"></Button><ImageView android:id="@+id/imageview01" android:layout_width="wrap_content" android:layout_height="wrap_content" android:cropToPadding="true" ></ImageView><TextView      android:id="@+id/textview01"    android:layout_width="fill_parent"     android:layout_height="wrap_content" /></LinearLayout>

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="http://schemas.android.com/apk/res/android"      android:versionCode="1"      android:versionName="1.0" package="com.bogotobogo.httpconnecta"><application android:icon="@drawable/icon"     android:label="@string/app_name"><activity android:name=".HttpURLConnectionA"                  android:label="@string/app_name"><intent-filter><action android:name="android.intent.action.MAIN" /><category android:name="android.intent.category.LAUNCHER" /></intent-filter></activity></application><uses-sdk android:minSdkVersion="7" /><uses-permission android:name="android.permission.INTERNET"></uses-permission></manifest>

Files used in this HTTP Connection example A,HttpURLConnectionA.zip

更多相关文章

随机推荐