android访问网络,可以使用java.net.HttpURLConnection类，下面的例子是在点一个button的时候，启动一个新的线程，下载一个网络图片数据，在ImageView上进行显示：
NetActivity.java
package Test.wangfu;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import android.app.Activity;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
public class NetActivity extends Activity {
??? private ImageView view=null;
??? private android.graphics.Bitmap bmp=null;
??? private TextView text=null;
??? @Override
??? protected void onCreate(Bundle savedInstanceState) {
??? ??? // TODO Auto-generated method stub
??? ??? super.onCreate(savedInstanceState);
??? ??? this.setContentView(R.layout.net);
??? ???
??? ??? Button btn=(Button)this.findViewById(R.id.downloadButton);
??? ??? view=(ImageView)this.findViewById(R.id.imageView2);
??? ??? text=(TextView)this.findViewById(R.id.label);
??? ??? btn.setOnClickListener(new OnClickListener(){
??? ??? ??? public void onClick(View arg0) {
??? ??? ??? ??? HandlerThread thread=new HandlerThread("netthread");
??? ??? ??? ??? thread.start();
??? ??? ??? ??? Handler r=new Handler(thread.getLooper());
??? ??? ??? ??? text.setText("启动下载图片线程");
??? ??? ??? ??? r.postDelayed(new java.lang.Runnable(){
??? ??? ??? ??? ??? public void run() {
??? ??? ??? ??? ??? ??? try {
??? ??? ??? ??? ??? ??? ??? java.net.URL url=new URL("http://www.taihe.com.cn/images/logo.gif");
??? ??? ??? ??? ??? ??? ??? try {
??? ??? ??? ??? ??? ??? ??? ??? java.net.HttpURLConnection conn=(HttpURLConnection)url.openConnection();
??? ??? ??? ??? ??? ??? ??? ??? NetActivity.this.runOnUiThread(new Runnable(){
??? ??? ??? ??? ??? ??? ??? ??? ??? public void run() {
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? // TODO Auto-generated method stub
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? text.setText("开始请求网络图片资源");
??? ??? ??? ??? ??? ??? ??? ??? ??? }});
??? ??? ??? ??? ??? ??? ??? ??? conn.setDoInput(true);
??? ??? ??? ??? ??? ??? ??? ??? conn.setUseCaches(false);
??? ??? ??? ??? ??? ??? ??? ??? conn.setRequestMethod("GET");
??? ??? ??? ??? ??? ??? ??? ??? conn.setConnectTimeout(5000);
??? ??? ??? ??? ??? ??? ??? ??? java.io.InputStream is=conn.getInputStream();
??? ??? ??? ??? ??? ??? ??? ??? final int length = (int) conn.getContentLength();
??? ??? ??? ??? ??? ??? ??? ??? NetActivity.this.runOnUiThread(new Runnable(){
??? ??? ??? ??? ??? ??? ??? ??? ??? public void run() {
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? // TODO Auto-generated method stub
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? text.setText("数据总长度为："+String.valueOf(length));
??? ??? ??? ??? ??? ??? ??? ??? ??? }});
??? ??? ??? ??? ??? ??? ??? ??? if (length != -1) {
??? ??? ??? ??? ??? ??? ??? ??? ??? byte[] imgData = new byte[length];
??? ??? ??? ??? ??? ??? ??? ??? ??? byte[] buffer = new byte[512];
??? ??? ??? ??? ??? ??? ??? ??? ??? int readLen = 0;
??? ??? ??? ??? ??? ??? ??? ??? ??? int destPos = 0;
??? ??? ??? ??? ??? ??? ??? ??? ??? while ((readLen = is.read(buffer)) > 0) {
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? System.arraycopy(buffer, 0, imgData, destPos, readLen);
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? destPos += readLen;
??? ??? ??? ??? ??? ??? ??? ??? ??? }
??? ??? ??? ??? ??? ??? ??? ??? ??? bmp = BitmapFactory.decodeByteArray(imgData, 0, imgData.length);
??? ??? ??? ??? ??? ??? ??? ??? ??? NetActivity.this.runOnUiThread(new Runnable(){
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? public void run() {
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? // TODO Auto-generated method stub
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? view.setImageBitmap(bmp);
??? ??? ??? ??? ??? ??? ??? ??? ??? ??? }});
??? ??? ??? ??? ??? ??? ??? ??? ???
??? ??? ??? ??? ??? ??? ??? ??? }
??? ??? ??? ??? ??? ??? ??? ??? conn.disconnect();
??? ??? ??? ??? ??? ??? ??? ???
??? ??? ??? ??? ??? ??? ??? } catch (final IOException ex) {
??? ??? ??? ??? ??? ??? ??? ??? ex.printStackTrace();
??? ??? ??? ??? ??? ??? ??? }
??? ??? ??? ??? ??? ??? } catch (final MalformedURLException e) {
????????????????????????????? e.printStackTrace();
??? ??? ??? ??? ??? ??? }
??? ??? ??? ??? ??? ???
??? ??? ??? ??? ??? }}, 1000);
??? ??? ??? ???
??? ??? ??? }});
??? }
}
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net.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
??? android:layout_width="match_parent"
??? android:layout_height="match_parent"
??? android:orientation="vertical" >
??? <TextView
??????? android:id="@+id/label"
??????? android:layout_width="fill_parent"
??????? android:layout_height="wrap_content"
??????? android:text="Enter Number to Dial:" />
???? <ImageView
??????? android:id="@+id/imageView2"
??????? android:layout_width="200dp"
??????? android:layout_height="wrap_content"
??????? android:layout_weight="0.89" />
????
???? <Button
??????? android:id="@+id/downloadButton"
??????? android:layout_width="wrap_content"
??????? android:layout_height="wrap_content"
??????? android:text="DownLoad" />
</LinearLayout>
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由于使用网络，需要在AndroidManifest.xml中添加权限。
<permission android:protectionLevel="normal" android:name="android.permission.INTERNET"></permission>
<uses-permission android:name="android.permission.INTERNET" />
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说明：
1、如果当网络访问直接在protected void onCreate(Bundle savedInstanceState)方法中执行的话，会出现android.os.NetworkOnMainThreadException错误，这是一个Android开发规范反馈的错误。
可以通过在onCreate中添加代码：
StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
??????? .detectDiskReads()
??????? .detectDiskWrites()
??????? .detectNetwork()?? // or .detectAll() for all detectable problems
??????? .penaltyLog()
??????? .build());
??????? StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder()
??????? .detectLeakedSqlLiteObjects()
??????? .detectLeakedClosableObjects()
??????? .penaltyLog()
??????? .penaltyDeath()
??????? .build());
来解决。一般来讲，网络访问需要一定的请求时间，不建议网络访问在主线程中执行。
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2、URL参数问题
java.net.URL url=new URL("http://www.taihe.com.cn/images/logo.gif");
如果java.net.URL不能正常解析参数的URL地址，可能会在 java.io.InputStream is=conn.getInputStream();这
语句的地方出错。测试时，可以先通过模拟器的浏览器先输入地址，看是否正常可以显示。
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