Question

我从网址下载了一张图片并显示在ImageView中。这个例子在2.3.3模拟器上工作正常但是当我在4.0或4.1上尝试它时，它崩溃了消息：

不幸的是，AndroidWebImageExample已停止。

LogCat显示我收到NullPointerException。有谁知道为什么它不适用于4.0或4.1？

MainActivity.java

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.widget.ImageView;

public class MainActivity extends Activity {

    String image_URL = "http://4.bp.blogspot.com/_C5a2qH8Y_jk/StYXDpZ9-WI/AAAAAAAAAJQ/sCgPx6jfWPU/S1600-R/android.png";

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ImageView bmImage = (ImageView) findViewById(R.id.image);
        BitmapFactory.Options bmOptions;
        bmOptions = new BitmapFactory.Options();
        bmOptions.inSampleSize = 1;
        Bitmap bm = LoadImage(image_URL, bmOptions);
        bmImage.setImageBitmap(bm);
    }

    private Bitmap LoadImage(String URL, BitmapFactory.Options options) {
        Bitmap bitmap = null;
        InputStream in = null;
        try {
            in = OpenHttpConnection(URL);
            bitmap = BitmapFactory.decodeStream(in, null, options);
            in.close();
        } catch (IOException e1) {
        }
        return bitmap;
    }

    private InputStream OpenHttpConnection(String strURL) throws IOException {
        InputStream inputStream = null;
        URL url = new URL(strURL);
        URLConnection conn = url.openConnection();

        try {
            HttpURLConnection httpConn = (HttpURLConnection) conn;
            httpConn.setRequestMethod("GET");
            httpConn.connect();

            if (httpConn.getResponseCode() == HttpURLConnection.HTTP_OK) {
                inputStream = httpConn.getInputStream();
            }
        } catch (Exception ex) {
        }
        return inputStream;
    }

}

activity_main.xml中

<?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"
   >
<TextView 
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:text="hello"
   />
<ImageView
   android:id="@+id/image"
   android:scaleType="centerCrop"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
/>
</LinearLayout>

logcat的

07-20 14:09:08.689: D/AndroidRuntime(1894): Shutting down VM
07-20 14:09:08.689: W/dalvikvm(1894): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
07-20 14:09:08.699: E/AndroidRuntime(1894): FATAL EXCEPTION: main
07-20 14:09:08.699: E/AndroidRuntime(1894): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidwebimageexample/com.example.androidwebimageexample.MainActivity}: java.lang.NullPointerException
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.os.Looper.loop(Looper.java:137)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.ActivityThread.main(ActivityThread.java:4745)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at java.lang.reflect.Method.invokeNative(Native Method)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at java.lang.reflect.Method.invoke(Method.java:511)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at dalvik.system.NativeStart.main(Native Method)
07-20 14:09:08.699: E/AndroidRuntime(1894): Caused by: java.lang.NullPointerException
07-20 14:09:08.699: E/AndroidRuntime(1894):     at com.example.androidwebimageexample.MainActivity.LoadImage(MainActivity.java:43)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at com.example.androidwebimageexample.MainActivity.onCreate(MainActivity.java:33)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.Activity.performCreate(Activity.java:5008)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
07-20 14:09:08.699: E/AndroidRuntime(1894):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
07-20 14:09:08.699: E/AndroidRuntime(1894):     ... 11 more

Answer 1

由于某种原因，InputStream为NULL，因此抛出NullPointerExeption。在你的OpenHttpConnection方法中，添加catch Log.i("OpenHttpConnection",ex.getMessage());并查看原因。

不要阻止UI线程，必须使用AsyncTask，Service或自定义线程。你想要这个班http://code.google.com/p/and-bookworm/source/browse/trunk/src/com/totsp/bookworm/data/HTTPRequestHelper.java?r=31。在构造函数中，传递一个自定义的ResponseHandler并进行所需的操作。

Answer 2

您正在主线程上执行（可能很慢）网络操作。如果您的目标SDK是11（Honeycomb）或更高，这将在Honeycomb或更高版本上抛出NetworkOnMainThreadException，因为此行为可能会阻止UI并导致应用程序无响应。即使是在Honeycomb之前的设备上this behaviour is discouraged。

您可以使用AsyncTask解决此问题，在doInBackground(..)中加载数据。

Answer 3

您应该将网络操作从UI线程移动到服务或AsyncTask

您可以在此处阅读更多内容：http://www.androiddesignpatterns.com/2012/06/app-force-close-honeycomb-ics.html

下载图像在2.3.3中工作，但在4.0或4.1中不起作用

3 个答案: