java - SingleView ImageLoader 显示 NullPointerException

Question

我对 Android 开发和 Java 总体来说是新手。我意识到这个问题可能看起来很基本，但尽管查看了许多论坛，阅读了有关异常的信息并调试了我的 Activity ，但我还是不理解。

我正在尝试将 JSON 图像数据解析到 GridView 中。

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Context;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.ImageView;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class JSONImageViewer extends Activity {

    JSONObject jsonobject;
    JSONArray jsonarray;
    ListViewAdapter adapter;
    ProgressDialog mProgressDialog;
    ArrayList<HashMap<String, String>> arraylist;

    static String TAG_IMG = "CarImageLink";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //Get the view from gridview_item.xml
        setContentView(R.layout.gridview_item);

        // Execute DownloadJSON AsyncTask
        new DownloadJSON().execute();
    }

    // DownloadJSON AsyncTask
    private class DownloadJSON extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Create a progressdialog
            mProgressDialog = new ProgressDialog(JSONImageViewer.this);

            // Set progressdialog message
            mProgressDialog.setMessage("Loading...");
            mProgressDialog.setIndeterminate(false);

            // Show progressdialog
            mProgressDialog.show();
        }

        @Override
        protected Void doInBackground(Void... params) {
            //Create an array
            arraylist = new ArrayList<HashMap<String, String>>();

            //Retrieve JSON Objects from the given URL address
            jsonobject = JSONfunctions
                    .getJSONfromURL("......");


            try {
                //Locate the array name in JSON
                jsonarray = jsonobject.getJSONArray("car_images");

                for (int i = 0; i < jsonarray.length(); i++) {
                    HashMap<String, String> map = new HashMap<String, String>();
                    jsonobject = jsonarray.getJSONObject(i);

                    //Retrieve JSON Object
                    map.put("CarImageLink", jsonobject.getString("CarImageLink"));

                    //Set the JSON Objects into the array
                    arraylist.add(map);
                }
            } catch (JSONException e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void args) {

            //Locate the Gridview in gridview_main.xml
            GridView gridview;
            gridview = (GridView) findViewById(R.id.gridview);

            adapter = new ListViewAdapter(JSONImageViewer.this, arraylist);

            //Set the adapter to the GridView
            gridview.setAdapter(adapter);

            //Close the progressdialog
            mProgressDialog.dismiss();
        }
    }

    public static class JSONfunctions {

        public static JSONObject getJSONfromURL(String url) {
            InputStream is = null;
            String result = "";
            JSONObject jArray = null;

            //Download JSON data from URL
            try {
                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost(url);
                HttpResponse response = httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();
                is = entity.getContent();

            } catch (Exception e) {
                Log.e("log_tag", "Error in http connection " + e.toString());
            }

            //Convert response to string
            try {
                BufferedReader reader = new BufferedReader(new InputStreamReader(is, "iso-8859-1"), 8);
                StringBuilder sb = new StringBuilder();
                String line = null;
                while ((line = reader.readLine()) != null) {
                    sb.append(line + "\n");
                }
                is.close();
                result = sb.toString();
            } catch (Exception e) {
                Log.e("log_tag", "Error converting result " + e.toString());
            }

            try {

                jArray = new JSONObject(result);
            } catch (JSONException e) {
                Log.e("log_tag", "Error parsing data " + e.toString());
            }

            return jArray;
        }
    }

    public class ListViewAdapter extends BaseAdapter {

        Context context;
        LayoutInflater inflater;
        ArrayList<HashMap<String, String>> data;
        ImageLoader imageLoader;
        HashMap<String, String> resultp = new HashMap<String, String>();
        public ListViewAdapter(Context context, ArrayList<HashMap<String, String>> arraylist) {
            this.context = context;
            data = arraylist;
            imageLoader = new ImageLoader(context);
        }

        @Override
        public int getCount() {
            return data.size();
        }

        @Override
        public Object getItem(int position) {
            return null;
        }

        @Override
        public long getItemId(int position) {
            return 0;
        }

        public View getView(final int position, View convertView, ViewGroup parent) {

            ImageView carimg;

            inflater = (LayoutInflater) context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

            View itemView = inflater.inflate(R.layout.gridview_item, parent, false);

            //Get the position
            resultp = data.get(position);
            //resultp.put(TAG_IMG,data.get(position).get("CarImageLink"));
            //resultp.put(TAG_IMG,data.get(position).get("CarImageLink))"));
            Log.v("resultp", resultp.toString());
            //Locate the ImageView in gridview_item.xml
            carimg = (ImageView) itemView.findViewById(R.id.car_img);
            //Capture position and set results to the ImageView
            //Passes images URL into ImageLoader.class
            int loader = R.drawable.temp_img;
            int stub_id = loader;
            imageLoader.DisplayImage(resultp.get(JSONImageViewer.TAG_IMG),stub_id, carimg);


            Log.v("resultp e1", (resultp.get(TAG_IMG).toString()));
            //Log.v("carimg", carimg.toString());
            Log.v("resultp e2", resultp.toString());


            //Capture ListView item click
            itemView.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View arg0) {
                    //Get the position
                    resultp = data.get(position);
                    //resultp.put(TAG_IMG,data.get(position).get("CarImageLink"));

                    Intent intent = new Intent(context, SingleItemView.class);

                    // Pass all data
                    intent.putExtra("CarImageLink", resultp.get(JSONImageViewer.TAG_IMG));

                    //Start SingleItemView Class
                    context.startActivity(intent);

                }
            });
            return itemView;
        }
    }

    public class ImageLoader {

        MemoryCache memoryCache=new MemoryCache();
        FileCache fileCache;
        private Map<ImageView, String> imageViews=Collections.synchronizedMap(new WeakHashMap<ImageView, String>());
        ExecutorService executorService;

        // Handler to display images in UI thread
        Handler handler = new Handler();

        public ImageLoader(Context context){
            fileCache=new FileCache(context);
            executorService=Executors.newFixedThreadPool(5);
        }

        int stub_id = R.drawable.temp_img;
        public void DisplayImage(String url, int loader, ImageView imageView)
        {
            try {

                stub_id = loader;
                imageViews.put(imageView, url);
                Bitmap bitmap=memoryCache.get(url);
                if(bitmap!=null) {
                    imageView.setImageBitmap(bitmap);
                    Log.v("Bitmap 3", bitmap.toString());
                }
                else
                {
                    queuePhoto(url, imageView);
                    imageView.setImageResource(stub_id);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }


        private void queuePhoto(String url, ImageView imageView) {
            PhotoToLoad p = new PhotoToLoad(url, imageView);
            executorService.submit(new PhotosLoader(p));
        }

        private Bitmap getBitmap(String url) {
            File f = fileCache.getFile(url);

            Bitmap b = decodeFile(f);
            if (b != null) {
                Log.v("bitmap 4", b.toString());
                return b;
            }
            //Download Images from the Internet
            try {
                Bitmap bitmap;
                // Log.v("Bitmap 5", bitmap.toString());
                Uri.Builder uri = Uri.parse("....").buildUpon();
                uri.appendPath(url);
                uri.build();
                Log.v("Build", uri.build().toString());
                Log.v("Uri", uri.toString());

                URL imageUrl = new URL ("http://"+ "....com" + url);
                Log.v("URL 3", imageUrl.toString());

                HttpURLConnection conn = (HttpURLConnection) imageUrl
                        .openConnection();
                conn.setConnectTimeout(30000);
                conn.setReadTimeout(30000);
                conn.setInstanceFollowRedirects(true);
                InputStream is = conn.getInputStream();
                OutputStream os = new FileOutputStream(f);
                Utils.CopyStream(is, os);
                os.close();
                conn.disconnect();
                bitmap = decodeFile(f);
                //Log.v("bitmap 6", bitmap.toString());
                return bitmap;
            } catch (Throwable ex) {
                ex.printStackTrace();
                if (ex instanceof OutOfMemoryError)
                    memoryCache.clear();
                return null;
            }
        }

        //Decodes image and scales it to reduce memory consumption
        private Bitmap decodeFile(File f) {
            try {
                //Decode image size
                BitmapFactory.Options o = new BitmapFactory.Options();
                o.inJustDecodeBounds = true;
                FileInputStream stream1 = new FileInputStream(f);
                BitmapFactory.decodeStream(stream1, null, o);
                stream1.close();

                //Find the correct scale value. It should be the power of 2.
                final int REQUIRED_SIZE = 70;
                int width_tmp = o.outWidth, height_tmp = o.outHeight;
                int scale = 1;
                while (true) {
                    if (width_tmp / 2 < REQUIRED_SIZE
                            || height_tmp / 2 < REQUIRED_SIZE)
                        break;
                    width_tmp /= 2;
                    height_tmp /= 2;
                    scale *= 2;
                }

                //Decode with inSampleSize
                BitmapFactory.Options o2 = new BitmapFactory.Options();
                o2.inSampleSize = scale;
                FileInputStream stream2 = new FileInputStream(f);
                Bitmap bitmap = BitmapFactory.decodeStream(stream2, null, o2);
                stream2.close();
                Log.v("bitmap 7", bitmap.toString());
                return bitmap;
            } catch (FileNotFoundException e) {
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }

        //Task for the queue
        private class PhotoToLoad {

            public String url;
            public ImageView imageView;

            public PhotoToLoad(String u, ImageView i) {
                url = u;
                imageView = i;
            }
        }

        class PhotosLoader implements Runnable {
            PhotoToLoad photoToLoad;

            PhotosLoader(PhotoToLoad photoToLoad) {
                this.photoToLoad = photoToLoad;
            }

            @Override
            public void run() {
                try {
                    //if (imageViewReused(photoToLoad))
                       // return;
                    Bitmap bmp = getBitmap(photoToLoad.url);
                    Log.v("URL 2", photoToLoad.url.toString());
                    Log.v("Bitmap  bmp", bmp.toString());
                    memoryCache.put(photoToLoad.url, bmp);

                    //if (imageViewReused(photoToLoad))
                        //return;
                    //BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);
                    //handler.post(bd);
                    //Log.v("bd", bd.toString());

                } catch (Throwable th) {
                    th.printStackTrace();
                }
            }
        }

       /* boolean imageViewReused(PhotoToLoad photoToLoad) {
            String tag = imageViews.get(photoToLoad.imageView);
            if (tag == null || !tag.equals(photoToLoad.url))
                return true;
            return false;
        }
*/
        class BitmapDisplayer implements Runnable {
            Bitmap bitmap;
            PhotoToLoad photoToLoad;
            public BitmapDisplayer(Bitmap b, PhotoToLoad p) {
                bitmap=b;
                Log.v("bitmap b", b.toString());
                photoToLoad=p;
            }
           public void run(){
              // if(imageViewReused(photoToLoad)) {
                   //Log.v("BITMAP", bitmap.toString());
                  // return;
               //}
               Log.v("BITMAP", bitmap.toString());
                if(bitmap != null) {
                    photoToLoad.imageView.setImageBitmap(bitmap);
                    Log.v("bitmap 2", bitmap.toString());
                }
                else {
                    photoToLoad.imageView.setImageResource(stub_id);
                }
            }
        }


        public void clearCache() {
            memoryCache.clear();
            fileCache.clear();
        }

    }
    public class MemoryCache {

        private static final String TAG = "MemoryCache";

        //Last argument true for LRU ordering
        private Map<String, Bitmap> cache = Collections
                .synchronizedMap(new LinkedHashMap<String, Bitmap>(10, 1.5f, true));

        //Current allocated size
        private long size = 0;

        //Max memory in bytes
        private long limit = 1000000;

        public MemoryCache() {
            //Use 25% of available heap size
            setLimit(Runtime.getRuntime().maxMemory() / 4);
        }

        public void setLimit(long new_limit) {
            limit = new_limit;
        }

        public Bitmap get(String id) {
            try {
                if (!cache.containsKey(id))
                    return null;
                return cache.get(id);
            } catch (NullPointerException ex) {
                ex.printStackTrace();
                return null;
            }
        }

        public void put(String id, Bitmap bitmap) {
            try {
                if (cache.containsKey(id))
                    size -= getSizeInBytes(cache.get(id));
                cache.put(id, bitmap);
                size += getSizeInBytes(bitmap);
                checkSize();
            } catch (Throwable th) {
                th.printStackTrace();
            }
        }

        private void checkSize() {
            Log.i(TAG, "cache size=" + size + " length=" + cache.size());
            if (size > limit) {

                //Least recently accessed item will be the first one iterated
                Iterator<Map.Entry<String, Bitmap>> iter = cache.entrySet().iterator();

                while (iter.hasNext()) {
                    Map.Entry<String, Bitmap> entry = iter.next();
                    size -= getSizeInBytes(entry.getValue());
                    iter.remove();
                    if (size <= limit)
                        break;
                }
                Log.i(TAG, "Clean cache. New size " + cache.size());
            }
        }

        public void clear() {
            try {
                cache.clear();
                size = 0;
            } catch (NullPointerException ex) {
                ex.printStackTrace();
            }
        }

        long getSizeInBytes(Bitmap bitmap) {
            if (bitmap == null)
                return 0;
            return bitmap.getRowBytes() * bitmap.getHeight();
        }
    }

    public class FileCache {

        private File cacheDir;

        public FileCache(Context context) {

            //Find the dir to save cached images
            if (android.os.Environment.getExternalStorageState().equals(
                    android.os.Environment.MEDIA_MOUNTED))
                cacheDir = new File(
                        android.os.Environment.getExternalStorageDirectory(),
                        "");
            else
                cacheDir = context.getCacheDir();
            if (!cacheDir.exists())
                cacheDir.mkdirs();
        }

        public File getFile(String url) {
            String filename = String.valueOf(url.hashCode());
            //String filename = URLEncoder.encode(url);
            File f = new File(cacheDir, filename);
            return f;

        }

        public void clear() {
            File[] files = cacheDir.listFiles();
            if (files == null)
                return;
            for (File f : files)
                f.delete();
        }

    }
    public static class Utils {
        public static void CopyStream(InputStream is, OutputStream os)
        {
            final int buffer_size=1024;
            try
            {
                byte[] bytes=new byte[buffer_size];
                for(;;)
                {
                    int count=is.read(bytes, 0, buffer_size);
                    if(count==-1)
                        break;
                    os.write(bytes, 0, count);
                }
            }
            catch(Exception ex){}
        }
    }

    public class SingleItemView extends Activity {

        String carimg;
        ImageLoader imageLoader = new ImageLoader(this);

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);

            //Get the view from singleitemview.xml
            setContentView(R.layout.singleitemview);

            Intent i = getIntent();
            //Get the result of CarImageLink
            carimg = i.getStringExtra("CarImageLink");
            Log.v("carimg", i.getStringExtra("CarImageLink").toString());

            int loader = R.drawable.temp_img;
            int stub_id = loader;
            //Locate the ImageView in singleitemview.xml
            ImageView img_car = (ImageView) findViewById(R.id.car_img);
            Log.v("Imageview img_car", img_car.toString());

            //Capture position and set results to the ImageView
            //Passes carimg images URL into ImageLoader.class
            imageLoader.DisplayImage(carimg, stub_id, img_car);
        }
    }

griview_item.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >

    <GridView
        android:id="@+id/gridview"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" />

</RelativeLayout>

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >


    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/cars"/>

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/car_id"
        android:text="@string/id"/>

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/car_vin"
        android:layout_below="@id/car_id"
        android:text="@string/vin"/>

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/model_img"
        android:layout_toRightOf="@id/car_id"/>

    <ListView
        android:id="@+id/list"
        android:layout_weight="1"
        android:layout_height="wrap_content"
        android:layout_width="fill_parent"/>

</LinearLayout>

singleitemview.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >

    <ImageView
        android:id="@+id/car_img"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:background="#000000"
        android:padding="1dp"
        android:src="@drawable/temp_img" />

</RelativeLayout>

任何有关如何解决这些错误的建议将不胜感激。谢谢。

更新:

java.lang.NullPointerException
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at com.example.justin.myapplication.JSONImageViewer$ImageLoader.DisplayImage(JSONImageViewer.java:290)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at com.example.justin.myapplication.JSONImageViewer$ListViewAdapter.getView(JSONImageViewer.java:229)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.AbsListView.obtainView(AbsListView.java:2143)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.GridView.makeAndAddView(GridView.java:1341)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.GridView.makeRow(GridView.java:341)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.GridView.fillDown(GridView.java:283)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.GridView.fillFromTop(GridView.java:417)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.GridView.layoutChildren(GridView.java:1229)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.AbsListView.onLayout(AbsListView.java:1994)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.view.View.layout(View.java:14008)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.view.ViewGroup.layout(ViewGroup.java:4373)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1021)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.view.View.layout(View.java:14008)
07-30 15:30:54.609  29426-29426/com.example.justin.myapplication W/System.err﹕ at android.view.ViewGroup.layout(ViewGroup.java:4373)

imageView.setImageResource(stub_id);对于(JSONImageViewer.java:290)imageLoader.DisplayImage(resultp.get(JSONImageViewer.TAG_IMG),stub_id, carimg);对于(JSONImageViewer.java:229)

Answer 1

Android 中的直接调试内容。

面对看似难以理解的异常堆栈跟踪，技巧是扫描例程列表，查找代码中第一次出现的源代码行。在那一行设置一个断点，然后看看实际发生了什么。执行此操作时，您还应该查找“由”引起的内部异常。当存在内部异常时，您将在内部异常的堆栈跟踪中找到问题。

在您的情况下，感兴趣的行是堆栈跟踪中的第一行:

at com.example.justin.myapplication.JSONImageViewer$ImageLoader.DisplayImage(JSONImageViewer.java:287)

JSONImageViewer.java第287行发生空指针异常。

在调试器中的该行上设置断点，然后查看发生了什么。您只需双击堆栈跟踪中的文件和行号即可直接转到源代码行。

我不知道哪一行是第287行。但这似乎与您正在构造的URL有关。一旦您在第 287 行设置断点，您应该能够很容易地弄清楚这一点。