从ListView中的url获取图像

我试图从url获取图像，并通过我的listviewadapter类将其设置在listview中。我得到NULLPOINTEREXCEPTION的openConnection()在getBitmapFromUrl方法结尾。这是我的代码：

ListViewAdapter.java

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

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

public class ListViewAdapter extends BaseAdapter {

    public String msgType[] = new String[8];
    public String msgData[] = new String[8];
    public String msgTimeStamp[] = new String[8];
    Bitmap[] myBitmap = new Bitmap[8];
    public Activity context;
    int positionX;
    URL urlX;
    public LayoutInflater inflater;

    public ListViewAdapter(Activity context, String[] type, String[] data,
            String[] timestamp) {
        super();

        this.context = context;
        this.msgType = type;
        this.msgData = data;
        this.msgTimeStamp = timestamp;

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

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return msgData.length;
    }

    @Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return 0;
    }

    public static class ViewHolder {
        ImageView imgView;
        TextView txtType;
        TextView txtData;
        TextView txtTimestamp;
        Bitmap myBitmap;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub
        ViewHolder holder;
        if (convertView == null) {
            holder = new ViewHolder();
            convertView = inflater.inflate(R.layout.list_item, null);

            holder.imgView = (ImageView) convertView.findViewById(R.id.image);
            holder.txtType = (TextView) convertView.findViewById(R.id.type);
            holder.txtData = (TextView) convertView.findViewById(R.id.data);
            holder.txtTimestamp = (TextView) convertView
                    .findViewById(R.id.timestamp);

            convertView.setTag(holder);
        } else
            holder = (ViewHolder) convertView.getTag();

        // holder.imgView.setImageBitmap(myBitmap[position]);
        holder.txtType.setText(msgType[position]);
        holder.txtTimestamp.setText(msgTimeStamp[position]);
        if (msgType[position].equals("0"))
            holder.txtData.setText(msgData[position]);
        else {
            holder.txtData.setText("");
            URL url = null;
            try {
                url = new URL(msgData[position]);
            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
urlX=url;
        positionX=position;
        try {
        Thread t = new Thread(new Runnable() { 
            public void run(){         
                myBitmap[positionX] = getBitmapFromUrl(urlX);
            } 
        });
        t.start();

            t.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
            holder.imgView.setImageBitmap(myBitmap[position]);
        }

        return convertView;
    }





    public Bitmap getBitmapFromUrl(URL url1) {
        Bitmap Bitmap = null;
        try {
            HttpURLConnection connection = (HttpURLConnection) url1
                    .openConnection();
            connection.setDoInput(true);

            connection.connect();
            InputStream input = connection.getInputStream();
            Bitmap = BitmapFactory.decodeStream(input);

        } catch (IOException e) {
            e.printStackTrace();
        }
        return Bitmap;
    }

}

编辑：

Thread t = new Thread(new Runnable() { 
            public void run(){         
                myBitmap[positionX] = getBitmapFromUrl(urlX);
            } 
        });
        t.start();
        try {
            t.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

LOGCAT（编辑后）：

04-03 12:01:26.860: E/AndroidRuntime(11944): FATAL EXCEPTION: Thread-1155
04-03 12:01:26.860: E/AndroidRuntime(11944): java.lang.NullPointerException
04-03 12:01:26.860: E/AndroidRuntime(11944):    at com.appquest.flatcassign.ListViewAdapter.getBitmapFromUrl(ListViewAdapter.java:127)
04-03 12:01:26.860: E/AndroidRuntime(11944):    at com.appquest.flatcassign.ListViewAdapter$1.run(ListViewAdapter.java:107)
04-03 12:01:26.860: E/AndroidRuntime(11944):    at java.lang.Thread.run(Thread.java:856)