加载网络图片（若SDCard有，则本地加载）

布局文件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    >    <Button        android:onClick="download"        android:id="@+id/button"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="下载图片"         android:layout_centerHorizontal="true"/>    <ImageView         android:id="@+id/image_view"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_centerHorizontal="true"        android:layout_below="@id/button"/></RelativeLayout>

MainActivity：

package com.example.zjday13_download_picture;import android.app.Activity;import android.graphics.Bitmap;import android.os.Bundle;import android.util.Log;import android.view.View;import android.widget.ImageView;import com.example.zjday13_download_picture.utils.FileUtil;import com.example.zjday13_download_picture.utils.MyTask;public class MainActivity extends Activity {    private ImageView imageView;    String path ="https://www.baidu.com/img/bd_logo1.png";    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        imageView=(ImageView) findViewById(R.id.image_view);    }    /**     * 下载图片     * @param v     */    public void download(View v){        //先判断，如果SD卡有图片，直接加载否则开启异步任务下载图片        Bitmap bitmap =FileUtil.readImage(path);        if(bitmap!=null){            imageView.setImageBitmap(bitmap);            Log.i("info", "本地有图片，从本地加载");        }else{            new MyTask(getApplicationContext(),imageView).execute(path);            Log.i("info", "从网络下载图片");        }    }}

文件的工具类：

package com.example.zjday13_download_picture.utils;import java.io.File;import java.io.FileOutputStream;import java.io.IOException;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Environment;public class FileUtil {    //保存图片的缓存路径    //Environment.getExternalStorageDirectory() 得到sd卡的根目录    private static final String PATH=Environment.getExternalStorageDirectory()            +"/downloadpicture/images";    public static boolean isMountend(){        //获取sd卡的状态        String state =Environment.getExternalStorageState();        return state.equals(Environment.MEDIA_MOUNTED);    }    //从图片路径获取图片的名字    public static String getFileName(String url){        return url.substring(url.lastIndexOf("/")+1);       }    public static void saveImage(String url, byte[] buffer) throws IOException {        //1.判断sd卡状态，如果sd卡不可用，则直接返回        if(!isMountend()) return;        //2.判断当前缓存的目录是否存在        File dir=new File(PATH);        if(!dir.exists()){            dir.mkdirs();//级联创建        }        //3.将图片的字节数写到指定的文件中(需要知道文件的缓存目录和图图片的名字)        FileOutputStream fos=new FileOutputStream(new File(dir, getFileName(url)));        fos.write(buffer);        fos.close();    }    /**     * 根据传入的图片的路径，获取sd卡中缓存的图片     * @param path2     * @return     */    public static Bitmap readImage(String url) {        if(!isMountend()) return null;        File imageFile =new File(PATH,getFileName(url));        if(imageFile.exists())        {            //把指定路径的文件转换成Bitmap格式            return BitmapFactory.decodeFile(imageFile.getAbsolutePath());        }        return null;    }}

异步下载任务：

package com.example.zjday13_download_picture.utils;import java.io.ByteArrayOutputStream;import java.io.InputStream;import java.net.HttpURLConnection;import java.net.URL;import android.content.Context;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.AsyncTask;import android.widget.ImageView;import android.widget.Toast;public class MyTask extends AsyncTask<String, Void, Bitmap> {    private Context context;    private ImageView imageView;    public MyTask(Context context,ImageView imageView) {        super();        this.context=context;        this.imageView = imageView;    }    @Override    protected Bitmap doInBackground(String... params) {        String path =params[0];        try {            URL url =new URL(path);            HttpURLConnection conn=(HttpURLConnection) url.openConnection();            conn.setRequestMethod("GET");            conn.connect();            if(conn.getResponseCode()==200){                InputStream inStream=conn.getInputStream();                ByteArrayOutputStream baso=new ByteArrayOutputStream();                int len =0;                byte[]buffer=new byte[1024];                while((len =inStream.read(buffer))!=-1){                    baso.write(buffer, 0, len);                }                buffer =baso.toByteArray();                //将获取的数据写入Sd卡中                FileUtil.saveImage(path,buffer);                Bitmap bitmap=BitmapFactory.decodeByteArray(buffer, 0, buffer.length);                return bitmap;            }        } catch (Exception e) {            e.printStackTrace();        }        return null;    }    @Override    protected void onPostExecute(Bitmap result) {        super.onPostExecute(result);        if(result!=null){            imageView.setImageBitmap(result);            Toast.makeText(context, "加载。。。。", Toast.LENGTH_LONG).show();        }    }}