缓存工具类

public class LruUtils {
    //实例化Lrucache对象
    private LruCache<String, Bitmap> lruCache;
    public LruUtils(){
        int l = (int)Runtime.getRuntime().maxMemory();
        lruCache=new LruCache<String,Bitmap>(l/8){
            //重写sizeOf方法返回每个对象的大小
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getByteCount();
            }
        };
    }
    //拿图片
    public Bitmap get(String k){
       return lruCache.get(k);
    }
    //存图片
    public void set(String k,Bitmap v){
        lruCache.put(k,v);
    }

}

SD卡读取工具类

public class SDcard {
    private static SDcard sDcard=new SDcard();
    private SDcard(){

    }
    public static SDcard getInstance(){
            return sDcard;
    }

    //存图片
    public static void setBitmap(String name, Bitmap bitmap) throws FileNotFoundException {
        if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
            File file=Environment.getExternalStorageDirectory();
            File file1=new File(file,name);

            bitmap.compress(Bitmap.CompressFormat.JPEG,100,new FileOutputStream(file1));
        }

    }

    //读图片
    public static Bitmap getBitmap(String name){
        //获取路径存储图片
        if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
            File file=Environment.getExternalStorageDirectory();
            File file1=new File(file,name);

            return BitmapFactory.decodeFile(file1.getAbsolutePath());
        }
        return null;
    }

}

网络下载图片类

public class Workthread extends AsyncTask<String,Void, Bitmap> {
    public static Bitmap getBitmap(String url) throws ExecutionException, InterruptedException {
        return new Workthread().execute(url).get();
    }
    @Override
    protected Bitmap doInBackground(String... strings) {
        try {
            URL url=new URL(strings[0]);
            HttpURLConnection http = (HttpURLConnection) url.openConnection();
            http.setConnectTimeout(5000);
            http.setRequestMethod("GET");
            http.connect();

            if(http.getResponseCode()==200){
                InputStream is = http.getInputStream();
                Bitmap bitmap = BitmapFactory.decodeStream(is);
                return bitmap;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

Main

public void click() throws Exception {
        //先从内存读
        Bitmap bitmap = lruUtils.get("aaa");
        if(bitmap!=null){
            imageView.setImageBitmap(bitmap);
            Toast.makeText(this,"图片来自内存",Toast.LENGTH_SHORT).show();
        }else{
            //从sd卡读
            Bitmap bitmap1 = SDcard.getBitmap("aaa.jpg");
            if(bitmap1!=null){
                imageView.setImageBitmap(bitmap1);
                Toast.makeText(this,"图片来自sd卡",Toast.LENGTH_SHORT).show();
                lruUtils.set("aaa",bitmap1);
            }else{
                Bitmap bitmap2 = Workthread.getBitmap("http://www.qubaobei.com/ios/cf/uploadfile/132/9/8289.jpg");
                if(bitmap2!=null){
                    imageView.setImageBitmap(bitmap2);
                    Toast.makeText(this,"图片来自网络",Toast.LENGTH_SHORT).show();

                    SDcard.setBitmap("aaa.jpg",bitmap2);
                    lruUtils.set("aaa",bitmap2);
                }else{
                    Toast.makeText(this,"查无此图片",Toast.LENGTH_SHORT).show();
                }

            }
        }
    }

效果图