内存缓存的使用

        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/btn_down"
        android:text="下载"/>

          android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:id="@+id/img"
       android:layout_below="@+id/btn_down"

       android:layout_centerInParent="true"/>

public class LruCacheUtils {
    //申明内存缓存
    private LruCache mLruCache;

    //在构造方法中进行初使化
    public LruCacheUtils(Context context) {
        //得到当前应用程序的内存
        int maxMemory=(int)Runtime.getRuntime().maxMemory();
        //内存缓存为当前应用程序的8分之1
        int cacheMemory=maxMemory/8;

        //进行初使化
        mLruCache=new LruCache(cacheMemory){
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getByteCount();//自定义bitmap数据大小的计算方式
            }
        };
    }

    /**
     * 保存图片到内存缓存
     * @param key 图片的url
     * @param bitmap 图片
     */
    public void savePicToMemory(String key,Bitmap bitmap){
        try {
            mLruCache.put(key,bitmap);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    /**
     * 通过key值得到缓存的图片
     * @param key 图片的url地址
     * @return Bitmap 或 null
     */
    public  Bitmap getPicFromMemory(String key){

        Bitmap bitmap=null;
        try {
            //通过key获取图片
            bitmap= mLruCache.get(key);
        } catch (Exception e) {
            e.printStackTrace();
        }

        return bitmap;
    }

}

public class MainActivity extends AppCompatActivity {

    private ImageView imageView;
    private LruCacheUtils lruCacheUtils;
    private String picUrl="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1513777051557&di=a6de503f11b6aa53a60e844e13c01f0c&imgtype=jpg&src=http%3A%2F%2Fimg3.imgtn.bdimg.com%2Fit%2Fu%3D3164275898%2C1502108289%26fm%3D214%26gp%3D0.jpg";
    private Button btnDown;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        imageView = findViewById(R.id.img);
        btnDown = findViewById(R.id.btn_down);

        //初使化内存缓存
        lruCacheUtils = new LruCacheUtils(this);

        //按钮的点击事件
        btnDown.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //首先判断内存缓存中有没有
                Bitmap bitmap=lruCacheUtils.getPicFromMemory(picUrl);

                if(bitmap!=null){
                    Log.d("zzz","从内存缓存中获取");
                    imageView.setImageBitmap(bitmap);
                }else {
                    Log.d("zzz","从网络中获取");
                    //进行网络下载图片
                    MyPicTask task=new MyPicTask();
                    task.execute(picUrl);
                }
            }
        });
    }

    class MyPicTask extends AsyncTask{

        @Override
        protected Bitmap doInBackground(String... strings) {

            try {
                //使用HttpUrlConnection
                URL url=new URL(strings[0]);
                HttpURLConnection connection=(HttpURLConnection) url.openConnection();
                connection.setRequestMethod("GET");
                connection.setReadTimeout(5000);
                connection.setConnectTimeout(5000);

                if(connection.getResponseCode()==200){
                    InputStream inputStream=connection.getInputStream();
                    return BitmapFactory.decodeStream(inputStream);
                }

            } catch (MalformedURLException e) {
                e.printStackTrace();
            }catch (IOException e){
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            imageView.setImageBitmap(bitmap);
            //保存在内存缓存中
            lruCacheUtils.savePicToMemory(picUrl,bitmap);
        }
    }

}