想起刚開始写代码的时候,领导叫我写一个头像下载的方法,当时屁颠屁颠就写了一个图片下载的,每次都要去网络上请求,最后直接被pass掉了
当时的思路是这种
后来渐渐地就知道了有二级缓存这东西。
自己也阅读过非常多关于双缓存的文章。
APP开发到越后面。对性能的要求越高。那么双缓存的优势就逐渐体现出来了。
所谓图片双缓存。首先到执行内存中请求,再到sd卡请求,最后到网络请求,流程图例如以下
那我们从第一部開始解析
1.先看 内存缓存的代码
[java] view
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- public class MemoryCache implements ImageCache {
- private static final String TAG = MemoryCache.class.getSimpleName();
- private LruCache<String,Bitmap> mMemoryCache;
- public MemoryCache(){
- init();
- }
- private void init(){
- final int maxMemory = (int) (Runtime.getRuntime().maxMemory()/1024);
- final int cacheSize = maxMemory/4;
- mMemoryCache = new LruCache<String,Bitmap>(cacheSize){
- @Override
- protected int sizeOf(String key, Bitmap value) {
- return value.getRowBytes()*value.getHeight()/1024;
- }
- };
- }
- @Override
- public Bitmap get(String key) {
- Bitmap bitmap = mMemoryCache.get(key);
- if (bitmap!=null){
- Log.i(TAG,"File is exist in memory");
- }
- return mMemoryCache.get(key);
- }
- @Override
- public void put(String key, Bitmap bitmap) {
- if (get(key)==null) {
- mMemoryCache.put(key, bitmap);
- }
- }
- }
[java] view
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- private void init()
init()方法中对一些变量进行初始化,mMemoryCache用于在内存中缓存图片
[java] view
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- public Bitmap get(String key) {}
get()方法用于从内存中获得缓存
[java] view
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- public void put(String key, Bitmap bitmap) {}
put()方法将下载好的图片缓存到内存中,方便下次使用
2.再看sd卡缓存
[java] view
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- public class DiskCache implements ImageCache {
- private static final String TAG = DiskCache.class.getSimpleName();
- static String mPath ;
- public DiskCache(Context context){
- init(context);
- }
- private void init(Context context){
- // 获取图片缓存路径
- mPath = getDiskCachePath(context,"bitmap");
- File cacheDir = new File(mPath);
- if (!cacheDir.exists()) {
- cacheDir.mkdirs();
- }
- }
- @Override
- public Bitmap get(String key) {
- File file = new File(mPath+key);
- if (file.exists()){
- return BitmapFactory.decodeFile(mPath+key);
- }
- return null;
- }
- @Override
- public void put(String key, Bitmap bitmap) {
- FileOutputStream fileOutputStream = null;
- try {
- File file = new File(mPath+key);
- if (file.exists()){
- Log.i(TAG,"File is exist on disk");
- }
- fileOutputStream = new FileOutputStream(mPath+key);
- bitmap.compress(Bitmap.CompressFormat.PNG,100,fileOutputStream);
- } catch (FileNotFoundException e) {
- e.printStackTrace();
- }finally {
- CloseUtils.closeQuietly(fileOutputStream);
- }
- }
- /**
- * 依据传入的dir获得路径
- * @param context
- * @param dir
- * @return
- */
- public String getDiskCachePath(Context context, String dir) {
- String cachePath;
- if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())
- || !Environment.isExternalStorageRemovable()) {
- cachePath = context.getExternalCacheDir().getPath();
- } else {
- cachePath = context.getCacheDir().getPath();
- }
- return cachePath + File.separator + dir;
- }
- }
相同
[java] view
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- private void init()
init()方法中对一些变量进行初始化,mMemoryCache用于在内存中缓存图片
[java] view
plaincopy
- public Bitmap get(String key) {}
get()方法用于从内存中获得缓存
[java] view
plaincopy
- public void put(String key, Bitmap bitmap) {}
put()方法将下载好的图片缓存到内存中,方便下次使用
接下来我们会在一个叫DoubleCache的类中对以上两种缓存方式进行管理
[java] view
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- public class DoubleCache implements ImageCache {
- private static final String TAG = DoubleCache.class.getSimpleName();
- private MemoryCache mMemoryCache = null;
- private DiskCache mDiskCache = null;
- public DoubleCache(Context context){
- mMemoryCache = new MemoryCache();
- mDiskCache = new DiskCache(context);
- }
- @Override
- public Bitmap get(String url) {
- String key = url2Key(url);
- Bitmap bitmap = mMemoryCache.get(key);
- if(bitmap==null){
- bitmap = mDiskCache.get(key);
- }else {
- }
- return bitmap;
- }
- @Override
- public void put(String url, Bitmap bitmap) {
- String key = url2Key(url);
- mMemoryCache.put(key,bitmap);
- mDiskCache.put(key,bitmap);
- }
- //url转key
- private String url2Key(String url){
- String key = MD5.hashKeyForDisk(url)+".jpg";
- return key;
- }
- }
我们在获取缓存的时候先从内存中获取。当内存中击中直接返回,当内存中没有击中,则訪问sd卡。
3.看到这里,小伙伴们一定急了,这仅仅有从缓存中和sd卡中取图片,并没有从网络获取,别急。立即就来
[java] view
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- public class ImageLoader {
- private static final String TAG = ImageLoader.class.getSimpleName();
- private static ImageLoader sInstance;
- private DoubleCache mDoubleCache = null;
- private ExecutorService mExecutorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
- private ImageLoader(Context context) {
- mDoubleCache = new DoubleCache(context);
- }
- public static ImageLoader getInstance(Context context) {
- if (sInstance == null) {
- synchronized (ImageLoader.class) {
- sInstance = new ImageLoader(context);
- }
- }
- return sInstance;
- }
- public void displayImage(String url, ImageView imageView) {
- Bitmap bitmap = mDoubleCache.get(url);
- if (bitmap != null) {
- imageView.setImageBitmap(bitmap);
- mDoubleCache.put(url,bitmap);
- return;
- }
- submitLoadRequest(url, imageView);
- }
- private void submitLoadRequest(final String url, final ImageView imageView) {
- Log.i(TAG,"Download,url:"+url);
- imageView.setTag(url);
- mExecutorService.submit(new Runnable() {
- @Override
- public void run() {
- final Bitmap bitmap = downloadImage(url);
- if (imageView.getTag().equals(url)) {
- imageView.post(new Runnable() {
- @Override
- public void run() {
- imageView.setImageBitmap(bitmap);
- }
- });
- }
- mDoubleCache.put(url, bitmap);
- }
- });
- }
- Handler handler = new Handler(){
- @Override
- public void handleMessage(Message msg) {
- }
- };
- public Bitmap downloadImage(String url) {
- Bitmap bitmap = null;
- HttpURLConnection conn = null;
- try {
- URL url1 = new URL(url);
- conn = (HttpURLConnection) url1.openConnection();
- bitmap = BitmapFactory.decodeStream(conn.getInputStream());
- if (bitmap!=null){
- mDoubleCache.put(url,bitmap);
- }
- } catch (MalformedURLException e) {
- e.printStackTrace();
- } catch (Exception e) {
- e.printStackTrace();
- } finally {
- if (conn != null) {
- conn.disconnect();
- }
- }
- return bitmap;
- }
从
[java] view
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- displayImage()
方法中能够看到。假设缓存中都没有才从网络中获取
[java] view
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- public Bitmap downloadImage(String url) {}
下载完毕之后。把图片放到缓存中。
到这里,我们的第二张流程图就走完了。是不是非常easy。
我们在看一下是怎样使用的
[java] view
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- private ImageView imageView;
- private ImageView imageView2;
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.activity_main);
- imageView = (ImageView) findViewById(R.id.image);
- imageView2 = (ImageView) findViewById(R.id.image2);
- imageView.setOnClickListener(new View.OnClickListener() {
- @Override
- public void onClick(View v) {
- ImageLoader.getInstance(MainActivity.this).displayImage("http://img.my.csdn.net/uploads/201407/26/1406383299_1976.jpg", imageView);
- }
- });
- imageView2.setOnClickListener(new View.OnClickListener() {
- @Override
- public void onClick(View v) {
- ImageLoader.getInstance(MainActivity.this).displayImage("http://img.my.csdn.net/uploads/201407/26/1406383299_1976.jpg", imageView2);
- }
- });
- }