Android 图片压缩

核心思想是通过BitmapFactory.Options来缩放图片，主要是用到了它的inSampleSize参数（采样率）

当inSampleSize为1的时候，采样后的图片大小为图片的原始大小；

当inSampleSize为2的时候，采样后的图片的宽和高是原来的1/2，也就是说，它的像素点是原来的1/4，占的内存自然就是原来的1/4了。以此类推。

当inSampleSize小于1的时候，效果和等于1的时候是一样的。

压缩流程如下：
1.BitmapFactory.Options 的inJustDecodeBounds参数设置为true（这个时候BitmapFactory只是解析图片的原始宽高，并不会去加载图片）。

2.从BitmapFactory.Options 中取出图片的原始宽高，outWidth，outHeight。

3.根据自己的需要设置合适的采样率。

4.BitmapFactory.Options 的inJustDecodeBounds参数设置为false，然后就可以加载图片了。

下面我们看代码：

public Bitmap decodeSampledBitmapFromBytes(byte[] bytes,int reqWidth,int reqHeight){
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeByteArray(bytes,0,bytes.length,options);
        options.inSampleSize = calculateInSampleSize(options,reqWidth,reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeByteArray(bytes,0,bytes.length,options);
    }


public int calculateInSampleSize(BitmapFactory.Options options,int reqWidth,int reqHeight){
        if(reqWidth == 0 || reqHeight == 0){
            return 1;
        }
        final int  width = options.outWidth;
        final int height = options.outHeight;
        int inSampleSize = 1;
        if( width > reqWidth || height > reqHeight){
            final int halfWidth  = width / 2;
            final int halfHeight = height / 2;
            while ((halfWidth / inSampleSize) >= reqWidth && (halfHeight / inSampleSize) >= reqHeight){
                inSampleSize *=2;
            }
        }
        return inSampleSize;
    }

如此一来，就完成了一张图片的压缩。另外，BitmapFactory还有其它的decode方法，我们也可以仿照上面的来写。

public Bitmap decodeSampledBitmapFromResource(Resources res,int resId,int reqWidth,int reqHeight){
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res,resId,options);
        options.inSampleSize = calculateInSampleSize(options,reqWidth,reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res,resId,options);
    }

public Bitmap decodeSampledBitmapFromDescrptor(FileDescriptor fd,int reqWidth,int reqHeight){
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFileDescriptor(fd,null,options);
        options.inSampleSize = calculateInSampleSize(options,reqWidth,reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeFileDescriptor(fd,null,options);
    }

接下来结合一个小demo来实现一个完整的流程

先把图片压缩类封装起来

public class ImageResizer {
    private static final String TAG = "ImageResizer";

    public ImageResizer(){}

    public Bitmap decodeSampledBitmapFromResource(Resources res,int resId,int reqWidth,int reqHeight){
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res,resId,options);
        options.inSampleSize = calculateInSampleSize(options,reqWidth,reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res,resId,options);
    }

    public Bitmap decodeSampledBitmapFromBytes(byte[] bytes,int reqWidth,int reqHeight){
        final BitmapFactory.Options options = new BitmapFactory.Options();
        Bitmap a = BitmapFactory.decodeByteArray(bytes,0,bytes.length,options);
        Log.d(TAG, "before bitmap : " + a.getRowBytes() * a.getHeight());
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeByteArray(bytes,0,bytes.length,options);
        options.inSampleSize = calculateInSampleSize(options,reqWidth,reqHeight);
        options.inJustDecodeBounds = false;
        Bitmap b = BitmapFactory.decodeByteArray(bytes,0,bytes.length,options);
        Log.d(TAG, "after bitmap : " +  b.getRowBytes() * b.getHeight());
        return b;
    }

    public Bitmap decodeSampledBitmapFromDescrptor(FileDescriptor fd,int reqWidth,int reqHeight){
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFileDescriptor(fd,null,options);
        options.inSampleSize = calculateInSampleSize(options,reqWidth,reqHeight);
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeFileDescriptor(fd,null,options);
    }

    public int calculateInSampleSize(BitmapFactory.Options options,int reqWidth,int reqHeight){
        if(reqWidth == 0 || reqHeight == 0){
            return 1;
        }
        final int  width = options.outWidth;
        final int height = options.outHeight;
        int inSampleSize = 1;
        if( width > reqWidth || height > reqHeight){
            final int halfWidth  = width / 2;
            final int halfHeight = height / 2;
            while ((halfWidth / inSampleSize) >= reqWidth && (halfHeight / inSampleSize) >= reqHeight){
                inSampleSize *=2;
            }
        }
        return inSampleSize;
    }
}

然后就可以拿来用了：

activity_main2.xml

"http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main2"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.yuan.test.Main2Activity">

    "@+id/main2Iv"
        android:layout_width="200dp"
        android:layout_height="200dp" />

</RelativeLayout>

Main2Activity.java

public class Main2Activity extends AppCompatActivity {

    private ImageView iv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        initView();
        testHttp(iv);
    }

    private void testHttp(final ImageView iv) {

        new Thread(new Runnable() {
            @Override
            public void run() {
                String urlString = "https://static.pexels.com/photos/295818/pexels-photo-295818.jpeg";
                HttpURLConnection urlConnection = null;
                InputStream in = null;
                ByteArrayOutputStream outStream = null;
                try {
                    URL url = new URL(urlString);
                    urlConnection = (HttpURLConnection) url.openConnection();
                    in = urlConnection.getInputStream();
                    byte[] buffer = new byte[1024];
                    int len;
                    outStream = new ByteArrayOutputStream();
                    while ((len = in.read(buffer)) != -1){
                        outStream.write(buffer,0,len);
                    }
                    final byte[] data = outStream.toByteArray();
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() { //在主线程加载UI
                            iv.setImageBitmap(new ImageResizer().decodeSampledBitmapFromBytes(data,200,200));
                        }
                    });
                } catch (IOException e) {
                    e.printStackTrace();
                }finally {
                    try {
                        if(urlConnection !=null){
                            urlConnection.disconnect();
                        }
                        if(in != null){
                            in.close();
                        }
                        if(outStream != null){
                            outStream.close();
                        }
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }

    private void initView() {
        iv = (ImageView) findViewById(R.id.main2Iv);
    }
}

最后记得获取网络权限

运行的结果：
压缩前后的bitmap大小对比