Android节省资源的使用.

public class Utils {
	
	// shell 命令查看程序内存使用情况
	//  :  adb shell dumpsys meminfo packageName
	
	//dalvik.system.VMRuntime
	
	/***
	 * 增强程序堆内存的处理效率.
	 *  private final static float TARGET_HEAP_UTILIZATION = 0.75f;
     *  在程序onCreate时就可以调用
     *   VMRuntime.getRuntime().setTargetHeapUtilization(TARGET_HEAP_UTILIZATION); 即可。
	 */

	/***
	 * 强制定义自己软件的对内存大小
	 * private final static int CWJ_HEAP_SIZE = 6* 1024* 1024 ;
	 * VMRuntime.getRuntime().setMinimumHeapSize(CWJ_HEAP_SIZE); //设置最小heap内存为6MB大小。 
	 */
	
	
	/**
	 * 计算原始图片与缩放图片的比例
	 * 
	 * @param options
	 * @param reqWidth
	 * @param reqHeight
	 * @return
	 */
	public static int calculateInSampleSize(BitmapFactory.Options options,
			int reqWidth, int reqHeight) {
		// Raw height and width of image
		final int height = options.outHeight;
		final int width = options.outWidth;
		int inSampleSize = 1;
		if (height > reqHeight || width > reqWidth) {
			if (width > height) {
				inSampleSize = Math.round((float) height / (float) reqHeight);
			} else {
				inSampleSize = Math.round((float) width / (float) reqWidth);
			}
		}
		return inSampleSize;
	}

	/***
	 * 设置合适的sampleSize后获取图片
	 * @param res
	 * @param resId
	 * @param reqWidth
	 * @param reqHeight
	 * @return
	 */
	public static 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);
		// Calculate inSampleSize
		options.inSampleSize = calculateInSampleSize(options, reqWidth,
				reqHeight);
		// Decode bitmap with inSampleSize set
		options.inJustDecodeBounds = false;
		return BitmapFactory.decodeResource(res, resId, options);
	}
	
	/***
	 *  以最省内存的方式读取本地资源的图片
	 * @param context
	 * @param resId
	 * @return
	 */
	public static Bitmap readBitmap(Context context,int resId){
		BitmapFactory.Options opt = new BitmapFactory.Options();  
		opt.inPreferredConfig = Bitmap.Config.RGB_565;
		opt.inPurgeable = true;
		opt.inInputShareable = true;
		//获取资源图片
		InputStream is = context.getResources().openRawResource(resId);
		return BitmapFactory.decodeStream(is,null,opt);

	
	}

	/**
 	 * Reads this input stream and returns contents as a byte[] 将流中的内容解释为byte[]
 	 * 数组 from:aspectjweaver.jar
 	 */
	 public static byte[] readAsByteArray(InputStream inStream)
   	throws IOException {
  		int size = 1024;
  		byte[] buff = new byte[size];
  		int readSoFar = 0;
  		while (true) {
   		int nRead = inStream.read(buff, readSoFar, size - readSoFar);
  		 if (nRead == -1) {
   		 break;
   		}
   		readSoFar += nRead;
   		if (readSoFar == size) {
    		int newSize = size * 2;
    		byte[] newBuff = new byte[newSize];
   		 System.arraycopy(buff, 0, newBuff, 0, size);
   		 buff = newBuff;
   		 size = newSize;
  		 }
  	}
 	 byte[] newBuff = new byte[readSoFar];
  	System.arraycopy(buff, 0, newBuff, 0, readSoFar);
  	return newBuff;
 	}		
}