最近有项需求是获取应用的缩略，用于在动画时显示。因此就对此块知识简要了解了一下。
在android中获取视频文件的缩略图有三种方法：
1.从媒体库中查询新视频增加后需要SDCard重新扫描才能给新增加的文件添加缩略图，灵活性差，而且不是很稳定，适合简单应用
2. android 2.2以后使用ThumbnailUtils类获取
实现简单，但2.2以前的版本不支持
3.调用jni文件，实现MediaMetadataRetriever类
实现复杂，但比较灵活，推荐使用
那么Android 中如何获取最近应用的缩略图，这块也有现成的函数，不过都被 @hide 掉了，所以需要利用java reflect 反射才能使用
核心代码如下：
package com.example.testondraw;import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.Method;import android.app.ActivityManager;import android.graphics.Bitmap;import android.util.Log;/** * 调用隐藏方法的助手类 */public class HideExploder { static final String TAG = "HideExploder"; private final static Object refLock = new Object(); private static Method getTaskThumbnails = null; private static Field taskThumbnailsBitmap = null; @SuppressWarnings("rawtypes") public static void listAllObject(String classFullName) { try { Log.i(TAG, "listAllObject classFullName: " + classFullName); Class<?> clazz = Class.forName(classFullName); // 反射属性字段 Field[] fields = clazz.getDeclaredFields(); // 反射方法字段 Method[] methods = clazz.getDeclaredMethods(); // 反射构造器 Constructor[] constuctors = clazz.getDeclaredConstructors(); Log.i(TAG, "FIELD========"); for (Field f : fields) { Log.i(TAG, "TYPE: " + f.getType() + " NAME: " + f.getName()); } Log.i(TAG, "METHOD========"); for (Method m : methods) { Log.i(TAG, "METHOD NAME: " + m.getName()); } Log.i(TAG, "CONSTUCTOR========"); for (Constructor c : constuctors) { Log.i(TAG, "NAME:" + c.getName()); Class[] clss = c.getParameterTypes(); Object o = null; // 产生实例 try { if (clss.length == 0) o = c.newInstance(); else if (clss.length > 0) { // TODO } Log.i(TAG, "object: " + o); } catch (Exception e) { e.printStackTrace(); } } } catch (Exception e) { e.printStackTrace(); } } /** * 调用逻辑 * * TaskThumbnails o = am.getTaskThumbnails(id) * Bitmap b = o.mainThumbnail; * * @param am ActivityManager 类实例 * @param id 标识值 * @return */ public static Bitmap getTaskThumbnailsBitmap(ActivityManager am, int id) { synchronized (refLock) { try { if (getTaskThumbnails == null) { getTaskThumbnails = am.getClass().getDeclaredMethod( "getTaskThumbnails", int.class); } if (getTaskThumbnails != null) { Object thumbnails = getTaskThumbnails.invoke(am, Integer.valueOf(id)); if (thumbnails != null) { if (taskThumbnailsBitmap == null) taskThumbnailsBitmap = thumbnails.getClass() .getField("mainThumbnail"); if (taskThumbnailsBitmap != null) { return (Bitmap) taskThumbnailsBitmap .get(thumbnails); } } } } catch (Exception e) { Log.w(TAG, "getTaskThumbnailsBitmap error:" + e); e.printStackTrace(); } } return null; }}测试用例如下：
// 应用缩略图测试 static final int MAX_RECENT_TASKS = 64; Bitmap loadThumbnail(int index) { ActivityManager am = (ActivityManager) this .getSystemService(Context.ACTIVITY_SERVICE); List<ActivityManager.RecentTaskInfo> recentTasks = am.getRecentTasks( MAX_RECENT_TASKS, ActivityManager.RECENT_IGNORE_UNAVAILABLE); int numTasks = recentTasks.size(); if (index < numTasks) { final ActivityManager.RecentTaskInfo info = recentTasks.get(index); if (info != null) { Bitmap b = HideExploderX.getTaskThumbnailsBitmap(am, info.persistentId); return b; } } return null; }调用 loadThumbnail(X) 【索引值为0就是上一次的缩略图】拿到Bitmap对象，利用 SurfaceView 方法画出来即可：
public void drawBitmap() { // 锁定画布，得到canvas if (mSurfaceHolder == null || this.mBitmap == null) return; Canvas canvas = mSurfaceHolder.lockCanvas(); if (canvas == null) { return; } // 绘图 Paint paint = new Paint(); paint.setAntiAlias(true); paint.setColor(Color.BLUE); canvas.drawBitmap(this.mBitmap, 0, 0, paint); // 绘制后解锁，绘制后必须解锁才能显示 mSurfaceHolder.unlockCanvasAndPost(canvas); }注意点：
1、添加应用权限
<uses-permission android:name="android.permission.GET_TASKS"/>
<uses-permission android:name="android.permission.READ_FRAME_BUFFER"/>
否则会报：
W/System.err( 3314): Caused by: java.lang.SecurityException: Permission Denial: getTaskThumbnails() from pid=3314, uid=10040 requires android.permission.READ_FRAME_BUFFER
2、有 eclipse 上调试运行时，即使加上权限了也还是会报上述错误，有两个方法解决，要么进行系统签名，要不放到linux上编译，利用platform 签名，Android.mk 如下：
LOCAL_PATH:= $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE_TAGS := optional# Only compile source java files in this apk.LOCAL_SRC_FILES := $(call all-java-files-under, src)$(warning build apk)LOCAL_PACKAGE_NAME := testLOCAL_CERTIFICATE := platforminclude $(BUILD_PACKAGE)include $(call all-makefiles-under,$(LOCAL_PATH))