转自：/xxm282828/article/details/49095839
Android启动过程中针对类和资源部分预加载耗时比较久，这个部分需要优化，主要涉及的文件：
./base/core/java/com/android/internal/os/ZygoteInit.java
主要采取三个措施：
1. 修改ZygoteInit.java 中预加载资源函数preload() , preloadClasses(); 与 preloadResources(); 并行加载。2. 修改读取配置信息过程中GC频率。3. 提升进程优先级
1、资源和类并行加载：
static void preload() {//Thread preloadRsThread = new Thread(new Runnable(){ @Overridepublic void run() {// TODO Auto-generated method stub//将该资源加载放在子线程中 。加载资源文件要比加载classes文件要快，因此这里不提升子线程优先级。 preloadResources();}}) ; preloadRsThread.start() ;preloadClasses();//wait preloadRes complete.try {preloadRsThread.join() ;} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();} //暴露什么问题。preloadOpenGL();}
2、减少GC的频繁调度：
/*** Performs Zygote process initialization. Loads and initializes* commonly used classes.** Most classes only cause a few hundred bytes to be allocated, but* a few will allocate a dozen Kbytes (in one case, 500+K).*/private static void preloadClasses() {final VMRuntime runtime = VMRuntime.getRuntime(); InputStream is = ClassLoader.getSystemClassLoader().getResourceAsStream(PRELOADED_CLASSES);if (is == null) {Log.e(TAG, "Couldn't find " + PRELOADED_CLASSES + ".");} else {Log.i(TAG, "Preloading classes...");long startTime = SystemClock.uptimeMillis();// Drop root perms while running static initializers.setEffectiveGroup(UNPRIVILEGED_GID);setEffectiveUser(UNPRIVILEGED_UID);// Alter the target heap utilization. With explicit GCs this// is not likely to have any effect.float defaultUtilization = runtime.getTargetHeapUtilization();runtime.setTargetHeapUtilization(0.8f);// Start with a clean slate.System.gc();runtime.runFinalizationSync();Debug.startAllocCounting(); try {BufferedReader br= new BufferedReader(new InputStreamReader(is), 256);int count = 0;String line;while ((line = br.readLine()) != null) {// Skip comments and blank lines.line = line.trim();if (line.startsWith("#") || line.equals("")) {continue;} try {if (false) {Log.v(TAG, "Preloading " + line + "...");}Class.forName(line);//减少GC频率，modify beginif (count%128==0&&Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {//endif (false) {Log.v(TAG," GC at " + Debug.getGlobalAllocSize());}System.gc();runtime.runFinalizationSync();Debug.resetGlobalAllocSize();}count++;} catch (ClassNotFoundException e) {Log.w(TAG, "Class not found for preloading: " + line);} catch (UnsatisfiedLinkError e) {Log.w(TAG, "Problem preloading " + line + ": " + e);} catch (Throwable t) {Log.e(TAG, "Error preloading " + line + ".", t);if (t instanceof Error) {throw (Error) t;}if (t instanceof RuntimeException) {throw (RuntimeException) t;}throw new RuntimeException(t);}} Log.i(TAG, "...preloaded " + count + " classes in "+ (SystemClock.uptimeMillis()-startTime) + "ms.");} catch (IOException e) {Log.e(TAG, "Error reading " + PRELOADED_CLASSES + ".", e);} finally {IoUtils.closeQuietly(is);// Restore default.runtime.setTargetHeapUtilization(defaultUtilization);// Fill in dex caches with classes, fields, and methods brought in by preloading.runtime.preloadDexCaches();Debug.stopAllocCounting();// Bring back root. We'll need it later.setEffectiveUser(ROOT_UID);setEffectiveGroup(ROOT_GID);}}}
3、提升进程的优先级
// ZygoteInit.java入口public static void main(String argv[]) {try { //优化开机速度 begin/* 0127 begin *///获取当前进程优先级int currentPriority = Process.getThreadPriority(Process.myPid()) ;//提升当前进程优先级。Process.setThreadPriority(Process.THREAD_PRIORITY_AUDIO) ;/* 0127 end */ // Start profiling the zygote initialization.SamplingProfilerIntegration.start(); //1.注册socket服务端registerZygoteSocket();EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,SystemClock.uptimeMillis());//5.加载资源preload();EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,SystemClock.uptimeMillis());// Finish profiling the zygote initialization.SamplingProfilerIntegration.writeZygoteSnapshot();// Do an initial gc to clean up after startupgc();/* 0127 begin */Process.setThreadPriority(currentPriority) ;/* 0127 end *///优化开机速度 end// Disable tracing so that forked processes do not inherit stale tracing tags from// Zygote.Trace.setTracingEnabled(false);// If requested, start system server directly from Zygoteif (argv.length != 2) {throw new RuntimeException(argv[0] + USAGE_STRING);}//2. 调用starySystemServer（）方法if (argv[1].equals("start-system-server")) {startSystemServer();} else if (!argv[1].equals("")) {throw new RuntimeException(argv[0] + USAGE_STRING);}Log.i(TAG, "Accepting command socket connections");//3.循环监听并接收客户端请求。runSelectLoop();//关闭socketcloseServerSocket();} catch (MethodAndArgsCaller caller) {//4 《深入理解Android卷1》作者说这里比较重要caller.run();} catch (RuntimeException ex) {Log.e(TAG, "Zygote died with exception", ex);closeServerSocket();throw ex;}}
4、修改gc调用阀值
/** when preloading, GC after allocating this many bytes * * 0127 优化开机速度*/ //- private static final int PRELOAD_GC_THRESHOLD = 50000;private static final int PRELOAD_GC_THRESHOLD = 64*1024*1024; /*0127 优化开机速度 end*/
另外，Android系统启动过程中，会预先加载资源文件，这些文件中，包含了很多系统应用会频繁使用到的资源文件，比如图片资源等。因此，我们自己的新添加其他的部分资源也可以预先加载到内存，这样不仅提升响应速度，而且也可以加快应用的冷启动速度（不过对于系统启动速度比较敏感的设备，不建议这么使用）。
5、关于进程优先级
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //设置线程优先级为后台，这样当多个线程并发后很多无关紧要的线程分配的CPU时间将会减少，有利于主线程的处理，有以下几种：
int THREAD_PRIORITY_AUDIO //标准音乐播放使用的线程优先级
int THREAD_PRIORITY_BACKGROUND //标准后台程序
int THREAD_PRIORITY_DEFAULT // 默认应用的优先级
int THREAD_PRIORITY_DISPLAY //标准显示系统优先级，主要是改善UI的刷新
int THREAD_PRIORITY_FOREGROUND //标准前台线程优先级
int THREAD_PRIORITY_LESS_FAVORABLE //低于favorable
int THREAD_PRIORITY_LOWEST //有效的线程最低的优先级
int THREAD_PRIORITY_MORE_FAVORABLE //高于favorable
int THREAD_PRIORITY_URGENT_AUDIO //标准较重要音频播放优先级
int THREAD_PRIORITY_URGENT_DISPLAY //标准较重要显示优先级，对于输入事件同样适用
Thread.setPriority:是JDK提供的设置线程优先级的方法.
android.os.Process.setThreadPriority: 是android.jar 专门针对dalvik来进行设置线程优先级的方法;此方法更适合Android上面设置优先级,它设置的优先级更直接的适应在dalvik(Linux) 上的进程的优先级
注意在使用进行优先级设置的时候，如果提升当前进行的优先级，如果主进程优先级比较低，那么在系统启动过程中当前进程共享主进程的资源直接导致系统启动延迟。
参考：/kingguary/blog/1573951
/xxm282828/article/details/49095839
