转自：https://blog.csdn.net/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(){
@Override
public 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 block
e.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 begin
if (count%128==0&&Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {//end
if (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
/* 20150127 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) ;
/* 20150127 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 startup
gc();
/* 20150127 begin */
Process.setThreadPriority(currentPriority) ;
/* 20150127 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 Zygote
if (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socket
closeServerSocket();
} 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
*
* 20150127 优化开机速度
*/
//- private static final int PRELOAD_GC_THRESHOLD = 50000;
private static final int PRELOAD_GC_THRESHOLD = 64*1024*1024;
/*20150127 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。