首先看一下Android系统的启动流程：
bootloader
引导程序
kernel
内核
init
init初始化（这个大家都比较熟悉了，不要多说）
- loads several daemons and services, including zygote
- see /init.rc and init.<platform>.rc
zygote
这个是占用时间最多的，重点修理对象
- preloads classes
装载了一千多个类，妈呀！！！ - starts package manager 扫描package(下面详细介绍)
service manager
- start services （启动多个服务）
从实际的测试数据来看，有两个地方时最耗时间的，一个是zygote的装载一千多个类和初始化堆栈的过程，用了20秒左右。另一个是扫描
/system/app,
/system/framework,
/data/app,
/data/app-private.
这几个目录下面的package用了大概10秒，所以我们重点能够修理的就是这两个老大的。
一、首先是调试工具的使用，可以测试哪些类和那些过程占用了多少时间，
主要工具为
stopwatch
Message loggers
grabserial
printk times
logcat
Android自带
bootchart
strace
AOSP 的一部分（Eclair及以上版本）
使用例子
在init.rc中为了调试zygote
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server改为
service zygote /system/xbin/strace -tt -o/data/boot.strace /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
method tracer*
ftrace*
详细使用可看提供的文档和网页介绍
上面的工具如果不用详细的分析不一定都用到，也可以使用logcat就可以,在代码中加一点计算时间和一些类的调试信息也可以达到很好效果。
二、zygote 装载1千多个类
首先，我们可以添加一点调试信息，以获得具体转载情况。
diff --git a/core/java/com/Android/internal/os/ZygoteInit.java b/core/java/com/android/internal/os/ZygoteInit.java
index 404c513..f2b573c 100644
--- a/core/java/com/Android/internal/os/ZygoteInit.java
+++ b/core/java/com/Android/internal/os/ZygoteInit.java
@@ -259,6 +259,8 @@ public class ZygoteInit {
} else {
Log.i(TAG, "Preloading classes...");
long startTime = SystemClock.uptimeMillis();
+ long lastTime = SystemClock.uptimeMillis();
+ long nextTime = SystemClock.uptimeMillis();
// Drop root perms while running static initializers.
setEffectiveGroup(UNPRIVILEGED_GID);
@@ -292,12 +294,24 @@ public class ZygoteInit {
if (Config.LOGV) {
Log.v(TAG, "Preloading " + line + "...");
}
+ //if (count%5==0) {
+ // Log.v(TAG, "Preloading " + line + "...");
+ //}
+ Log.v(TAG, "Preloading " + line + "...");
Class.forName(line);
+ nextTime = SystemClock.uptimeMillis();
+ if (nextTime-lastTime >50) {
+ Log.i(TAG, "Preloading " + line + "... took " + (nextTime-lastTime) + "ms.");
+ }
+ lastTime = nextTime;
+
if (Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {
if (Config.LOGV) {
Log.v(TAG,
" GC at " + Debug.getGlobalAllocSize());
}
+ Log.i(TAG,
+ " GC at " + Debug.getGlobalAllocSize());
runtime.gcSoftReferences();
runtime.runFinalizationSync();
Debug.resetGlobalAllocSize();上面+代表添加的代码，这样就可以很容易的得到在装载类的过程中具体装载了哪些类，耗费了多久。具体装载的类在文件platform/frameworks/base/ preloaded-classes
内容类似：
Android.R$styleable
Android.accounts.AccountMonitor
Android.accounts.AccountMonitor$AccountUpdater
Android.app.Activity
Android.app.ActivityGroup
Android.app.ActivityManager$MemoryInfo$1
Android.app.ActivityManagerNative
Android.app.ActivityManagerProxy
Android.app.ActivityThread
Android.app.ActivityThread$ActivityRecord
Android.app.ActivityThread$AppBindData
Android.app.ActivityThread$ApplicationThread
Android.app.ActivityThread$ContextCleanupInfo
Android.app.ActivityThread$GcIdler
Android.app.ActivityThread$H
Android.app.ActivityThread$Idler而这个文件是由文件WritePreloadedClassFile.java中的WritePreloadedClassFile类自动生成
/**
* Writes /frameworks/base/preloaded-classes. Also updates
* {@link LoadedClass#preloaded} fields and writes over compiled log file.
*/
public class WritePreloadedClassFile
/**
* Preload any class that take longer to load than MIN_LOAD_TIME_MICROS us.
*/
static final int MIN_LOAD_TIME_MICROS = 1250;//这个代表了装载时间小于1250us即1.25ms的类将不予装载，也许可以改这个参数减少一下类的装载
//这里可以看到什么样的类会被装载
A:启动必须装载的类，比如系统级的类
B：刚才说的装载时间大于1.25ms的类
C：被使用一次以上或被应用装载的类
仔细看看筛选类的具体实现，可以帮助我们认识哪些类比较重要，哪些可以去掉。
筛选规则是
第一 isPreloadable，
/**Reports if the given class should be preloaded. */
public static boolean isPreloadable(LoadedClass clazz) { return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name);
}
意思是指除了EXCLUDED_CLASSES包含的类之外的所有系统装载的类。
EXCLUDED_CLASSES包含
/**
* Classes which we shouldn't load from the Zygote.
*/
private static final Set<String> EXCLUDED_CLASSES
= new HashSet<String>(Arrays.asList(
// Binders
"Android.app.AlarmManager",
"Android.app.SearchManager",
"Android.os.FileObserver",
"com.Android.server.PackageManagerService$AppDirObserver", // Threads
"Android.os.AsyncTask",
"Android.pim.ContactsAsyncHelper",
"java.lang.ProcessManager"
));目前是跟Binders跟Threads有关的不会被预装载。
第二 clazz.medianTimeMicros() > MIN_LOAD_TIME_MICROS装载时间大于1.25ms。
第三 names.size() > 1 ，既是被processes一次以上的。
上面的都是指的system class,另外还有一些application class需要被装载
规则是fromZygote而且不是服务
proc.fromZygote() && !Policy.isService(proc.name)
fromZygote指的除了com.Android.development的zygote类
public boolean fromZygote() {
return parent != null && parent.name.equals("zygote")
&& !name.equals("com.Android.development");
}
/除了常驻内存的服务
/**
* Long running services. These are restricted in their contribution to the
* preloader because their launch time is less critical.
*/
// TODO: Generate this automatically from package manager.
private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList(
"system_server",
"com.google.process.content",
"Android.process.media",
"com.Android.bluetooth",
"com.Android.calendar",
"com.Android.inputmethod.latin",
"com.Android.phone",
"com.google.Android.apps.maps.FriendService", // pre froyo
"com.google.Android.apps.maps:FriendService", // froyo
"com.google.Android.apps.maps.LocationFriendService",
"com.google.Android.deskclock",
"com.google.process.gapps",
"Android.tts"
));好了。要转载的就是这些类了。虽然preloaded- classes是在下载源码的时候已经确定了的，也就是对我们来说WritePreloadedClassFile类是没用到的，我们可以做的就是在 preloaded-classes文件中，把不预装载的类去掉，试了把所有类去掉，启动确实很快跳过那个地方，但是启动HOME的时候就会很慢了。所以最好的方法就是只去掉那些没怎么用到的，不过要小心处理。至于该去掉哪些，还在摸索，稍后跟大家分享。有兴趣的朋友可以先把preloaded- classes这个文件里面全部清空，启动快了很多，但在启动apk的时候会慢了点。当然了，也可以把Android相关的类全部去掉，剩下java的类，试过了也是可以提高速度。
三，系统服务初始化和package 扫描
在启动系统服务的init2()时会启动应用层（Java层）的所有服务。
public static void main(String[] args) {
System.loadLibrary("Android_servers");
init1(args); //init1 初始化，完成之后会回调init2()
}
在init2()中会启动一个线程来启动所有服务
public static final void init2() {
Log.i(TAG, "Entered the Android system server!");
Thread thr = new ServerThread();
thr.setName("Android.server.ServerThread");
thr.start();
}
class ServerThread extends Thread {
。。。
public void run() {
。。。
关键服务：
ServiceManager.addService("entropy", new EntropyService());
ServiceManager.addService(Context.POWER_SERVICE, power);
context = ActivityManagerService.main(factoryTest);
ServiceManager.addService("telephony.registry", new TelephonyRegistry(context));
PackageManagerService.main(context,
factoryTest != SystemServer.FACTORY_TEST_OFF);//apk扫描的服务 ServiceManager.addService(Context.ACCOUNT_SERVICE,
new AccountManagerService(context)); ContentService.main(context,
factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL); battery = new BatteryService(context);
ServiceManager.addService("battery", battery);
hardware = new HardwareService(context);
ServiceManager.addService("hardware", hardware); AlarmManagerService alarm = new AlarmManagerService(context);
ServiceManager.addService(Context.ALARM_SERVICE, alarm); ServiceManager.addService(Context.SENSOR_SERVICE, new SensorService(context));
WindowManagerService.main(context, power,
factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL);
ServiceManager.addService(Context.WINDOW_SERVICE, wm);
上面这些都是关键服务，不建议进行裁剪。
下面的这些不是很关键，可以进行裁剪，当是必须相应的修改framework部分的代码，工作量比较大和复杂。我去掉了20个服务，大概需要相应修改大概20多个文件。
statusBar = new StatusBarService(context);
ServiceManager.addService("statusbar", statusBar);
ServiceManager.addService("clipboard", new ClipboardService(context));
imm = new InputMethodManagerService(context, statusBar);
ServiceManager.addService(Context.INPUT_METHOD_SERVICE, imm);
ServiceManager.addService("netstat", new NetStatService(context));
connectivity = ConnectivityService.getInstance(context);
ServiceManager.addService(Context.CONNECTIVITY_SERVICE, connectivity);
ServiceManager.addService(Context.ACCESSIBILITY_SERVICE,
new AccessibilityManagerService(context));
notification = new NotificationManagerService(context, statusBar, hardware);
ServiceManager.addService(Context.NOTIFICATION_SERVICE, notification);
ServiceManager.addService("mount", new MountService(context));
ServiceManager.addService(DeviceStorageMonitorService.SERVICE,
new DeviceStorageMonitorService(context));
ServiceManager.addService(Context.LOCATION_SERVICE, new LocationManagerService(context));
ServiceManager.addService( Context.SEARCH_SERVICE, new SearchManagerService(context) );
if (INCLUDE_DEMO) {
Log.i(TAG, "Installing demo data...");
(new DemoThread(context)).start();
}
Intent intent = new Intent().setComponent(new ComponentName(
"com.google.Android.server.checkin",
"com.google.Android.server.checkin.CheckinService"));
ServiceManager.addService("checkin", new FallbackCheckinService(context));
wallpaper = new WallpaperManagerService(context);
ServiceManager.addService(Context.WALLPAPER_SERVICE, wallpaper);
ServiceManager.addService(Context.AUDIO_SERVICE, new AudioService(context));
headset = new HeadsetObserver(context);
dock = new DockObserver(context, power);
ServiceManager.addService(Context.BACKUP_SERVICE, new BackupManagerService(context));
ServiceManager.addService(Context.APPWIDGET_SERVICE, appWidget); package 扫描部分，整个流程为callstatic("com/Android/server/SystemServer","init2")
init2()
main(context, factoryTest)
PackageManagerService(context, factoryTest)
scanDirLI(dir, flags, mode)
scanPackageLI(file, file resfile, flags, mode)
parsePackage(file, dest filename, metrics, flags)
parsePackage(file, dest, metrics, flags)
openXmlResourceParser(cookie, filename) (“AndroidManifest.xml“)
openXmlBlockAsset(cookie, filename)
openXmlAssetNative(cookie, filename)
Android_content_AssetManager_openXmlAssetNative
openNonAsset(cookie, filename, flag)
openNonAssetInPathLocked(filename, mode, asset_path)
getZipFileLocked(asset_path)
getZip(ap.path)
get(path)
getZip()
SharedZip(path, modWhen)
ZipFileRO::open(path)
parseZipArchive(void)
get4LE(ptr)
(memory access)最终的zip文件（apk）读取是在下面这两个函数：
/*
* Open the specified file read-only. We memory-map the entire thing and
* close the file before returning.
*/
status_t ZipFileRO::open(const char* zipFileName)
{
int fd = -1;
off_t length; assert(mFileMap == NULL);
LOGD("opening zip '%s'\n", zipFileName);
/*
* Open and map the specified file.
*/ fd = ::open(zipFileName, O_RDONLY);
if (fd < 0) {
LOGW("Unable to open zip '%s': %s\n", zipFileName, strerror(errno));
return NAME_NOT_FOUND;
} length = lseek(fd, 0, SEEK_END);
if (length < 0) {
close(fd);
return UNKNOWN_ERROR;
}
mFileMap = new FileMap();
if (mFileMap == NULL) {
close(fd);
return NO_MEMORY;
}
if (!mFileMap->create(zipFileName, fd, 0, length, true)) {
LOGW("Unable to map '%s': %s\n", zipFileName, strerror(errno));
close(fd);
return UNKNOWN_ERROR;
}
mFd = fd;
/*
* Got it mapped, verify it and create data structures for fast access.
*/
if (!parseZipArchive()) {
mFileMap->release();
mFileMap = NULL;
return UNKNOWN_ERROR;
}
LOGD("done opening zip\n");
return OK;
}
/*
* Parse the Zip archive, verifying its contents and initializing internal
* data structures.
*/
bool ZipFileRO::parseZipArchive(void)
{
#define CHECK_OFFSET(_off) { \
if ((unsigned int) (_off) >= maxOffset) { \
LOGE("ERROR: bad offset %u (max %d): %s\n", \
(unsigned int) (_off), maxOffset, #_off); \
goto bail; \
} \
}
const unsigned char* basePtr = (const unsigned char*)mFileMap->getDataPtr();
const unsigned char* ptr;
size_t length = mFileMap->getDataLength();
bool result = false;
unsigned int i, numEntries, cdOffset;
unsigned int val; /*
* The first 4 bytes of the file will either be the local header
* signature for the first file (kLFHSignature) or, if the archive doesn't
* have any files in it, the end-of-central-directory signature
* (kEOCDSignature).
*/
val = get4LE(basePtr);
if (val == kEOCDSignature) {
LOGI("Found Zip archive, but it looks empty\n");
goto bail;
} else if (val != kLFHSignature) {
LOGV("Not a Zip archive (found 0x%08x)\n", val);
goto bail;
} /*
* Find the EOCD. We'll find it immediately unless they have a file
* comment.
*/
ptr = basePtr + length - kEOCDLen; while (ptr >= basePtr) {
if (*ptr == (kEOCDSignature & 0xff) && get4LE(ptr) == kEOCDSignature)
break;
ptr--;
}
if (ptr < basePtr) {
LOGI("Could not find end-of-central-directory in Zip\n");
goto bail;
} /*
* There are two interesting items in the EOCD block: the number of
* entries in the file, and the file offset of the start of the
* central directory.
*
* (There's actually a count of the #of entries in this file, and for
* all files which comprise a spanned archive, but for our purposes
* we're only interested in the current file. Besides, we expect the
* two to be equivalent for our stuff.)
*/
numEntries = get2LE(ptr + kEOCDNumEntries);
cdOffset = get4LE(ptr + kEOCDFileOffset); /* valid offsets are [0,EOCD] */
unsigned int maxOffset;
maxOffset = (ptr - basePtr) +1; LOGV("+++ numEntries=%d cdOffset=%d\n", numEntries, cdOffset);
if (numEntries == 0 || cdOffset >= length) {
LOGW("Invalid entries=%d offset=%d (len=%zd)\n",
numEntries, cdOffset, length);
goto bail;
} /*
* Create hash table. We have a minimum 75% load factor, possibly as
* low as 50% after we round off to a power of 2.
*/
mNumEntries = numEntries;
mHashTableSize = roundUpPower2(1 + ((numEntries * 4) / 3));
mHashTable = (HashEntry*) calloc(1, sizeof(HashEntry) * mHashTableSize); /*
* Walk through the central directory, adding entries to the hash
* table.
*/
ptr = basePtr + cdOffset;
for (i = 0; i < numEntries; i++) {
unsigned int fileNameLen, extraLen, commentLen, localHdrOffset;
const unsigned char* localHdr;
unsigned int hash; if (get4LE(ptr) != kCDESignature) {
LOGW("Missed a central dir sig (at %d)\n", i);
goto bail;
}
if (ptr + kCDELen > basePtr + length) {
LOGW("Ran off the end (at %d)\n", i);
goto bail;
} localHdrOffset = get4LE(ptr + kCDELocalOffset);
CHECK_OFFSET(localHdrOffset);
fileNameLen = get2LE(ptr + kCDENameLen);
extraLen = get2LE(ptr + kCDEExtraLen);
commentLen = get2LE(ptr + kCDECommentLen); //LOGV("+++ %d: localHdr=%d fnl=%d el=%d cl=%d\n",
// i, localHdrOffset, fileNameLen, extraLen, commentLen);
//LOGV(" '%.*s'\n", fileNameLen, ptr + kCDELen); /* add the CDE filename to the hash table */
hash = computeHash((const char*)ptr + kCDELen, fileNameLen);
addToHash((const char*)ptr + kCDELen, fileNameLen, hash);
// localHdr = basePtr + localHdrOffset;
// if (get4LE(localHdr) != kLFHSignature) {
// LOGW("Bad offset to local header: %d (at %d)\n",
// localHdrOffset, i);
// goto bail;
// } ptr += kCDELen + fileNameLen + extraLen + commentLen;
CHECK_OFFSET(ptr - basePtr);
} result = true;
bail:
return result;
#undef CHECK_OFFSET
}
