在Andorid开发中我们要设置文字或图片显示，都直接通过Api一步调用就完成了，不仅是我们工程下res资源以及系统自带的framwork资源也可以，那这些资源打包成Apk之后是如何被系统加载从而显示出来的呢。
这里我要从Apk安装之后启动流程开始讲起，在桌面应用click事件之后
会通过Binder机制通知ActivityManagerService启动,具体由ActivityManagerNative.getDefault返回ActivityManagerService的远程接口代理对象ActivityManagerProxy，通知ActivityManagerService执行startActivity进入启动流程. 该Service会进行些获取目标内容，检查权限之后，再检查对应进程的ProcessRecord是否存在了.如果ProcessRecord是null, ActivityManagerService会创建新的进程来实例化目标activity从而把App启动了起来，详细的App启动流程可以参考老罗的文章：http://blog.csdn.net/luoshengyang/article/details/6689748。
进程的创建及绑定Application
创建进程
ActivityManagerService调用startProcessLocked()方法来创建新的进程, 该方法会通过前面讲到的socket通道传递参数给Zygote进程. Zygote孵化自身, 并调用ZygoteInit.main()方法来实例化ActivityThread对象并最终返回新进程的pid。代码如下
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......
private final void startProcessLocked(ProcessRecord app,
String hostingType, String hostingNameStr) {
......
try {
int uid = app.info.uid;
int[] gids = null;
try {
gids = mContext.getPackageManager().getPackageGids(
app.info.packageName);
} catch (PackageManager.NameNotFoundException e) {
......
}
int debugFlags = 0;
......
int pid = Process.start("android.app.ActivityThread",
mSimpleProcessManagement ? app.processName : null, uid, uid,
gids, debugFlags, null);
......
} catch (RuntimeException e) {
......
}
}
......
}这里主要是调用Process.start接口来创建一个新的进程，新的进程会导入android.app.ActivityThread类，并且执行它的main函数。
绑定Application
这个是通过调用上文的ActivityThread对象中调用handleBindApplication方法完成的. 我们可以通过Thread.dumpStack()来查看流程：
at java.lang.Thread.dumpStack(Thread.java:505)06-21 17:42:18.448 8620-8620/cn.terminal.egame.myphone W/System.err: at cn.terminal.egame.myphone.MyApplication.onCreate(MyApplication.java:13)06-21 17:42:18.448 8620-8620/cn.terminal.egame.myphone W/System.err: at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1015)06-21 17:42:18.448 8620-8620/cn.terminal.egame.myphone W/System.err: at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4793)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at android.app.ActivityThread.access$1600(ActivityThread.java:165)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1437)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at android.os.Looper.loop(Looper.java:150)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at android.app.ActivityThread.main(ActivityThread.java:5621)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at java.lang.reflect.Method.invoke(Native Method)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794)06-21 17:42:18.449 8620-8620/cn.terminal.egame.myphone W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:684)123456789101112
那么ActivityThread的handleBindApplication又是谁来调用的（这里就简单介绍下，不能偏离主题太远，深入太远，最后不知所云，没有了方向）：
在ActivityThread的main方法中会执行ActivityThread对象的attach方法，会调用用了ActivityManagerService的远程接口本地代理对象ActivityManagerProxy的attachApplication函数通知attachApplication，并传入参数是mAppThread，这是ApplicationThread类型的Binder对象，用来接受ActivityManagerService的进程间消息。
public final class ActivityThread {
...... public static void main(String[] args) {
.....
ActivityThread thread = new ActivityThread();
thread.attach(false);
.....
} private void attach(boolean system) ｛
....... final IActivityManager mgr = ActivityManagerNative.getDefault(); try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) { // Ignore
}
....
}
}ActivityManagerService在接受到attachApplication函数调用远程消息之后，一系列处理之后，会有两个重要Binder通信，一个就是通过传来的参数Binder参数ApplicationThread来通知ActivityThread中mAppThread远程中调用bindApplication()，另一个是scheduleLaunchActivity。在Ams中收到attachApplication时代码如下：
AMS @Override
public final void attachApplication(IApplicationThread thread) { synchronized (this) { //获取applicationThread的进程id(也就是淘宝应用进程)
int callingPid = Binder.getCallingPid(); final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
} private final boolean attachApplicationLocked(IApplicationThread thread, int pid) { // Find the application record that is being attached... either via
// the pid if we are running in multiple processes, or just pull the
// next app record if we are emulating process with anonymous threads.
ProcessRecord app; if (pid != MY_PID && pid >= 0) { synchronized (mPidsSelfLocked) {
app = mPidsSelfLocked.get(pid);
}
} else {
app = null;
} //因为进程由AMS启动，所以在AMS中一定会有ProcessRecord（进程记录）
//如果没有ProcessRecord，则需要杀死该进程并退出
if (app == null) { ``````
return false;
} // If this application record is still attached to a previous
// process, clean it up now.
if (app.thread != null) { //如果从ProcessRecord中获取的IApplicationThread不为空，则需要处理该IApplicationThread
//因为有可能此Pid为复用，旧应用进程刚释放，内部IApplicationThread尚未清空，
//同时新进程又刚好使用了此Pid
handleAppDiedLocked(app, true, true);
} //创建死亡代理（进程kill后通知AMS）
AppDeathRecipient adr = new AppDeathRecipient(app, pid, thread); //进程注册成功，移除超时通知
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app); ``````
try { //******绑定Application******
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode,
mBinderTransactionTrackingEnabled, enableTrackAllocation,
isRestrictedBackupMode || !normalMode, app.persistent, new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
updateLruProcessLocked(app, false, null);
} catch (Exception e) { ``````
//bindApplication失败后，重启进程
startProcessLocked(app, "bind fail", processName); return false;
} try { //******启动Activity(启动应用MainActivity)******
if (mStackSupervisor.attachApplicationLocked(app)) {
didSomething = true;//didSomething表示是否有启动四大组件
}
} catch (Exception e) {
badApp = true;
} ``````
//绑定service和Broadcast的Application
if (badApp) { //如果以上组件启动出错，则需要杀死进程并移除记录
app.kill("error during init", true);
handleAppDiedLocked(app, false, true); return false;
} //如果以上没有启动任何组件，那么didSomething为false
if (!didSomething) { //调整进程的oom_adj值， oom_adj相当于一种优先级
//如果应用进程没有运行任何组件，那么当内存出现不足时，该进程是最先被系统“杀死”
updateOomAdjLocked();
} return true;
}从上午可以看到在attachApplicationLocked中有两个比较重要的方法函数：
thread.bindApplication（…） : 绑定Application到ActivityThread mStackSupervisor.attachApplicationLocked（app） : 启动Activity（7.0前为mMainStack.realStartActivityLocked()）123
bindApplication
通过AIDL接口IApplicationThread远程通知到ApplicationThreadNative的onTransact方法指定执行BIND_APPLICATION_TRANSACTION方法，而ActivityThread的内部类ApplicationThread实现ApplicationThreadNative抽象类bindApplication()，由于bindApplication()是运行在服务端Binder的线程池中，所以bindApplication会通过Handler发送BIND_APPLICATION的Message消息,ActivityThread中handler接受到之后调用handleBindApplication。
public final class ActivityThread {private class ApplicationThread extends ApplicationThreadNative {
..... public final void bindApplication(String processName, ApplicationInfo appInfo,
List<ProviderInfo> providers, ComponentName instrumentationName,
ProfilerInfo profilerInfo, Bundle instrumentationArgs,
IInstrumentationWatcher instrumentationWatcher,
IUiAutomationConnection instrumentationUiConnection, int debugMode, boolean enableOpenGlTrace, boolean trackAllocation, boolean isRestrictedBackupMode, boolean persistent, Configuration config, CompatibilityInfo compatInfo,
Map<String, IBinder> services, Bundle coreSettings) {
.........
AppBindData data = new AppBindData();
......
sendMessage(H.BIND_APPLICATION, data);
}
.....
} private class H extends Handler {
..... public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
.... case BIND_APPLICATION:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break;
.....
}
....
}
}初始化ContextImpl加载Apk资源
在handleBindApplication的具体实现中就可以看到资源加载：
private void handleBindApplication(AppBindData data) { //..........
// Context初始化(ContextImpl)
final ContextImpl appContext = ContextImpl.createAppContext(this/*ActivityThread*/, data.info/*LoadedApk*/); //........
}最终会调用到ContextImpl这个构造函数：
private ContextImpl(ContextImpl container, ActivityThread mainThread,
LoadedApk packageInfo, IBinder activityToken, UserHandle user, boolean restricted,
Display display, Configuration overrideConfiguration, int createDisplayWithId) { //.......
// LoadedApk赋值
mPackageInfo = packageInfo;
mResourcesManager = ResourcesManager.getInstance(); // resources初始化:通过LoadedApk.getResources来创建一个Resources实例
Resources resources = packageInfo.getResources(mainThread); if (resources != null) { if (displayId != Display.DEFAULT_DISPLAY
|| overrideConfiguration != null
|| (compatInfo != null && compatInfo.applicationScale
!= resources.getCompatibilityInfo().applicationScale)) {
resources = mResourcesManager.getTopLevelResources(packageInfo.getResDir(),
packageInfo.getSplitResDirs(), packageInfo.getOverlayDirs(),
packageInfo.getApplicationInfo().sharedLibraryFiles, displayId,
overrideConfiguration, compatInfo);
}
}
mResources = resources;// 赋值
//......
mContentResolver = new ApplicationContentResolver(this, mainThread, user);
}其中 packageInfo.getResources(mainThread)是指 LoadedApk.getResources()：
public Resources getResources(ActivityThread mainThread) { if (mResources == null) { // ActivityThread.getTopLevelResources()
mResources = mainThread.getTopLevelResources(mResDir/*APK文件位置*/, mSplitResDirs, mOverlayDirs,
mApplicationInfo.sharedLibraryFiles, Display.DEFAULT_DISPLAY, null, this);
} return mResources;
}即而又调用到ActivityThread.getTopLevelResources()：
/**
* Creates the top level resources for the given package.
*/
Resources getTopLevelResources(String resDir, String[] splitResDirs, String[] overlayDirs,
String[] libDirs, int displayId, Configuration overrideConfiguration,
LoadedApk pkgInfo) { return mResourcesManager.getTopLevelResources(resDir, splitResDirs, overlayDirs, libDirs,
displayId, overrideConfiguration, pkgInfo.getCompatibilityInfo());
}mResourcesManager是ResourcesManager的实例,最后资源加载是交给ResourcesManager来完成。
ResourcesManager.getTopLevelResources：
Resources getTopLevelResources(String resDir, String[] splitResDirs,
String[] overlayDirs, String[] libDirs, int displayId,
Configuration overrideConfiguration, CompatibilityInfo compatInfo) {
final float scale = compatInfo.applicationScale;
Configuration overrideConfigCopy = (overrideConfiguration != null)
? new Configuration(overrideConfiguration) : null;
ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfigCopy, scale);
Resources r;
synchronized (this) { // Resources is app scale dependent.
if (DEBUG) Slog.w(TAG, "getTopLevelResources: " + resDir + " / " + scale); // Resources是以ResourcesKey为key以弱应用的方式保存在mActiveResources这个Map中
WeakReference<Resources> wr = mActiveResources.get(key);
r = wr != null ? wr.get() : null; //if (r != null) Log.i(TAG, "isUpToDate " + resDir + ": " + r.getAssets().isUpToDate());
if (r != null && r.getAssets().isUpToDate()) {/ // 缓存里面有，并且是最新的
if (DEBUG) Slog.w(TAG, "Returning cached resources " + r + " " + resDir
+ ": appScale=" + r.getCompatibilityInfo().applicationScale
+ " key=" + key + " overrideConfig=" + overrideConfiguration); return r;
}
} //if (r != null) {
// Log.w(TAG, "Throwing away out-of-date resources!!!! "
// + r + " " + resDir);
//}
// AssetManager创建
AssetManager assets = new AssetManager(); // resDir can be null if the 'android' package is creating a new Resources object.
// This is fine, since each AssetManager automatically loads the 'android' package
// already.
//加载apk资源
if (resDir != null) { if (assets.addAssetPath(resDir) == 0) { return null;
}
}
...... if (libDirs != null) { for (String libDir : libDirs) { if (libDir.endsWith(".apk")) { // Avoid opening files we know do not have resources,
// like code-only .jar files.
if (assets.addAssetPath(libDir) == 0) {
Log.w(TAG, "Asset path '" + libDir + "' does not exist or contains no resources.");
}
}
}
} //Log.i(TAG, "Resource: key=" + key + ", display metrics=" + metrics);
DisplayMetrics dm = getDisplayMetricsLocked(displayId);
Configuration config;
...... //config初始化赋值
..... // 创建Resources
r = new Resources(assets, dm, config, compatInfo); //缓存Resources
synchronized (this) { // 可能其他线程已经创建好了，则直接返回
WeakReference<Resources> wr = mActiveResources.get(key);
Resources existing = wr != null ? wr.get() : null; if (existing != null && existing.getAssets().isUpToDate()) { // Someone else already created the resources while we were
// unlocked; go ahead and use theirs.
r.getAssets().close(); return existing;
} // XXX need to remove entries when weak references go away
// 把最新的对象保存到缓存中
mActiveResources.put(key, new WeakReference<>(r)); if (DEBUG) Slog.v(TAG, "mActiveResources.size()=" + mActiveResources.size()); return r;
}
}可以看到ResourcesManager先从缓存找已经加载好的资源Resource，如果没有就重新加载，通过初始化AssetManager和Resources来完成，并缓存。
其中关键函数就是AssetManager的addAssetPath(resDir)来完成加载并交给Resource来暴露接口。
先看下AssetManager初始化其，构造函数：
/**
* Create a new AssetManager containing only the basic system assets.
* Applications will not generally use this method, instead retrieving the
* appropriate asset manager with {@link Resources#getAssets}. Not for
* use by applications.
* {@hide}
*/
public AssetManager() { synchronized (this) { //......
init(false); // 确保有能够访问系统资源的AssetManager对象
ensureSystemAssets();
}
}init是个native方法，实现如下：
android_util_AssetManager.android_content_AssetManager_init()
static void android_content_AssetManager_init(JNIEnv* env, jobject clazz, jboolean isSystem)
{ if (isSystem) {// false
verifySystemIdmaps();
}
AssetManager* am = new AssetManager(); if (am == NULL) {
jniThrowException(env, "java/lang/OutOfMemoryError", ""); return;
}
am->addDefaultAssets();
ALOGV("Created AssetManager %p for Java object %p\n", am, clazz);
env->SetLongField(clazz, gAssetManagerOffsets.mObject, reinterpret_cast<jlong>(am));
}AssetManager.cpp:addDefaultAssets()是添加系统默认资源路径：/system/framework/framework-res.apk
bool AssetManager::addDefaultAssets()
{ // root = /system/
const char* root = getenv("ANDROID_ROOT");
LOG_ALWAYS_FATAL_IF(root == NULL, "ANDROID_ROOT not set");
String8 path(root); // path = /system/framework/framework-res.apk
path.appendPath(kSystemAssets); return addAssetPath(path, NULL);
}再通过addAssetPath添加资源路径到mAssetPaths并加载openNonAssetInPathLocked：
bool AssetManager::addAssetPath(const String8& path, int32_t* cookie)
{
AutoMutex _l(mLock);
asset_path ap;
String8 realPath(path); if (kAppZipName) { // 如果kAppZipName不为NULL(classes.jar),这里这个值是为NULL的
realPath.appendPath(kAppZipName);
}
ap.type = ::getFileType(realPath.string()); if (ap.type == kFileTypeRegular) {// kAppZipName不为NULL
ap.path = realPath;
} else { // kAppZipName为NULL
ap.path = path;//ap.path指向APK文件
ap.type = ::getFileType(path.string()); if (ap.type != kFileTypeDirectory && ap.type != kFileTypeRegular) {
ALOGW("Asset path %s is neither a directory nor file (type=%d).",
path.string(), (int)ap.type); return false;
}
} // Skip if we have it already.
for (size_t i=0; i<mAssetPaths.size(); i++) { if (mAssetPaths[i].path == ap.path) { if (cookie) {
*cookie = static_cast<int32_t>(i+1);
} return true;
}
}
ALOGV("In %p Asset %s path: %s", this,
ap.type == kFileTypeDirectory ? "dir" : "zip", ap.path.string()); // Check that the path has an AndroidManifest.xml
Asset* manifestAsset = const_cast<AssetManager*>(this)->openNonAssetInPathLocked(
kAndroidManifest, Asset::ACCESS_BUFFER, ap); if (manifestAsset == NULL) { // This asset path does not contain any resources.
delete manifestAsset; return false;
} delete manifestAsset;
mAssetPaths.add(ap); // new paths are always added at the end
if (cookie) {
*cookie = static_cast<int32_t>(mAssetPaths.size());
}#ifdef __ANDROID__
// Load overlays, if any
asset_path oap; for (size_t idx = 0; mZipSet.getOverlay(ap.path, idx, &oap); idx++) {
mAssetPaths.add(oap);
}#endif
...... return true;
}初始化完成AssetManager之后就是初始Resource,其作用就是缓存mAssets，并暴露接口对外加载资源，实际都是通过AssetManager来完成的：
public class Resources {
..... public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config,
CompatibilityInfo compatInfo) {
mAssets = assets;
mMetrics.setToDefaults(); if (compatInfo != null) {
mCompatibilityInfo = compatInfo;
} // 设备相关配置信息更新处理
updateConfiguration(config, metrics); // 创建字符串资源池
assets.ensureStringBlocks();
} //实际加载都是转给mAssets即AssetManager
public CharSequence getText(@StringRes int id) throws NotFoundException {
CharSequence res = mAssets.getResourceText(id); if (res != null) { return res;
} throw new NotFoundException("String resource ID #0x"
+ Integer.toHexString(id));
} //加载图片
private Drawable loadDrawableForCookie(TypedValue value, int id, Theme theme) {
final Drawable dr;
Trace.traceBegin(Trace.TRACE_TAG_RESOURCES, file); try { if (file.endsWith(".xml")) {
final XmlResourceParser rp = loadXmlResourceParser(
file, id, value.assetCookie, "drawable");
dr = Drawable.createFromXml(this, rp, theme);
rp.close();
} else {
final InputStream is = mAssets.openNonAsset( value.assetCookie, file, AssetManager.ACCESS_STREAMING);
dr = Drawable.createFromResourceStream(this, value, is, file, null); is.close();
}
} catch (Exception e) {
Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);
final NotFoundException rnf = new NotFoundException( "File " + file + " from drawable resource ID #0x" + Integer.toHexString(id));
rnf.initCause(e); throw rnf;
}
Trace.traceEnd(Trace.TRACE_TAG_RESOURCES); return dr;
}
......
}从上那么多可以看到开发中不论是设啥资源都是如此AssetManager完成的，到这里就讲完了整个Apk资源加载。
总结
通过上文就发现资源apk（resDir）通过AssetManager.addAssetPath(resDir)来完成加载，并初始化resource来处理暴露资源加载的流程。那么我们就可以通过自定义资源的加载，使用AssetManager来加载我们的单独资源apk不就可以了么，请看我的另一篇文章：打造自己的框架-实现动态加载两种方式 以及Resource是如何暴露出资源加载的流程，系统如何加载显示res下资源。
随时随地看视频
