Android 插件化开发有两方面，一是代码的加载，二是资源的加载。
基于上一篇Android activity的启动方式先对代码的加载说一下，下一篇说一下资源的的加载。
插件化：将一个未安装的apk下载到本地，在未安装的情况下，宿主app可以打开apk 的activity，严格的插件化和组件化需要大家百度科普一下。上一篇<activity的启动>中提到最后是调用ActivityThread的中的performLaunchActivity方法，
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
}
方法中的r.packageInfo.getClassLoader()，packaginfo是LoadApk的对象，方法是怎么实现的具体看一下：
public ClassLoader getClassLoader() {
synchronized (this) {
if (mClassLoader == null) {
createOrUpdateClassLoaderLocked(null /*addedPaths*/);
}
return mClassLoader;
}
} 通过getClassLoader获取了一个ClassLoader,
LoadApk，文件的相关信息，诸如Apk文件的代码和资源，甚至代码里面的Activity，Service等四大组件的信息我们都可以通过此对象获取。看一下如何获取：
if (mPackageName.equals("android")) {
// Note: This branch is taken for system server and we don't need to setup
// jit profiling support.
if (mClassLoader != null) {
// nothing to update
return;
}
if (mBaseClassLoader != null) {
mClassLoader = mBaseClassLoader;
} else {
mClassLoader = ClassLoader.getSystemClassLoader();
}
return;
}
// Avoid the binder call when the package is the current application package.
// The activity manager will perform ensure that dexopt is performed before
// spinning up the process.
if (!Objects.equals(mPackageName, ActivityThread.currentPackageName())) {
VMRuntime.getRuntime().vmInstructionSet();
try {
ActivityThread.getPackageManager().notifyPackageUse(mPackageName,
PackageManager.NOTIFY_PACKAGE_USE_CROSS_PACKAGE);
} catch (RemoteException re) {
throw re.rethrowFromSystemServer();
}
}
if (mRegisterPackage) {
try {
ActivityManagerNative.getDefault().addPackageDependency(mPackageName);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
// Lists for the elements of zip/code and native libraries.
//
// Both lists are usually not empty. We expect on average one APK for the zip component,
// but shared libraries and splits are not uncommon. We expect at least three elements
// for native libraries (app-based, system, vendor). As such, give both some breathing
// space and initialize to a small value (instead of incurring growth code).
final List<String> zipPaths = new ArrayList<>(10);
final List<String> libPaths = new ArrayList<>(10);
makePaths(mActivityThread, mApplicationInfo, zipPaths, libPaths);
final boolean isBundledApp = mApplicationInfo.isSystemApp()
&& !mApplicationInfo.isUpdatedSystemApp();
String libraryPermittedPath = mDataDir;
if (isBundledApp) {
// This is necessary to grant bundled apps access to
// libraries located in subdirectories of /system/lib
libraryPermittedPath += File.pathSeparator +
System.getProperty("java.library.path");
}
final String librarySearchPath = TextUtils.join(File.pathSeparator, libPaths);
// If we're not asked to include code, we construct a classloader that has
// no code path included. We still need to set up the library search paths
// and permitted path because NativeActivity relies on it (it attempts to
// call System.loadLibrary() on a classloader from a LoadedApk with
// mIncludeCode == false).
if (!mIncludeCode) {
if (mClassLoader == null) {
StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();
mClassLoader = ApplicationLoaders.getDefault().getClassLoader(
"" /* codePath */, mApplicationInfo.targetSdkVersion, isBundledApp,
librarySearchPath, libraryPermittedPath, mBaseClassLoader);
StrictMode.setThreadPolicy(oldPolicy);
}
return;
}
/*
* With all the combination done (if necessary, actually create the java class
* loader and set up JIT profiling support if necessary.
*
* In many cases this is a single APK, so try to avoid the StringBuilder in TextUtils.
*/
final String zip = (zipPaths.size() == 1) ? zipPaths.get(0) :
TextUtils.join(File.pathSeparator, zipPaths);
if (ActivityThread.localLOGV)
Slog.v(ActivityThread.TAG, "Class path: " + zip +
", JNI path: " + librarySearchPath);
boolean needToSetupJitProfiles = false;
if (mClassLoader == null) {
// Temporarily disable logging of disk reads on the Looper thread
// as this is early and necessary.
StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();
mClassLoader = ApplicationLoaders.getDefault().getClassLoader(zip,
mApplicationInfo.targetSdkVersion, isBundledApp, librarySearchPath,
libraryPermittedPath, mBaseClassLoader);
StrictMode.setThreadPolicy(oldPolicy);
// Setup the class loader paths for profiling.
needToSetupJitProfiles = true;
}
if (addedPaths != null && addedPaths.size() > 0) {
final String add = TextUtils.join(File.pathSeparator, addedPaths);
ApplicationLoaders.getDefault().addPath(mClassLoader, add);
// Setup the new code paths for profiling.
needToSetupJitProfiles = true;
}
// Setup jit profile support.
//
// It is ok to call this multiple times if the application gets updated with new splits.
// The runtime only keeps track of unique code paths and can handle re-registration of
// the same code path. There's no need to pass `addedPaths` since any new code paths
// are already in `mApplicationInfo`.
//
// It is NOT ok to call this function from the system_server (for any of the packages it
// loads code from) so we explicitly disallow it there.
if (needToSetupJitProfiles && !ActivityThread.isSystem()) {
setupJitProfileSupport();
}
} 拿到classLoader后，将activity加载进来。大家应该还记得这个，发送消息给ActivityThread的中的H，Handler对象，重写HandlerMessage的方法里面的，其中handlerLauncherActivity里面调用的是performLaunchActivity方法。重点看一下getPackaeInfoNoCheck()方法，看一下
LoadApk是如何产生的。
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, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;public final LoadedApk getPackageInfoNoCheck(ApplicationInfo ai,
CompatibilityInfo compatInfo) {
return getPackageInfo(ai, compatInfo, null, false, true, false);
}private LoadedApk getPackageInfo(ApplicationInfo aInfo, CompatibilityInfo compatInfo,
ClassLoader baseLoader, boolean securityViolation, boolean includeCode,
boolean registerPackage) {
final boolean differentUser = (UserHandle.myUserId() != UserHandle.getUserId(aInfo.uid));
synchronized (mResourcesManager) {
WeakReference<LoadedApk> ref;
if (differentUser) {
// Caching not supported across users
ref = null;
} else if (includeCode) {
ref = mPackages.get(aInfo.packageName);
} else {
ref = mResourcePackages.get(aInfo.packageName);
}
LoadedApk packageInfo = ref != null ? ref.get() : null;
if (packageInfo == null || (packageInfo.mResources != null
&& !packageInfo.mResources.getAssets().isUpToDate())) {
if (localLOGV) Slog.v(TAG, (includeCode ? "Loading code package "
: "Loading resource-only package ") + aInfo.packageName
+ " (in " + (mBoundApplication != null
? mBoundApplication.processName : null)
+ ")");
packageInfo =
new LoadedApk(this, aInfo, compatInfo, baseLoader,
securityViolation, includeCode &&
(aInfo.flags&ApplicationInfo.FLAG_HAS_CODE) != 0, registerPackage);
if (mSystemThread && "android".equals(aInfo.packageName)) {
packageInfo.installSystemApplicationInfo(aInfo,
getSystemContext().mPackageInfo.getClassLoader());
}
if (differentUser) {
// Caching not supported across users
} else if (includeCode) {
mPackages.put(aInfo.packageName,
new WeakReference<LoadedApk>(packageInfo));
} else {
mResourcePackages.put(aInfo.packageName,
new WeakReference<LoadedApk>(packageInfo));
}
}
return packageInfo;大家注意一下，第三个参数，在调用的时候是穿的参数是null即baseLoader，所以走的是这个：
packageInfo =
new LoadedApk(this, aInfo, compatInfo, baseLoader,
securityViolation, includeCode &&
(aInfo.flags&ApplicationInfo.FLAG_HAS_CODE) != 0, registerPackage); 跟进：
public LoadedApk(ActivityThread activityThread, ApplicationInfo aInfo,
CompatibilityInfo compatInfo, ClassLoader baseLoader,
boolean securityViolation, boolean includeCode, boolean registerPackage) {
mActivityThread = activityThread;
setApplicationInfo(aInfo);
mPackageName = aInfo.packageName;
mBaseClassLoader = baseLoader;
mSecurityViolation = securityViolation;
mIncludeCode = includeCode;
mRegisterPackage = registerPackage;
mDisplayAdjustments.setCompatibilityInfo(compatInfo);
} 文章前面的getClassLoader方法里面调用的是：
private void createOrUpdateClassLoaderLocked(List<String> addedPaths) {
if (mPackageName.equals("android")) {
// Note: This branch is taken for system server and we don't need to setup
// jit profiling support.
if (mClassLoader != null) {
// nothing to update
return;
}
if (mBaseClassLoader != null) {
mClassLoader = mBaseClassLoader;
} else {
mClassLoader = ClassLoader.getSystemClassLoader();
}
return;
}
因为mBaseClassLoader是null直接调ClassLoader.getSystemClassLoader()
@CallerSensitive
public static ClassLoader getSystemClassLoader() {
return SystemClassLoader.loader;
}static private class SystemClassLoader {
public static ClassLoader loader = ClassLoader.createSystemClassLoader();
}private static ClassLoader createSystemClassLoader() {
String classPath = System.getProperty("java.class.path", ".");
String librarySearchPath = System.getProperty("java.library.path", "");
// String[] paths = classPath.split(":");
// URL[] urls = new URL[paths.length];
// for (int i = 0; i < paths.length; i++) {
// try {
// urls[i] = new URL("file://" + paths[i]);
// }
// catch (Exception ex) {
// ex.printStackTrace();
// }
// }
//
// return new java.net.URLClassLoader(urls, null);
// TODO Make this a java.net.URLClassLoader once we have those?
return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());
}最后new 了一个PathClassLoader，也就是LoaderAPK最后返回的是是一个PathClassLoader的对象去加载
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);public Activity newActivity(ClassLoader cl, String className,
Intent intent)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
return (Activity)cl.loadClass(className).newInstance();
} 里面的cl即使PathClassLoader对象，进一步看一下PathClassLoader：
public class PathClassLoader extends BaseDexClassLoader {
public PathClassLoader(String dexPath, ClassLoader parent) {
super((String)null, (File)null, (String)null, (ClassLoader)null);
throw new RuntimeException("Stub!");
}
public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
super((String)null, (File)null, (String)null, (ClassLoader)null);
throw new RuntimeException("Stub!");
}
}public class BaseDexClassLoader extends ClassLoader {
public BaseDexClassLoader(String dexPath, File optimizedDirectory, String librarySearchPath, ClassLoader parent) {
throw new RuntimeException("Stub!");
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new RuntimeException("Stub!");
}
protected URL findResource(String name) {
throw new RuntimeException("Stub!");
}
protected Enumeration<URL> findResources(String name) {
throw new RuntimeException("Stub!");
}
public String findLibrary(String name) {
throw new RuntimeException("Stub!");
}
protected synchronized Package getPackage(String name) {
throw new RuntimeException("Stub!");
}
public String toString() {
throw new RuntimeException("Stub!");
}
}//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package dalvik.system;
import java.io.File;
public class DexClassLoader extends BaseDexClassLoader {
public DexClassLoader(String dexPath, String optimizedDirectory, String librarySearchPath, ClassLoader parent) {
super((String)null, (File)null, (String)null, (ClassLoader)null);
throw new RuntimeException("Stub!");
}
}
PathClassLoader和DexClassLoader，
它们都继承自BaseDexClassLoader，这两个类有什么区别呢？其实看一下它们的源码注释就一目了然了。
Android系统通过PathClassLoader来加载系统类和主dex中的类。
而DexClassLoader则用于加载其他dex文件中的类。
他们都是继承自BaseDexClassLoader，具体的加载方法是findClass。
@Override
protected Class<?> loadClass(String className, boolean resolve)
throws ClassNotFoundException {
Class<?> clazz = findLoadedClass(className);
if (clazz == null) {
clazz = findClass(className);
}
return clazz;
} 在PathClassLoader和BaseDexClassLoader都米有找到loadClass,最后在ClassLoader中找见，调的是findClass,父类里面没有实现，子类PathClassLoader也没有实现，BaseDexClassLoader也没有实现，我是不是看到假的源码了，我看的API是25的。
ClassLoader的方法，findClass方法，PathClassLoader和BaseDexClassLoader都木有重写是什么鬼？求解释。
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
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