这里说明下：BootClassLoader的创建，是在zygote进程创建的时候，在ZygoteInit的main方法中，调用preLoad预加载类的时候，通过反射的方式创建BootClassLoader，BootClassLoader是一个单例的。

一、创建应用程序时创建PathClassLoader

在创建应用程序进程的时候，最终会执行到ActivityThread#handleBindApplication方法，而在ActivityThread#handleBindApplication方法中，会调用下面这段代码：

data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);

在这里就会执行创建该应用程序进程的PathClassLoader的过程。

1.ActivityThread#getPackageInfoNoCheck

    @Override
    @UnsupportedAppUsage
    public final LoadedApk getPackageInfoNoCheck(ApplicationInfo ai,
            CompatibilityInfo compatInfo) {
        return getPackageInfo(ai, compatInfo, null, false, true, false);
    }

2.ActivityThread#getPackageInfo

    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) {
                if (!isLoadedApkResourceDirsUpToDate(packageInfo, aInfo)) {
                    packageInfo.updateApplicationInfo(aInfo, null);
                }

                return packageInfo;
            }

            if (localLOGV) {
                Slog.v(TAG, (includeCode ? "Loading code package "
                        : "Loading resource-only package ") + aInfo.packageName
                        + " (in " + (mBoundApplication != null
                        ? mBoundApplication.processName : null)
                        + ")");
            }
            // 创建当前应用程序进程的package信息
            packageInfo =
                    new LoadedApk(this, aInfo, compatInfo, baseLoader,
                            securityViolation, includeCode
                            && (aInfo.flags & ApplicationInfo.FLAG_HAS_CODE) != 0, registerPackage);

            if (mSystemThread && "android".equals(aInfo.packageName)) {
                // 加载系统应用程序信息
                // getSystemContext().mPackageInfo其实就是获取LoadedApk对象
                // 这是获取系统进程的LoadedApk信息
                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;
        }
    }

3.LoadedApk#installSystemApplicationInfo

在调用installSystemApplicationInfo方法的时候，会传入一个默认的ClassLoader，这个默认的ClassLoader其实就是通过调用LoadedApk的getClassLoader创建的。

void installSystemApplicationInfo(ApplicationInfo info, ClassLoader classLoader) {
    assert info.packageName.equals("android");
    mApplicationInfo = info;
    // 设置默认的ClassLoader（系统进程的ClassLoader）
    mDefaultClassLoader = classLoader;
    mAppComponentFactory = createAppFactory(info, mDefaultClassLoader);
    // 根据当前应用程序进程的applicationInfo实例化ClassLoader，这是通过工厂进行实例化的
    mClassLoader = mAppComponentFactory.instantiateClassLoader(mDefaultClassLoader,
            new ApplicationInfo(mApplicationInfo));
}

4.AppComponentFactory#instantiateClassLoader

通过工厂实例化ClassLoader，这个工厂方法其实并没有实现任何操作，而是直接返回了mDefaultClassLoader这个默认的ClassLoader

    public @NonNull ClassLoader instantiateClassLoader(@NonNull ClassLoader cl,
            @NonNull ApplicationInfo aInfo) {
        return cl;
    }

5.LoadedApk#getClassLoader

    @UnsupportedAppUsage
    public ClassLoader getClassLoader() {
        synchronized (this) {
            if (mClassLoader == null) {
                createOrUpdateClassLoaderLocked(null /*addedPaths*/);
            }
            return mClassLoader;
        }
    }

6.LoadedApk#createOrUpdateClassLoaderLocked

    private void createOrUpdateClassLoaderLocked(List<String> addedPaths) {
        // 这个if判断是用于系统服务器，也就是系统进程的ClassLoader的创建的
        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) {
                mDefaultClassLoader = mBaseClassLoader;
            } else {
                mDefaultClassLoader = ClassLoader.getSystemClassLoader();
            }
            mAppComponentFactory = createAppFactory(mApplicationInfo, mDefaultClassLoader);
            mClassLoader = mAppComponentFactory.instantiateClassLoader(mDefaultClassLoader,
                    new ApplicationInfo(mApplicationInfo));
            return;
        }

        // 当程序包是当前应用程序程序包时，避免进行binder调用。 活动管理器将执行确保在加速过程之前执行dexopt。 
        // 同样，当我们没有ActivityThread对象时，也不要调用binder。
        if (mActivityThread != null
                && !Objects.equals(mPackageName, ActivityThread.currentPackageName())
                && mIncludeCode) {
            try {
                ActivityThread.getPackageManager().notifyPackageUse(mPackageName,
                        PackageManager.NOTIFY_PACKAGE_USE_CROSS_PACKAGE);
            } catch (RemoteException re) {
                throw re.rethrowFromSystemServer();
            }
        }

        if (mRegisterPackage) {
            try {
                ActivityManager.getService().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);

        boolean isBundledApp = mApplicationInfo.isSystemApp()
                && !mApplicationInfo.isUpdatedSystemApp();

        // Vendor apks are treated as bundled only when /vendor/lib is in the default search
        // paths. If not, they are treated as unbundled; access to system libs is limited.
        // Having /vendor/lib in the default search paths means that all system processes
        // are allowed to use any vendor library, which in turn means that system is dependent
        // on vendor partition. In the contrary, not having /vendor/lib in the default search
        // paths mean that the two partitions are separated and thus we can treat vendor apks
        // as unbundled.
        final String defaultSearchPaths = System.getProperty("java.library.path");
        final boolean treatVendorApkAsUnbundled = !defaultSearchPaths.contains("/vendor/lib");
        if (mApplicationInfo.getCodePath() != null
                && mApplicationInfo.isVendor() && treatVendorApkAsUnbundled) {
            isBundledApp = false;
        }

        makePaths(mActivityThread, isBundledApp, mApplicationInfo, zipPaths, libPaths);

        String libraryPermittedPath = mDataDir;

        if (isBundledApp) {
            // For bundled apps, add the base directory of the app (e.g.,
            // /system/app/Foo/) to the permitted paths so that it can load libraries
            // embedded in module apks under the directory. For now, GmsCore is relying
            // on this, but this isn't specific to the app. Also note that, we don't
            // need to do this for unbundled apps as entire /data is already set to
            // the permitted paths for them.
            libraryPermittedPath += File.pathSeparator
                    + Paths.get(getAppDir()).getParent().toString();

            // This is necessary to grant bundled apps access to
            // libraries located in subdirectories of /system/lib
            libraryPermittedPath += File.pathSeparator + defaultSearchPaths;
        }

        final String librarySearchPath = TextUtils.join(File.pathSeparator, libPaths);

        // 如果不要求包含代码，则构建一个不包含代码路径的类加载器
        // 但是我们仍然需要设置库搜索路径和允许的路径，因为NativeActivity依赖于它
        // （它试图通过mIncludeCode == false从LoadedApk调用类加载器上的System.loadLibrary（））。
        if (!mIncludeCode) {
            if (mDefaultClassLoader == null) {
                StrictMode.ThreadPolicy oldPolicy = allowThreadDiskReads();
                mDefaultClassLoader = ApplicationLoaders.getDefault().getClassLoader(
                        "" /* codePath */, mApplicationInfo.targetSdkVersion, isBundledApp,
                        librarySearchPath, libraryPermittedPath, mBaseClassLoader,
                        null /* classLoaderName */);
                setThreadPolicy(oldPolicy);
                mAppComponentFactory = AppComponentFactory.DEFAULT;
            }

            if (mClassLoader == null) {
                mClassLoader = mAppComponentFactory.instantiateClassLoader(mDefaultClassLoader,
                        new ApplicationInfo(mApplicationInfo));
            }

            return;
        }

        /*
         * 完成所有组合后（如有必要，请实际创建Java类加载器并在必要时设置JIT分析支持。
         * 在许多情况下，这是一个APK，因此请避免在TextUtils中使用StringBuilder。
         */
        final String zip = (zipPaths.size() == 1) ? zipPaths.get(0) :
                TextUtils.join(File.pathSeparator, zipPaths);

        if (DEBUG) Slog.v(ActivityThread.TAG, "Class path: " + zip +
                    ", JNI path: " + librarySearchPath);

        boolean needToSetupJitProfiles = false;
        if (mDefaultClassLoader == null) {
            // Temporarily disable logging of disk reads on the Looper thread
            // as this is early and necessary.
            StrictMode.ThreadPolicy oldPolicy = allowThreadDiskReads();

            List<ClassLoader> sharedLibraries = createSharedLibrariesLoaders(
                    mApplicationInfo.sharedLibraryInfos, isBundledApp, librarySearchPath,
                    libraryPermittedPath);
            // 创建mDefaultClassLoader，这个方法最终是调用了ApplicationLoaders#getClassLoader
            mDefaultClassLoader = ApplicationLoaders.getDefault().getClassLoaderWithSharedLibraries(
                    zip, mApplicationInfo.targetSdkVersion, isBundledApp, librarySearchPath,
                    libraryPermittedPath, mBaseClassLoader,
                    mApplicationInfo.classLoaderName, sharedLibraries);
            mAppComponentFactory = createAppFactory(mApplicationInfo, mDefaultClassLoader);

            setThreadPolicy(oldPolicy);
            // Setup the class loader paths for profiling.
            needToSetupJitProfiles = true;
        }

        if (!libPaths.isEmpty() && SystemProperties.getBoolean(PROPERTY_NAME_APPEND_NATIVE, true)) {
            // Temporarily disable logging of disk reads on the Looper thread as this is necessary
            StrictMode.ThreadPolicy oldPolicy = allowThreadDiskReads();
            try {
                ApplicationLoaders.getDefault().addNative(mDefaultClassLoader, libPaths);
            } finally {
                setThreadPolicy(oldPolicy);
            }
        }

        // /aepx/com.android.runtime/lib, /vendor/lib, /odm/lib and /product/lib
        // are added to the native lib search paths of the classloader.
        // Note that this is done AFTER the classloader is
        // created by ApplicationLoaders.getDefault().getClassLoader(...). The
        // reason is because if we have added the paths when creating the classloader
        // above, the paths are also added to the search path of the linker namespace
        // 'classloader-namespace', which will allow ALL libs in the paths to apps.
        // Since only the libs listed in <partition>/etc/public.libraries.txt can be
        // available to apps, we shouldn't add the paths then.
        //
        // However, we need to add the paths to the classloader (Java) though. This
        // is because when a native lib is requested via System.loadLibrary(), the
        // classloader first tries to find the requested lib in its own native libs
        // search paths. If a lib is not found in one of the paths, dlopen() is not
        // called at all. This can cause a problem that a vendor public native lib
        // is accessible when directly opened via dlopen(), but inaccesible via
        // System.loadLibrary(). In order to prevent the problem, we explicitly
        // add the paths only to the classloader, and not to the native loader
        // (linker namespace).
        List<String> extraLibPaths = new ArrayList<>(4);
        String abiSuffix = VMRuntime.getRuntime().is64Bit() ? "64" : "";
        if (!defaultSearchPaths.contains("/apex/com.android.runtime/lib")) {
            extraLibPaths.add("/apex/com.android.runtime/lib" + abiSuffix);
        }
        if (!defaultSearchPaths.contains("/vendor/lib")) {
            extraLibPaths.add("/vendor/lib" + abiSuffix);
        }
        if (!defaultSearchPaths.contains("/odm/lib")) {
            extraLibPaths.add("/odm/lib" + abiSuffix);
        }
        if (!defaultSearchPaths.contains("/product/lib")) {
            extraLibPaths.add("/product/lib" + abiSuffix);
        }
        if (!extraLibPaths.isEmpty()) {
            StrictMode.ThreadPolicy oldPolicy = allowThreadDiskReads();
            try {
                ApplicationLoaders.getDefault().addNative(mDefaultClassLoader, extraLibPaths);
            } finally {
                setThreadPolicy(oldPolicy);
            }
        }

        if (addedPaths != null && addedPaths.size() > 0) {
            final String add = TextUtils.join(File.pathSeparator, addedPaths);
            ApplicationLoaders.getDefault().addPath(mDefaultClassLoader, 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.
        //
        // It is not ok to call this in a zygote context where mActivityThread is null.
        if (needToSetupJitProfiles && !ActivityThread.isSystem() && mActivityThread != null) {
            setupJitProfileSupport();
        }

        // Call AppComponentFactory to select/create the main class loader of this app.
        // Since this may call code in the app, mDefaultClassLoader must be fully set up
        // before invoking the factory.
        // Invoke with a copy of ApplicationInfo to protect against the app changing it.
        if (mClassLoader == null) {
            mClassLoader = mAppComponentFactory.instantiateClassLoader(mDefaultClassLoader,
                    new ApplicationInfo(mApplicationInfo));
        }
    }

7.ApplicationLoaders#getClassLoader

    private ClassLoader getClassLoader(String zip, int targetSdkVersion, boolean isBundled,
                                       String librarySearchPath, String libraryPermittedPath,
                                       ClassLoader parent, String cacheKey,
                                       String classLoaderName, List<ClassLoader> sharedLibraries) {
        /*
         * 这里理论上是系统类加载器，其实就是BootClassLoader
         */
        ClassLoader baseParent = ClassLoader.getSystemClassLoader().getParent();

        synchronized (mLoaders) {
            if (parent == null) {
                parent = baseParent;
            }

            /*
             * If we're one step up from the base class loader, find
             * something in our cache.  Otherwise, we create a whole
             * new ClassLoader for the zip archive.
             */
            if (parent == baseParent) {
                ClassLoader loader = mLoaders.get(cacheKey);
                if (loader != null) {
                    return loader;
                }

                Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, zip);

                ClassLoader classloader = ClassLoaderFactory.createClassLoader(
                        zip,  librarySearchPath, libraryPermittedPath, parent,
                        targetSdkVersion, isBundled, classLoaderName, sharedLibraries);

                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

                Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "setLayerPaths");
                GraphicsEnvironment.getInstance().setLayerPaths(
                        classloader, librarySearchPath, libraryPermittedPath);
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

                if (cacheKey != null) {
                    mLoaders.put(cacheKey, classloader);
                }
                return classloader;
            }

            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, zip);
            ClassLoader loader = ClassLoaderFactory.createClassLoader(
                    zip, null, parent, classLoaderName, sharedLibraries);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            return loader;
        }
    }

8.ClassLoaderFactory#createClassLoader

这里就是创建应用程序进程的PathClassLoader对象的具体位置

    public static ClassLoader createClassLoader(String dexPath,
            String librarySearchPath, ClassLoader parent, String classloaderName,
            List<ClassLoader> sharedLibraries) {
        ClassLoader[] arrayOfSharedLibraries = (sharedLibraries == null)
                ? null
                : sharedLibraries.toArray(new ClassLoader[sharedLibraries.size()]);
        if (isPathClassLoaderName(classloaderName)) {
            // 创建PathClassLoader对象，并且返回
            return new PathClassLoader(dexPath, librarySearchPath, parent, arrayOfSharedLibraries);
        } else if (isDelegateLastClassLoaderName(classloaderName)) {
            return new DelegateLastClassLoader(dexPath, librarySearchPath, parent,
                    arrayOfSharedLibraries);
        }

        throw new AssertionError("Invalid classLoaderName: " + classloaderName);
    }

二、创建SystemServer进程时创建PathClassLoader

创建SystemServer进程的时候，也会创建该进程对应的PathClassLoader，从ZygoteInit#startSystemServer方法开始创建SystemServer进程，在通过fork zygote进程创建完成SystemServer进程之后，就会返回一个pid，当pid=0，表示就是fork子进程成功，则执行对应的子进程需要的操作，在pid=0的if条件分支中，会调用handleSystemServerProcess方法，在这里会创建SystemServer的PathClassLoader