Android对第三方类库运行时加载

首先，把需要运行时加载的类库，放到项目的其他目录，如新建一个thirdlibs的目录。然后用dx命令，对下面的类库进行压缩，做成dex文件。

dx --dex --output=../assets/ *.jar

这会在assets下面生成一个classes.dex文件，压缩成classes.zip文件。

新建一个类，用来加载类库

import java.io.File;import java.io.FileOutputStream;import java.io.FilenameFilter;import java.io.IOException;import java.io.InputStream;import java.io.OutputStream;import java.lang.reflect.Array;import java.lang.reflect.Field;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.util.ArrayList;import java.util.Arrays;import java.util.HashSet;import java.util.List;import java.util.ListIterator;import java.util.Set;import java.util.zip.ZipFile;import android.content.Context;import android.content.pm.ApplicationInfo;import android.content.pm.PackageManager;import android.content.pm.PackageManager.NameNotFoundException;import android.content.res.AssetManager;import android.os.Build;import android.util.Log;import dalvik.system.DexFile;public class AssetsDex {static final String TAG = "AssetsDex";private static final int MAX_SUPPORTED_SDK_VERSION = 20;private static final int MIN_SDK_VERSION = 4;private static final Set installedApk = new HashSet();private static boolean installed = false;private AssetsDex() {}public static void install(Context context) {if (installed) {return;}ensureLibs(context);Log.i(TAG, "install");if (Build.VERSION.SDK_INT < MIN_SDK_VERSION) {throw new RuntimeException("Multi dex installation failed. SDK "+ Build.VERSION.SDK_INT+ " is unsupported. Min SDK version is " + MIN_SDK_VERSION+ ".");}try {ApplicationInfo applicationInfo = getApplicationInfo(context);if (applicationInfo == null) {// Looks like running on a test Context, so just return without// patching.return;}synchronized (installedApk) {String apkPath = applicationInfo.sourceDir;if (installedApk.contains(apkPath)) {return;}installedApk.add(apkPath);if (Build.VERSION.SDK_INT > MAX_SUPPORTED_SDK_VERSION) {Log.w(TAG,"MultiDex is not guaranteed to work in SDK version "+ Build.VERSION.SDK_INT+ ": SDK version higher than "+ MAX_SUPPORTED_SDK_VERSION+ " should be backed by "+ "runtime with built-in multidex capabilty but it's not the "+ "case here: java.vm.version=\""+ System.getProperty("java.vm.version")+ "\"");}/* * The patched class loader is expected to be a descendant of * dalvik.system.BaseDexClassLoader. We modify its * dalvik.system.DexPathList pathList field to append additional * DEX file entries. */ClassLoader loader;try {loader = context.getClassLoader();} catch (RuntimeException e) {/* * Ignore those exceptions so that we don't break tests * relying on Context like a android.test.mock.MockContext * or a android.content.ContextWrapper with a null base * Context. */Log.w(TAG,"Failure while trying to obtain Context class loader. "+ "Must be running in test mode. Skip patching.",e);return;}if (loader == null) {// Note, the context class loader is null when running// Robolectric tests.Log.e(TAG,"Context class loader is null. Must be running in test mode. "+ "Skip patching.");return;}File dexDir = context.getDir("outdex", Context.MODE_PRIVATE);File[] szFiles = dexDir.listFiles(new FilenameFilter() {@Overridepublic boolean accept(File dir, String filename) {return filename.endsWith(".zip");}});List files = new ArrayList();for (File f : szFiles) {Log.i(TAG, "load file:" + f.getName());files.add(f);}Log.i(TAG, "loader before:" + context.getClassLoader());installSecondaryDexes(loader, dexDir, files);Log.i(TAG, "loader end:" + context.getClassLoader());}} catch (Exception e) {Log.e(TAG, "Multidex installation failure", e);throw new RuntimeException("Multi dex installation failed ("+ e.getMessage() + ").");}installed = true;Log.i(TAG, "install done");}private static ApplicationInfo getApplicationInfo(Context context)throws NameNotFoundException {PackageManager pm;String packageName;try {pm = context.getPackageManager();packageName = context.getPackageName();} catch (RuntimeException e) {/* * Ignore those exceptions so that we don't break tests relying on * Context like a android.test.mock.MockContext or a * android.content.ContextWrapper with a null base Context. */Log.w(TAG,"Failure while trying to obtain ApplicationInfo from Context. "+ "Must be running in test mode. Skip patching.", e);return null;}if (pm == null || packageName == null) {// This is most likely a mock context, so just return without// patching.return null;}ApplicationInfo applicationInfo = pm.getApplicationInfo(packageName,PackageManager.GET_META_DATA);return applicationInfo;}public static void ensureLibs(Context context) {AssetManager assetManager = context.getAssets();InputStream in = null;OutputStream out = null;try {File outdex = context.getDir("outdex", Context.MODE_PRIVATE);outdex.mkdir();File dex = context.getDir("outdex", Context.MODE_PRIVATE);dex.mkdir();in = assetManager.open("classes.zip");File f = new File(dex, "classes.zip");if (f.exists() && f.length() == in.available()) {Log.i(TAG, "classes.zip no change");return;}Log.i(TAG, "classes.zip chaneged");out = new FileOutputStream(f);byte[] buffer = new byte[102400];int read;while ((read = in.read(buffer)) != -1) {out.write(buffer, 0, read);}in.close();in = null;out.flush();out.close();out = null;Log.i(TAG, "classes.zip copy over");} catch (Exception e) {e.printStackTrace();}}private static void installSecondaryDexes(ClassLoader loader, File dexDir,List files) throws IllegalArgumentException,IllegalAccessException, NoSuchFieldException,InvocationTargetException, NoSuchMethodException, IOException {if (!files.isEmpty()) {if (Build.VERSION.SDK_INT >= 19) {V19.install(loader, files, dexDir);} else if (Build.VERSION.SDK_INT >= 14) {V14.install(loader, files, dexDir);} else {V4.install(loader, files);}}}/** * Locates a given field anywhere in the class inheritance hierarchy. * * @param instance *            an object to search the field into. * @param name *            field name * @return a field object * @throws NoSuchFieldException *             if the field cannot be located */private static Field findField(Object instance, String name)throws NoSuchFieldException {for (Class<?> clazz = instance.getClass(); clazz != null; clazz = clazz.getSuperclass()) {try {Field field = clazz.getDeclaredField(name);if (!field.isAccessible()) {field.setAccessible(true);}return field;} catch (NoSuchFieldException e) {// ignore and search next}}throw new NoSuchFieldException("Field " + name + " not found in "+ instance.getClass());}/** * Locates a given method anywhere in the class inheritance hierarchy. * * @param instance *            an object to search the method into. * @param name *            method name * @param parameterTypes *            method parameter types * @return a method object * @throws NoSuchMethodException *             if the method cannot be located */private static Method findMethod(Object instance, String name,Class<?>... parameterTypes) throws NoSuchMethodException {for (Class<?> clazz = instance.getClass(); clazz != null; clazz = clazz.getSuperclass()) {try {Method method = clazz.getDeclaredMethod(name, parameterTypes);if (!method.isAccessible()) {method.setAccessible(true);}return method;} catch (NoSuchMethodException e) {// ignore and search next}}throw new NoSuchMethodException("Method " + name + " with parameters "+ Arrays.asList(parameterTypes) + " not found in "+ instance.getClass());}/** * Replace the value of a field containing a non null array, by a new array * containing the elements of the original array plus the elements of * extraElements. *  * @param instance *            the instance whose field is to be modified. * @param fieldName *            the field to modify. * @param extraElements *            elements to append at the end of the array. */private static void expandFieldArray(Object instance, String fieldName,Object[] extraElements) throws NoSuchFieldException,IllegalArgumentException, IllegalAccessException {Field jlrField = findField(instance, fieldName);Object[] original = (Object[]) jlrField.get(instance);Object[] combined = (Object[]) Array.newInstance(original.getClass().getComponentType(), original.length + extraElements.length);System.arraycopy(original, 0, combined, 0, original.length);System.arraycopy(extraElements, 0, combined, original.length,extraElements.length);Log.i(TAG, "install 4");jlrField.set(instance, combined);Log.i(TAG, "install 5");}/** * Installer for platform versions 19. */private static final class V19 {private static void install(ClassLoader loader,List additionalClassPathEntries, File optimizedDirectory)throws IllegalArgumentException, IllegalAccessException,NoSuchFieldException, InvocationTargetException,NoSuchMethodException {/* * The patched class loader is expected to be a descendant of * dalvik.system.BaseDexClassLoader. We modify its * dalvik.system.DexPathList pathList field to append additional DEX * file entries. */Log.i(TAG, "install 1");Field pathListField = findField(loader, "pathList");Object dexPathList = pathListField.get(loader);Log.i(TAG, "install 2");ArrayList suppressedExceptions = new ArrayList();expandFieldArray(dexPathList,"dexElements",makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory,suppressedExceptions));Log.i(TAG, "install 3");if (suppressedExceptions.size() > 0) {for (IOException e : suppressedExceptions) {Log.w(TAG, "Exception in makeDexElement", e);}Field suppressedExceptionsField = findField(loader,"dexElementsSuppressedExceptions");IOException[] dexElementsSuppressedExceptions = (IOException[]) suppressedExceptionsField.get(loader);if (dexElementsSuppressedExceptions == null) {dexElementsSuppressedExceptions = suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);} else {IOException[] combined = new IOException[suppressedExceptions.size() + dexElementsSuppressedExceptions.length];suppressedExceptions.toArray(combined);System.arraycopy(dexElementsSuppressedExceptions, 0,combined, suppressedExceptions.size(),dexElementsSuppressedExceptions.length);dexElementsSuppressedExceptions = combined;}suppressedExceptionsField.set(loader,dexElementsSuppressedExceptions);}}/** * A wrapper around * {@code private static final dalvik.system.DexPathList#makeDexElements} * . */private static Object[] makeDexElements(Object dexPathList,ArrayList files, File optimizedDirectory,ArrayList suppressedExceptions)throws IllegalAccessException, InvocationTargetException,NoSuchMethodException {Log.i(TAG, "install 9");Method makeDexElements = findMethod(dexPathList, "makeDexElements",ArrayList.class, File.class, ArrayList.class);return (Object[]) makeDexElements.invoke(dexPathList, files,optimizedDirectory, suppressedExceptions);}}/** * Installer for platform versions 14, 15, 16, 17 and 18. */private static final class V14 {private static void install(ClassLoader loader,List additionalClassPathEntries, File optimizedDirectory)throws IllegalArgumentException, IllegalAccessException,NoSuchFieldException, InvocationTargetException,NoSuchMethodException {/* * The patched class loader is expected to be a descendant of * dalvik.system.BaseDexClassLoader. We modify its * dalvik.system.DexPathList pathList field to append additional DEX * file entries. */Field pathListField = findField(loader, "pathList");Object dexPathList = pathListField.get(loader);expandFieldArray(dexPathList,"dexElements",makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory));}/** * A wrapper around * {@code private static final dalvik.system.DexPathList#makeDexElements} * . */private static Object[] makeDexElements(Object dexPathList,ArrayList files, File optimizedDirectory)throws IllegalAccessException, InvocationTargetException,NoSuchMethodException {Method makeDexElements = findMethod(dexPathList, "makeDexElements",ArrayList.class, File.class);return (Object[]) makeDexElements.invoke(dexPathList, files,optimizedDirectory);}}/** * Installer for platform versions 4 to 13. */private static final class V4 {private static void install(ClassLoader loader,List additionalClassPathEntries)throws IllegalArgumentException, IllegalAccessException,NoSuchFieldException, IOException {/* * The patched class loader is expected to be a descendant of * dalvik.system.DexClassLoader. We modify its fields mPaths, * mFiles, mZips and mDexs to append additional DEX file entries. */int extraSize = additionalClassPathEntries.size();Field pathField = findField(loader, "path");StringBuilder path = new StringBuilder((String) pathField.get(loader));String[] extraPaths = new String[extraSize];File[] extraFiles = new File[extraSize];ZipFile[] extraZips = new ZipFile[extraSize];DexFile[] extraDexs = new DexFile[extraSize];for (ListIterator iterator = additionalClassPathEntries.listIterator(); iterator.hasNext();) {File additionalEntry = iterator.next();String entryPath = additionalEntry.getAbsolutePath();path.append(':').append(entryPath);int index = iterator.previousIndex();extraPaths[index] = entryPath;extraFiles[index] = additionalEntry;extraZips[index] = new ZipFile(additionalEntry);extraDexs[index] = DexFile.loadDex(entryPath, entryPath+ ".dex", 0);}pathField.set(loader, path.toString());expandFieldArray(loader, "mPaths", extraPaths);expandFieldArray(loader, "mFiles", extraFiles);expandFieldArray(loader, "mZips", extraZips);expandFieldArray(loader, "mDexs", extraDexs);}}}

在程序初始化的地方，如Application或者启动的Activity上，用AssetsDex.install(context)，加载类库，一定要在用到类库之前就加载完毕。如果类库过大，第一次加载可能会比较慢。用这种方法的好处是：1、突破64K限制 2、Eclipse下运行速度快（需要每次打包的lib减少了）

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