【安卓笔记】热修复_addAssetPath不同版本区别原因

在做热修复功能时Java层通过反射调用addAssetPath在Android5.0及以上系统没有问题，在Android 4.x版本找不到资源。

addAssetPath方法:

/**
 * Add an additional set of assets to the asset manager.  This can be
 * either a directory or ZIP file.  Not for use by applications.  Returns
 * the cookie of the added asset, or 0 on failure.
 * {@hide}
 */
public final int addAssetPath(String path) {
    int res = addAssetPathNative(path);
    return res;
}
private native final int addAssetPathNative(String path);

addAssetPath具体的实现方法是native层的。

AssetManager.cpp源码

4.4及以前的版本调用addAssetPath方法时，只是把补丁包的路径添加到mAssetPath中，不会去重新解析，真正解析的代码是在app第一次执行AssetManager.getResTable()`方法的时候。
一旦解析完一次后，mResource对象就不为nil，以后就会直接return掉，不会重新解析。

bool AssetManager::addAssetPath(const String8& path, void** cookie)
{
    AutoMutex _l(mLock);
    asset_path ap;
    String8 realPath(path);
    if (kAppZipName) {
        realPath.appendPath(kAppZipName);
    }
    ap.type = ::getFileType(realPath.string());
    if (ap.type == kFileTypeRegular) {
        ap.path = realPath;
    } else {
        ap.path = path;
        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 = (void*)(i+1);
            }
            return true;
        }
    }
    ALOGV("In %p Asset %s path: %s", this,
         ap.type == kFileTypeDirectory ? "dir" : "zip", ap.path.string());
    mAssetPaths.add(ap);
    // new paths are always added at the end
    if (cookie) {
        *cookie = (void*)mAssetPaths.size();
    }
    // add overlay packages for /system/framework; apps are handled by the
    // (Java) package manager
    if (strncmp(path.string(), "/system/framework/", 18) == 0) {
        // When there is an environment variable for /vendor, this
        // should be changed to something similar to how ANDROID_ROOT
        // and ANDROID_DATA are used in this file.
        String8 overlayPath("/vendor/overlay/framework/");
        overlayPath.append(path.getPathLeaf());
        if (TEMP_FAILURE_RETRY(access(overlayPath.string(), R_OK)) == 0) {
            asset_path oap;
            oap.path = overlayPath;
            oap.type = ::getFileType(overlayPath.string());
            bool addOverlay = (oap.type == kFileTypeRegular); // only .apks supported as overlay
            if (addOverlay) {
                oap.idmap = idmapPathForPackagePath(overlayPath);
                if (isIdmapStaleLocked(ap.path, oap.path, oap.idmap)) {
                    addOverlay = createIdmapFileLocked(ap.path, oap.path, oap.idmap);
                }
            }
            if (addOverlay) {
                mAssetPaths.add(oap);
            } else {
                ALOGW("failed to add overlay package %s\n", overlayPath.string());
            }
        }
    }
    return true;
}

const ResTable* AssetManager::getResTable(bool required) const
{
    // 执行该方法第一次之后，就都会return
    ResTable* rt = mResources;
    if (rt) {
        return rt;
    }
    // Iterate through all asset packages, collecting resources from each.
    AutoMutex _l(mLock);
    if (mResources != NULL) {
        return mResources;
    }
    if (required) {
        LOG_FATAL_IF(mAssetPaths.size() == 0, "No assets added to AssetManager");
    }
    if (mCacheMode != CACHE_OFF && !mCacheValid)
        const_cast<AssetManager*>(this)->loadFileNameCacheLocked();
    const size_t N = mAssetPaths.size();
    // 真正解析package的地方
    for (size_t i=0; i<N; i++) {
        Asset* ass = NULL;
        ResTable* sharedRes = NULL;
        bool shared = true;
        const asset_path& ap = mAssetPaths.itemAt(i);
        MY_TRACE_BEGIN(ap.path.string());
        Asset* idmap = openIdmapLocked(ap);
        ALOGV("Looking for resource asset in '%s'\n", ap.path.string());
        if (ap.type != kFileTypeDirectory) {
            if (i == 0) {
                // The first item is typically the framework resources,
                // which we want to avoid parsing every time.
                sharedRes = const_cast<AssetManager*>(this)->
                    mZipSet.getZipResourceTable(ap.path);
            }
            if (sharedRes == NULL) {
                ass = const_cast<AssetManager*>(this)->
                    mZipSet.getZipResourceTableAsset(ap.path);
                if (ass == NULL) {
                    ALOGV("loading resource table %s\n", ap.path.string());
                    ass = const_cast<AssetManager*>(this)->
                        openNonAssetInPathLocked("resources.arsc",
                                                 Asset::ACCESS_BUFFER,
                                                 ap);
                    if (ass != NULL && ass != kExcludedAsset) {
                        ass = const_cast<AssetManager*>(this)->
                            mZipSet.setZipResourceTableAsset(ap.path, ass);
                    }
                }
                
                if (i == 0 && ass != NULL) {
                    // If this is the first resource table in the asset
                    // manager, then we are going to cache it so that we
                    // can quickly copy it out for others.
                    ALOGV("Creating shared resources for %s", ap.path.string());
                    sharedRes = new ResTable();
                    sharedRes->add(ass, (void*)(i+1), false, idmap);
                    sharedRes = const_cast<AssetManager*>(this)->
                        mZipSet.setZipResourceTable(ap.path, sharedRes);
                }
            }
        } else {
            ALOGV("loading resource table %s\n", ap.path.string());
            Asset* ass = const_cast<AssetManager*>(this)->
                openNonAssetInPathLocked("resources.arsc",
                                         Asset::ACCESS_BUFFER,
                                         ap);
            shared = false;
        }
        if ((ass != NULL || sharedRes != NULL) && ass != kExcludedAsset) {
            if (rt == NULL) {
                mResources = rt = new ResTable();
                updateResourceParamsLocked();
            }
            ALOGV("Installing resource asset %p in to table %p\n", ass, mResources);
            if (sharedRes != NULL) {
                ALOGV("Copying existing resources for %s", ap.path.string());
                rt->add(sharedRes);
            } else {
                ALOGV("Parsing resources for %s", ap.path.string());
                rt->add(ass, (void*)(i+1), !shared, idmap);
            }
            if (!shared) {
                delete ass;
            }
        }
        if (idmap != NULL) {
            delete idmap;
        }
        MY_TRACE_END();
    }
    if (required && !rt) ALOGW("Unable to find resources file resources.arsc");
    if (!rt) {
        mResources = rt = new ResTable();
    }
    return rt;
}


const ResTable& AssetManager::getResources(bool required) const
{
    const ResTable* rt = getResTable(required);
    return *rt;
}

而当我们执行加载补丁的代码的时候，getResTable已经执行过多次了，Android Framework里面的代码会多次调用该方法。所以即使是使用addAssetPath，也只是添加到了mAssetPath，并不会发生解析，所以补丁包里面的资源就是完全不生效的。

而在android 5.0及以上的代码中:

Android 5.0 AssetManager.cpp源码

bool AssetManager::addAssetPath(const String8& path, int32_t* cookie)
{
    AutoMutex _l(mLock);
    asset_path ap;
    String8 realPath(path);
    if (kAppZipName) {
        realPath.appendPath(kAppZipName);
    }
    ap.type = ::getFileType(realPath.string());
    if (ap.type == kFileTypeRegular) {
        ap.path = realPath;
    } else {
        ap.path = path;
        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 HAVE_ANDROID_OS
    // Load overlays, if any
    asset_path oap;
    for (size_t idx = 0; mZipSet.getOverlay(ap.path, idx, &oap); idx++) {
        mAssetPaths.add(oap);
    }
#endif
    if (mResources != NULL) {
    // 重新调用该方法去解析package
        appendPathToResTable(ap);
    }
    return true;
}

bool AssetManager::appendPathToResTable(const asset_path& ap) const {
    Asset* ass = NULL;
    ResTable* sharedRes = NULL;
    bool shared = true;
    bool onlyEmptyResources = true;
    MY_TRACE_BEGIN(ap.path.string());
    Asset* idmap = openIdmapLocked(ap);
    size_t nextEntryIdx = mResources->getTableCount();
    ALOGV("Looking for resource asset in '%s'\n", ap.path.string());
    if (ap.type != kFileTypeDirectory) {
        if (nextEntryIdx == 0) {
            // The first item is typically the framework resources,
            // which we want to avoid parsing every time.
            sharedRes = const_cast<AssetManager*>(this)->
                mZipSet.getZipResourceTable(ap.path);
            if (sharedRes != NULL) {
                // skip ahead the number of system overlay packages preloaded
                nextEntryIdx = sharedRes->getTableCount();
            }
        }
        if (sharedRes == NULL) {
            ass = const_cast<AssetManager*>(this)->
                mZipSet.getZipResourceTableAsset(ap.path);
            if (ass == NULL) {
                ALOGV("loading resource table %s\n", ap.path.string());
                ass = const_cast<AssetManager*>(this)->
                    openNonAssetInPathLocked("resources.arsc",
                                             Asset::ACCESS_BUFFER,
                                             ap);
                if (ass != NULL && ass != kExcludedAsset) {
                    ass = const_cast<AssetManager*>(this)->
                        mZipSet.setZipResourceTableAsset(ap.path, ass);
                }
            }
            
            if (nextEntryIdx == 0 && ass != NULL) {
                // If this is the first resource table in the asset
                // manager, then we are going to cache it so that we
                // can quickly copy it out for others.
                ALOGV("Creating shared resources for %s", ap.path.string());
                sharedRes = new ResTable();
                sharedRes->add(ass, idmap, nextEntryIdx + 1, false);
#ifdef HAVE_ANDROID_OS
                const char* data = getenv("ANDROID_DATA");
                LOG_ALWAYS_FATAL_IF(data == NULL, "ANDROID_DATA not set");
                String8 overlaysListPath(data);
                overlaysListPath.appendPath(kResourceCache);
                overlaysListPath.appendPath("overlays.list");
                addSystemOverlays(overlaysListPath.string(), ap.path, sharedRes, nextEntryIdx);
#endif
                sharedRes = const_cast<AssetManager*>(this)->
                    mZipSet.setZipResourceTable(ap.path, sharedRes);
            }
        }
    } else {
        ALOGV("loading resource table %s\n", ap.path.string());
        ass = const_cast<AssetManager*>(this)->
            openNonAssetInPathLocked("resources.arsc",
                                     Asset::ACCESS_BUFFER,
                                     ap);
        shared = false;
    }
    if ((ass != NULL || sharedRes != NULL) && ass != kExcludedAsset) {
        ALOGV("Installing resource asset %p in to table %p\n", ass, mResources);
        if (sharedRes != NULL) {
            ALOGV("Copying existing resources for %s", ap.path.string());
            mResources->add(sharedRes);
        } else {
            ALOGV("Parsing resources for %s", ap.path.string());
            mResources->add(ass, idmap, nextEntryIdx + 1, !shared);
        }
        onlyEmptyResources = false;
        if (!shared) {
            delete ass;
        }
    } else {
        ALOGV("Installing empty resources in to table %p\n", mResources);
        mResources->addEmpty(nextEntryIdx + 1);
    }
    if (idmap != NULL) {
        delete idmap;
    }
    MY_TRACE_END();
    return onlyEmptyResources;
}
const ResTable* AssetManager::getResTable(bool required) const
{
    ResTable* rt = mResources;
    if (rt) {
        return rt;
    }
    // Iterate through all asset packages, collecting resources from each.
    AutoMutex _l(mLock);
    if (mResources != NULL) {
        return mResources;
    }
    if (required) {
        LOG_FATAL_IF(mAssetPaths.size() == 0, "No assets added to AssetManager");
    }
    if (mCacheMode != CACHE_OFF && !mCacheValid) {
        const_cast<AssetManager*>(this)->loadFileNameCacheLocked();
    }
    mResources = new ResTable();
    updateResourceParamsLocked();
    bool onlyEmptyResources = true;
    const size_t N = mAssetPaths.size();
    // 也是调用appendPathToResTable去解析
    for (size_t i=0; i<N; i++) {
        bool empty = appendPathToResTable(mAssetPaths.itemAt(i));
        onlyEmptyResources = onlyEmptyResources && empty;
    }
    if (required && onlyEmptyResources) {
        ALOGW("Unable to find resources file resources.arsc");
        delete mResources;
        mResources = NULL;
    }
    return mResources;
}

也就是说在Android5.0及以上的系统中native层的addAssetPath方法会再调用appendPathToResTable去解析，所以在5.0及以上系统通过反射调用addAssetPath方法就不会有问题。

解决方案就是根据Google Instant Run实现的原理:创建一个新的AssetManager，然后加入完整的新资源包，替换掉原有的AssetManager。

具体可参考:深度理解Android InstantRun原理以及源码分析中的2.2 monkeyPatchExistingResources部分

通过aapt工具编译apk包，package id是0x7f，系统的资源包(framework-res.jar)，package id为0x01，如果addAssetPath补丁包中的package id也是0x7f，就会使得同一个pakcage id的包被加载两次，在Android L后，会把后来的包添加到之前的包的同一个PackageGoup下，但是在get资源时，会从前往后便利，也就是说先得到原有安装包里的资源，补丁中的资源就永远无法生效。可以构建一个package id为0x66的资源包。这样就不会与已经加载的0x7f冲突。