Android更新資原始檔淺思考

public final class AssetManager {
......
/*package*/ static AssetManager sSystem = null;
private native final void init(boolean isSystem);
......
//構造方法
public AssetManager() {
synchronized (this) {
......
init(false);
......
//每個AssetManager例項都會初始化系統的資源
ensureSystemAssets();
}
}
private static void ensureSystemAssets() {
synchronized (sSync) {
if (sSystem == null) {
AssetManager system = new AssetManager(true);
system.makeStringBlocks(false);
sSystem = system;
}
}
}
......
//新增資源路徑
public final int addAssetPath(String path) {
synchronized (this) {
int res = addAssetPathNative(path);
if (mStringBlocks != null) {
makeStringBlocks(mStringBlocks);
}
return res;
}
}
private native final int addAssetPathNative(String path);

/*package*/ final CharSequence getResourceText(int ident) {
synchronized (this) {
TypedValue tmpValue = mValue;
int block = loadResourceValue(ident, (short) 0, tmpValue, true);
if (block >= 0) {
if (tmpValue.type == TypedValue.TYPE_STRING) {
return mStringBlocks[block].get(tmpValue.data);
}
return tmpValue.coerceToString();
}
}
return null;
}
//查詢並載入資源
private native final int loadResourceValue(int ident, short density, TypedValue outValue,
boolean resolve);
}

//frameworks/base/core/jni/android_util_AssetManager.cpp
static void android_content_AssetManager_init(JNIEnv* env, jobject clazz, jboolean isSystem)
{
if (isSystem) {
verifySystemIdmaps();
}
//構造C++層的AssetManager的物件
AssetManager* am = new AssetManager();
if (am == NULL) {
jniThrowException(env, "java/lang/OutOfMemoryError", "");
return;
}
//新增系統資源
am->addDefaultAssets();
ALOGV("Created AssetManager %p for Java object %p\n", am, clazz);
env->SetLongField(clazz, gAssetManagerOffsets.mObject, reinterpret_cast<jlong>(am));
}

//frameworks/base/core/jni/android_util_AssetManager.cpp
static jint android_content_AssetManager_loadResourceValue(JNIEnv* env, jobject clazz, jint ident, jshort density, jobject outValue, jboolean resolve)
{
/***部分程式碼省略***/
//這行程式碼最重要，通過獲取C層的AssetManager的成員變數ResTable來獲取資源
const ResTable& res(am->getResources());
Res_value value;
ResTable_config config;
uint32_t typeSpecFlags;
ssize_t block = res.getResource(ident, &value, false, density, &typeSpecFlags, &config);
/***部分程式碼省略***/
if (block >= 0) {
return copyValue(env, outValue, &res, value, ref, block, typeSpecFlags, &config);
}
return static_cast<jint>(block);
}
//getResources實際獲取的是ResTable
const ResTable& AssetManager::getResources(bool required) const
{
const ResTable* rt = getResTable(required);
return *rt;
}

//frameworks/base/libs/androidfw/AssetManager.cpp
const ResTable* AssetManager::getResTable(bool required) const
{
ResTable* rt = mResources;
if (rt) {
return rt;
}
/***部分程式碼省略***/
const size_t N = mAssetPaths.size();
for (size_t i=0; i<N; i++) {
//遍歷mAssetPaths將其ResTable中
/***部分程式碼省略***/
}
/***部分程式碼省略***/
return rt;
}

//frameworks/base/libs/androidfw/AssetManager.cpp
bool AssetManager::addDefaultAssets()
{
const char* root = getenv("ANDROID_ROOT");
LOG_ALWAYS_FATAL_IF(root == NULL, "ANDROID_ROOT not set");
String8 path(root);
path.appendPath(kSystemAssets);
String8 pathCM(root);
pathCM.appendPath(kCMSDKAssets);
return addAssetPath(path, NULL) & addAssetPath(pathCM, NULL);
}


bool AssetManager::addAssetPath(const String8& path, int32_t* cookie)
{
AutoMutex _l(mLock);
asset_path ap;
String8 realPath(path);
if (kAppZipName) {
realPath.appendPath(kAppZipName);
}
/***部分程式碼省略***/
//將path新增到mAssetPaths中
mAssetPaths.add(ap);
if (mResources != NULL) {
size_t index = mAssetPaths.size() - 1;
//新增到ResTable中
appendPathToResTable(ap, &index);
}
// new paths are always added at the end
if (cookie) {
*cookie = static_cast<int32_t>(mAssetPaths.size());
}
/***部分程式碼省略***/
return true;
}

//frameworks/base/libs/androidfw/AssetManager.cpp
bool AssetManager::addAssetPath(const String8& path, void** cookie)
{
AutoMutex _l(mLock);
asset_path ap;
String8 realPath(path);
if (kAppZipName) {
realPath.appendPath(kAppZipName);
}
/***部分程式碼省略***/
//將path新增到mAssetPaths中
mAssetPaths.add(ap);
// new paths are always added at the end
if (cookie) {
*cookie = (void*)mAssetPaths.size();
}
/***部分程式碼省略***/
return true;
}

public final class AssetManager {
/***部分程式碼省略***/
//構造方法
public AssetManager() {
synchronized (this) {
......
init(false);
......
//每個AssetManager例項都會初始化系統的資源
ensureSystemAssets();
}
}
private static void ensureSystemAssets() {
synchronized (sSync) {
if (sSystem == null) {
AssetManager system = new AssetManager(true);
//初始化系統的字串塊
system.makeStringBlocks(false);
sSystem = system;
}
}
}
/*package*/ final void makeStringBlocks(StringBlock[] seed) {
final int seedNum = (seed != null) ? seed.length : 0;
final int num = getStringBlockCount();
mStringBlocks = new StringBlock[num];
if (localLOGV) Log.v(TAG, "Making string blocks for " + this
+ ": " + num);
for (int i=0; i<num; i++) {
if (i < seedNum) {
mStringBlocks[i] = seed[i];
} else {
mStringBlocks[i] = new StringBlock(getNativeStringBlock(i), true);
}
}
}
private native final int getStringBlockCount();
/***部分程式碼省略***/
}

//frameworks/base/core/jni/android_util_AssetManager.cpp
static jint android_content_AssetManager_getStringBlockCount(JNIEnv* env, jobject clazz)
{
AssetManager* am = assetManagerForJavaObject(env, clazz);
if (am == NULL) {
return 0;
}
//初次呼叫資源，進行初始化ResTable
return am->getResources().getTableCount();
}