Android培訓班(67)dex檔案開啟流程
從上面可知呼叫函式Dalvik_dalvik_system_DexFile_openDexFile來開啟Ｄex檔案，這個函式的原始碼如下：
staticvoid Dalvik_dalvik_system_DexFile_openDexFile(const u4* args,
JValue* pResult)
{
StringObject* sourceNameObj =(StringObject*) args[0];
這行是輸入的Jar或Ｄex檔名引數。
StringObject* outputNameObj =(StringObject*) args[1];
這行是輸出的檔名引數。
int flags = args[2];
這行是處理的標示。
DexOrJar* pDexOrJar = NULL;
JarFile* pJarFile;
RawDexFile* pRawDexFile;
char* sourceName;
char* outputName;
if (sourceNameObj == NULL) {
dvmThrowException("Ljava/lang/NullPointerException;",NULL);
RETURN_VOID();
}
這段程式碼是當輸入檔名稱為空物件時，就丟擲異常。
sourceName =dvmCreateCstrFromString(sourceNameObj);
這行程式碼呼叫函式dvmCreateCstrFromString把java字串轉換Ｃ字串，由於Ｊava物件表示的字串並不能立即就使用到Ｃ語言裡，所以需要轉換才能使用。
if (outputNameObj != NULL)
outputName =dvmCreateCstrFromString(outputNameObj);
else
outputName = NULL;
這段程式碼是同樣把輸出字串轉換Ｃ字串。
/*
* We have to deal with the possibility thatsomebody might try to
* open one of our bootstrap class DEXfiles. The set of dependencies
* will be different, and hence the resultsof optimization might be
* different, which means we'd actually needto have two versions of
* the optimized DEX: one that only knowsabout part of the boot class
* path, and one that knows about everythingin it. The latter might
* optimize field/method accesses based on aclass that appeared later
* in the class path.
*
* We can't let the user-defined classloader open it and start using
* the classes, since the optimized form ofthe code skips some of
* the method and field resolution that wewould ordinarily do, and
* we'd have the wrong semantics.
*
* We have to reject attempts to manuallyopen a DEX file from the boot
* class path. The easiest way to do thisis by filename, which works
* out because variations in name (e.g."/system/framework/./ext.jar")
* result in us hitting a differentdalvik-cache entry. It's also fine
* if the caller specifies their own outputfile.
*/
if(dvmClassPathContains(gDvm.bootClassPath, sourceName)) {
LOGW("Refusing to reopen boot DEX'%s'\n", sourceName);
dvmThrowException("Ljava/io/IOException;",
"Re-opening BOOTCLASSPATH DEXfiles is not allowed");
free(sourceName);
RETURN_VOID();
}
這段程式碼是判斷使用者是否載入系統目錄下面的Ｄex檔案，如果載入就要拒絕這樣的操作，因為系統啟動時已經載入了一份這樣的優化程式碼，沒有必要再次載入一次。
/*
* Try to open it directly as a DEX. Ifthat fails, try it as a Zip
* with a "classes.dex" inside.
*/
if (dvmRawDexFileOpen(sourceName,outputName, &pRawDexFile, false) == 0) {
LOGV("Opening DEX file '%s'(DEX)\n", sourceName);
pDexOrJar = (DexOrJar*)malloc(sizeof(DexOrJar));
pDexOrJar->isDex = true;
pDexOrJar->pRawDexFile = pRawDexFile;
這段程式碼是嘗試載入Ｄex檔案，但基本不存在直接加Ｄex檔案的情況，因此在函式dvmRawDexFileOpen還是空函式，沒有實際的內容。
}else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false)== 0) {
LOGV("Opening DEX file '%s'(Jar)\n", sourceName);
pDexOrJar = (DexOrJar*)malloc(sizeof(DexOrJar));
pDexOrJar->isDex = false;
pDexOrJar->pJarFile = pJarFile;
這段程式碼是載入Jar檔案，就是從這裡載入Dex檔案到快取裡。
}else {
LOGV("Unable to open DEX file'%s'\n", sourceName);
dvmThrowException("Ljava/io/IOException;","unable to open DEX file");
}
if (pDexOrJar != NULL) {
pDexOrJar->fileName = sourceName;
這行程式碼儲存檔名稱到Dex檔案物件裡。
/* add to hash table */
u4 hash = dvmComputeUtf8Hash(sourceName);
這行程式碼通過檔名稱計算ＨＡＳＨ串，加速對檔案的查詢速度。
void* result;
dvmHashTableLock(gDvm.userDexFiles);
result =dvmHashTableLookup(gDvm.userDexFiles, hash, pDexOrJar,
hashcmpDexOrJar, true);
dvmHashTableUnlock(gDvm.userDexFiles);
這段程式碼新增ＨＡＳＨ表裡，以便後面查詢使用。
if (result != pDexOrJar) {
LOGE("Pointer has already beenadded?\n");
dvmAbort();
}
pDexOrJar->okayToFree = true;
}else
free(sourceName);
RETURN_PTR(pDexOrJar);
這行程式碼返回開啟的檔案物件。
}
這個函式是通過JAVA呼叫時輸入Ｄex檔名稱，然後載入Ｄex檔案，最後把這個檔名稱放到ＨＡＳＨ表裡，然後返回開啟的物件。