dexFd = open(fileName, O_RDONLY);

verifyMagicAndGetAdler32(dexFd, &adler32) < 0

            result = dvmOptimizeDexFile(optFd, dexOffset, fileSize,
                fileName, modTime, adler32, isBootstrap);

/*
 * 打开一个未优化的 DEX 文件.
 */

/* 请参阅页眉中的文档注释. */
int dvmRawDexFileOpen(const char* fileName, const char* odexOutputName,
    RawDexFile** ppRawDexFile, bool isBootstrap)
{
    /*
     * TODO: 这复制了JarFile.c 中 dvmJarFileOpen() 的大量代码。这应该被重构。
     */

    DvmDex* pDvmDex = NULL;
    char* cachedName = NULL;
    int result = -1;
    int dexFd = -1;
    int optFd = -1;
    u4 modTime = 0;
    u4 adler32 = 0;
    size_t fileSize = 0;
    bool newFile = false;
    bool locked = false;

	// 调用 open 函数 以只读形式 , 打开了 DEX 文件
    dexFd = open(fileName, O_RDONLY);
    if (dexFd < 0) goto bail;

    /* If we fork/exec into dexopt, don't let it inherit the open fd. */
    dvmSetCloseOnExec(dexFd);

	// 校验 DEX 文件 
    if (verifyMagicAndGetAdler32(dexFd, &adler32) < 0) {
        ALOGE("Error with header for %s", fileName);
        goto bail;
    }

    if (getModTimeAndSize(dexFd, &modTime, &fileSize) < 0) {
        ALOGE("Error with stat for %s", fileName);
        goto bail;
    }

	/*
	 * 查看缓存的文件是否匹配。如果是这样，optFd将成为一个参考
	 * 到缓存文件，并将被查找到刚刚超过“opt”
	 * 标题。
	 */

    if (odexOutputName == NULL) {
    	// 优化的过程 
        cachedName = dexOptGenerateCacheFileName(fileName, NULL);
        if (cachedName == NULL)
            goto bail;
    } else {
        cachedName = strdup(odexOutputName);
    }

    ALOGV("dvmRawDexFileOpen: Checking cache for %s (%s)",
            fileName, cachedName);

    optFd = dvmOpenCachedDexFile(fileName, cachedName, modTime,
        adler32, isBootstrap, &newFile, /*createIfMissing=*/true);

    if (optFd < 0) {
        ALOGI("Unable to open or create cache for %s (%s)",
                fileName, cachedName);
        goto bail;
    }
    locked = true;

    /*
     * 如果optFd指向一个新文件（因为没有缓存
     * 版本，或缓存的版本已过时），生成
     * 优化的DEX。返回的文件描述符仍处于锁定状态，
     * 并定位在刚好超过优化标题的位置。
     */
    if (newFile) {
        u8 startWhen, copyWhen, endWhen;
        bool result;
        off_t dexOffset;

        dexOffset = lseek(optFd, 0, SEEK_CUR);
        result = (dexOffset > 0);

        if (result) {
            startWhen = dvmGetRelativeTimeUsec();
            result = copyFileToFile(optFd, dexFd, fileSize) == 0;
            copyWhen = dvmGetRelativeTimeUsec();
        }

        if (result) {
            result = dvmOptimizeDexFile(optFd, dexOffset, fileSize,
                fileName, modTime, adler32, isBootstrap);
        }

        if (!result) {
            ALOGE("Unable to extract+optimize DEX from '%s'", fileName);
            goto bail;
        }

        endWhen = dvmGetRelativeTimeUsec();
        ALOGD("DEX prep '%s': copy in %dms, rewrite %dms",
            fileName,
            (int) (copyWhen - startWhen) / 1000,
            (int) (endWhen - copyWhen) / 1000);
    }

	/*
	 * 映射缓存的版本。这会立即倒带fd，因此
	 * 不需要在任何地方寻找。
	 */
    if (dvmDexFileOpenFromFd(optFd, &pDvmDex) != 0) {
        ALOGI("Unable to map cached %s", fileName);
        goto bail;
    }

    if (locked) {
        /* unlock the fd */
        if (!dvmUnlockCachedDexFile(optFd)) {
            /* uh oh -- this process needs to exit or we'll wedge the system */
            ALOGE("Unable to unlock DEX file");
            goto bail;
        }
        locked = false;
    }

    ALOGV("Successfully opened '%s'", fileName);

    *ppRawDexFile = (RawDexFile*) calloc(1, sizeof(RawDexFile));
    (*ppRawDexFile)->cacheFileName = cachedName;
    (*ppRawDexFile)->pDvmDex = pDvmDex;
    cachedName = NULL;      // don't free it below
    result = 0;

bail:
    free(cachedName);
    if (dexFd >= 0) {
        close(dexFd);
    }
    if (optFd >= 0) {
        if (locked)
            (void) dvmUnlockCachedDexFile(optFd);
        close(optFd);
    }
    return result;
}