Android培训班 68 dex文件打开流程

最新推荐文章于 2024-04-22 18:11:58 发布

原创最新推荐文章于 2024-04-22 18:11:58 发布 · 369 阅读

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本文深入解析了DVM如何通过dvmJarFileOpen函数找到并加载classes.dex文件到内存的过程，包括查找优化后的Dex文件、从压缩包中提取Dex文件、优化Dex文件及缓存管理等关键步骤。

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在上面的函数里，提到使用dvmJarFileOpen函数找到classes.dex文件，并加载到内存里，然后提供后面的函数使用。现在就来分析这个函数的代码，如下：

intdvmJarFileOpen(const char* fileName, const char* odexOutputName,

JarFile** ppJarFile, bool isBootstrap)

{

在这里提供四个参数，第一个参数fileName是输入的Ｊar的文件名称；第二个参数odexOutputName是进行优化后的Ｄex输出文件；第三个参数ppJarFile是已经打开并缓存到内存里的文件对象；第四个参数isBootstrap是指示是否系统里Ｄex文件。

ZipArchive archive;

DvmDex* pDvmDex = NULL;

char* cachedName = NULL;

bool archiveOpen = false;

bool locked = false;

int fd = -1;

int result = -1;

/* Even if we're not going to look at thearchive, we need to

* open it so we can stuff it intoppJarFile.

if (dexZipOpenArchive(fileName, &archive)!= 0)

goto bail;

archiveOpen = true;

这段代码是调用前面介绍的函数dexZipOpenArchive来打开Ｚip文件，并缓存到系统内存里。

/* If we fork/exec into dexopt, don't letit inherit the archive's fd.

dvmSetCloseOnExec(dexZipGetArchiveFd(&archive));

这行代码设置当执行完成后，关闭这个文件句柄。

/* First, look for a ".odex"alongside the jar file. It will

* have the same name/path except for theextension.

fd = openAlternateSuffix(fileName, "odex",O_RDONLY, &cachedName);

if (fd >= 0) {

这里优先处理优化的Dex文件。

LOGV("Using alternate file (odex)for %s ...\n", fileName);

if (!dvmCheckOptHeaderAndDependencies(fd,false, 0, 0, true, true)) {

LOGE("%s odex has staledependencies\n", fileName);

free(cachedName);

close(fd);

fd = -1;

goto tryArchive;

} else {

LOGV("%s odex has gooddependencies\n", fileName);

//TODO: make sure that the .odexactually corresponds

// to the classes.dex inside thearchive (if present).

// For typical use there will beno classes.dex.

}

}else {

ZipEntry entry;

tryArchive:

* Pre-created .odex absent or stale. Look inside the jar for a

* "classes.dex".

entry = dexZipFindEntry(&archive,kDexInJarName);

这行代码是从压缩包里找到Dex文件，然后打开这个文件。

if (entry != NULL) {

bool newFile = false;

* We've found the one we want. Seeif there's an up-to-date copy

* in the cache.

* On return, "fd" will beseeked just past the "opt" header.

* If a stale .odex file is presentand classes.dex exists in

* the archive, this will *not*return an fd pointing to the

* .odex file; the fd will point intodalvik-cache like any

* other jar.

if (odexOutputName == NULL) {

cachedName =dexOptGenerateCacheFileName(fileName,

kDexInJarName);

if (cachedName == NULL)

goto bail;

这段代码是把Dex文件进行优化处理，并输出到指定的文件。

} else {

cachedName =strdup(odexOutputName);

}

LOGV("dvmDexCacheStatus:Checking cache for %s (%s)\n",

fileName, cachedName);

fd = dvmOpenCachedDexFile(fileName,cachedName,

dexGetZipEntryModTime(&archive,entry),

dexGetZipEntryCrc32(&archive,entry),

isBootstrap, &newFile,/*createIfMissing=*/true);

这段代码创建缓存的优化文件。

if (fd < 0) {

LOGI("Unable to open orcreate cache for %s (%s)\n",

fileName, cachedName);

goto bail;

}

locked = true;

* If fd points to a new file(because there was no cached version,

* or the cached version was stale),generate the optimized DEX.

* The file descriptor returned isstill locked, and is positioned

* just past the optimization header.

if (newFile) {

u8 startWhen, extractWhen,endWhen;

bool result;

off_t dexOffset, fileLen;

dexOffset = lseek(fd, 0,SEEK_CUR);

result = (dexOffset > 0);

if (result) {

startWhen =dvmGetRelativeTimeUsec();

result =dexZipExtractEntryToFile(&archive, entry, fd);

extractWhen =dvmGetRelativeTimeUsec();

这段代码调用函数dexZipExtractEntryToFile从压缩包里解压文件出来。

}

if (result) {

result =dvmOptimizeDexFile(fd, dexOffset,

dexGetZipEntryUncompLen(&archive,entry),

fileName,

dexGetZipEntryModTime(&archive,entry),

dexGetZipEntryCrc32(&archive,entry),

isBootstrap);

这段代码调用函数dvmOptimizeDexFile对Ｄex文件进行优化处理。

}

if (!result) {

LOGE("Unable toextract+optimize DEX from '%s'\n",

fileName);

goto bail;

}

endWhen =dvmGetRelativeTimeUsec();

LOGD("DEX prep '%s': unzipin %dms, rewrite %dms\n",

fileName,

(int) (extractWhen -startWhen) / 1000,

(int) (endWhen - extractWhen)/ 1000);

}

} else {

LOGI("Zip is good, but no %sinside, and no valid .odex "

"file in the samedirectory\n", kDexInJarName);

goto bail;

}

* Map the cached version. This immediatelyrewinds the fd, so it

* doesn't have to be seeked anywhere inparticular.

if (dvmDexFileOpenFromFd(fd, &pDvmDex)!= 0) {

LOGI("Unable to map %s in %s\n",kDexInJarName, fileName);

goto bail;

}

这段代码是调用函数dvmDexFileOpenFromFd来缓存Ｄex文件，并分析文件的内容。比如标记是否优化的文件，通过签名检查Dex文件是否合法。

if (locked) {

/* unlock the fd */

if (!dvmUnlockCachedDexFile(fd)) {

/* uh oh -- this process needs toexit or we'll wedge the system */

LOGE("Unable to unlock DEXfile\n");

goto bail;

}

locked = false;

}

这段代码是保存文件到缓存里，标记这个文件句柄已经保存到缓存。

LOGV("Successfully opened '%s' in'%s'\n", kDexInJarName, fileName);

*ppJarFile = (JarFile*) calloc(1,sizeof(JarFile));

(*ppJarFile)->archive = archive;

(*ppJarFile)->cacheFileName =cachedName;

(*ppJarFile)->pDvmDex = pDvmDex;

cachedName = NULL; // don't free itbelow

result = 0;

这段代码已经成功打开压缩包，并读取Dex文件到内存缓存，这里只是设置一些相关信息返回前面的函数处理。

bail:

/* clean up, closing the open file */

if (archiveOpen && result != 0)

dexZipCloseArchive(&archive);

free(cachedName);

if (fd >= 0) {

if (locked)

(void) dvmUnlockCachedDexFile(fd);

close(fd);

}

return result;

}

本函数主要流程是打开压缩文件，然后找到Ｄex文件，读取出来，并进行分析，然后拷贝到缓存里，再返回给调用函数。

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