[Android]Dalvik的BOOTCLASSPATH和dexopt流程

BOOTCLASSPATH简介
1.BOOTCLASSPATH是Android Linux的一个环境变量，可以在adb shell下用$BOOTCLASSPATH看到。
2.BOOTCLASSPATH于/init.rc文件中export，如果没有找到的话，可以在init.rc中import的文件里找到（如import /init.environ.rc）。
3.init.rc文件存在于boot.img的ramdisk映像中。如果仅仅是修改/init.rc文件，重启后会被ramdisk恢复，所以直接修改是没有效果的。
4.boot.img是一种特殊的Android定制格式，由boot header，kernel，ramdisk以及second stage loader（可选）组成，详见android/system/core/mkbootimg/bootimg.h。

boot.img空间结构:

** +-----------------+
** | boot header     | 1 page
** +-----------------+
** | kernel          | n pages
** +-----------------+
** | ramdisk         | m pages
** +-----------------+
** | second stage    | o pages
** +-----------------+

典型的ramdisk文件结构:
./init.trout.rc
./default.prop
./proc
./dev
./init.rc
./init
./sys
./init.goldfish.rc
./sbin
./sbin/adbd
./system
./data

BOOTCLASSPATH的作用
以Android4.4手机的BOOTCLASSPATH为例：
export BOOTCLASSPATH /system/framework/core.jar:/system/framework/conscrypt.jar:/system/framework/okhttp.jar...
当kernel启动时1号进程init解析init.rc，将/system/framework下的jar包路径export出来。
Dalvik虚拟机在初始化过程中，会读取环境变量BOOTCLASSPATH，用于之后的类加载和优化。

Dalvik虚拟机的启动和dexopt流程
从Dalvik虚拟机的启动过程分析一文可以知道，Zygote会在启动后创建Dalvik虚拟机实例，并进行初始化。

那我们就接着Dalvik虚拟机初始化后开始探究它是如何通过BOOTCLASSPATH来进行dex优化的：

1.1. VM initialization

android/dalvik/vm/Init.cpp

std::string dvmStartup(int argc, const char* const argv[],
bool ignoreUnrecognized, JNIEnv* pEnv)
{
	...
	ALOGV("VM init args (%d):", argc);
	...

	setCommandLineDefaults(); // ---> 读取BOOTCLASSPATH
	...
	if (!dvmClassStartup()) { // ---> 初始化bootstrap class loader
		return "dvmClassStartup failed";
	}
}

1.2. 读取BOOTCLASSPATH

static void setCommandLineDefaults()
{
	const char* envStr = getenv("CLASSPATH");
	if (envStr != NULL) {
		gDvm.classPathStr = strdup(envStr);
	} else {
		gDvm.classPathStr = strdup(".");
	}
	envStr = getenv("BOOTCLASSPATH"); // 读取到BOOTCLASSPATH环境变量
	if (envStr != NULL) {
		gDvm.bootClassPathStr = strdup(envStr);
	} else {
		gDvm.bootClassPathStr = strdup(".");
	}
	...
}

就这样，BOOTCLASSPATH的值被保存到gDvm.bootClassPathStr中。

2.1. 初始化bootstrap class loader

android/dalvik/vm/oo/Class.cpp

bool dvmClassStartup()
{
	...

	/*
	* Process the bootstrap class path. This means opening the specified
	* DEX or Jar files and possibly running them through the optimizer.
	*/
	assert(gDvm.bootClassPath == NULL);
	processClassPath(gDvm.bootClassPathStr, true); // 下一步

	if (gDvm.bootClassPath == NULL)
		return false;
}

2.2. 将路径、Zip文件和Dex文件的list转换到ClassPathEntry结构体当中

static ClassPathEntry* processClassPath(const char* pathStr, bool isBootstrap)
{
	ClassPathEntry* cpe = NULL;
	...
	　　　　if (!prepareCpe(&tmp, isBootstrap)) {}
}

2.3. 根据cpe打开文件

static bool prepareCpe(ClassPathEntry* cpe, bool isBootstrap)
{
    ...

    if ((strcmp(suffix, "jar") == 0) || (strcmp(suffix, "zip") == 0) ||
    (strcmp(suffix, "apk") == 0)) {
        JarFile* pJarFile = NULL;
        /* 打开jar包，找到class.dex或jar包旁边的.odex文件 */
        if (dvmJarFileOpen(cpe->fileName, NULL, &pJarFile, isBootstrap) == 0) {
            cpe->kind = kCpeJar;
            cpe->ptr = pJarFile;
            return true;
        }
    } else if (strcmp(suffix, "dex") == 0) {
        RawDexFile* pRawDexFile = NULL;
        /* 与dvmJarFileOpen函数作用类似，是由它复制过来重构的 */
        if (dvmRawDexFileOpen(cpe->fileName, NULL, &pRawDexFile, isBootstrap) == 0) {
            cpe->kind = kCpeDex;
            cpe->ptr = pRawDexFile;
            return true;
        }
    } else {
        ALOGE("Unknown type suffix '%s'", suffix);
    }
    ...
}

3. 打开jar包，找到class.dex或jar包旁边的.odex文件

android/dalvik/vm/JarFile.cpp

int dvmJarFileOpen(const char* fileName, const char* odexOutputName,
    JarFile** ppJarFile, bool isBootstrap)
{
    ...

    /* Even if we're not going to look at the archive, we need to
     * open it so we can stuff it into ppJarFile.
     */
    if (dexZipOpenArchive(fileName, &archive) != 0)
        goto bail;
    archiveOpen = true;

    /* If we fork/exec into dexopt, don't let it inherit the archive's fd.
     */
    dvmSetCloseOnExec(dexZipGetArchiveFd(&archive));

    /* First, look for a ".odex" alongside the jar file.  It will
     * have the same name/path except for the extension.
     */
    fd = openAlternateSuffix(fileName, "odex", O_RDONLY, &cachedName);
    if (fd >= 0) {
		ALOGV("Using alternate file (odex) for %s ...", fileName);
		/* 读、验证header和dependencies */
		if (!dvmCheckOptHeaderAndDependencies(fd, false, 0, 0, true, true)) {
            ALOGE("%s odex has stale dependencies", fileName);
            free(cachedName);
            cachedName = NULL;
            close(fd);
            fd = -1;
            goto tryArchive;
        } else {
            ALOGV("%s odex has good dependencies", fileName);
            //TODO: make sure that the .odex actually corresponds
            //      to the classes.dex inside the archive (if present).
            //      For typical use there will be no classes.dex.
        }
    } else {
        ZipEntry entry;

tryArchive:
        /*
         * Pre-created .odex absent or stale.  Look inside the jar for a
         * "classes.dex".
         */
        ...
}

4.读、验证opt的header，读、验证dependencies

android/dalvik/vm/analysis/DexPrepare.cpp

bool dvmCheckOptHeaderAndDependencies(int fd, bool sourceAvail, u4 modWhen,
u4 crc, bool expectVerify, bool expectOpt)
{
     ...
    /*
     * Verify dependencies on other cached DEX files.  It must match
     * exactly with what is currently defined in the bootclasspath.
     */
    ClassPathEntry* cpe;
    u4 numDeps;

    numDeps = read4LE(&ptr);
    ALOGV("+++ DexOpt: numDeps = %d", numDeps);
    for (cpe = gDvm.bootClassPath; cpe->ptr != NULL; cpe++) {
        const char* cacheFileName =
            dvmPathToAbsolutePortion(getCacheFileName(cpe));
        assert(cacheFileName != NULL); /* guaranteed by Class.c */

        const u1* signature = getSignature(cpe);
        size_t len = strlen(cacheFileName) +1;
        u4 storedStrLen;

        if (numDeps == 0) {
            /* more entries in bootclasspath than in deps list */
            ALOGI("DexOpt: not all deps represented");
            goto bail;
        }

        storedStrLen = read4LE(&ptr);
        if (len != storedStrLen ||
            strcmp(cacheFileName, (const char*) ptr) != 0)
        {
            ALOGI("DexOpt: mismatch dep name: '%s' vs. '%s'",
                cacheFileName, ptr);
            goto bail;
        }

        ptr += storedStrLen;

        if (memcmp(signature, ptr, kSHA1DigestLen) != 0) {
            ALOGI("DexOpt: mismatch dep signature for '%s'", cacheFileName);
            goto bail;
        }
        ptr += kSHA1DigestLen;

        ALOGV("DexOpt: dep match on '%s'", cacheFileName);

        numDeps--;
    }

    if (numDeps != 0) {
        /* more entries in deps list than in classpath */
        ALOGI("DexOpt: Some deps went away");
        goto bail;
    }
	...
}

实际应用
打通了Dalvik dexopt的这个流程，那这到底又有什么用呢？
让我们看看实际开发过程中的手机升级binary后无法boot到Home界面的log：

 AndroidRuntime >>>>>> AndroidRuntime START com.android.internal.os.ZygoteInit <<<<<<
 AndroidRuntime CheckJNI is ON
 dalvikvm DexOpt: Some deps went away
 dalvikvm /system/framework/core-junit.jar odex has stale dependencies
 dalvikvm DexOpt: --- BEGIN 'core-junit.jar' (bootstrap=) ---
 dalvikvm DexOpt: load 42ms, verify+opt 25ms,  bytes
 dalvikvm DexOpt: --- END 'core-junit.jar' (success) ---
 dalvikvm DEX prep '/system/framework/core-junit.jar': unzip in 1ms, rewrite 126ms
 dalvikvm DexOpt: mismatch dep name: '/data/dalvik-cache/system@framework@core-junit.jar@classes.dex' vs. '/system/framework/conscrypt.odex'
 dalvikvm /system/framework/bouncycastle.jar odex has stale dependencies
 dalvikvm DexOpt: --- BEGIN 'bouncycastle.jar' (bootstrap=) ---
 dalvikvm DexOpt: Some deps went away
 dalvikvm /system/framework/core-junit.jar odex has stale dependencies
 dalvikvm DexOpt: load 33ms, verify+opt 350ms,  bytes
 dalvikvm DexOpt: --- END 'bouncycastle.jar' (success) ---
 dalvikvm DEX prep '/system/framework/bouncycastle.jar': unzip in 57ms, rewrite 548ms
 dalvikvm DexOpt: mismatch dep name: '/data/dalvik-cache/system@framework@core-junit.jar@classes.dex' vs. '/system/framework/conscrypt.odex'
 dalvikvm /system/framework/ext.jar odex has stale dependencies
 dalvikvm DexOpt: --- BEGIN 'ext.jar' (bootstrap=) ---
 dalvikvm DexOpt: Some deps went away
 dalvikvm /system/framework/core-junit.jar odex has stale dependencies
 dalvikvm DexOpt: mismatch dep name: '/data/dalvik-cache/system@framework@core-junit.jar@classes.dex' vs. '/system/framework/conscrypt.odex'
 dalvikvm /system/framework/bouncycastle.jar odex has stale dependencies

根据前面的流程，结合log我们就可以分析出，DexOpt: mismatch dep name: '/data/dalvik-cache/system@framework@core-junit.jar@classes.dex' vs. '/system/framework/conscrypt.odex'是错误所在，是由于data/dalvik-cache/下的dex cache文件和system/framework/下的jar文件验证依赖关系时候对应不上。

从函数dvmCheckOptHeaderAndDependencies()可以得知，BOOTCLASSPATH和cache必须是完全一致的
尝试删除所有cache文件，重启还是不行。那么应该想到BOOTCLASSPATH和实际的system/framework/的jar包不一致，才会导致和其生成的cache不一致。
对比一下果然不一致，issue trouble-shooted.

解决方法：把对应boot.img也烧进去，这样BOOTCLASSPATH就能更新一致，dex优化就能正确进行下去。