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。