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
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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
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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<br>
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
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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
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static
ClassPathEntry* processClassPath(const
char* pathStr, bool
isBootstrap)
{ ClassPathEntry* cpe = NULL;
...
if
(!prepareCpe(&tmp, isBootstrap)) {}
} |
2.3. 根据cpe打开文件
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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
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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
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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：
1 AndroidRuntime >>>>>> AndroidRuntime START com.android.internal.os.ZygoteInit <<<<<< 2 AndroidRuntime CheckJNI is ON 3 dalvikvm DexOpt: Some deps went away 4 dalvikvm /system/framework/core-junit.jar odex has stale dependencies 5 dalvikvm DexOpt: --- BEGIN ‘core-junit.jar‘ (bootstrap=1) --- 6 dalvikvm DexOpt: load 42ms, verify+opt 25ms, 143956 bytes 7 dalvikvm DexOpt: --- END ‘core-junit.jar‘ (success) --- 8 dalvikvm DEX prep ‘/system/framework/core-junit.jar‘: unzip in 1ms, rewrite 126ms 9 dalvikvm DexOpt: mismatch dep name: ‘/data/dalvik-cache/system@framework@core-junit.jar@classes.dex‘ vs. ‘/system/framework/conscrypt.odex‘ 10 dalvikvm /system/framework/bouncycastle.jar odex has stale dependencies 11 dalvikvm DexOpt: --- BEGIN ‘bouncycastle.jar‘ (bootstrap=1) --- 12 dalvikvm DexOpt: Some deps went away 13 dalvikvm /system/framework/core-junit.jar odex has stale dependencies 14 dalvikvm DexOpt: load 33ms, verify+opt 350ms, 681812 bytes 15 dalvikvm DexOpt: --- END ‘bouncycastle.jar‘ (success) --- 16 dalvikvm DEX prep ‘/system/framework/bouncycastle.jar‘: unzip in 57ms, rewrite 548ms 17 dalvikvm DexOpt: mismatch dep name: ‘/data/dalvik-cache/system@framework@core-junit.jar@classes.dex‘ vs. ‘/system/framework/conscrypt.odex‘ 18 dalvikvm /system/framework/ext.jar odex has stale dependencies 19 dalvikvm DexOpt: --- BEGIN ‘ext.jar‘ (bootstrap=1) --- 20 dalvikvm DexOpt: Some deps went away 21 dalvikvm /system/framework/core-junit.jar odex has stale dependencies 22 dalvikvm DexOpt: mismatch dep name: ‘/data/dalvik-cache/system@framework@core-junit.jar@classes.dex‘ vs. ‘/system/framework/conscrypt.odex‘ 23 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。