因为ClassLoader一定与虚拟机的启动有关系，那么必须从Zygote的启动开始看代码。下面就分析一下这些代码，行数不多:

int main(int argc, const char* const argv[])
{
// These are global variables in ProcessState.cpp
//ProcessState.cpp中可能要用到一些main函数。
mArgC = argc;
mArgV = argv;
mArgLen = 0;
for (int i=0; i<argc; i++) {
mArgLen += strlen(argv[i]) + 1;
}
mArgLen--;
AppRuntime runtime;
const char* argv0 = argv[0];
// Process command line arguments
// ignore argv[0]
argc--;
argv++;
// Everything up to '--' or first non '-' arg goes to the vm
int i = runtime.addVmArguments(argc, argv);
// Parse runtime arguments.  Stop at first unrecognized option.
bool zygote = false;
bool startSystemServer = false;
bool application = false;
const char* parentDir = NULL;
const char* niceName = NULL;
const char* className = NULL;
while (i < argc) {
const char* arg = argv[i++];
if (!parentDir) {
parentDir = arg;
} else if (strcmp(arg, "--zygote") == 0) {
zygote = true;
niceName = "zygote";
} else if (strcmp(arg, "--start-system-server") == 0) {
startSystemServer = true;
} else if (strcmp(arg, "--application") == 0) {
application = true;
} else if (strncmp(arg, "--nice-name=", 12) == 0) {
niceName = arg + 12;
} else {
className = arg;
break;
}
}
if (niceName && *niceName) {
setArgv0(argv0, niceName);
set_process_name(niceName);
}
runtime.mParentDir = parentDir;
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer ? "start-system-server" : "");
} else if (className) {
// Remainder of args get passed to startup class main()
runtime.mClassName = className;
runtime.mArgC = argc - i;
runtime.mArgV = argv + i;
runtime.start("com.android.internal.os.RuntimeInit",
application ? "application" : "tool");
} else {
fprintf(stderr, "Error: no class name or --zygote supplied.\n");
app_usage();
LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
return 10;
}
}

分析完之后发现如下参数规律：

1. argv[0]：用这个修改了进程名称。

2. 虚拟机参数：前面的选项参数都是以“-”打头。被放入了runtime。这些参数被称为是虚拟机参数。

3.“--”打头的参数是zygote参数。有如下几种，排列顺序如下：

-runtimearg[0]

-runtimearg[1]

。。。。

parentDir //这个也是runtime使用的，也就是VM使用的。

className//这个也是runtime使用的，也就是VM使用的。

--zygote

--start-system-server

--application

--nice-name=

然后，如果是zygote，那么进入下面这句话

   runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer ? "start-system-server" : "");

如果有类名，那么进入下面这句话：

  runtime.mClassName = className;
runtime.mArgC = argc - i; //className，包括className以后的参数个数。
runtime.mArgV = argv + i; //截止到className的参数个数
runtime.start("com.android.internal.os.RuntimeInit",
application ? "application" : "tool");

第一部分：那么开机第一次启动的就一定是，

runtime.start("com.android.internal.os.ZygoteInit", startSystemServer ? "start-system-server" : "");

其中startSystemServer 由init.rc指定，在目录android40\system\core\rootdir中的init.rc.

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart media
    onrestart restart netd

第二部分：从ActivityManagerService可以看出，--application并没有指定，这句话也就相当于：

runtime.start("com.android.internal.os.RuntimeInit", "tool");

现在代码分成了两部分。

那么先分析第一部分。

那么zygote启动到底配置了那些参数呢，我们就看一看：

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

根据上面说的参数序列图，可以看出。

runtime.mParentDir  为/system/bin

runtime的一个arg为-Xzygote

那么这个这个start函数就变成：

runtime.start("com.android.internal.os.ZygoteInit",  "start-system-server");

代码进入到base/core/jni目录的AndroidRuntime.cpp里面。这个函数还不算长，就直接贴出来看一下，注意注释，由此可以看出这个就是启动虚拟机的代码所在啊。那么既然Zygote进程也是这么启动的，那么我们就有理由断定Zygote也是个Dalvik虚拟机！事情是不是这样呢？那么就带着这个疑问去分析一下：

/*
* Start the Android runtime.  This involves starting the virtual machine
* and calling the "static void main(String[] args)" method in the class
* named by "className".
*
* Passes the main function two arguments, the class name and the specified
* options string.
*/
void AndroidRuntime::start(const char* className, const char* options)
{
LOGD("\n>>>>>> AndroidRuntime START %s <<<<<<\n",
className != NULL ? className : "(unknown)");
blockSigpipe();
/*
* 'startSystemServer == true' means runtime is obsolete and not run from
* init.rc anymore, so we print out the boot start event here.
*/
if (strcmp(options, "start-system-server") == 0) {
/* track our progress through the boot sequence */
const int LOG_BOOT_PROGRESS_START = 3000;
LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,
ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
}
const char* rootDir = getenv("ANDROID_ROOT");
if (rootDir == NULL) {
rootDir = "/system";
if (!hasDir("/system")) {
LOG_FATAL("No root directory specified, and /android does not exist.");
return;
}
setenv("ANDROID_ROOT", rootDir, 1);
}
//const char* kernelHack = getenv("LD_ASSUME_KERNEL");
//LOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
/* start the virtual machine */
JNIEnv* env;
if (startVm(&mJavaVM, &env) != 0) {
return;
}
onVmCreated(env);
/*
* Register android functions.
*/
if (startReg(env) < 0) {
LOGE("Unable to register all android natives\n");
return;
}
/*
* We want to call main() with a String array with arguments in it.
* At present we have two arguments, the class name and an option string.
* Create an array to hold them.
*/
jclass stringClass;
jobjectArray strArray;
jstring classNameStr;
jstring optionsStr;
stringClass = env->FindClass("java/lang/String");
assert(stringClass != NULL);
strArray = env->NewObjectArray(2, stringClass, NULL);
assert(strArray != NULL);
classNameStr = env->NewStringUTF(className);
assert(classNameStr != NULL);
env->SetObjectArrayElement(strArray, 0, classNameStr);
optionsStr = env->NewStringUTF(options);
env->SetObjectArrayElement(strArray, 1, optionsStr);
/*
* Start VM.  This thread becomes the main thread of the VM, and will
* not return until the VM exits.
*/
char* slashClassName = toSlashClassName(className);
jclass startClass = env->FindClass(slashClassName);
if (startClass == NULL) {
LOGE("JavaVM unable to locate class '%s'\n", slashClassName);
/* keep going */
} else {
jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
"([Ljava/lang/String;)V");
if (startMeth == NULL) {
LOGE("JavaVM unable to find main() in '%s'\n", className);
/* keep going */
} else {
env->CallStaticVoidMethod(startClass, startMeth, strArray);
#if 0
if (env->ExceptionCheck())
threadExitUncaughtException(env);
#endif
}
}
free(slashClassName);
LOGD("Shutting down VM\n");
if (mJavaVM->DetachCurrentThread() != JNI_OK)
LOGW("Warning: unable to detach main thread\n");
if (mJavaVM->DestroyJavaVM() != 0)
LOGW("Warning: VM did not shut down cleanly\n");
}

linux的POSIX (Portable Operating System Interface of Unix)我不懂。但是从直观上看，可能是一种禁止打断进程的方法：

    LOGD("\n>>>>>> AndroidRuntime START %s <<<<<<\n",
className != NULL ? className : "(unknown)");
blockSigpipe();

下面这句话毫无意义，就是打印log

if (strcmp(options, "start-system-server") == 0) {
/* track our progress through the boot sequence */
const int LOG_BOOT_PROGRESS_START = 3000;
LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,
ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
}

下面这句话定义androidroot的目录

   const char* rootDir = getenv("ANDROID_ROOT");
if (rootDir == NULL) {
rootDir = "/system";
if (!hasDir("/system")) {
LOG_FATAL("No root directory specified, and /android does not exist.");
return;
}
setenv("ANDROID_ROOT", rootDir, 1);
}

对照init.rc可以知道，就是/system

# setup the global environment
export PATH /sbin:/vendor/bin:/system/sbin:/system/bin:/system/xbin
export LD_LIBRARY_PATH /vendor/lib:/system/lib
export ANDROID_BOOTLOGO 1
export ANDROID_ROOT /system
export ANDROID_ASSETS /system/app
export ANDROID_DATA /data
export ASEC_MOUNTPOINT /mnt/asec
export LOOP_MOUNTPOINT /mnt/obb
export BOOTCLASSPATH /system/framework/core.jar:/system/framework/core-junit.jar:/system/framework/bouncycastle.jar:/system/framework/ext.jar:/system/framework/framework.jar:/system/framework/android.policy.jar:/system/framework/services.jar:/system/framework/apache-xml.jar:/system/framework/filterfw.jar

主要是下面这两句话

  /* start the virtual machine */
JNIEnv* env;
if (startVm(&mJavaVM, &env) != 0) {
return;
}
onVmCreated(env);
/*
* Register android functions.
*/
if (startReg(env) < 0) {
LOGE("Unable to register all android natives\n");
return;
}

一个启动虚拟机，一个启动注册安卓本地方法。虚拟机的启动流程，最终调用的是

JNI_CreateJavaVM 在framework/base/core/jni/AndroidRuntime.cpp下。JNI_CreateJavaVM 调用的是：

然后调用dalvik/vm/Jni.cpp的JNI_CreateJavaVM 方法：

jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
const JavaVMInitArgs* args = (JavaVMInitArgs*) vm_args;
if (args->version < JNI_VERSION_1_2) {
return JNI_EVERSION;
}
// TODO: don't allow creation of multiple VMs -- one per customer for now
/* zero globals; not strictly necessary the first time a VM is started */
memset(&gDvm, 0, sizeof(gDvm));
/*
* Set up structures for JNIEnv and VM.
*/
JavaVMExt* pVM = (JavaVMExt*) malloc(sizeof(JavaVMExt));
memset(pVM, 0, sizeof(JavaVMExt));
pVM->funcTable = &gInvokeInterface;
pVM->envList = NULL;
dvmInitMutex(&pVM->envListLock);
UniquePtr<const char*[]> argv(new const char*[args->nOptions]);
memset(argv.get(), 0, sizeof(char*) * (args->nOptions));
/*
* Convert JNI args to argv.
*
* We have to pull out vfprintf/exit/abort, because they use the
* "extraInfo" field to pass function pointer "hooks" in.  We also
* look for the -Xcheck:jni stuff here.
*/
int argc = 0;
for (int i = 0; i < args->nOptions; i++) {
const char* optStr = args->options[i].optionString;
if (optStr == NULL) {
dvmFprintf(stderr, "ERROR: CreateJavaVM failed: argument %d was NULL\n", i);
return JNI_ERR;
} else if (strcmp(optStr, "vfprintf") == 0) {
gDvm.vfprintfHook = (int (*)(FILE *, const char*, va_list))args->options[i].extraInfo;
} else if (strcmp(optStr, "exit") == 0) {
gDvm.exitHook = (void (*)(int)) args->options[i].extraInfo;
} else if (strcmp(optStr, "abort") == 0) {
gDvm.abortHook = (void (*)(void))args->options[i].extraInfo;
} else if (strcmp(optStr, "sensitiveThread") == 0) {
gDvm.isSensitiveThreadHook = (bool (*)(void))args->options[i].extraInfo;
} else if (strcmp(optStr, "-Xcheck:jni") == 0) {
gDvmJni.useCheckJni = true;
} else if (strncmp(optStr, "-Xjniopts:", 10) == 0) {
char* jniOpts = strdup(optStr + 10);
size_t jniOptCount = 1;
for (char* p = jniOpts; *p != 0; ++p) {
if (*p == ',') {
++jniOptCount;
*p = 0;
}
}
char* jniOpt = jniOpts;
for (size_t i = 0; i < jniOptCount; ++i) {
if (strcmp(jniOpt, "warnonly") == 0) {
gDvmJni.warnOnly = true;
} else if (strcmp(jniOpt, "forcecopy") == 0) {
gDvmJni.forceCopy = true;
} else if (strcmp(jniOpt, "logThirdPartyJni") == 0) {
gDvmJni.logThirdPartyJni = true;
} else {
dvmFprintf(stderr, "ERROR: CreateJavaVM failed: unknown -Xjniopts option '%s'\n",
jniOpt);
return JNI_ERR;
}
jniOpt += strlen(jniOpt) + 1;
}
free(jniOpts);
} else {
/* regular option */
argv[argc++] = optStr;
}
}
if (gDvmJni.useCheckJni) {
dvmUseCheckedJniVm(pVM);
}
if (gDvmJni.jniVm != NULL) {
dvmFprintf(stderr, "ERROR: Dalvik only supports one VM per process\n");
return JNI_ERR;
}
gDvmJni.jniVm = (JavaVM*) pVM;
/*
* Create a JNIEnv for the main thread.  We need to have something set up
* here because some of the class initialization we do when starting
* up the VM will call into native code.
*/
JNIEnvExt* pEnv = (JNIEnvExt*) dvmCreateJNIEnv(NULL);
/* Initialize VM. */
gDvm.initializing = true;
std::string status =
dvmStartup(argc, argv.get(), args->ignoreUnrecognized, (JNIEnv*)pEnv);
gDvm.initializing = false;
if (!status.empty()) {
free(pEnv);
free(pVM);
LOGW("CreateJavaVM failed: %s", status.c_str());
return JNI_ERR;
}
/*
* Success!  Return stuff to caller.
*/
dvmChangeStatus(NULL, THREAD_NATIVE);
*p_env = (JNIEnv*) pEnv;
*p_vm = (JavaVM*) pVM;
LOGV("CreateJavaVM succeeded");
return JNI_OK;
}

然后调用Jni.cpp中的

/*
* Create a new JNIEnv struct and add it to the VM's list.
*
* "self" will be NULL for the main thread, since the VM hasn't started
* yet; the value will be filled in later.
*/
JNIEnv* dvmCreateJNIEnv(Thread* self) {
JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;
//if (self != NULL)
//    LOGI("Ent CreateJNIEnv: threadid=%d %p", self->threadId, self);
assert(vm != NULL);
JNIEnvExt* newEnv = (JNIEnvExt*) calloc(1, sizeof(JNIEnvExt));
newEnv->funcTable = &gNativeInterface;
if (self != NULL) {
dvmSetJniEnvThreadId((JNIEnv*) newEnv, self);
assert(newEnv->envThreadId != 0);
} else {
/* make it obvious if we fail to initialize these later */
newEnv->envThreadId = 0x77777775;
newEnv->self = (Thread*) 0x77777779;
}
if (gDvmJni.useCheckJni) {
dvmUseCheckedJniEnv(newEnv);
}
ScopedPthreadMutexLock lock(&vm->envListLock);
/* insert at head of list */
newEnv->next = vm->envList;
assert(newEnv->prev == NULL);
if (vm->envList == NULL) {
// rare, but possible
vm->envList = newEnv;
} else {
vm->envList->prev = newEnv;
}
vm->envList = newEnv;
//if (self != NULL)
//    LOGI("Xit CreateJNIEnv: threadid=%d %p", self->threadId, self);
return (JNIEnv*) newEnv;
}

好吧，这些全是些乱七八糟的东西。真正启动的是这句话，Jni.cpp中：

 std::string status =
dvmStartup(argc, argv.get(), args->ignoreUnrecognized, (JNIEnv*)pEnv);

在Dalvik/vm/Init.cpp中

*
* VM initialization.  Pass in any options provided on the command line.
* Do not pass in the class name or the options for the class.
*
* Returns 0 on success.
*/
std::string dvmStartup(int argc, const char* const argv[],
bool ignoreUnrecognized, JNIEnv* pEnv)
{
ScopedShutdown scopedShutdown;
assert(gDvm.initializing);
LOGV("VM init args (%d):", argc);
for (int i = 0; i < argc; i++) {
LOGV("  %d: '%s'", i, argv[i]);
}
setCommandLineDefaults();
/*
* Process the option flags (if any).
*/
int cc = processOptions(argc, argv, ignoreUnrecognized);
if (cc != 0) {
if (cc < 0) {
dvmFprintf(stderr, "\n");
usage("dalvikvm");
}
return "syntax error";
}
#if WITH_EXTRA_GC_CHECKS > 1
/* only "portable" interp has the extra goodies */
if (gDvm.executionMode != kExecutionModeInterpPortable) {
LOGI("Switching to 'portable' interpreter for GC checks");
gDvm.executionMode = kExecutionModeInterpPortable;
}
#endif
/* Configure group scheduling capabilities */
if (!access("/dev/cpuctl/tasks", F_OK)) {
LOGV("Using kernel group scheduling");
gDvm.kernelGroupScheduling = 1;
} else {
LOGV("Using kernel scheduler policies");
}
/* configure signal handling */
if (!gDvm.reduceSignals)
blockSignals();
/* verify system page size */
if (sysconf(_SC_PAGESIZE) != SYSTEM_PAGE_SIZE) {
return StringPrintf("expected page size %d, got %d",
SYSTEM_PAGE_SIZE, (int) sysconf(_SC_PAGESIZE));
}
/* mterp setup */
LOGV("Using executionMode %d", gDvm.executionMode);
dvmCheckAsmConstants();
/*
* Initialize components.
*/
dvmQuasiAtomicsStartup();
if (!dvmAllocTrackerStartup()) {
return "dvmAllocTrackerStartup failed";
}
if (!dvmGcStartup()) {
return "dvmGcStartup failed";
}
if (!dvmThreadStartup()) {
return "dvmThreadStartup failed";
}
if (!dvmInlineNativeStartup()) {
return "dvmInlineNativeStartup";
}
if (!dvmRegisterMapStartup()) {
return "dvmRegisterMapStartup failed";
}
if (!dvmInstanceofStartup()) {
return "dvmInstanceofStartup failed";
}
if (!dvmClassStartup()) {
return "dvmClassStartup failed";
}
/*
* At this point, the system is guaranteed to be sufficiently
* initialized that we can look up classes and class members. This
* call populates the gDvm instance with all the class and member
* references that the VM wants to use directly.
*/
if (!dvmFindRequiredClassesAndMembers()) {
return "dvmFindRequiredClassesAndMembers failed";
}
if (!dvmStringInternStartup()) {
return "dvmStringInternStartup failed";
}
if (!dvmNativeStartup()) {
return "dvmNativeStartup failed";
}
if (!dvmInternalNativeStartup()) {
return "dvmInternalNativeStartup failed";
}
if (!dvmJniStartup()) {
return "dvmJniStartup failed";
}
if (!dvmProfilingStartup()) {
return "dvmProfilingStartup failed";
}
/*
* Create a table of methods for which we will substitute an "inline"
* version for performance.
*/
if (!dvmCreateInlineSubsTable()) {
return "dvmCreateInlineSubsTable failed";
}
/*
* Miscellaneous class library validation.
*/
if (!dvmValidateBoxClasses()) {
return "dvmValidateBoxClasses failed";
}
/*
* Do the last bits of Thread struct initialization we need to allow
* JNI calls to work.
*/
if (!dvmPrepMainForJni(pEnv)) {
return "dvmPrepMainForJni failed";
}
/*
* Explicitly initialize java.lang.Class.  This doesn't happen
* automatically because it's allocated specially (it's an instance
* of itself).  Must happen before registration of system natives,
* which make some calls that throw assertions if the classes they
* operate on aren't initialized.
*/
if (!dvmInitClass(gDvm.classJavaLangClass)) {
return "couldn't initialized java.lang.Class";
}
/*
* Register the system native methods, which are registered through JNI.
*/
if (!registerSystemNatives(pEnv)) {
return "couldn't register system natives";
}
/*
* Do some "late" initialization for the memory allocator.  This may
* allocate storage and initialize classes.
*/
if (!dvmCreateStockExceptions()) {
return "dvmCreateStockExceptions failed";
}
/*
* At this point, the VM is in a pretty good state.  Finish prep on
* the main thread (specifically, create a java.lang.Thread object to go
* along with our Thread struct).  Note we will probably be executing
* some interpreted class initializer code in here.
*/
if (!dvmPrepMainThread()) {
return "dvmPrepMainThread failed";
}
/*
* Make sure we haven't accumulated any tracked references.  The main
* thread should be starting with a clean slate.
*/
if (dvmReferenceTableEntries(&dvmThreadSelf()->internalLocalRefTable) != 0)
{
LOGW("Warning: tracked references remain post-initialization");
dvmDumpReferenceTable(&dvmThreadSelf()->internalLocalRefTable, "MAIN");
}
/* general debugging setup */
if (!dvmDebuggerStartup()) {
return "dvmDebuggerStartup failed";
}
if (!dvmGcStartupClasses()) {
return "dvmGcStartupClasses failed";
}
/*
* Init for either zygote mode or non-zygote mode.  The key difference
* is that we don't start any additional threads in Zygote mode.
*/
if (gDvm.zygote) {
if (!initZygote()) {
return "initZygote failed";
}
} else {
if (!dvmInitAfterZygote()) {
return "dvmInitAfterZygote failed";
}
}
#ifndef NDEBUG
if (!dvmTestHash())
LOGE("dvmTestHash FAILED");
if (false /*noisy!*/ && !dvmTestIndirectRefTable())
LOGE("dvmTestIndirectRefTable FAILED");
#endif
if (dvmCheckException(dvmThreadSelf())) {
dvmLogExceptionStackTrace();
return "Exception pending at end of VM initialization";
}
scopedShutdown.disarm();
return "";
}

代码真长。寻找其中最具价值的部分

插入代码：

   if (!dvmAllocTrackerStartup()) {
return "dvmAllocTrackerStartup failed";
}
if (!dvmGcStartup()) {
return "dvmGcStartup failed";
}
if (!dvmThreadStartup()) {
return "dvmThreadStartup failed";
}
if (!dvmInlineNativeStartup()) {
return "dvmInlineNativeStartup";
}
if (!dvmRegisterMapStartup()) {
return "dvmRegisterMapStartup failed";
}
if (!dvmInstanceofStartup()) {
return "dvmInstanceofStartup failed";
}
if (!dvmClassStartup()) {
return "dvmClassStartup failed";
}

经分析，这些都没有建立gc线程，gc线程的建立是在如下方法：

dvmInitAfterZygote

由于跟得太深，东西很多，就不一一列举。仅仅跟一下dvmClassStartup，最终调用到了dalvik/vm/oo/Class.cpp中的方法：

/*
* Initialize the bootstrap class loader.
*
* Call this after the bootclasspath string has been finalized.
*/
bool dvmClassStartup()
{
/* make this a requirement -- don't currently support dirs in path */
if (strcmp(gDvm.bootClassPathStr, ".") == 0) {
LOGE("ERROR: must specify non-'.' bootclasspath");
return false;
}
gDvm.loadedClasses =
dvmHashTableCreate(256, (HashFreeFunc) dvmFreeClassInnards);
gDvm.pBootLoaderAlloc = dvmLinearAllocCreate(NULL);
if (gDvm.pBootLoaderAlloc == NULL)
return false;
if (false) {
linearAllocTests();
exit(0);
}
/*
* Class serial number.  We start with a high value to make it distinct
* in binary dumps (e.g. hprof).
*/
gDvm.classSerialNumber = INITIAL_CLASS_SERIAL_NUMBER;
/*
* Set up the table we'll use for tracking initiating loaders for
* early classes.
* If it's NULL, we just fall back to the InitiatingLoaderList in the
* ClassObject, so it's not fatal to fail this allocation.
*/
gDvm.initiatingLoaderList = (InitiatingLoaderList*)
calloc(ZYGOTE_CLASS_CUTOFF, sizeof(InitiatingLoaderList));
/*
* Create the initial classes. These are the first objects constructed
* within the nascent VM.
*/
if (!createInitialClasses()) {
return false;
}
/*
* 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);
if (gDvm.bootClassPath == NULL)
return false;
return true;
}

根据注释，Initialize the bootstrap class loader.
这个函数告诉我们，他建立了boottrap classloader。

createInitialClasses加载了9大基本类型。而后的processClassPath则建立了基本的classloader。分析过后，比较失望。可能是为后续的boottrapclassloader做一些前期准备工作。

startVM就到这里。

好吧。现在又回到了最初的App_main.cpp中。进入了com.android.internal.os.ZygoteInit.java的main

    public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
registerZygoteSocket();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gc();
// If requested, start system server directly from Zygote
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
if (argv[1].equals("start-system-server")) {
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
if (ZYGOTE_FORK_MODE) {
runForkMode();
} else {
runSelectLoopMode();
}
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}

回过头来继续看一下ZygoteInit.java这个类是如何初始化的，看如下代码：

/*
* Create a new JNIEnv struct and add it to the VM's list.
*
* "self" will be NULL for the main thread, since the VM hasn't started
* yet; the value will be filled in later.
*/
JNIEnv* dvmCreateJNIEnv(Thread* self) {
JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;
//if (self != NULL)
//    LOGI("Ent CreateJNIEnv: threadid=%d %p", self->threadId, self);
assert(vm != NULL);
JNIEnvExt* newEnv = (JNIEnvExt*) calloc(1, sizeof(JNIEnvExt));
newEnv->funcTable = &gNativeInterface;
if (self != NULL) {
dvmSetJniEnvThreadId((JNIEnv*) newEnv, self);
assert(newEnv->envThreadId != 0);
} else {
/* make it obvious if we fail to initialize these later */
newEnv->envThreadId = 0x77777775;
newEnv->self = (Thread*) 0x77777779;
}
if (gDvmJni.useCheckJni) {
dvmUseCheckedJniEnv(newEnv);
}
ScopedPthreadMutexLock lock(&vm->envListLock);
/* insert at head of list */
newEnv->next = vm->envList;
assert(newEnv->prev == NULL);
if (vm->envList == NULL) {
// rare, but possible
vm->envList = newEnv;
} else {
vm->envList->prev = newEnv;
}
vm->envList = newEnv;
//if (self != NULL)
//    LOGI("Xit CreateJNIEnv: threadid=%d %p", self->threadId, self);
return (JNIEnv*) newEnv;
}

最重要的是gNativeInterface 我们看定义，太长了，我们找到其中的FindClass。

static jclass FindClass(JNIEnv* env, const char* name) {
ScopedJniThreadState ts(env);
const Method* thisMethod = dvmGetCurrentJNIMethod();
assert(thisMethod != NULL);
Object* loader;
Object* trackedLoader = NULL;
if (ts.self()->classLoaderOverride != NULL) {
/* hack for JNI_OnLoad */
assert(strcmp(thisMethod->name, "nativeLoad") == 0);
loader = ts.self()->classLoaderOverride;
} else if (thisMethod == gDvm.methDalvikSystemNativeStart_main ||
thisMethod == gDvm.methDalvikSystemNativeStart_run) {
/* start point of invocation interface */
if (!gDvm.initializing) {
loader = trackedLoader = dvmGetSystemClassLoader();
} else {
loader = NULL;
}
} else {
loader = thisMethod->clazz->classLoader;
}
char* descriptor = dvmNameToDescriptor(name);
if (descriptor == NULL) {
return NULL;
}
ClassObject* clazz = dvmFindClassNoInit(descriptor, loader);
free(descriptor);
jclass jclazz = (jclass) addLocalReference(ts.self(), (Object*) clazz);
dvmReleaseTrackedAlloc(trackedLoader, ts.self());
return jclazz;
}

在dalvik/vm/Init.cpp中的方法对gVM的bootpath进行了初始化：

static void setCommandLineDefaults()
{
const char* envStr = getenv("CLASSPATH");
if (envStr != NULL) {
gDvm.classPathStr = strdup(envStr);
} else {
gDvm.classPathStr = strdup(".");
}
envStr = getenv("BOOTCLASSPATH");
if (envStr != NULL) {
gDvm.bootClassPathStr = strdup(envStr);
} else {
gDvm.bootClassPathStr = strdup(".");
}
gDvm.properties = new std::vector<std::string>();
/* Defaults overridden by -Xms and -Xmx.
* TODO: base these on a system or application-specific default
*/
gDvm.heapStartingSize = 2 * 1024 * 1024;  // Spec says 16MB; too big for us.
gDvm.heapMaximumSize = 16 * 1024 * 1024;  // Spec says 75% physical mem
gDvm.heapGrowthLimit = 0;  // 0 means no growth limit
gDvm.stackSize = kDefaultStackSize;
gDvm.concurrentMarkSweep = true;
/* gDvm.jdwpSuspend = true; */

现在明白了，在init.rc中指定的BOOTCLASSPATH赋值给了gDvm.bootClassPathStr 。

而下面这个地方，则对FindClass进行了初始化。还是在dalvik/vm/Init.cpp中

     */
if (!dvmFindRequiredClassesAndMembers()) {
return "dvmFindRequiredClassesAndMembers failed";
}

initDirectMethodReferences 把方法gDvm.methDalvikSystemNativeStart_main与NativeStart进行了对应。

 { &gDvm.methDalvikSystemNativeStart_main, "Ldalvik/system/NativeStart;", "main", "([Ljava/lang/String;)V" },

主要分析FindClass方法：

nativeLoad标示从System.loadlibrary加载。那么Zygote的dvmGetCurrentJNIMethod是哪个呢。我猜测，这个一定是没有的，也就是里面的成员变量全为空。哈哈。这意味着，最终调用到了

dvmFindClassNoInit方法中。

然后是ClassObject* dvmFindSystemClassNoInit(const char* descriptor)
{
    return findClassNoInit(descriptor, NULL, NULL);
}
然后是findClassNoInit

ZygoteInit这个类根本是没有ClassLoader的。而是直接从包里面查找得到的。

进入ZygoteInit后。就是java代码了。