//app_main.cpp
int main(int argc, char* const argv[])
{
    //AppRuntime继承自AndroidRuntime, 构造函数中没有什么有用信息，可以不关注
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    //...
    if (zygote) {
        //调用到AndroidRuntime.start函数启动zygote进程的虚拟机
        //传入的第一个参数是java类ZygoteInit
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } else if (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
    } else {
        fprintf(stderr, "Error: no class name or --zygote supplied.\n");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
    }
}

//AndroidRuntime.cpp

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
    ......
    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);  // 1. 加载虚拟机动态库，并加载库函数符号
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != 0) { // 2. 启动虚拟机
        return;
    }
    onVmCreated(env);

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
    ......

    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        //调用com.android.internal.os.ZygoteInit的main函数，从此便进入了Java的世界
        jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
            "([Ljava/lang/String;)V");
        if (startMeth == NULL) {
            ALOGE("JavaVM unable to find main() in '%s'\n", className);
            /* keep going */
        } else {
            env->CallStaticVoidMethod(startClass, startMeth, strArray);
        }
    }
}

//JniInvocation.cpp

bool JniInvocation::Init(const char* library) {
  ......
  library = GetLibrary(library, buffer); //返回“libart.so” art虚拟机库
  ......
  handle_ = dlopen(library, kDlopenFlags);  //动态加载虚拟机so库
  ......
  //下面的三个if语句都是通过dlsym来获取虚拟机so库中的函数符号
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetDefaultJavaVMInitArgs_),
                  "JNI_GetDefaultJavaVMInitArgs")) {
    return false;
  }
  //无比重要的函数，获取虚拟机构造函数的函数符号，创建虚拟机全靠JNI_CreateJavaVM
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_CreateJavaVM_),
                  "JNI_CreateJavaVM")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetCreatedJavaVMs_),
                  "JNI_GetCreatedJavaVMs")) {
    return false;
  }
  return true;
}
jint JniInvocation::JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
  return JNI_CreateJavaVM_(p_vm, p_env, vm_args);
}

//该函数就是最终创建虚拟机的函数
extern "C" jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
  return JniInvocation::GetJniInvocation().JNI_CreateJavaVM(p_vm, p_env, vm_args);
}

/*
 * Start the Dalvik Virtual Machine.
 */
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
{
    ...... //组装虚拟机启动参数
    /*
     * Initialize the VM.
     *
     * The JavaVM* is essentially per-process, and the JNIEnv* is per-thread.
     * If this call succeeds, the VM is ready, and we can start issuing
     * JNI calls.
     */
    if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
        ALOGE("JNI_CreateJavaVM failed\n");
        return -1;
    }

    return 0;
}