Init函数主要作用是初始化JNI，具体工作是首先通过dlopen加载libart.so获得其句柄，然后调用dlsym从libart.so中找到JNI_GetDefaultJavaVMInitArgs、JNI_CreateJavaVM、JNI_GetCreatedJavaVMs三个函数地址，赋值给对应成员属性，这三个函数会在后续虚拟机创建中调用。
bool JniInvocation::Init(const char* library) { #ifdef __ANDROID__ char buffer[PROP_VALUE_MAX]; #else char* buffer = NULL; #endif library = GetLibrary(library, buffer); // 默认返回 libart.so // Load with RTLD_NODELETE in order to ensure that libart.so is not unmapped when it is closed. // This is due to the fact that it is possible that some threads might have yet to finish // exiting even after JNI_DeleteJavaVM returns, which can lead to segfaults if the library is // unloaded. const int kDlopenFlags = RTLD_NOW | RTLD_NODELETE; /* * 1.dlopen功能是以指定模式打开指定的动态链接库文件，并返回一个句柄 * 2.RTLD_NOW表示需要在dlopen返回前，解析出所有未定义符号，如果解析不出来，在dlopen会返回NULL * 3.RTLD_NODELETE表示在dlclose()期间不卸载库，并且在以后使用dlopen()重新加载库时不初始化库中的静态变量 */ handle_ = dlopen(library, kDlopenFlags); // 获取libart.so的句柄 if (handle_ == NULL) { // 获取失败打印错误日志并尝试再次打开libart.so if (strcmp(library, kLibraryFallback) == 0) { // Nothing else to try. ALOGE("Failed to dlopen %s: %s", library, dlerror()); return false; } // Note that this is enough to get something like the zygote // running, we can't property_set here to fix this for the future // because we are root and not the system user. See // RuntimeInit.commonInit for where we fix up the property to // avoid future fallbacks. ALOGW("Falling back from %s to %s after dlopen error: %s", library, kLibraryFallback, dlerror()); library = kLibraryFallback; handle_ = dlopen(library, kDlopenFlags); if (handle_ == NULL) { ALOGE("Failed to dlopen %s: %s", library, dlerror()); return false; } } /* * 1.FindSymbol函数内部实际调用的是dlsym * 2.dlsym作用是根据 动态链接库 操作句柄(handle)与符号(symbol)，返回符号对应的地址 * 3.这里实际就是从libart.so中将JNI_GetDefaultJavaVMInitArgs等对应的地址存入&JNI_GetDefaultJavaVMInitArgs_中 */ if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetDefaultJavaVMInitArgs_), "JNI_GetDefaultJavaVMInitArgs")) { return false; } if (!FindSymbol(reinterpret_cast<void**>(&JNI_CreateJavaVM_), "JNI_CreateJavaVM")) { return false; } if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetCreatedJavaVMs_), "JNI_GetCreatedJavaVMs")) { return false; } return true; }其次，我们再跟一下startVm()：
这个函数特别长，但是里面做的事情很单一，其实就是从各种系统属性中读取一些参数，然后通过addOption设置到AndroidRuntime的mOptions数组中存起来，另外就是调用之前从libart.so中找到JNI_CreateJavaVM函数，并将这些参数传入，由于本篇主要讲zygote启动流程，因此关于虚拟机的实现就不深入探究了。
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote) { JavaVMInitArgs initArgs; char propBuf[PROPERTY_VALUE_MAX]; char stackTraceFileBuf[sizeof("-Xstacktracefile:")-1 + PROPERTY_VALUE_MAX]; char jniOptsBuf[sizeof("-Xjniopts:")-1 + PROPERTY_VALUE_MAX]; ... ... /* route exit() to our handler */ addOption("exit", (void*) runtime_exit); // 将参数放入mOptions数组中 ... ... initArgs.version = JNI_VERSION_1_4; initArgs.options = mOptions.editArray(); // 将mOptions赋值给initArgs initArgs.nOptions = mOptions.size(); initArgs.ignoreUnrecognized = JNI_FALSE; /* * 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) { // 调用libart.so的JNI_CreateJavaVM函数 ALOGE("JNI_CreateJavaVM failed\n"); return -1; } return 0; } 注册JNI函数