涉及源码（Android 4.4.2）：
frameworks/base/cmds/app_process/app_main.cpp
/frameworks/base/core/jni/AndroidRuntime.cpp
/libnativehelper/JniInvocation.cpp
/dalvik/vm/Jni.cpp
/dalvik/vm/JniInternal.h
/libnativehelper/include/nativehelper/jni.h
首先来看看app_main.cpp的main方法，它是启动虚拟机的入口方法。
frameworks/base/cmds/app_process/app_main.cpp
int main(int argc, char* const argv[])
{
// zygote进程是由init进程通过fork而来
AppRuntime runtime;
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer ? "start-system-server" : "");
}
}
/frameworks/base/core/jni/AndroidRuntime.cpp
void AndroidRuntime::start(const char* className, const char* options)
{
/* start the virtual machine */
JniInvocation jni_invocation;
// 1、初始化JNI相关的函数
jni_invocation.Init(NULL);
JNIEnv* env;
// 2、启动虚拟机
if (startVm(&mJavaVM, &env) != 0) {
return;
}
onVmCreated(env);
// 3、注册JNI函数
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
}
一、初始化JNI相关的函数
/libnativehelper/JniInvocation.cpp
bool JniInvocation::Init(const char* library) {
#ifdef HAVE_ANDROID_OS
char default_library[PROPERTY_VALUE_MAX];
property_get(kLibrarySystemProperty, default_library, kLibraryFallback);
#else
const char* default_library = kLibraryFallback;
#endif
if (library == NULL) {
library = default_library;
}
// 通过dlopen加载libdvm.so，libdvm.so是dalvik vm的核心库
handle_ = dlopen(library, RTLD_NOW);
//从libdvm.so中找到JNI_GetDefaultJavaVMInitArgs、NI_CreateJavaVM和JNI_GetCreateJavaVMs这三个函数指针。
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;
}
二、启动虚拟机
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv)
{
// 前面是初始化操作，重点是整个方法
if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
ALOGE("JNI_CreateJavaVM failed\n");
goto bail;
}
}
JNI_CreateJavaVM方法就是前面libdvm.so中的方法，它就是创建虚拟机的方法。
下面来看看这个JNI_CreateJavaVM方法
/dalvik/vm/Jni.cpp
jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
const JavaVMInitArgs* args = (JavaVMInitArgs*) vm_args;
//1、gDvm是一个静态的数据结构，数据类型为结构体DvmGlobals
// 虚拟机的大部分数据都是有它来存放
memset(&gDvm, 0, sizeof(gDvm));
//2、创建JavaVMExt对象，其实也是JavaVM对象
JavaVMExt* pVM = (JavaVMExt*) calloc(1, sizeof(JavaVMExt));
pVM->funcTable = &gInvokeInterface;
pVM->envList = NULL;
dvmInitMutex(&pVM->envListLock);
gDvmJni.jniVm = (JavaVM*) pVM;
// 3、创建JNIEnvExt对象，其实对应的也是JNIEnv对象
JNIEnvExt* pEnv = (JNIEnvExt*) dvmCreateJNIEnv(NULL);
//4、启动虚拟机
gDvm.initializing = true;
std::string status =
dvmStartup(argc, argv.get(), args->ignoreUnrecognized, (JNIEnv*)pEnv);
gDvm.initializing = false;
*p_env = (JNIEnv*) pEnv;
*p_vm = (JavaVM*) pVM;
}
1、重置gDvm结构
DvmGlobals，该结构体是Dalvik的核心数据结构，几乎所有的重要成员等都通过gDvm来管理。
2、创建JavaVMExt对象
JavaVMExt对象的funcTable成员用来保存Dalvik虚拟机实例函数表，从代码可以看到，这个函数表为gInvokeInterface，gInvokeInterface是一个类型为JNIInvokeInterface的结构体。
/dalvik/vm/jni.cpp
static const struct JNIInvokeInterface gInvokeInterface = {
NULL,
NULL,
NULL,
DestroyJavaVM,
AttachCurrentThread,
DetachCurrentThread,
GetEnv,
AttachCurrentThreadAsDaemon,
};
3、创建JNIEnvExt对象
下面来看看dvmCreateJNIEnv函数
JNIEnv* dvmCreateJNIEnv(Thread* self) {
JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;
// 1、创建JNIEnvExt对象
// Dalvik虚拟机里JNI的所有函数都封装在gNativeInterface中
JNIEnvExt* newEnv = (JNIEnvExt*) calloc(1, sizeof(JNIEnvExt));
newEnv->funcTable = &gNativeInterface;
if (self != NULL) {
dvmSetJniEnvThreadId((JNIEnv*) newEnv, self);
assert(newEnv->envThreadId != 0);
} else {
newEnv->envThreadId = 0x77777775;
newEnv->self = (Thread*) 0x77777779;
}
if (gDvmJni.useCheckJni) {
dvmUseCheckedJniEnv(newEnv);
}
ScopedPthreadMutexLock lock(&vm->envListLock);
// 2、将JNIEnvExt插入到JavaVMExt对象的成员变量envList列表的头部
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;
return (JNIEnv*) newEnv;
}
（1）结构体JNINativeInterface用来描述一个本地接口表。当我们需要在C/C++代码在中调用Java函数，就要用到这个本地接口表
static const struct JNINativeInterface gNativeInterface = {
......
FindClass,
......
GetMethodID,
......
CallObjectMethod,
......
GetFieldID,
......
SetIntField,
......
RegisterNatives,
UnregisterNatives,
......
GetJavaVM,
......
};
（2）每一个Dalvik虚拟机实例还有一个JNI环境列表,这个列表保存JavaVMExt对象的成员变量envList中。JavaVMExt的JNI环境列表就是JNIEnvExt列表。每一个Attach到Dalvik虚拟机中去的线程都有一个对应的JNIEnvExt，用来描述它的JNI环境。
他们之间的关系图如下图所示：
/dalvik/vm/JniInternal.h
/libnativehelper/include/nativehelper/jni.h
vmstart.png
4、初始化并启动虚拟机
dalvik/vm/init.cpp
std::string dvmStartup(int argc, const char* const argv[],
bool ignoreUnrecognized, JNIEnv* pEnv)
{
/*
* 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";
}
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";
}
if (!dvmCreateInlineSubsTable()) {
return "dvmCreateInlineSubsTable failed";
}
if (!dvmValidateBoxClasses()) {
return "dvmValidateBoxClasses failed";
}
if (!dvmPrepMainForJni(pEnv)) {
return "dvmPrepMainForJni failed";
}
if (!dvmInitClass(gDvm.classJavaLangClass)) {
return "couldn't initialized java.lang.Class";
}
if (!registerSystemNatives(pEnv)) {
return "couldn't register system natives";
}
if (!dvmCreateStockExceptions()) {
return "dvmCreateStockExceptions failed";
}
if (!dvmPrepMainThread()) {
return "dvmPrepMainThread failed";
}
if (dvmReferenceTableEntries(&dvmThreadSelf()->internalLocalRefTable) != 0)
{
ALOGW("Warning: tracked references remain post-initialization");
dvmDumpReferenceTable(&dvmThreadSelf()->internalLocalRefTable, "MAIN");
}
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";
}
}
}
主要用来初始化Dalvik虚拟机
三、注册JNI函数
/frameworks/base/core/jni/AndroidRuntime.cpp
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
// 注册JNI函数，gRegJNI是一个全局数组
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
env->PopLocalFrame(NULL);
return -1;
}
env->PopLocalFrame(NULL);
return 0;
}
static const RegJNIRec gRegJNI[] = {
REG_JNI(register_android_debug_JNITest),
REG_JNI(register_com_android_internal_os_RuntimeInit),
REG_JNI(register_android_os_SystemClock),
REG_JNI(register_android_util_EventLog),
REG_JNI(register_android_util_Log),
REG_JNI(register_android_util_FloatMath),
REG_JNI(register_android_text_format_Time),
REG_JNI(register_android_content_AssetManager),
REG_JNI(register_android_content_StringBlock),
REG_JNI(register_android_content_XmlBlock),
REG_JNI(register_android_emoji_EmojiFactory),
REG_JNI(register_android_text_AndroidCharacter),
REG_JNI(register_android_text_AndroidBidi),
REG_JNI(register_android_view_InputDevice),
REG_JNI(register_android_view_KeyCharacterMap),
REG_JNI(register_android_os_Process),
REG_JNI(register_android_os_SystemProperties),
REG_JNI(register_android_os_Binder),
// ...还是很多
}
参考文章：
http://www.infoq.com/cn/articles/android-in-depth-dalvik
http://blog.csdn.net/luoshengyang/article/details/8885792
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