Dalvik虚拟机启动过程

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(&JNI_GetDefaultJavaVMInitArgs_),
                  "JNI_GetDefaultJavaVMInitArgs")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast(&JNI_CreateJavaVM_),
                  "JNI_CreateJavaVM")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast(&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