Dalvik学习笔记--运行过程

最新推荐文章于 2024-04-05 10:13:00 发布

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本文详细解析了Android应用程序的启动流程，重点关注Dalvik虚拟机如何通过JNI接口调用Java层的main方法，以及Dalvik虚拟机内部如何解释执行Java字节码。

在AndroidRuntime.start函数中，在经过startVm和startReg两个函数对Dalvik虚拟机实例初始化之后（启动过程）

会执行如下代码

<span style="font-size:14px;">    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        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);</span>

也就是说会通过CallStaticVoidMethod来启动入口函数。

CallStaticVoidMethod来自于struct _JNIEnv结构中

    void CallVoidMethod(jobject obj, jmethodID methodID, ...)
    {
        va_list args;
        va_start(args, methodID);
        functions->CallVoidMethodV(this, obj, methodID, args);
        va_end(args);
    }

当中的functions是一个const struct JNINativeInterface*成员，指向一个回调函数表

<span style="font-size:14px;">void        (*CallStaticVoidMethodV)(JNIEnv*, jclass, jmethodID, va_list);</span>

从启动过程的分析看到，在JNI_CreateJavaVM函数中，返回的JNIEnv** p_env实际上是由强制类型转化得来的

*p_env = (JNIEnv*) pEnv;

而这个pEnv本身则是一个 JNIEnvExt *对象

pEnv = (JNIEnvExt*) dvmCreateJNIEnv(NULL);

所以，这里JNIEnv的functions实际上是JNIEnvExt结构中的第一个成员

struct JNIEnvExt {
    const struct JNINativeInterface* funcTable;     /* must be first */

    const struct JNINativeInterface* baseFuncTable;

    u4      envThreadId;
    Thread* self;

    /* if nonzero, we are in a "critical" JNI call */
    int     critical;

    struct JNIEnvExt* prev;
    struct JNIEnvExt* next;
};

同样是JNINativeInterface结构的funcTable指针

在启动时，由dvmCreateJNIEnv创建这个pEnv（其实它的返回值就是JNIEnv结构）

JNIEnv* dvmCreateJNIEnv(Thread* self) {
    JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;

    //if (self != NULL)
    //    ALOGI("Ent CreateJNIEnv: threadid=%d %p", self->threadId, self);

    assert(vm != NULL);

    JNIEnvExt* newEnv = (JNIEnvExt*) calloc(1, sizeof(JNIEnvExt));
    newEnv->funcTable = &gNativeInterface;			//在这里
    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)
    //    ALOGI("Xit CreateJNIEnv: threadid=%d %p", self->threadId, self);
    return (JNIEnv*) newEnv;
}

所以这个funcTable指向了gNativeInterface

在gNativeInterface中有一个CallStaticVoidMethodV，它的定义在同一个文件中，是通过宏定义的

    static _ctype CallStatic##_jname##MethodV(JNIEnv* env, jclass jclazz,   \
        jmethodID methodID, va_list args)                                   \
    {                                                                       \
        UNUSED_PARAMETER(jclazz);                                           \
        ScopedJniThreadState ts(env);                                       \
        JValue result;                                                      \
        dvmCallMethodV(ts.self(), (Method*)methodID, NULL, true, &result, args);\
        if (_isref && !dvmCheckException(ts.self()))                        \
            result.l = (Object*)addLocalReference(ts.self(), result.l);           \
        return _retok;                                                      \
    }

终于找到了，也就是说最开始的env->CallStaticVoidMethod(startClass, startMeth, strArray);

其实是调用的这个宏（真的好绕）。

这个宏内容很简单，调用dvmCallMethonV执行下一步

void dvmCallMethodV(Thread* self, const Method* method, Object* obj,
    bool fromJni, JValue* pResult, va_list args)
{
    const char* desc = &(method->shorty[1]); // [0] is the return type.
    int verifyCount = 0;
    ClassObject* clazz;
    u4* ins;

    clazz = callPrep(self, method, obj, false);
    if (clazz == NULL)
        return;

    /* "ins" for new frame start at frame pointer plus locals */
    ins = ((u4*)self->interpSave.curFrame) +
           (method->registersSize - method->insSize);

    //ALOGD("  FP is %p, INs live at >= %p", self->interpSave.curFrame, ins);

    /* put "this" pointer into in0 if appropriate */
    if (!dvmIsStaticMethod(method)) {
#ifdef WITH_EXTRA_OBJECT_VALIDATION
        assert(obj != NULL && dvmIsHeapAddress(obj));
#endif
        *ins++ = (u4) obj;
        verifyCount++;
    }

    while (*desc != '\0') {
        switch (*(desc++)) {
            case 'D': case 'J': {
                u8 val = va_arg(args, u8);
                memcpy(ins, &val, 8);       // EABI prevents direct store
                ins += 2;
                verifyCount += 2;
                break;
            }
            case 'F': {
                /* floats were normalized to doubles; convert back */
                float f = (float) va_arg(args, double);
                *ins++ = dvmFloatToU4(f);
                verifyCount++;
                break;
            }
            case 'L': {     /* 'shorty' descr uses L for all refs, incl array */
                void* arg = va_arg(args, void*);
                assert(obj == NULL || dvmIsHeapAddress(obj));
                jobject argObj = reinterpret_cast<jobject>(arg);
                if (fromJni)
                    *ins++ = (u4) dvmDecodeIndirectRef(self, argObj);
                else
                    *ins++ = (u4) argObj;
                verifyCount++;
                break;
            }
            default: {
                /* Z B C S I -- all passed as 32-bit integers */
                *ins++ = va_arg(args, u4);
                verifyCount++;
                break;
            }
        }
    }

#ifndef NDEBUG
    if (verifyCount != method->insSize) {
        ALOGE("Got vfycount=%d insSize=%d for %s.%s", verifyCount,
            method->insSize, clazz->descriptor, method->name);
        assert(false);
        goto bail;
    }
#endif

    //dvmDumpThreadStack(dvmThreadSelf());

    if (dvmIsNativeMethod(method)) {
        TRACE_METHOD_ENTER(self, method);
        /*
         * Because we leave no space for local variables, "curFrame" points
         * directly at the method arguments.
         */
        (*method->nativeFunc)((u4*)self->interpSave.curFrame, pResult,
                              method, self);
        TRACE_METHOD_EXIT(self, method);
    } else {
        dvmInterpret(self, method, pResult);
    }

#ifndef NDEBUG
bail:
#endif
    dvmPopFrame(self);
}

函数有点长，不过没关系，直接看最后

dvmIsNativeMethod判断这个method是不是native，如果是，通过method的nativeFunc保存的地址就可以

直接进行调用。如果不是，也就是Java函数，那么调用dvmInterpert

dvmInterpert函数挺长的，而主要需要看的地方在这里

    if (gDvm.executionMode == kExecutionModeInterpFast)
        stdInterp = dvmMterpStd;
#if defined(WITH_JIT)
    else if (gDvm.executionMode == kExecutionModeJit ||
             gDvm.executionMode == kExecutionModeNcgO0 ||
             gDvm.executionMode == kExecutionModeNcgO1)
        stdInterp = dvmMterpStd;
#endif
    else
        stdInterp = dvmInterpretPortable;

    // Call the interpreter
    (*stdInterp)(self);

Dalvik虚拟机有三种执行模式，fast、jit、protable
fast会针对本地平台进行优化，可以更快速的执行Java代码

jit会将Java代码动态编译为Native代码，然后执行

protable则是可移植模式，也就是通用模式

根据模式不同会选择dvmMterpStd和dvmInterpretProtable当中的一个执行。

void dvmMterpStd(Thread* self)
{
    /* configure mterp items */
    self->interpSave.methodClassDex = self->interpSave.method->clazz->pDvmDex;

    IF_LOGVV() {
        char* desc = dexProtoCopyMethodDescriptor(
                         &self->interpSave.method->prototype);
        LOGVV("mterp threadid=%d : %s.%s %s",
            dvmThreadSelf()->threadId,
            self->interpSave.method->clazz->descriptor,
            self->interpSave.method->name,
            desc);
        free(desc);
    }
    //ALOGI("self is %p, pc=%p, fp=%p", self, self->interpSave.pc,
    //      self->interpSave.curFrame);
    //ALOGI("first instruction is 0x%04x", self->interpSave.pc[0]);

    /*
     * Handle any ongoing profiling and prep for debugging
     */
    if (self->interpBreak.ctl.subMode != 0) {
        TRACE_METHOD_ENTER(self, self->interpSave.method);
        self->debugIsMethodEntry = true;   // Always true on startup
    }

    dvmMterpStdRun(self);

#ifdef LOG_INSTR
    ALOGD("|-- Leaving interpreter loop");
#endif
}

继续调用dvmMterStdRun

void dvmMterpStdRun(Thread* self)
{
    jmp_buf jmpBuf;

    self->interpSave.bailPtr = &jmpBuf;

    /* We exit via a longjmp */
    if (setjmp(jmpBuf)) {
        LOGVV("mterp threadid=%d returning", dvmThreadSelf()->threadId);
        return;
    }

    /* run until somebody longjmp()s out */
    while (true) {
        typedef void (*Handler)(Thread* self);

        u2 inst = /*self->interpSave.*/pc[0];
        /*
         * In mterp, dvmCheckBefore is handled via the altHandlerTable,
         * while in the portable interpreter it is part of the handler
         * FINISH code.  For allstubs, we must do an explicit check
         * in the interpretation loop.
         */
        if (self->interpBreak.ctl.subMode) {
            dvmCheckBefore(pc, fp, self);
        }
        Handler handler = (Handler) gDvmMterpHandlers[inst & 0xff];
        (void) gDvmMterpHandlerNames;   /* avoid gcc "defined but not used" */
        LOGVV("handler %p %s",
            handler, (const char*) gDvmMterpHandlerNames[inst & 0xff]);
        (*handler)(self);
    }
}

获取指令然后处理

看下mterp下的目录

这些不同的平台文件夹下都是.S汇编文件，通过这种方法来提高程序运行速度