【分析】dalvik虚拟机启动过程（二）

源码版本：Android-4.4.4_r2

提示：大部分分析直接注释在代码内。

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接着上一篇【分析】dalvik虚拟机启动过程（一）

JNI_CreateJavaVM函数调用dvmCreateJNIEnv创建JNIEnv后，接着又调用了dvmStartup函数初始化VM：

/* * VM 初始化。 * VM initialization.  Pass in any options provided on the command line. * Do not pass in the class name or the options for the class. * * 如果成功的话，则返回空字符串。 * Returns 0 on success. *  * argc:参数个数。 * argv:参数数组。 */std::string dvmStartup(int argc, const char* const argv[],        bool ignoreUnrecognized, JNIEnv* pEnv){    ScopedShutdown scopedShutdown;    assert(gDvm.initializing);    ALOGV("VM init args (%d):", argc);    for (int i = 0; i < argc; i++) {        ALOGV("  %d: '%s'", i, argv[i]);    }    // 设置默认值。    setCommandLineDefaults();    // 处理选项标志（如果有的话）。    /*     * Process the option flags (if any).     */    int cc = processOptions(argc, argv, ignoreUnrecognized);    if (cc != 0) {        if (cc < 0) {            dvmFprintf(stderr, "\n");            usage("dalvikvm");        }        return "syntax error";    }    ......    // 验证系统页大小。    /* verify system page size */    if (sysconf(_SC_PAGESIZE) != SYSTEM_PAGE_SIZE) {        return StringPrintf("expected page size %d, got %d",                SYSTEM_PAGE_SIZE, (int) sysconf(_SC_PAGESIZE));    }    // 验证用于mterp解释器的常量。    /* mterp setup */    ALOGV("Using executionMode %d", gDvm.executionMode);    dvmCheckAsmConstants();    // 初始化组件。    /*     * Initialize components.     */    dvmQuasiAtomicsStartup();    if (!dvmAllocTrackerStartup()) {        return "dvmAllocTrackerStartup failed";    }    if (!dvmGcStartup()) {        return "dvmGcStartup failed";    }    // 初始化线程。    if (!dvmThreadStartup()) {        return "dvmThreadStartup failed";    }    ......}

dvmSartup函数中调用了dvmThreadStartup函数初始化了线程，这个函数在dalvik/vm/Thread.cpp文件中：

/* * 初始化线程列表和主线程的环境。 * 我们需要设置一些东西，当我们开始加载类时dvmThreadSelf()将会工作。 *  * Initialize thread list and main thread's environment.  We need to set * up some basic stuff so that dvmThreadSelf() will work when we start * loading classes (e.g. to check for exceptions). */bool dvmThreadStartup(){    Thread* thread;    // 分配一个线程局部存储。    /* allocate a TLS slot */    if (pthread_key_create(&gDvm.pthreadKeySelf, threadExitCheck) != 0) {        ALOGE("ERROR: pthread_key_create failed");        return false;    }    /* test our pthread lib */    if (pthread_getspecific(gDvm.pthreadKeySelf) != NULL)        ALOGW("WARNING: newly-created pthread TLS slot is not NULL");    // 准备与线程相关的锁和条件。    /* prep thread-related locks and conditions */    dvmInitMutex(&gDvm.threadListLock);    pthread_cond_init(&gDvm.threadStartCond, NULL);    pthread_cond_init(&gDvm.vmExitCond, NULL);    dvmInitMutex(&gDvm._threadSuspendLock);    dvmInitMutex(&gDvm.threadSuspendCountLock);    pthread_cond_init(&gDvm.threadSuspendCountCond, NULL);    // 专用于监听Thread.sleep()。    //     /*     * Dedicated monitor for Thread.sleep().     * TODO: change this to an Object* so we don't have to expose this     * call, and we interact better with JDWP monitor calls.  Requires     * deferring the object creation to much later (e.g. final "main"     * thread prep) or until first use.     */    gDvm.threadSleepMon = dvmCreateMonitor(NULL);    /* 创建线程Id映射。     * 返回的BitVector结构可以保存的数据为 (kMaxThreadId + 31) >> 5 个位，     * 详见下面的dvmAllocBitVector函数说明。     *      * gDvm.threadIdMap中保存所有线程的线程Id。     * 如果线程Id为1，那么第1位置1，如果线程Id为而，则第2位置1，以此类推。     * 位索引从0开始。保留0作为无效的线程Id。     * 这里所说的线程Id，与pthread_self()的返回值不是一回事。     *      * gettid()是内核中的线程的ID。（linux使用进程模拟线程，gettid 函数返回实际的进程ID，这么展开话就长了……）     * pthread_self()获取的是POSIX thread ID。     * 所以我认为，gDvm.threadIdMap内的线程Id，代表的是Android虚拟机内的线程Id。     *      * #define kMaxThreadId        ((1 << 16) - 1)，即65535     * kMaxThreadId表示线程id的最大个数。     */    gDvm.threadIdMap = dvmAllocBitVector(kMaxThreadId, false);    // 动态分配和初始化一个Thread结构。    thread = allocThread(gDvm.mainThreadStackSize);    if (thread == NULL)        return false;    // 线程状态：正在运行。    /* switch mode for when we run initializers */    thread->status = THREAD_RUNNING;    // 完成一个Thread结构的初始化。    /*     * We need to assign the threadId early so we can lock/notify     * object monitors.  We'll set the "threadObj" field later.     */    prepareThread(thread);    gDvm.threadList = thread;#ifdef COUNT_PRECISE_METHODS    gDvm.preciseMethods = dvmPointerSetAlloc(200);#endif    return true;}

dvmAllocBitVector函数：

/* * Allocate a bit vector with enough space to hold at least the specified * number of bits. *  * expandable:当BitVector中的位使用完以后，是否扩展。true:扩展。false:不扩展。 */BitVector* dvmAllocBitVector(unsigned int startBits, bool expandable){    BitVector* bv;    unsigned int count;    assert(sizeof(bv->storage[0]) == 4);        /* assuming 32-bit units */    bv = (BitVector*) malloc(sizeof(BitVector));    // count代表数组元素个数。    count = (startBits + 31) >> 5;    bv->storageSize = count;    bv->expandable = expandable;    bv->storage = (u4*) calloc(count, sizeof(u4));    return bv;}

allocThread函数动态分配和初始化一个Thread，这个函数在dalvik/vm/Thread.cpp文件中：

/* * 分配和初始化一个线程结构。 * Alloc and initialize a Thread struct. * * 不要创建任何对象，...... * Does not create any objects, just stuff on the system (malloc) heap. */static Thread* allocThread(int interpStackSize){    Thread* thread;    u1* stackBottom;    // 从堆中分配内存给Thread结构。    thread = (Thread*) calloc(1, sizeof(Thread));    if (thread == NULL)        return NULL;    /* Check sizes and alignment */    assert((((uintptr_t)&thread->interpBreak.all) & 0x7) == 0);    assert(sizeof(thread->interpBreak) == sizeof(thread->interpBreak.all));    ......    return thread;}

prepareThread函数完成一个Thread结构的初始化，这个函数在dalvik/vm/Thread.cpp文件中：

/* * 完成一个Thread结构的初始化。 * Finish initialization of a Thread struct. * * 必须同时在新的线程中执行调用，但是在线程被添加到线程列表之前。 * This must be called while executing in the new thread, but before the * thread is added to the thread list. * * 注意：threadListLock必须由调用者维护（需要assignThreadId()）。 * NOTE: The threadListLock must be held by the caller (needed for * assignThreadId()). */static bool prepareThread(Thread* thread){    assignThreadId(thread); // 分配一个线程Id。其实就是为thread->threadId赋值。    thread->handle = pthread_self();    thread->systemTid = dvmGetSysThreadId();    //ALOGI("SYSTEM TID IS %d (pid is %d)", (int) thread->systemTid,    //    (int) getpid());    // 将thread设置到线程局部存储。    // 如果我们通过 dvmAttachCurrentThread 调用，self值已经正确的成为"thread"。    /*     * If we were called by dvmAttachCurrentThread, the self value is     * already correctly established as "thread".     */    setThreadSelf(thread);    ALOGV("threadid=%d: interp stack at %p",        thread->threadId, thread->interpStackStart - thread->interpStackSize);    ......}

assignThreadId函数为Thread结构分配了一个线程Id，这个线程Id与gettid和pthread_self函数返回的线程Id不同，我认为这个线程Id代表这个线程在dalvik虚拟机内的线程Id，这个函数在dalvik/vm/Thread.cpp文件中：

/* * 分配一个线程ID。这需要... * Assign the threadId.  This needs to be a small integer so that our * "thin" locks fit in a small number of bits. * * 我们保留零用作无效的ID。 * We reserve zero for use as an invalid ID. * * This must be called with threadListLock held. */static void assignThreadId(Thread* thread){    // 在threadIdMap中为线程Id分配一位。返回的(num+1)代表线程Id。    /*     * Find a small unique integer.  threadIdMap is a vector of     * kMaxThreadId bits;  dvmAllocBit() returns the index of a     * bit, meaning that it will always be < kMaxThreadId.     */    int num = dvmAllocBit(gDvm.threadIdMap);    if (num < 0) {        ALOGE("Ran out of thread IDs");        dvmAbort();     // TODO: make this a non-fatal error result    }    thread->threadId = num + 1;    assert(thread->threadId != 0);}

dvmAllocBit函数是实际分配线程Id的函数，这个函数在dalvik/vm/BitVector.cpp文件中：

/* * 分配bitmap中第一个可用的位。 * "Allocate" the first-available bit in the bitmap. * * 这是不同步的。调用者被期望持有某种锁，以防止多个线程在dvmAllocBit/ dvmFreeBit同时执行。 * This is not synchronized.  The caller is expected to hold some sort of * lock that prevents multiple threads from executing simultaneously in * dvmAllocBit/dvmFreeBit. */int dvmAllocBit(BitVector* pBits){    unsigned int word, bit;retry:    for (word = 0; word < pBits->storageSize; word++) {        // 0xffffffff代表32位均置位为1。        if (pBits->storage[word] != 0xffffffff) {            /*             * 在word中有一个位还未分配。返回找到的第一个未分配的位。             * There are unallocated bits in this word.  Return the first.             */            bit = ffs(~(pBits->storage[word])) -1;            assert(bit < 32);            pBits->storage[word] |= 1 << bit;   // 对未分配的位置1。            // (word << 5) => word * 2^5 => word * 32。            return (word << 5) | bit;   // 返回第几位被置位。        }    }    // 如果位都用完了，那么进行下面的判断来决定是否扩展。    /*     * Ran out of space, allocate more if we're allowed to.     */    if (!pBits->expandable)        return -1;    pBits->storage = (u4*)realloc(pBits->storage,                    (pBits->storageSize + kBitVectorGrowth) * sizeof(u4));    memset(&pBits->storage[pBits->storageSize], 0x00,        kBitVectorGrowth * sizeof(u4));    pBits->storageSize += kBitVectorGrowth;    goto retry;}

上面函数中参数pBits其实是gDvm.threadIdMap，关于gDvm.threadIdMap可以参考上面dvmThreadStartup函数中相关的介绍。