源码版本：Android-4.4.4_r2
提示：大部分分析直接注释在代码内。
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_METHODSgDvm.preciseMethods = dvmPointerSetAlloc(200);#endifreturn 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。
蚁穴虽小，溃之千里。
