安卓高手之路之ClassLoader(二)

因为ClassLoader一定与虚拟机的启动有关系，那么必须从Zygote的启动开始看代码。下面就分析一下这些代码，行数不多:

int main(int argc, const char* const argv[])
{
    // These are global variables in ProcessState.cpp
    //ProcessState.cpp中可能要用到一些main函数。
    mArgC = argc;
    mArgV = argv;

    mArgLen = 0;
    for (int i=0; i<argc; i++) {
        mArgLen += strlen(argv[i]) + 1;
    }
    mArgLen--;

    AppRuntime runtime;
    const char* argv0 = argv[0];

    // Process command line arguments
    // ignore argv[0]
    argc--;
    argv++;

    // Everything up to '--' or first non '-' arg goes to the vm

    int i = runtime.addVmArguments(argc, argv);

    // Parse runtime arguments.  Stop at first unrecognized option.
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    const char* parentDir = NULL;
    const char* niceName = NULL;
    const char* className = NULL;
    while (i < argc) {
        const char* arg = argv[i++];
        if (!parentDir) {
            parentDir = arg;
        } else if (strcmp(arg, "--zygote") == 0) {
            zygote = true;
            niceName = "zygote";
        } else if (strcmp(arg, "--start-system-server") == 0) {
            startSystemServer = true;
        } else if (strcmp(arg, "--application") == 0) {
            application = true;
        } else if (strncmp(arg, "--nice-name=", 12) == 0) {
            niceName = arg + 12;
        } else {
            className = arg;
            break;
        }
    }

    if (niceName && *niceName) {
        setArgv0(argv0, niceName);
        set_process_name(niceName);
    }

    runtime.mParentDir = parentDir;

    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit",
                startSystemServer ? "start-system-server" : "");
    } else if (className) {
        // Remainder of args get passed to startup class main()
        runtime.mClassName = className;
        runtime.mArgC = argc - i;
        runtime.mArgV = argv + i;
        runtime.start("com.android.internal.os.RuntimeInit",
                application ? "application" : "tool");
    } else {
        fprintf(stderr, "Error: no class name or --zygote supplied.
");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
        return 10;
    }
}

 分析完之后发现如下参数规律：

    1. argv[0]：用这个修改了进程名称。

    2. 虚拟机参数：前面的选项参数都是以“-”打头。被放入了runtime。这些参数被称为是虚拟机参数。

    3.“--”打头的参数是zygote参数。有如下几种，排列顺序如下：

          -runtimearg[0]

         -runtimearg[1]

         。。。。

          parentDir //这个也是runtime使用的，也就是VM使用的。

          className//这个也是runtime使用的，也就是VM使用的。

          --zygote

          --start-system-server

          --application

          --nice-name=

然后，如果是zygote，那么进入下面这句话

    

runtime.start("com.android.internal.os.ZygoteInit",
             startSystemServer ? "start-system-server" : "");

如果有类名，那么进入下面这句话：

   

runtime.mClassName = className;
   runtime.mArgC = argc - i; //className，包括className以后的参数个数。
 runtime.mArgV = argv + i; //截止到className的参数个数
runtime.start("com.android.internal.os.RuntimeInit",
              application ? "application" : "tool");

第一部分：那么开机第一次启动的就一定是，

  

runtime.start("com.android.internal.os.ZygoteInit", startSystemServer ? "start-system-server" : "");

 其中startSystemServer 由init.rc指定，在目录android40systemcore ootdir中的init.rc.

   service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart media
    onrestart restart netd

第二部分：从ActivityManagerService可以看出，--application并没有指定，这句话也就相当于：

runtime.start("com.android.internal.os.RuntimeInit", "tool");

 现在代码分成了两部分。

那么先分析第一部分。

那么zygote启动到底配置了那些参数呢，我们就看一看：

   service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

根据上面说的参数序列图，可以看出。

   runtime.mParentDir  为/system/bin

   runtime的一个arg为-Xzygote

那么这个这个start函数就变成：

runtime.start("com.android.internal.os.ZygoteInit",  "start-system-server");

代码进入到base/core/jni目录的AndroidRuntime.cpp里面。这个函数还不算长，就直接贴出来看一下，注意注释，由此可以看出这个就是启动虚拟机的代码所在啊。那么既然Zygote进程也是这么启动的，那么我们就有理由断定Zygote也是个Dalvik虚拟机！事情是不是这样呢？那么就带着这个疑问去分析一下：

/*
 * Start the Android runtime.  This involves starting the virtual machine
 * and calling the "static void main(String[] args)" method in the class
 * named by "className".
 *
 * Passes the main function two arguments, the class name and the specified
 * options string.
 */
void AndroidRuntime::start(const char* className, const char* options)
{
    LOGD("
>>>>>> AndroidRuntime START %s <<<<<<
",
            className != NULL ? className : "(unknown)");

    blockSigpipe();

    /*
     * 'startSystemServer == true' means runtime is obsolete and not run from
     * init.rc anymore, so we print out the boot start event here.
     */
    if (strcmp(options, "start-system-server") == 0) {
        /* track our progress through the boot sequence */
        const int LOG_BOOT_PROGRESS_START = 3000;
        LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,
                       ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
    }

    const char* rootDir = getenv("ANDROID_ROOT");
    if (rootDir == NULL) {
        rootDir = "/system";
        if (!hasDir("/system")) {
            LOG_FATAL("No root directory specified, and /android does not exist.");
            return;
        }
        setenv("ANDROID_ROOT", rootDir, 1);
    }

    //const char* kernelHack = getenv("LD_ASSUME_KERNEL");
    //LOGD("Found LD_ASSUME_KERNEL='%s'
", kernelHack);

    /* start the virtual machine */
    JNIEnv* env;
    if (startVm(&mJavaVM, &env) != 0) {
        return;
    }
    onVmCreated(env);

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {
        LOGE("Unable to register all android natives
");
        return;
    }

    /*
     * We want to call main() with a String array with arguments in it.
     * At present we have two arguments, the class name and an option string.
     * Create an array to hold them.
     */
    jclass stringClass;
    jobjectArray strArray;
    jstring classNameStr;
    jstring optionsStr;

    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    strArray = env->NewObjectArray(2, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);
    optionsStr = env->NewStringUTF(options);
    env->SetObjectArrayElement(strArray, 1, optionsStr);

    /*
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
    char* slashClassName = toSlashClassName(className);
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        LOGE("JavaVM unable to locate class '%s'
", slashClassName);
        /* keep going */
    } else {
        jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
            "([Ljava/lang/String;)V");
        if (startMeth == NULL) {
            LOGE("JavaVM unable to find main() in '%s'
", className);
            /* keep going */
        } else {
            env->CallStaticVoidMethod(startClass, startMeth, strArray);

#if 0
            if (env->ExceptionCheck())
                threadExitUncaughtException(env);
#endif
        }
    }
    free(slashClassName);

    LOGD("Shutting down VM
");
    if (mJavaVM->DetachCurrentThread() != JNI_OK)
        LOGW("Warning: unable to detach main thread
");
    if (mJavaVM->DestroyJavaVM() != 0)
        LOGW("Warning: VM did not shut down cleanly
");
}

linux的POSIX (Portable Operating System Interface of Unix)我不懂。但是从直观上看，可能是一种禁止打断进程的方法：

LOGD("
>>>>>> AndroidRuntime START %s <<<<<<
",
        className != NULL ? className : "(unknown)");

blockSigpipe();

 下面这句话毫无意义，就是打印log

  

if (strcmp(options, "start-system-server") == 0) {
        /* track our progress through the boot sequence */
        const int LOG_BOOT_PROGRESS_START = 3000;
        LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,
                       ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
    }

 下面这句话定义androidroot的目录

  

const char* rootDir = getenv("ANDROID_ROOT");
 if (rootDir == NULL) {
     rootDir = "/system";
     if (!hasDir("/system")) {
         LOG_FATAL("No root directory specified, and /android does not exist.");
         return;
     }
     setenv("ANDROID_ROOT", rootDir, 1);
 }

 对照init.rc可以知道，就是/system

# setup the global environment
    export PATH /sbin:/vendor/bin:/system/sbin:/system/bin:/system/xbin
    export LD_LIBRARY_PATH /vendor/lib:/system/lib
    export ANDROID_BOOTLOGO 1
    export ANDROID_ROOT /system
    export ANDROID_ASSETS /system/app
    export ANDROID_DATA /data
    export ASEC_MOUNTPOINT /mnt/asec
    export LOOP_MOUNTPOINT /mnt/obb
    export BOOTCLASSPATH /system/framework/core.jar:/system/framework/core-junit.jar:/system/framework/bouncycastle.jar:/system/framework/ext.jar:/system/framework/framework.jar:/system/framework/android.policy.jar:/system/framework/services.jar:/system/framework/apache-xml.jar:/system/framework/filterfw.jar

主要是下面这两句话

/* start the virtual machine */
  JNIEnv* env;
  if (startVm(&mJavaVM, &env) != 0) {
      return;
  }
  onVmCreated(env);

  /*
   * Register android functions.
   */
  if (startReg(env) < 0) {
      LOGE("Unable to register all android natives
");
      return;
  }

 一个启动虚拟机，一个启动注册安卓本地方法。虚拟机的启动流程，最终调用的是

JNI_CreateJavaVM 在framework/base/core/jni/AndroidRuntime.cpp下。JNI_CreateJavaVM 调用的是：

 然后调用dalvik/vm/Jni.cpp的JNI_CreateJavaVM 方法：

jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
    const JavaVMInitArgs* args = (JavaVMInitArgs*) vm_args;
    if (args->version < JNI_VERSION_1_2) {
        return JNI_EVERSION;
    }

    // TODO: don't allow creation of multiple VMs -- one per customer for now

    /* zero globals; not strictly necessary the first time a VM is started */
    memset(&gDvm, 0, sizeof(gDvm));

    /*
     * Set up structures for JNIEnv and VM.
     */
    JavaVMExt* pVM = (JavaVMExt*) malloc(sizeof(JavaVMExt));
    memset(pVM, 0, sizeof(JavaVMExt));
    pVM->funcTable = &gInvokeInterface;
    pVM->envList = NULL;
    dvmInitMutex(&pVM->envListLock);

    UniquePtr<const char*[]> argv(new const char*[args->nOptions]);
    memset(argv.get(), 0, sizeof(char*) * (args->nOptions));

    /*
     * Convert JNI args to argv.
     *
     * We have to pull out vfprintf/exit/abort, because they use the
     * "extraInfo" field to pass function pointer "hooks" in.  We also
     * look for the -Xcheck:jni stuff here.
     */
    int argc = 0;
    for (int i = 0; i < args->nOptions; i++) {
        const char* optStr = args->options[i].optionString;
        if (optStr == NULL) {
            dvmFprintf(stderr, "ERROR: CreateJavaVM failed: argument %d was NULL
", i);
            return JNI_ERR;
        } else if (strcmp(optStr, "vfprintf") == 0) {
            gDvm.vfprintfHook = (int (*)(FILE *, const char*, va_list))args->options[i].extraInfo;
        } else if (strcmp(optStr, "exit") == 0) {
            gDvm.exitHook = (void (*)(int)) args->options[i].extraInfo;
        } else if (strcmp(optStr, "abort") == 0) {
            gDvm.abortHook = (void (*)(void))args->options[i].extraInfo;
        } else if (strcmp(optStr, "sensitiveThread") == 0) {
            gDvm.isSensitiveThreadHook = (bool (*)(void))args->options[i].extraInfo;
        } else if (strcmp(optStr, "-Xcheck:jni") == 0) {
            gDvmJni.useCheckJni = true;
        } else if (strncmp(optStr, "-Xjniopts:", 10) == 0) {
            char* jniOpts = strdup(optStr + 10);
            size_t jniOptCount = 1;
            for (char* p = jniOpts; *p != 0; ++p) {
                if (*p == ',') {
                    ++jniOptCount;
                    *p = 0;
                }
            }
            char* jniOpt = jniOpts;
            for (size_t i = 0; i < jniOptCount; ++i) {
                if (strcmp(jniOpt, "warnonly") == 0) {
                    gDvmJni.warnOnly = true;
                } else if (strcmp(jniOpt, "forcecopy") == 0) {
                    gDvmJni.forceCopy = true;
                } else if (strcmp(jniOpt, "logThirdPartyJni") == 0) {
                    gDvmJni.logThirdPartyJni = true;
                } else {
                    dvmFprintf(stderr, "ERROR: CreateJavaVM failed: unknown -Xjniopts option '%s'
",
                            jniOpt);
                    return JNI_ERR;
                }
                jniOpt += strlen(jniOpt) + 1;
            }
            free(jniOpts);
        } else {
            /* regular option */
            argv[argc++] = optStr;
        }
    }

    if (gDvmJni.useCheckJni) {
        dvmUseCheckedJniVm(pVM);
    }

    if (gDvmJni.jniVm != NULL) {
        dvmFprintf(stderr, "ERROR: Dalvik only supports one VM per process
");
        return JNI_ERR;
    }
    gDvmJni.jniVm = (JavaVM*) pVM;

    /*
     * Create a JNIEnv for the main thread.  We need to have something set up
     * here because some of the class initialization we do when starting
     * up the VM will call into native code.
     */
    JNIEnvExt* pEnv = (JNIEnvExt*) dvmCreateJNIEnv(NULL);

    /* Initialize VM. */
    gDvm.initializing = true;
    std::string status =
            dvmStartup(argc, argv.get(), args->ignoreUnrecognized, (JNIEnv*)pEnv);
    gDvm.initializing = false;

    if (!status.empty()) {
        free(pEnv);
        free(pVM);
        LOGW("CreateJavaVM failed: %s", status.c_str());
        return JNI_ERR;
    }

    /*
     * Success!  Return stuff to caller.
     */
    dvmChangeStatus(NULL, THREAD_NATIVE);
    *p_env = (JNIEnv*) pEnv;
    *p_vm = (JavaVM*) pVM;
    LOGV("CreateJavaVM succeeded");
    return JNI_OK;
}

  然后调用Jni.cpp中的

/*
 * Create a new JNIEnv struct and add it to the VM's list.
 *
 * "self" will be NULL for the main thread, since the VM hasn't started
 * yet; the value will be filled in later.
 */
JNIEnv* dvmCreateJNIEnv(Thread* self) {
    JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;

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

    assert(vm != NULL);

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

   好吧，这些全是些乱七八糟的东西。真正启动的是这句话，Jni.cpp中：

  

std::string status =
           dvmStartup(argc, argv.get(), args->ignoreUnrecognized, (JNIEnv*)pEnv);

 在Dalvik/vm/Init.cpp中

*
 * VM initialization.  Pass in any options provided on the command line.
 * Do not pass in the class name or the options for the class.
 *
 * Returns 0 on success.
 */
std::string dvmStartup(int argc, const char* const argv[],
        bool ignoreUnrecognized, JNIEnv* pEnv)
{
    ScopedShutdown scopedShutdown;

    assert(gDvm.initializing);

    LOGV("VM init args (%d):", argc);
    for (int i = 0; i < argc; i++) {
        LOGV("  %d: '%s'", i, argv[i]);
    }
    setCommandLineDefaults();

    /*
     * Process the option flags (if any).
     */
    int cc = processOptions(argc, argv, ignoreUnrecognized);
    if (cc != 0) {
        if (cc < 0) {
            dvmFprintf(stderr, "
");
            usage("dalvikvm");
        }
        return "syntax error";
    }

#if WITH_EXTRA_GC_CHECKS > 1
    /* only "portable" interp has the extra goodies */
    if (gDvm.executionMode != kExecutionModeInterpPortable) {
        LOGI("Switching to 'portable' interpreter for GC checks");
        gDvm.executionMode = kExecutionModeInterpPortable;
    }
#endif

    /* Configure group scheduling capabilities */
    if (!access("/dev/cpuctl/tasks", F_OK)) {
        LOGV("Using kernel group scheduling");
        gDvm.kernelGroupScheduling = 1;
    } else {
        LOGV("Using kernel scheduler policies");
    }

    /* configure signal handling */
    if (!gDvm.reduceSignals)
        blockSignals();

    /* 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));
    }

    /* mterp setup */
    LOGV("Using executionMode %d", gDvm.executionMode);
    dvmCheckAsmConstants();

    /*
     * 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";
    }

    /*
     * At this point, the system is guaranteed to be sufficiently
     * initialized that we can look up classes and class members. This
     * call populates the gDvm instance with all the class and member
     * references that the VM wants to use directly.
     */
    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";
    }

    /*
     * Create a table of methods for which we will substitute an "inline"
     * version for performance.
     */
    if (!dvmCreateInlineSubsTable()) {
        return "dvmCreateInlineSubsTable failed";
    }

    /*
     * Miscellaneous class library validation.
     */
    if (!dvmValidateBoxClasses()) {
        return "dvmValidateBoxClasses failed";
    }

    /*
     * Do the last bits of Thread struct initialization we need to allow
     * JNI calls to work.
     */
    if (!dvmPrepMainForJni(pEnv)) {
        return "dvmPrepMainForJni failed";
    }

    /*
     * Explicitly initialize java.lang.Class.  This doesn't happen
     * automatically because it's allocated specially (it's an instance
     * of itself).  Must happen before registration of system natives,
     * which make some calls that throw assertions if the classes they
     * operate on aren't initialized.
     */
    if (!dvmInitClass(gDvm.classJavaLangClass)) {
        return "couldn't initialized java.lang.Class";
    }

    /*
     * Register the system native methods, which are registered through JNI.
     */
    if (!registerSystemNatives(pEnv)) {
        return "couldn't register system natives";
    }

    /*
     * Do some "late" initialization for the memory allocator.  This may
     * allocate storage and initialize classes.
     */
    if (!dvmCreateStockExceptions()) {
        return "dvmCreateStockExceptions failed";
    }

    /*
     * At this point, the VM is in a pretty good state.  Finish prep on
     * the main thread (specifically, create a java.lang.Thread object to go
     * along with our Thread struct).  Note we will probably be executing
     * some interpreted class initializer code in here.
     */
    if (!dvmPrepMainThread()) {
        return "dvmPrepMainThread failed";
    }

    /*
     * Make sure we haven't accumulated any tracked references.  The main
     * thread should be starting with a clean slate.
     */
    if (dvmReferenceTableEntries(&dvmThreadSelf()->internalLocalRefTable) != 0)
    {
        LOGW("Warning: tracked references remain post-initialization");
        dvmDumpReferenceTable(&dvmThreadSelf()->internalLocalRefTable, "MAIN");
    }

    /* general debugging setup */
    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";
        }
    }


#ifndef NDEBUG
    if (!dvmTestHash())
        LOGE("dvmTestHash FAILED");
    if (false /*noisy!*/ && !dvmTestIndirectRefTable())
        LOGE("dvmTestIndirectRefTable FAILED");
#endif

    if (dvmCheckException(dvmThreadSelf())) {
        dvmLogExceptionStackTrace();
        return "Exception pending at end of VM initialization";
    }

    scopedShutdown.disarm();
    return "";
}

 代码真长。寻找其中最具价值的部分

       

插入代码：

          

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";
 }

 经分析，这些都没有建立gc线程，gc线程的建立是在如下方法：

dvmInitAfterZygote

  由于跟得太深，东西很多，就不一一列举。仅仅跟一下dvmClassStartup，最终调用到了dalvik/vm/oo/Class.cpp中的方法：

/*
 * Initialize the bootstrap class loader.
 *
 * Call this after the bootclasspath string has been finalized.
 */
bool dvmClassStartup()
{
    /* make this a requirement -- don't currently support dirs in path */
    if (strcmp(gDvm.bootClassPathStr, ".") == 0) {
        LOGE("ERROR: must specify non-'.' bootclasspath");
        return false;
    }

    gDvm.loadedClasses =
        dvmHashTableCreate(256, (HashFreeFunc) dvmFreeClassInnards);

    gDvm.pBootLoaderAlloc = dvmLinearAllocCreate(NULL);
    if (gDvm.pBootLoaderAlloc == NULL)
        return false;

    if (false) {
        linearAllocTests();
        exit(0);
    }

    /*
     * Class serial number.  We start with a high value to make it distinct
     * in binary dumps (e.g. hprof).
     */
    gDvm.classSerialNumber = INITIAL_CLASS_SERIAL_NUMBER;

    /*
     * Set up the table we'll use for tracking initiating loaders for
     * early classes.
     * If it's NULL, we just fall back to the InitiatingLoaderList in the
     * ClassObject, so it's not fatal to fail this allocation.
     */
    gDvm.initiatingLoaderList = (InitiatingLoaderList*)
        calloc(ZYGOTE_CLASS_CUTOFF, sizeof(InitiatingLoaderList));

    /*
     * Create the initial classes. These are the first objects constructed
     * within the nascent VM.
     */
    if (!createInitialClasses()) {
        return false;
    }

    /*
     * Process the bootstrap class path.  This means opening the specified
     * DEX or Jar files and possibly running them through the optimizer.
     */
    assert(gDvm.bootClassPath == NULL);
    processClassPath(gDvm.bootClassPathStr, true);

    if (gDvm.bootClassPath == NULL)
        return false;

    return true;
}

 根据注释，Initialize the bootstrap class loader.
这个函数告诉我们，他建立了boottrap classloader。

createInitialClasses加载了9大基本类型。而后的processClassPath则建立了基本的classloader。分析过后，比较失望。可能是为后续的boottrapclassloader做一些前期准备工作。

startVM就到这里。

好吧。现在又回到了最初的App_main.cpp中。进入了com.android.internal.os.ZygoteInit.java的main

public static void main(String argv[]) {
    try {
        // Start profiling the zygote initialization.
        SamplingProfilerIntegration.start();

        registerZygoteSocket();
        EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
            SystemClock.uptimeMillis());
        preload();
        EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
            SystemClock.uptimeMillis());

        // Finish profiling the zygote initialization.
        SamplingProfilerIntegration.writeZygoteSnapshot();

        // Do an initial gc to clean up after startup
        gc();

        // If requested, start system server directly from Zygote
        if (argv.length != 2) {
            throw new RuntimeException(argv[0] + USAGE_STRING);
        }

        if (argv[1].equals("start-system-server")) {
            startSystemServer();
        } else if (!argv[1].equals("")) {
            throw new RuntimeException(argv[0] + USAGE_STRING);
        }

        Log.i(TAG, "Accepting command socket connections");

        if (ZYGOTE_FORK_MODE) {
            runForkMode();
        } else {
            runSelectLoopMode();
        }

        closeServerSocket();
    } catch (MethodAndArgsCaller caller) {
        caller.run();
    } catch (RuntimeException ex) {
        Log.e(TAG, "Zygote died with exception", ex);
        closeServerSocket();
        throw ex;
    }
}

 回过头来继续看一下ZygoteInit.java这个类是如何初始化的，看如下代码：

/*
 * Create a new JNIEnv struct and add it to the VM's list.
 *
 * "self" will be NULL for the main thread, since the VM hasn't started
 * yet; the value will be filled in later.
 */
JNIEnv* dvmCreateJNIEnv(Thread* self) {
    JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;

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

    assert(vm != NULL);

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

 最重要的是gNativeInterface 我们看定义，太长了，我们找到其中的FindClass。

static jclass FindClass(JNIEnv* env, const char* name) {
    ScopedJniThreadState ts(env);

    const Method* thisMethod = dvmGetCurrentJNIMethod();
    assert(thisMethod != NULL);

    Object* loader;
    Object* trackedLoader = NULL;
    if (ts.self()->classLoaderOverride != NULL) {
        /* hack for JNI_OnLoad */
        assert(strcmp(thisMethod->name, "nativeLoad") == 0);
        loader = ts.self()->classLoaderOverride;
    } else if (thisMethod == gDvm.methDalvikSystemNativeStart_main ||
               thisMethod == gDvm.methDalvikSystemNativeStart_run) {
        /* start point of invocation interface */
        if (!gDvm.initializing) {
            loader = trackedLoader = dvmGetSystemClassLoader();
        } else {
            loader = NULL;
        }
    } else {
        loader = thisMethod->clazz->classLoader;
    }

    char* descriptor = dvmNameToDescriptor(name);
    if (descriptor == NULL) {
        return NULL;
    }
    ClassObject* clazz = dvmFindClassNoInit(descriptor, loader);
    free(descriptor);

    jclass jclazz = (jclass) addLocalReference(ts.self(), (Object*) clazz);
    dvmReleaseTrackedAlloc(trackedLoader, ts.self());
    return jclazz;
}

在dalvik/vm/Init.cpp中的方法对gVM的bootpath进行了初始化：

static void setCommandLineDefaults()
{
    const char* envStr = getenv("CLASSPATH");
    if (envStr != NULL) {
        gDvm.classPathStr = strdup(envStr);
    } else {
        gDvm.classPathStr = strdup(".");
    }
    envStr = getenv("BOOTCLASSPATH");
    if (envStr != NULL) {
        gDvm.bootClassPathStr = strdup(envStr);
    } else {
        gDvm.bootClassPathStr = strdup(".");
    }

    gDvm.properties = new std::vector<std::string>();

    /* Defaults overridden by -Xms and -Xmx.
     * TODO: base these on a system or application-specific default
     */
    gDvm.heapStartingSize = 2 * 1024 * 1024;  // Spec says 16MB; too big for us.
    gDvm.heapMaximumSize = 16 * 1024 * 1024;  // Spec says 75% physical mem
    gDvm.heapGrowthLimit = 0;  // 0 means no growth limit
    gDvm.stackSize = kDefaultStackSize;

    gDvm.concurrentMarkSweep = true;

    /* gDvm.jdwpSuspend = true; */

现在明白了，在init.rc中指定的BOOTCLASSPATH赋值给了gDvm.bootClassPathStr 。

而下面这个地方，则对FindClass进行了初始化。还是在dalvik/vm/Init.cpp中

*/
f (!dvmFindRequiredClassesAndMembers()) {
   return "dvmFindRequiredClassesAndMembers failed";

initDirectMethodReferences 把方法gDvm.methDalvikSystemNativeStart_main与NativeStart进行了对应。

{ &gDvm.methDalvikSystemNativeStart_main, "Ldalvik/system/NativeStart;", "main", "([Ljava/lang/String;)V" },

主要分析FindClass方法：

 nativeLoad标示从System.loadlibrary加载。那么Zygote的dvmGetCurrentJNIMethod是哪个呢。我猜测，这个一定是没有的，也就是里面的成员变量全为空。哈哈。这意味着，最终调用到了

dvmFindClassNoInit方法中。

然后是ClassObject* dvmFindSystemClassNoInit(const char* descriptor)
{
    return findClassNoInit(descriptor, NULL, NULL);
}
然后是findClassNoInit

ZygoteInit这个类根本是没有ClassLoader的。而是直接从包里面查找得到的。

进入ZygoteInit后。就是java代码了。