1.虛擬機啟動過程

Android系統(tǒng)在啟動的時候，會創(chuàng)建一個Zygote進程，而Zygote進程會創(chuàng)建一個虛擬機實例，并加載java運行庫等等。當有新的進程要啟動時，系統(tǒng)會從Zygote中fork一個新的進程出來，并且將Zygote的虛擬機實例復制過去。

首先要從Zygote系統(tǒng)的啟動說起。Zygote進程源碼在frameworks\base\cmds\app_process\app_main.cpp里。zygote啟動腳本：

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

app_process后的內(nèi)容為命令行參數(shù)

下面看下這個app_main.cpp的main函數(shù):

int main(int argc, char* const argv[])
{
    ...
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    ...
    //從命令行獲取進程啟動參數(shù)
    ...
    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } else if (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
    } else {
        fprintf(stderr, "Error: no class name or --zygote supplied.\n");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
        return 10;
    }
}

這個函數(shù)主要做的事情就是，新建一個AppRuntime實例，然后從命令行參數(shù)中獲取進程啟動的參數(shù)，然后調(diào)用AppRuntime類的start。
AppRuntime類定義在App_main.cpp文件中，繼承自AndroidRuntime類。而AppRuntime沒有定義start方法，所以這個start方法屬于它的父類AndroidRuntime。
AndroidRuntime定義在frameworks\base\include\android_runtime\AndroidRuntime.cpp中。

/*
 * 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 Vector<String8>& options, bool zygote)
{
    ...
    
    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);
    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        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;
    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);
    for (size_t i = 0; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        assert(optionsStr != NULL);
        env->SetObjectArrayElement(strArray, i + 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) {
        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);
#if 0
            if (env->ExceptionCheck())
                threadExitUncaughtException(env);
#endif
        }
    }
    free(slashClassName);
    ALOGD("Shutting down VM\n");
    if (mJavaVM->DetachCurrentThread() != JNI_OK)
        ALOGW("Warning: unable to detach main thread\n");
    if (mJavaVM->DestroyJavaVM() != 0)
        ALOGW("Warning: VM did not shut down cleanly\n");
}

第一步

啟動虛擬機startVm，并把啟動的虛擬機實例保存在全局變量mJavaVM里

/*
 * Start the Dalvik Virtual Machine.
 *
 * Various arguments, most determined by system properties, are passed in.
 * The "mOptions" vector is updated.
 *
 * CAUTION: when adding options in here, be careful not to put the
 * char buffer inside a nested scope.  Adding the buffer to the
 * options using mOptions.add() does not copy the buffer, so if the
 * buffer goes out of scope the option may be overwritten.  It's best
 * to put the buffer at the top of the function so that it is more
 * unlikely that someone will surround it in a scope at a later time
 * and thus introduce a bug.
 *
 * Returns 0 on success.
 */
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
{
    JavaVMInitArgs initArgs;
    ...
    中間一堆虛擬機配置信息
    ...
    ...
    initArgs.version = JNI_VERSION_1_4;
    initArgs.options = mOptions.editArray();
    initArgs.nOptions = mOptions.size();
    initArgs.ignoreUnrecognized = JNI_FALSE;
    /*
     * Initialize the VM.
     *
     * The JavaVM* is essentially per-process, and the JNIEnv* is per-thread.
     * If this call succeeds, the VM is ready, and we can start issuing
     * JNI calls.
     */
    if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
        ALOGE("JNI_CreateJavaVM failed\n");
        return -1;
    }
    return 0;
}

startVm中調(diào)用了jni.h中的JNI_CreateJavaVM方法。

** JNI_CreateJavaVM　**

/*
 * Create a new VM instance.
 *
 * The current thread becomes the main VM thread.  We return immediately,
 * which effectively means the caller is executing in a native method.
 */
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;
    }
  
    memset(&gDvm, 0, sizeof(gDvm));
    /*
     * Set up structures for JNIEnv and VM.
     */
    JavaVMExt* pVM = (JavaVMExt*) calloc(1, 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\n", 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'\n",
                            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\n");
        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);
        ALOGW("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;
    ALOGV("CreateJavaVM succeeded");
    return JNI_OK;
}

第二步

調(diào)用startReg方法，注冊一些android核心類的jni方法

static const RegJNIRec gRegJNI[] = {
    ...
    REG_JNI(register_android_view_InputChannel),
    REG_JNI(register_android_view_InputEventReceiver),
    REG_JNI(register_android_view_InputEventSender),
    REG_JNI(register_android_view_InputQueue),
    REG_JNI(register_android_view_KeyEvent),
    REG_JNI(register_android_view_MotionEvent),
    REG_JNI(register_android_view_PointerIcon),
    REG_JNI(register_android_view_VelocityTracker),
    ...
};
/*
 * Register android native functions with the VM.
 */
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
    ATRACE_NAME("RegisterAndroidNatives");
    /*
     * This hook causes all future threads created in this process to be
     * attached to the JavaVM.  (This needs to go away in favor of JNI
     * Attach calls.)
     */
    androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);
    ALOGV("--- registering native functions ---\n");
    /*
     * Every "register" function calls one or more things that return
     * a local reference (e.g. FindClass).  Because we haven't really
     * started the VM yet, they're all getting stored in the base frame
     * and never released.  Use Push/Pop to manage the storage.
     */
    env->PushLocalFrame(200);
    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL);
        return -1;
    }
    env->PopLocalFrame(NULL);
    //createJavaThread("fubar", quickTest, (void*) "hello");
    return 0;
}

gRegJNI數(shù)組里保存了一堆注冊android核心類jni方法的函數(shù)指針，在startReg方法中調(diào)用了register_jni_procs方法，來注冊jni方法。

static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env)
{
    for (size_t i = 0; i < count; i++) {
        if (array[i].mProc(env) < 0) {
#ifndef NDEBUG
            ALOGD("----------!!! %s failed to load\n", array[i].mName);
#endif
            return -1;
        }
    }
    return 0;
}

register_jni_procs方法，把gRegJNI數(shù)組遍歷了一遍，然后把函數(shù)指針一次執(zhí)行了一遍。

第三步

找到main方法，然后在虛擬機實例中運行起來，如果main方法不返回，那么虛擬機就不會退出。

 if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);

從start方法的這段代碼可以知道，當初次啟動zygote進程時，入口類為ZygoteInit

public static void main(String argv[]) {
        // Mark zygote start. This ensures that thread creation will throw
        // an error.
        ZygoteHooks.startZygoteNoThreadCreation();
        try {
            ...
            registerZygoteSocket(socketName);
            ...
            runSelectLoop(abiList);
            closeServerSocket();
        } catch (MethodAndArgsCaller caller) {
            caller.run();
        } catch (RuntimeException ex) {
            Log.e(TAG, "Zygote died with exception", ex);
            closeServerSocket();
            throw ex;
        }
    }

main方法中，做了一些初始化的工作，啟動了系統(tǒng)服務，然后注冊了一個本地socket，用于其他的系統(tǒng)服務和zygote進程通信，在runSelectLoop方法中，開啟socket循環(huán)。只要這個循環(huán)不退出，那么這個main方法就不會結(jié)束。

第四步

最后一步就是關閉虛擬機了。當?shù)谌降膍ain函數(shù)結(jié)束時，虛擬機將會關閉，虛擬機的實例被銷毀。