Android Zygote进程启动（一）

启动流程

Zygote进程本身是一个native的程序，在init进程启动后，会执行init.rc中的内容:

   service zygote /system/bin/app_process64 -Xzygote 
                              /system/bin --  zygote 
                              --start-system-server --socket-name=zygote
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart audioserver
    onrestart restart cameraserver
    onrestart restart media
    onrestart restart netd
    writepid /dev/cpuset/foreground/tasks

1.第一行启动native service 名字为 “zygote”
2.关键字class 标示，service的类型是main
3.关键字socket，标示zygote进程会创建一个名称为“zygote”的socket
4.关键字onrestart 表示重启需要执行的命令

Zygote进程对应的主文件是app_main.cpp，当它被启动后会进入app_main.cpp 的main方法：

int main(int argc, char* const argv[]){
       ......

      AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));

       ......

      while (i < argc) {
            const char* arg = argv[i++];
            if (strcmp(arg, "--zygote") == 0) {
                 zygote = true;
                 niceName = ZYGOTE_NICE_NAME;
                 // 在zygote进程中重新命名进程名
            } else if (strcmp(arg, "--start-system-server") == 0) {
                  startSystemServer = true;
                  // 代表启动SystemServer
            } else if (strcmp(arg, "--application") == 0) {
                  application = true;
            } else if (strncmp(arg, "--nice-name=", 12) == 0) {
                  niceName.setTo(arg + 12);
            } else if (strncmp(arg, "--", 2) != 0) {
                  className.setTo(arg);
                  break;
            } else {
                  --i;
                  break;
        }
    }

    ......
    
    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;
    }
}

代码最后会调用runtime的start() 函数，从最初的代码中可以看到runtime是一个AppRuntime对象.

    class AppRuntime : public AndroidRuntime
    {
        ......
    }

AppRuntime是一个继承于AndroidRuntime，runtime的start()函数将调用到AndroidRuntime::start()函数：

void AndroidRuntime::start(const char* className
            , const Vector& 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;
    }
    
    .......

    /*
     * 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
        }
    }
    
}

AndroidRuntime 方法中主要做了三件事：

1. 调用startVM()启动虚拟机，并且对JNIEnv的指针进行赋值，为注册jni代码做准备；

int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote){
      ......
          if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
        ALOGE("JNI_CreateJavaVM failed\n");
        return -1;
    }
    .......
}

2. 调用startReq()注册JNI方法

/*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;
}

3. 调用ZygoteInit类的main方法，上面代码中className 是由上一步传递过来的"com.android.internal.os.ZygoteInit" toSlashClassName方法是将.置换成/，将className内容替换成了"com/android/internal/os/ZygoteInit"
   最终CallStaticVoidMethod调用ZygoteInit main 方法进入java的世界，ZygoteInit中的执行流程，将在