《深入了解Android 卷1》讀書筆記 （二）—— Android zygote淺析

1. Zygote —— Java世界的建立者

Zygote 原意為“受精卵”，是Android裡面Java世界的起源。 它建立了第一個Java虛拟機，也通過Fork産生了

framework的核心system_server程序。個人覺得《深入了解Android 卷1》一書裡對此部分的介紹比較詳盡，

我們通過看書，結合Android的源碼就可窺知一二。

Zygote建立Java世界的步驟為：

（1）建立AppRuntime對象，并調用它的start。（frameworks\base\cmds\app_process\app_main.c裡的Main函數）

參數是在Init那邊啟動zygote時傳過來的。

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);
           

解析部分參數，并将剩餘未解析的儲存，設定Process name。

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);
    }
           

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

（2）調動startVm建立Java虛拟機，然後調用startReg來注冊JNI函數。

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

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

對于startReg，這裡稍作說明：

(register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0)
           

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;
}
           

static const RegJNIRec gRegJNI[] = {
    REG_JNI(register_android_debug_JNITest),
    REG_JNI(register_com_android_internal_os_RuntimeInit),
    REG_JNI(register_android_os_SystemClock),
    REG_JNI(register_android_util_EventLog),
    REG_JNI(register_android_util_Log),
    REG_JNI(register_android_util_FloatMath),
    REG_JNI(register_android_text_format_Time),
    REG_JNI(register_android_content_AssetManager),
    REG_JNI(register_android_content_StringBlock),
    REG_JNI(register_android_content_XmlBlock),
...
...
}
           

就是執行數組中的那些JNI注冊函數，如register_android_hardware_Camera （這個後面介紹Android Camera流程的時候

會再提到）

（3）通過JNI調用com.android.internal.os.ZygoteInit類的main函數，從此進入了Java世界。

/*
     * 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
        }
    }
           

（4）調用registerZygoteSocket。通過這個函數，他可以響應子孫後代的請求。調用

preloadClasses和preloadResources預加載類和資源。

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();

            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false);

            // 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");

            runSelectLoop();

            closeServerSocket();
           

2. SystemServer

此處還是将書中的那個圖抄過來吧，讀者有時間的話，還是看下代碼。