概述

在Android系统中，所有的应用程序进程，以及用来运行系统关键服务的System进程都是由zygote进程负责创建的。因此，我们将它称为进程孵化器。zygote进程是通过复制自身的方式来创建System进程和应用程序进程的。由于zygote进程在启动时会在内部创建一个虚拟机实例，因此，通过复制zygote进程而得到的System进程和应用程序进程可以快速地在内部获得一个虚拟机实例拷贝。
zygote进程在启动完成之后，会马上将System进程启动起来，以便它可以将系统的关键服务启动起来。下面我们将介绍zygote进程的启动脚本，然后分析它和System进程的启动过程。

zygote分析

zygote进程的启动脚本如下：

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

在我之前的一篇博客中已经分析了init进程是如何启动service服务了，需要了解的同学可以参考这篇文章：Android init进程——解析配置文件

通过zygote服务的启动脚本，我们可以知道，zygote进程的实际是二进制文件app_process的调用，我们就从这个应用程序的main函数入手去分析一下zygote进程的启动过程，源码如下（/frameworks/base/cmds/app_process/app_main.cpp）：

/** * 将-Xzygote加入到JavaVMOption中，返回/system/bin参数指向的下标 */int AndroidRuntime::addVmArguments(int argc, const char* const argv[]){    int i;    for (i = 0; i < argc; i ++) {        if (argv[i][0] != '-') {            return i;        }        if (argv[i][1] == '-' && argv[i][2] == 0) {            return i + 1;        }        JavaVMOption opt;        memset(&opt, 0, sizeof(opt));        opt.optionString = (char*)argv[i];        mOptions.add(opt);    }    return i;}int main(int argc, char* const argv[]){    // zygote call parameters    // /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server    // These are global variables in ProcessState.cpp    mArgC = argc;    mArgV = argv;    mArgLen = 0;    for (int i = 0; i < argc; i ++) {        mArgLen += strlen(argv[i]) + 1;    }    // 去除末尾的空格    mArgLen--;    AppRuntime runtime;    const char* argv0 = argv[0];    // Process command line arguments    // ignore argv[0]    argc --;    argv ++;    // Everything up tp '--' 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)) {            niceName = arg + 12;        } else {            className = arg;            break;        }    }    if (niceName && *niceName) {        setArgv0(argv0, niceName);        set_process_name(niceName);    }    runtime.mParentDir = parentDir;    if (zygote) {        // 进入到AppRuntime的start函数        runtime.start("com.android.internal.os.ZygoteInit",            startSystemServer? "start-system-server" : "");    } else if (className) {        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.\n");        app_usage();        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied");        return 10;    }}

在zygote的main函数中，通过AppRuntime runtime代码创建了一个AppRuntime对象runtime，接下来Zygote进程就是通过它来进一步启动的。
init.rc中关于启动zygote命令中包含了–zygote参数，所以在if(strcmp(arg, “–zygote”) == 0)判断的时候，会将niceName赋值为”zygote”,然后通过set_process_name(niceName)函数将当前进程的名称设置为zygote。这也是为什么调用的脚本为/system/bin/app_process，而进程名为zygote的原因。set_process_name函数的源码如下(/system/core/libcutils/process_name.c)：

static const char* process_name = "unknown";void set_process_name(const char* new_name){    if (new_name == NULL) {        return;    }    int len = strlen(new_name);    char* copy = (char*)malloc(len + 1);    strcpy(copy, new_name);    process_name = (const char*) copy;}

从init.rc文件中关于zygote进程的配置参数可知，Zygote进程传递给应用程序app_process的启动参数arg还包含一个”–start-system-server”选项。因此，在调用AppRuntime对象runtime的成员函数start时，第二个参数为”start-system-server”，表示zygote进程启动完成之后，需要将system进程启动起来。

AppRuntime分析

AppRuntime类的成员函数start是从父类AndroidRuntime继承下来的，因此，接下来我们就继续分析AndroidRuntime类的成员函数start的实现，函数源码位置：/frameworks/base/core/jni/AndroidRuntime.cpp：

char* AndroidRuntime::toSlashClassName(const char* className){    char* result = strdup(className);    for (char* cp = result; *cp != '\0'; cp ++) {        if (*cp == '.') {            *cp = '/';        }    }    return result;}/** * Start the Android runtime. This involves starting the virtual machine * and calling the "static void main(String[] args)" method int the class * named by "className". * * 这两个参数的值分别为: * const char* className = "com.android.internal.os.ZygoteInit"; * const char* options = "start-system-server"; */void AndroidRuntime::start(const char* className, const char* options){    ALOGD("\n>>>>> AndroidRuntime START %s <<<<<<\n",        className != NULL ? className : "(unknown)");    /** * '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) {        const int LOG_BOOT_PROGRESS_START = 3000;        LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START, ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));    }    // 设置ANDROID_ROOT环境变量    const char* rootDir = getenv("ANDROID_ROOT");    if (rootDir == NULL) {        rootDir = "/system";        if (!hasDir("/system")) {            LOG_FATAL("No root directory specified, and /android dose not exist.");            return;        }        setenv("ANDROID_ROOT", rootDir, 1);    }    JniInvocation jni_invocation;    jni_invocation.Init(NULL);    JNIEnv* env;    // 1. 创建虚拟机    if (startVm(&mJavaVM, &env) != 0) {        return;    }    onVmCreated(env);    // 2. 注册JNI函数    if (startReg(env) < 0) {        ALOGE("Unable to register all android natives\n");        return;    }    jclass stringClass;    jobjectArray strArray;    jstring classNameStr;    jstring optionsStr;    stringClass = env->FindClass("java/lang/String");    assert(stringClass != NULL);    // 创建一个有两个元素的String数组，用Java代码表示为:String[] strArray = new String[2];    strArray = env->NewObjectArray(2, stringClass, NULL);    assert(strArray != NULL);    classNameStr = env->NewStringUTF(className);    assert(classNameStr != NULL);    // 设置第一个元素为"com.android.internal.os.ZygoteInit"    env->SetObjectArrayElement(strArray, 0, classNameStr);    optionsStr = env->NewStringUTF(options);    // 设置第二个元素为"start-system-server"    env->SetObjectArrayElement(strArray, 1, optionsStr);    // 将字符串"com.android.internal.os.ZygoteInit"转换为"com/android/internal/os/ZygoteInit"    char* slashClassName = toSlashClassName(className);    jclass startClass = env->FindClass(slashClassName);    if (startClass == NULL) {        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);    } else {        jmethodID startMeth = env->GetStaticMethodID(startClass, "main", "([Ljava/lang/String;)V");        if (startMeth == NULL) {            ALOGE("JavaVM unable to find main() in '%s\n'", className);        } else {            // 3.            // 通过JNI调用java函数，注意调用的是main函数，所属的类是"com.android.internal.os.ZygoteInit".            // 传递的参数是"com.android.internal.os.ZygoteInit true"            env->CallStaticVoidMethod(startClass, startMeth, strArray);        }    }    free(slashClassName);    ALOGD("Shutting down VM\n");    if (mJavaVM->DetachCurrentThread() != JNI_OK) {        ALOGW("Warning: unable to detach main thread\n");    }    if (mJavaVM->DestoryJavaVM() != 0) {        ALOGW("Warning: VM did not shut down cleanly\n");    }}

上述代码有几处关键点，分别是：

创建虚拟机。
注册JNI函数。
进入Java世界。

接下来，我们分别分析这三个关键点。

创建虚拟机——startVm

startVm并没有特别之处，就是调用JNI的虚拟机创建函数，但是创建虚拟机时的一些参数却是在startVm中确定的，其源码如下：

#define PROPERTY_VALUE_MAX 92/** * Start the Dalvik Virtual Machine. * * Various arguments, most determined by system properties, are passed in. * The "mOptions" vector is updated. * * Returns 0 on success. */int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIENV** pEnv){    int result = -1;    JavaVMInitArgs initArgs;    JavaVMOption opt;    char propBuf[PROPERTY_VALUE_MAX];    char stackTraceFileBuf[PROPERTY_VALUE_MAX];    char dexoptFlagsBuf[PROPERTY_VALUE_MAX];    char enableAssertBuf[sizeof("-ea:")-1 + PROPERTY_VALUE_MAX];    char jniOptsBuf[sizeof("-Xjniopts:")-1 + PROPERTY_VALUE_MAX];    char heapstartsizeOptsBuf[sizeof("-Xms")-1 + PROPERTY_VALUE_MAX];    char heapsizeOptsBuf[sizeof("-Xms")-1 + PROPERTY_VALUE_MAX];    char heapgrowthlimitOptsBuf[sizeof("-XX:HeapGrowthLimit=")-1 + PROPERTY_VALUE_MAX];    char heapminfreeOptsBuf[sizeof("-XX:HeapMinFree=")-1 + PROPERTY_VALUE_MAX];    char heapmaxfreeOptsBuf[sizeof("-XX:HeapMaxFree=")-1 + PROPERTY_VALUE_MAX];    char heaptargetutilizationOptsBuf[sizeof("-XX:HeapTargetUtilization=")-1 + PROPERTY_VALUE_MAX];    char jitcodecachesizeOptsBuf[sizeof("-Xjitcodecachesize:")-1 + PROPERTY_VALUE_MAX];    char extraOptsBuf[PROPERTY_VALUE_MAX];    char* stackTraceFile = NULL;    bool checkJni = false;    bool checkDexSum = false;    bool logStdio = false;    enum {        KEMDefault,        KEMIntPortable,        KEMIntFast,        KEMJitCompiler,    } executionMode = KEMDefault;    /** * 这段代码是用了设置JNI_check选项的。JNI_check指的是Native层调用JNI函数时，系统所做的一些检查动作。 * 这个选项虽然能增加可靠性，但是还有一些副作用： * 1. 因为检查工作比较耗时，所以会影响系统运行速度。 * 2. 有些检查工作比较耗时，一旦出错，整个进程会abort。 * 所以，JNI_check选项一般只在eng版本设置。 */    property_get("dalvik.vm.checkjni", propBuf, "");    if (strcmp(propBuf, "true") == 0) {        checkJni = true;    } else if (strcmp(propBuf, "false") != 0) {        property_get("ro.kernel.android.checkjni", propBuf, "");        if (propBuf[0] == '1') {            checkJni = true;        }    }    property_get("dalvik.vm.execution-mode", propBuf, "");    if (strcmp(propBuf, "int:portable") == 0) {        executionMode = KEMIntPortable;    } else if (strcmp(propBuf, "int:fast") == 0) {        executionMode = KEMIntFast;       } else if (strcmp(propBuf, "int:jit") == 0) {        executionMode = KEMJitCompiler;    }    // ... 省略大部分参数设置    /** * 设置虚拟机的heapsize，默认为16m。绝大多数厂商都会在build.prop文件里修改这个属性，一般是256m。 * heapsize不能设置得过小，否则在操作大尺寸的图片时无法分配所需的内存。 */    strcpy(heapsizeOptsBuf, "-Xmx");    property_get("dalvik.vm.heapsize", heapsizeOptsBuf+4, "16m");    opt.optionString = heapsizeOptsBuf;    mOptions.add(opt);    // ......    if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {        ALOGE("JNI_CreateJavaVM failed\n");        goto bail;    }    result = 0;bail:    free(stackTraceFile);    return result;}

更多虚拟机参数的设置，我这里就不做特殊说明了，大家感兴趣可以自行google。(ps:因为我不太懂虚拟机这一块…)

注册JNI函数——startReg

上面讲了如何创建虚拟机，接下来需要给这个虚拟机注册一些JNI函数。正是因为后续的Java世界用到的一些函数是采用native方式实现的，所以才必须提前注册这些函数。

接下来，我们来看一下startReg函数的源码实现：

int AndroidRuntime::startReg(JNIEnv* env){    // 设置Thread类的线程创建函数为javaCreateThreadEtc    androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);    ALOGV("--- registering native functions ---\n");    env->PushLocalFrame(200);    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {        env->PopLocalFrame(NULL);        return -1;    }    env->PopLocalFrame(NULL);    return 0;}

关键是需要注册JNI函数，具体实现是由register_jni_procs函数实现的，我们来看一下这个函数的具体实现(/frameworks/base/core/jni/AndroidRuntime.cpp)：

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;}

通过源码，我们可以看到，register_jni_procs只是对array数组的mProc函数的封装，而array数组指向的是gRegJNI数组，我们来看一下这个数组的实现：

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),    REG_JNI(register_android_emoji_EmojiFactory),    REG_JNI(register_android_text_AndroidCharacter),    REG_JNI(register_android_text_AndroidBidi),    REG_JNI(register_android_view_InputDevice),    REG_JNI(register_android_view_KeyCharacterMap),    REG_JNI(register_android_os_Process),    REG_JNI(register_android_os_SystemProperties),    REG_JNI(register_android_os_Binder),    REG_JNI(register_android_os_Parcel),    REG_JNI(register_android_view_DisplayEventReceiver),    REG_JNI(register_android_nio_utils),    REG_JNI(register_android_graphics_Graphics),    REG_JNI(register_android_view_GraphicBuffer),    REG_JNI(register_android_view_GLES20DisplayList),    REG_JNI(register_android_view_GLES20Canvas),    REG_JNI(register_android_view_HardwareRenderer),    REG_JNI(register_android_view_Surface),    REG_JNI(register_android_view_SurfaceControl),    REG_JNI(register_android_view_SurfaceSession),    REG_JNI(register_android_view_TextureView),    REG_JNI(register_com_google_android_gles_jni_EGLImpl),    REG_JNI(register_com_google_android_gles_jni_GLImpl),    REG_JNI(register_android_opengl_jni_EGL14),    REG_JNI(register_android_opengl_jni_EGLExt),    REG_JNI(register_android_opengl_jni_GLES10),    REG_JNI(register_android_opengl_jni_GLES10Ext),    REG_JNI(register_android_opengl_jni_GLES11),    REG_JNI(register_android_opengl_jni_GLES11Ext),    REG_JNI(register_android_opengl_jni_GLES20),    REG_JNI(register_android_opengl_jni_GLES30),    REG_JNI(register_android_graphics_Bitmap),    REG_JNI(register_android_graphics_BitmapFactory),    REG_JNI(register_android_graphics_BitmapRegionDecoder),    REG_JNI(register_android_graphics_Camera),    REG_JNI(register_android_graphics_CreateJavaOutputStreamAdaptor),    REG_JNI(register_android_graphics_Canvas),    REG_JNI(register_android_graphics_ColorFilter),    REG_JNI(register_android_graphics_DrawFilter),    REG_JNI(register_android_graphics_Interpolator),    REG_JNI(register_android_graphics_LayerRasterizer),    REG_JNI(register_android_graphics_MaskFilter),    REG_JNI(register_android_graphics_Matrix),    REG_JNI(register_android_graphics_Movie),    REG_JNI(register_android_graphics_NinePatch),    REG_JNI(register_android_graphics_Paint),    REG_JNI(register_android_graphics_Path),    REG_JNI(register_android_graphics_PathMeasure),    REG_JNI(register_android_graphics_PathEffect),    REG_JNI(register_android_graphics_Picture),    REG_JNI(register_android_graphics_PorterDuff),    REG_JNI(register_android_graphics_Rasterizer),    REG_JNI(register_android_graphics_Region),    REG_JNI(register_android_graphics_Shader),    REG_JNI(register_android_graphics_SurfaceTexture),    REG_JNI(register_android_graphics_Typeface),    REG_JNI(register_android_graphics_Xfermode),    REG_JNI(register_android_graphics_YuvImage),    REG_JNI(register_android_graphics_pdf_PdfDocument),    REG_JNI(register_android_database_CursorWindow),    REG_JNI(register_android_database_SQLiteConnection),    REG_JNI(register_android_database_SQLiteGlobal),    REG_JNI(register_android_database_SQLiteDebug),    REG_JNI(register_android_os_Debug),    REG_JNI(register_android_os_FileObserver),    REG_JNI(register_android_os_MessageQueue),    REG_JNI(register_android_os_SELinux),    REG_JNI(register_android_os_Trace),    REG_JNI(register_android_os_UEventObserver),    REG_JNI(register_android_net_LocalSocketImpl),    REG_JNI(register_android_net_NetworkUtils),    REG_JNI(register_android_net_TrafficStats),    REG_JNI(register_android_net_wifi_WifiNative),    REG_JNI(register_android_os_MemoryFile),    REG_JNI(register_com_android_internal_os_ZygoteInit),    REG_JNI(register_android_hardware_Camera),    REG_JNI(register_android_hardware_camera2_CameraMetadata),    REG_JNI(register_android_hardware_SensorManager),    REG_JNI(register_android_hardware_SerialPort),    REG_JNI(register_android_hardware_UsbDevice),    REG_JNI(register_android_hardware_UsbDeviceConnection),    REG_JNI(register_android_hardware_UsbRequest),    REG_JNI(register_android_media_AudioRecord),    REG_JNI(register_android_media_AudioSystem),    REG_JNI(register_android_media_AudioTrack),    REG_JNI(register_android_media_JetPlayer),    REG_JNI(register_android_media_RemoteDisplay),    REG_JNI(register_android_media_ToneGenerator),    REG_JNI(register_android_opengl_classes),    REG_JNI(register_android_server_NetworkManagementSocketTagger),    REG_JNI(register_android_server_Watchdog),    REG_JNI(register_android_ddm_DdmHandleNativeHeap),    REG_JNI(register_android_backup_BackupDataInput),    REG_JNI(register_android_backup_BackupDataOutput),    REG_JNI(register_android_backup_FileBackupHelperBase),    REG_JNI(register_android_backup_BackupHelperDispatcher),    REG_JNI(register_android_app_backup_FullBackup),    REG_JNI(register_android_app_ActivityThread),    REG_JNI(register_android_app_NativeActivity),    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),    REG_JNI(register_android_content_res_ObbScanner),    REG_JNI(register_android_content_res_Configuration),    REG_JNI(register_android_animation_PropertyValuesHolder),    REG_JNI(register_com_android_internal_content_NativeLibraryHelper),    REG_JNI(register_com_android_internal_net_NetworkStatsFactory),};#ifdef NDEBUG    #define REG_JNI(name) {name}    struct RegJNIRec {        int (*mProc)(JNIEnv*);    };#else    #define REG_JNI(name) {name, #name}    struct RegJNIRec {        int (*mProc)(JNIEnv*);        const char* mName;    };#endif

可以看到，REG_JNI是一个宏，宏里面包括的就是那个参数为JNIEnv*,返回值为int的函数指针mProc，我们以register_android_debug_JNITest为例，源码位置为/frameworks/base/core/jni/android_debug_JNITest.cpp：

#define NELEM(x) (sizeof(x)/sizeof(*(x)))int register_android_debug_JNITest(JNIEnv* env){    return jniRegisterNativeMethods(env, "android/debug/JNITest", gMethods, NELEM(gMethods));}

可以看到，mProc其实就是为Java类注册JNI函数。

进入JAVA世界

可以看到CallStaticVoidMethod最终将调用com.android.internal.os.ZygoteInit的main函数，下面就来看一下这个Java世界的入口函数。源码位置：/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java，源码如下：

public static void main(String argv[]){    try {        SamplingProfilerIntegration.start();        // 1. 注册zygote用的socket        registerZygoteSocket();        EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis());        // 2. 预加载类和资源        preload();        EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis());        SamplingProfilerIntegration.writeZygoteSnapshot();        // 强制执行一次垃圾收集        gc();        Trace.setTracingEnabled(false);        if (argv.length != 2) {            throw new RuntimeException(argv[0] + USAGE_STRING);        }        if (argv[1].equals("start-system-server")) {            // 3. 启动system-server            startSystemServer();        } else if (!argv[1].equals("")) {            throw new RuntimeException(argv[0] + USAGE_STRING);        }        Log.i(TAG, "Accepting command socket connections");        // 4. 进入请求应答模式        runSelectLoop();        closeServerSocket();    } catch(MethodAndArgsCaller caller) {        caller.run();    } catch(RuntimeException ex) {        Log.e(TAG, "Zygote died with exception", ex);        closeServerSocket();        throw ex;    }}

上述代码中有5个重要的点，我已经通过标号标记出来了，接下来我们分别分析一下这5点函数的具体实现。

建立IPC通信服务端——registerZygoteSocket

zygote及系统中其他程序的通信没有使用Binder，而是采用了基于AF_UNIX类型的socket。registerZygoteSocket函数的使命正是建立这个Socket，实现代码如下：

private static void registerZygoteSocket(){    if (sServerSocket == null) {        int fileDesc;        try {            String env = System.getenv(ANDROID_SOCKET_ENV);            fileDesc = Integer.parseInt(env);        } catch (RuntimeException ex) {            throw new RuntimeException(ANDROID_SOCKET_ENV + " unset or invalid", ex);        }        try {               sServerSocket = new LocalServerSocket(createFileDescriptor(fileDesc));        } catch(IOException ex) {               throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);        }    }}public class LocalServerSocket {    private final LocalSocketImpl impl;    private final LocalSocketAddress localAddress;    private static final int LISTEN_BACKLOG = 50;    /** * Create a LocalServerSocket from a file descriptor that's already * been created and bound. listen() will be called immediately on it. * Used for cases where file descriptors are passed in via environment * variables. */    public LocalServerSocket(FileDescriptor fd) throws IOException {        impl = new LocalSocketImpl(fd);        impl.listen(LISTEN_BACKLOG);        localAddress = impl.getSockAddress();    }}

registerZygoteSocket很简单，就是创建一个服务端的socket。

预加载类和资源——preload

我们先来看一下preload函数实现：

static void preload(){    preloadClasses();    preloadResources();    preloadOpenGL();}

preload函数里面分别调用了三个预加载函数，我们分别来分析一下这几个函数的实现。

首先是preloadClasses，函数实现如下：

private static final int UNPRIVILEGED_UID = 9999;private static final int UNPRIVILEGED_GID = 9999;private static final int ROOT_UID = 0;private static final int ROOT_GID = 0;private static void preloadClasses(){    final VMRuntime runtime = VMRuntime.getRuntime();    InputStream is = ClassLoader.getSystemClassLoader().getResourceAsStream(PRELOADED_CLASSES);    if (is == null) {        Log.e(TAG, "Couldn't find " + PRELOADED_CLASSES + ".");    } else {        Log.i(TAG, "Preloading classes...");        long startTime = SystemClock.uptimeMillis();        setEffectiveGroup(UNPRIVILEGED_GID);        setEffectiveGroup(UNPRIVILEGED_UID);        float defaultUtilization = runtime.getTargetHeapUtilization();        runtime.setTargetHeapUtilization(0.8f);        System.gc();        runtime.runFinalizationSync();        Debug.startAllocCounting();        try {            // 创建一个缓冲区为256字符的输入流            BufferedReader br = new BufferdReader(new InputStreamReader(is), 256);            int count = 0;            String line;            while ((line = br.readLine()) != null) {                // skip comments and blank lines.                line = line.trim();                if (line.startsWith("#") || line.equals("")) {                    continue;                }                try {                    if (false) {                        Log.v(TAG, "Preloading " + line + "...");                    }                    Class.forName(line);                    count ++;                } catch (ClassNotFoundException e) {                    Log.w(TAG, "Class not found for preloading: " + line);                } catch (UnsatisfiedLinkError e) {                    Log.w(TAG, "Problem preloading " + line + ": " + e);                } catch(Throwable t) {                    Log.e(TAG, "Error preloading " + line + ".", t);                }            }            Log.i(TAG, "...preloaded " + count + " classes in " + (SystemClock.uptimeMillis()-startTime) + "ms.");        } catch (IOException e) {            Log.e(TAG, "Error reading " + PRELOADED_CLASSES + ".", e);        } finally {            IoUtils.closeQuietly(is);            runtime.setTargetHeapUtilization(defaultUtilization);            runtime.preloadDexCaches();            Debug.stopAllocCounting();            setEffectiveUser(ROOT_UID);            setEffectiveGroup(ROOT_GID);        }    }}

preloadClasses看起来很简单，但是实际上它有很多的类需要加载。可以查看一下/frameworks/base/preloaded-classes文件，这里面都是需要预加载的类。

接下来，分析一下preloadResources函数的源码：

private static final boolean PRELOAD_RESOURCES = true;private static void preloadResources(){    final VMRuntime runtime = VMRuntime.getRuntime();    Debug.startAllocCounting();    try {        System.gc();        runtime.runFinalizationSync();        mResources = Resources.getSystem();        mResources.startPreloading();        if (PRELOAD_RESOURCES) {            Log.i(TAG, "Preloading resources...");            long startTime = SystemClock.uptimeMillis();            TypedArray ar = mResources.obtainTypedArray(com.android.internal.R.array.preloaded_drawables);            int N = preloadDrawables(runtime, ar);            ar.recycle();            Log.i(TAG, "...preloaded " + N + " resources in " + (SystemClock.uptimeMillis()-startTime) + "ms.");            startTime = SystemClock.uptimeMillis();            ar = mResources.obtainTypedArray(com.android.internal.R.array.preloaded_color_state_lists);            N = preloadColorstateLists(runtime, ar);            ar.recycle();            Log.i(TAG, "...preloaded " + N + " resources in " + (SystemClock.uptimeMillis() - startTime) + "ms.");        }        mResources.finishPreloading();    } catch (RuntimeException e) {        Log.w(TAG, "Failure preloading resources", e);    } finally {        Debug.stopAllocCounting();    }}

接下来，是预加载OpenGL。源码如下：

private static void preloadOpenGL(){    if (!SystemProperties.getBoolean(PROPERTY_DISABLE_OPENGL_PRELOADING, false)) {        EGL14.eglGetDisplay(EGL14.EGL_DEFAULT_DISPLAY);    }}

启动system_server

现在我们要分析第三个关键点：startSystemServer。这个函数会创建java世界中系统Service所驻留的进程system_server，该进程是framework的核心。如何system_server挂掉，会导致zygote自杀。我们来看一下startSystemServer()实现源码。

/** * Prepare the arguments and fork for the system server process. */private static boolean startSystemServer() throws MethodAndArgsCaller, RuntimeException{    long capabilities = posixCapabilitiesAsBits(        OsConstants.CAP_KILL,        OsConstants.CAP_NET_ADMIN,        OsConstants.CAP_NET_BIND_SERVICE,        OsConstants.CAP_NET_BROADCAST,        OsConstants.CAP_NET_RAW,        OsConstants.CAP_SYS_MODULE,        OsConstants.CAP_SYS_NICE,        OsConstants.CAP_SYS_RESOURCE,        OsConstants.CAP_SYS_TIME,        OsConstants.CAP_SYS_TTY_CONFIG    );    // 设置参数    String args[] = {        "--setuid=1000",        "--setgid=1000",        "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1032,3001,3002,3003,3006,3007",        "--capabilities=" + capabilities + "," + capabilities,        "--runtime-init",        "--nice-name=system_server", // 进程名为system_server        "com.android.server.SystemServer",    };    ZygoteConnection.Arguments parsedArgs = null;    int pid;    try {        parsedArgs = new ZygoteConnection.Arguments(args);        ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);        ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);        /* Request to fork the system server process */        pid = Zygote.forkSystemServer(                parsedArgs.uid, parsedArgs.gid,                parsedArgs.gids,                parsedArgs.debugFlags,                null,                parsedArgs.permittedCapabilities,                parsedArgs.effectiveCapabilities        );    } catch (IllegalArgumentException ex) {        throw new RuntimeException(ex);    }    /* For child process */    if (pid == 0) {        handleSystemServerProcess(parsedArgs);    }    return true;}

有求必应之等待请求——runSelectLoop

zygote从startSystemServer返回后，将进入第四个关键的函数:runSelectLoop。我们来看一下这个函数的实现：

static final int GC_LOOP_COUNT = 10;private static void runSelectLoop() throws MethodAndArgsCaller {    ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();    ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();    FileDescriptor[] fdArray = new FileDescriptor[4];    fds.add(sServerSocket.getFileDescriptor());    peers.add(null);    int loopCount = GC_LOOP_COUNT;    while (true) {        int index;        if (loopCount <= 0) {            gc();            loopCount = GC_LOOP_COUNT;        } else {            loopCount --;        }        try {            fdArray = fds.toArray(fdArray);            index = selectReadable(fdArray);        } catch(IOException ex) {            throw new RuntimeException("Error in select()", ex);        }        if (index < 0) {            throw new RuntimeException("Error in select()");        } else if (index == 0) {            ZygoteConnection newPeer = acceptCommandPeer();            peers.add(newPeer);        }    }}

Android(安卓)Zygote源码分析

目录

概述