Zygote进程【1】——Zygote的诞生

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在Android中存在着C和Java两个完全不同的世界，前者直接建立在Linux的基础上，后者直接建立在JVM的基础上。zygote的中文名字为“受精卵”，这个名字很好的诠释了zygote进程的作用。作为java世界的孵化者，zygote本身是一个native程序，是由init根据init.rc文件中的配置项创建的。

@/system/core/rootdir/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

关于init是如何解析和创建zygote进程的，这里不再赘述，不明的同学可以参考 init进程【2】——解析配置文件一文。这里解析一下上面的第一行：service是rc脚本中的一种SECTION，zygote表示service的名字，/system/bin/app_process表示service的路径，-Xzygote /system/bin --zygote --start-system-server则表示传入的参数。

zygote的实现在app_main.cpp中：

@frameworks/base/cmds/app_process/app_main.cpp

int main(int argc, char* const argv[])
{
//针对ARM平台的特殊逻辑
#ifdef __arm__
    /*
     * b/7188322 - Temporarily revert to the compat memory layout
     * to avoid breaking third party apps.
     *
     * THIS WILL GO AWAY IN A FUTURE ANDROID RELEASE.
     *
     * http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=7dbaa466
     * changes the kernel mapping from bottom up to top-down.
     * This breaks some programs which improperly embed
     * an out of date copy of Android's linker.
     */
    char value[PROPERTY_VALUE_MAX];
    property_get("ro.kernel.qemu", value, "");
    bool is_qemu = (strcmp(value, "1") == 0);
    if ((getenv("NO_ADDR_COMPAT_LAYOUT_FIXUP") == NULL) && !is_qemu) {
        int current = personality(0xFFFFFFFF);
        if ((current & ADDR_COMPAT_LAYOUT) == 0) {
            personality(current | ADDR_COMPAT_LAYOUT);
            setenv("NO_ADDR_COMPAT_LAYOUT_FIXUP", "1", 1);
            execv("/system/bin/app_process", argv);
            return -1;
        }
    }
    unsetenv("NO_ADDR_COMPAT_LAYOUT_FIXUP");
#endif

    // 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;
    }
    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) {//根据传入的参数，初始化启动zygote所需的参数
        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);//设置本进程的名称为zygote，至此进程有app_process变为了zygote
    }

    runtime.mParentDir = parentDir;

    if (zygote) {//根据我们传入的参考，这里的zygote值为TRUE
        runtime.start("com.android.internal.os.ZygoteInit",
                startSystemServer ? "start-system-server" : "");
    } else if (className) {//可以看出除了zygote，RuntimeInit也是在这里启动的
        // 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.\n");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
        return 10;
    }
}

通过对main()函数的分析，可以看出main()主要根据传入的参数初始化启动参数，具体的启动过程是由AppRuntime完成的。AppRuntime的声明和实现都在app_main.cpp中，它继承自AndroidRuntime，AppRuntime的实现如下：

可以看出start是AndroidRuntime中的方法。通过start函数前面的注释我们了解到它的主要作用是：启动Android运行时环境，包括启动虚拟机和调用className参数所指定的类的main()方法（即：Java中的main方法）。

/*
 * 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)
{
    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) {
        /* 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)));
    }

    //环境变量ANDROID_ROOT是否已经设置，如果未设置，则设置其值为"/system"
    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");
    //ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);

    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env) != 0) {//启动Java虚拟机
        return;
    }
    onVmCreated(env);//空函数

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {//注册Android JNI函数
        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;
    jstring optionsStr;

    stringClass = env->FindClass("java/lang/String");//JNI中调用java中的String类
    assert(stringClass != NULL);
    //创建包含2个元素的String数组，这里相当于Java中的String strArray[] = new String[2]
    strArray = env->NewObjectArray(2, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);//classNameStr的值为"com.android.internal.os.ZygoteInit"
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);
    optionsStr = env->NewStringUTF(options);//optionsStr的值为"start-system-server"
    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);//将"com.android.internal.os.ZygoteInit"中的"."替换成"/"供JNI调用
    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");//ZygoteInit类中的main()方法
        if (startMeth == NULL) {
            ALOGE("JavaVM unable to find main() in '%s'\n", className);
            /* keep going */
        } else {
            env->CallStaticVoidMethod(startClass, startMeth, strArray);//通过JNI调用main()方法

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

    //如果JVM退出。这两句代码一般来说执行不到
    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");
}

通过上面对start()函数的分析可以发现，在start()中主要完成了如下三项工作：

1. 启动JVM。
2. 注册Android JNI函数。
3. 调用ZygoteInit的main()方法。

创建Java虚拟机

start()中与创建虚拟机相关的代码如下：

    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env) != 0) {//启动Java虚拟机
        return;
    }
    onVmCreated(env);//空函数

这里代码中  创建一个JniInvocation实例，并且调用它的成员函数init来初始化JNI环境：

@/libnativehelper/jniInvocation.cpp

bool JniInvocation::Init(const char* library) {
#ifdef HAVE_ANDROID_OS
  char default_library[PROPERTY_VALUE_MAX];
  property_get("persist.sys.dalvik.vm.lib", default_library, "libdvm.so");
#else
  const char* default_library = "libdvm.so";
#endif
  if (library == NULL) {
    library = default_library;
  }

  handle_ = dlopen(library, RTLD_NOW);
  if (handle_ == NULL) {
    ALOGE("Failed to dlopen %s: %s", library, dlerror());
    return false;
  }
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetDefaultJavaVMInitArgs_),
                  "JNI_GetDefaultJavaVMInitArgs")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_CreateJavaVM_),
                  "JNI_CreateJavaVM")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetCreatedJavaVMs_),
                  "JNI_GetCreatedJavaVMs")) {
    return false;
  }
  return true;
}

JniInvocation类的成员函数init所做的事情很简单。它首先是读取系统属性persist.sys.dalvik.vm.lib的值。系统属性persist.sys.dalvik.vm.lib的值要么等于libdvm.so，要么等于libart.so，这两个so库分别对应着Dalvik虚拟机和ART虚拟机环境。
在初始化完虚拟机环境后，接下来调用startVm()来创建虚拟机。

@/frameworks/base/core/jni/AndroidRuntime.cpp

/*
 * 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("-Xmx")-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;


    property_get("dalvik.vm.checkjni", propBuf, "");
    if (strcmp(propBuf, "true") == 0) {
        checkJni = true;
    } else if (strcmp(propBuf, "false") != 0) {
        /* property is neither true nor false; fall back on kernel parameter */
        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;
    }

    property_get("dalvik.vm.stack-trace-file", stackTraceFileBuf, "");

    property_get("dalvik.vm.check-dex-sum", propBuf, "");
    if (strcmp(propBuf, "true") == 0) {
        checkDexSum = true;
    }

    property_get("log.redirect-stdio", propBuf, "");
    if (strcmp(propBuf, "true") == 0) {
        logStdio = true;
    }

    strcpy(enableAssertBuf, "-ea:");
    property_get("dalvik.vm.enableassertions", enableAssertBuf+4, "");

    strcpy(jniOptsBuf, "-Xjniopts:");
    property_get("dalvik.vm.jniopts", jniOptsBuf+10, "");

    /* route exit() to our handler */
    opt.extraInfo = (void*) runtime_exit;
    opt.optionString = "exit";
    mOptions.add(opt);

    /* route fprintf() to our handler */
    opt.extraInfo = (void*) runtime_vfprintf;
    opt.optionString = "vfprintf";
    mOptions.add(opt);

    /* register the framework-specific "is sensitive thread" hook */
    opt.extraInfo = (void*) runtime_isSensitiveThread;
    opt.optionString = "sensitiveThread";
    mOptions.add(opt);

    opt.extraInfo = NULL;

    /* enable verbose; standard options are { jni, gc, class } */
    //options[curOpt++].optionString = "-verbose:jni";
    opt.optionString = "-verbose:gc";
    mOptions.add(opt);
    //options[curOpt++].optionString = "-verbose:class";

    /*
     * The default starting and maximum size of the heap.  Larger
     * values should be specified in a product property override.
     */
    strcpy(heapstartsizeOptsBuf, "-Xms");
    property_get("dalvik.vm.heapstartsize", heapstartsizeOptsBuf+4, "4m");
    opt.optionString = heapstartsizeOptsBuf;
    mOptions.add(opt);
    strcpy(heapsizeOptsBuf, "-Xmx");
    property_get("dalvik.vm.heapsize", heapsizeOptsBuf+4, "16m");
    opt.optionString = heapsizeOptsBuf;
    mOptions.add(opt);

    // Increase the main thread's interpreter stack size for bug 6315322.
    opt.optionString = "-XX:mainThreadStackSize=24K";
    mOptions.add(opt);

    // Set the max jit code cache size.  Note: size of 0 will disable the JIT.
    strcpy(jitcodecachesizeOptsBuf, "-Xjitcodecachesize:");
    property_get("dalvik.vm.jit.codecachesize", jitcodecachesizeOptsBuf+19,  NULL);
    if (jitcodecachesizeOptsBuf[19] != '\0') {
      opt.optionString = jitcodecachesizeOptsBuf;
      mOptions.add(opt);
    }

    strcpy(heapgrowthlimitOptsBuf, "-XX:HeapGrowthLimit=");
    property_get("dalvik.vm.heapgrowthlimit", heapgrowthlimitOptsBuf+20, "");
    if (heapgrowthlimitOptsBuf[20] != '\0') {
        opt.optionString = heapgrowthlimitOptsBuf;
        mOptions.add(opt);
    }

    strcpy(heapminfreeOptsBuf, "-XX:HeapMinFree=");
    property_get("dalvik.vm.heapminfree", heapminfreeOptsBuf+16, "");
    if (heapminfreeOptsBuf[16] != '\0') {
        opt.optionString = heapminfreeOptsBuf;
        mOptions.add(opt);
    }

    strcpy(heapmaxfreeOptsBuf, "-XX:HeapMaxFree=");
    property_get("dalvik.vm.heapmaxfree", heapmaxfreeOptsBuf+16, "");
    if (heapmaxfreeOptsBuf[16] != '\0') {
        opt.optionString = heapmaxfreeOptsBuf;
        mOptions.add(opt);
    }

    strcpy(heaptargetutilizationOptsBuf, "-XX:HeapTargetUtilization=");
    property_get("dalvik.vm.heaptargetutilization", heaptargetutilizationOptsBuf+26, "");
    if (heaptargetutilizationOptsBuf[26] != '\0') {
        opt.optionString = heaptargetutilizationOptsBuf;
        mOptions.add(opt);
    }

    property_get("ro.config.low_ram", propBuf, "");
    if (strcmp(propBuf, "true") == 0) {
      opt.optionString = "-XX:LowMemoryMode";
      mOptions.add(opt);
    }

    /*
     * Enable or disable dexopt features, such as bytecode verification and
     * calculation of register maps for precise GC.
     */
    property_get("dalvik.vm.dexopt-flags", dexoptFlagsBuf, "");
    if (dexoptFlagsBuf[0] != '\0') {
        const char* opc;
        const char* val;

        opc = strstr(dexoptFlagsBuf, "v=");     /* verification */
        if (opc != NULL) {
            switch (*(opc+2)) {
            case 'n':   val = "-Xverify:none";      break;
            case 'r':   val = "-Xverify:remote";    break;
            case 'a':   val = "-Xverify:all";       break;
            default:    val = NULL;                 break;
            }

            if (val != NULL) {
                opt.optionString = val;
                mOptions.add(opt);
            }
        }

        opc = strstr(dexoptFlagsBuf, "o=");     /* optimization */
        if (opc != NULL) {
            switch (*(opc+2)) {
            case 'n':   val = "-Xdexopt:none";      break;
            case 'v':   val = "-Xdexopt:verified";  break;
            case 'a':   val = "-Xdexopt:all";       break;
            case 'f':   val = "-Xdexopt:full";      break;
            default:    val = NULL;                 break;
            }

            if (val != NULL) {
                opt.optionString = val;
                mOptions.add(opt);
            }
        }

        opc = strstr(dexoptFlagsBuf, "m=y");    /* register map */
        if (opc != NULL) {
            opt.optionString = "-Xgenregmap";
            mOptions.add(opt);

            /* turn on precise GC while we're at it */
            opt.optionString = "-Xgc:precise";
            mOptions.add(opt);
        }
    }

    /* enable debugging; set suspend=y to pause during VM init */
    /* use android ADB transport */
    opt.optionString =
        "-agentlib:jdwp=transport=dt_android_adb,suspend=n,server=y";
    mOptions.add(opt);

    ALOGD("CheckJNI is %s\n", checkJni ? "ON" : "OFF");
    if (checkJni) {
        /* extended JNI checking */
        opt.optionString = "-Xcheck:jni";
        mOptions.add(opt);

        /* set a cap on JNI global references */
        opt.optionString = "-Xjnigreflimit:2000";
        mOptions.add(opt);

        /* with -Xcheck:jni, this provides a JNI function call trace */
        //opt.optionString = "-verbose:jni";
        //mOptions.add(opt);
    }

    char lockProfThresholdBuf[sizeof("-Xlockprofthreshold:") + sizeof(propBuf)];
    property_get("dalvik.vm.lockprof.threshold", propBuf, "");
    if (strlen(propBuf) > 0) {
      strcpy(lockProfThresholdBuf, "-Xlockprofthreshold:");
      strcat(lockProfThresholdBuf, propBuf);
      opt.optionString = lockProfThresholdBuf;
      mOptions.add(opt);
    }

    /* Force interpreter-only mode for selected opcodes. Eg "1-0a,3c,f1-ff" */
    char jitOpBuf[sizeof("-Xjitop:") + PROPERTY_VALUE_MAX];
    property_get("dalvik.vm.jit.op", propBuf, "");
    if (strlen(propBuf) > 0) {
        strcpy(jitOpBuf, "-Xjitop:");
        strcat(jitOpBuf, propBuf);
        opt.optionString = jitOpBuf;
        mOptions.add(opt);
    }

    /* Force interpreter-only mode for selected methods */
    char jitMethodBuf[sizeof("-Xjitmethod:") + PROPERTY_VALUE_MAX];
    property_get("dalvik.vm.jit.method", propBuf, "");
    if (strlen(propBuf) > 0) {
        strcpy(jitMethodBuf, "-Xjitmethod:");
        strcat(jitMethodBuf, propBuf);
        opt.optionString = jitMethodBuf;
        mOptions.add(opt);
    }

    if (executionMode == kEMIntPortable) {
        opt.optionString = "-Xint:portable";
        mOptions.add(opt);
    } else if (executionMode == kEMIntFast) {
        opt.optionString = "-Xint:fast";
        mOptions.add(opt);
    } else if (executionMode == kEMJitCompiler) {
        opt.optionString = "-Xint:jit";
        mOptions.add(opt);
    }

    if (checkDexSum) {
        /* perform additional DEX checksum tests */
        opt.optionString = "-Xcheckdexsum";
        mOptions.add(opt);
    }

    if (logStdio) {
        /* convert stdout/stderr to log messages */
        opt.optionString = "-Xlog-stdio";
        mOptions.add(opt);
    }

    if (enableAssertBuf[4] != '\0') {
        /* accept "all" to mean "all classes and packages" */
        if (strcmp(enableAssertBuf+4, "all") == 0)
            enableAssertBuf[3] = '\0';
        ALOGI("Assertions enabled: '%s'\n", enableAssertBuf);
        opt.optionString = enableAssertBuf;
        mOptions.add(opt);
    } else {
        ALOGV("Assertions disabled\n");
    }

    if (jniOptsBuf[10] != '\0') {
        ALOGI("JNI options: '%s'\n", jniOptsBuf);
        opt.optionString = jniOptsBuf;
        mOptions.add(opt);
    }

    if (stackTraceFileBuf[0] != '\0') {
        static const char* stfOptName = "-Xstacktracefile:";

        stackTraceFile = (char*) malloc(strlen(stfOptName) +
            strlen(stackTraceFileBuf) +1);
        strcpy(stackTraceFile, stfOptName);
        strcat(stackTraceFile, stackTraceFileBuf);
        opt.optionString = stackTraceFile;
        mOptions.add(opt);
    }

    /* extra options; parse this late so it overrides others */
    property_get("dalvik.vm.extra-opts", extraOptsBuf, "");
    parseExtraOpts(extraOptsBuf);

    /* Set the properties for locale */
    {
        char langOption[sizeof("-Duser.language=") + 3];
        char regionOption[sizeof("-Duser.region=") + 3];
        strcpy(langOption, "-Duser.language=");
        strcpy(regionOption, "-Duser.region=");
        readLocale(langOption, regionOption);
        opt.extraInfo = NULL;
        opt.optionString = langOption;
        mOptions.add(opt);
        opt.optionString = regionOption;
        mOptions.add(opt);
    }

    /*
     * We don't have /tmp on the device, but we often have an SD card.  Apps
     * shouldn't use this, but some test suites might want to exercise it.
     */
    opt.optionString = "-Djava.io.tmpdir=/sdcard";
    mOptions.add(opt);

    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");
        goto bail;
    }

    result = 0;

bail:
    free(stackTraceFile);
    return result;
}

可以看出这个函数的绝大部分都是在设置Java虚拟机的各项参数，没有什么好说。看到下面这一段变量定义，不知道大家有没有去思考过，这里为什么用PROPERTY_VALUE_MAX作为初始大小？

    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("-Xmx")-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];

下面是 PROPERTY_VALUE_MAX的定义：

@system/core/include/cutils/properties.h

/* System properties are *small* name value pairs managed by the
** property service.  If your data doesn't fit in the provided
** space it is not appropriate for a system property.
**
** WARNING: system/bionic/include/sys/system_properties.h also defines
**          these, but with different names.  (TODO: fix that)
*/
#define PROPERTY_KEY_MAX   PROP_NAME_MAX
#define PROPERTY_VALUE_MAX  PROP_VALUE_MAX

所以，没错， PROPERTY_VALUE_MAX是Android中属性value的最大长度，而java虚拟机的这些参数都是通过Android属性赋值和控制的，所以他们的值得大小肯定不能超过属性的最大长度。下面是我的小米2S手机中的一部分Java参数。Android中所有属性都可以通过getprop命令来查看。

[dalvik.vm.heapconcurrentstart]: [2097152]
[dalvik.vm.heapgrowthlimit]: [96m]
[dalvik.vm.heapidealfree]: [8388608]
[dalvik.vm.heapsize]: [384m]
[dalvik.vm.heapstartsize]: [8m]
[dalvik.vm.heaputilization]: [0.25]
[dalvik.vm.stack-trace-file]: [/data/anr/traces.txt]

下面来看一下startVm()中的最后几句：

    /*
     * 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");
        goto bail;
    }

startVm()在最后会调用JNI_CreateJavaVM()来创建虚拟机。这里顺便看一下JNI_CreateJavaVM（）的这段说明：”Java虚拟机对象JavaVm对象每个进程有一个，JNI环境变量JNIEnv每个线程有一个“。这里也告诉我们，在写JNI代码时要注意： JNIEnv不能在任意线程中使用，必须是原本就是Java线程（Java代码通过JNI调用native代码时，发起调用的那个肯定是Java线程），或者是让已有的native线程通过JNI来attach到Java环境。具体这里不做详细介绍，感兴趣的读者可以参考Oracle官方文档http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/jniTOC.html。在JNI_CreateJavaVM（）调用成功后虚拟机VM就已经创建好了，接下来就可以进行JNI相关调用了。

注册JNI函数

创建好了虚拟机，接下来要给虚拟机注册一些JNI函数。不知道各位读者有没有想过，这里为什么要注册JNI函数呢？没错，因为在基于虚拟机的Java世界里，Java并不是万能的，它用到的很多方法（例如：音频、视频相关、Binder等）都需要以native的方式实现，而这些方法就需要虚拟机以JNI的方式进行加载。

@/frameworks/base/core/jni/AndroidRuntime.cpp

/*
 * Register android native functions with the VM.
 */
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
    /*
     * 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.)
     */
<span style="white-space:pre">	</span>//设置Thread类的线程创建函数为javaCreateThreadEtc
    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;
}

我们来看一下register_jni_procs（）的代码：

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;
}

看一下从startReg()传过来的参数gRegJNI:

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_FileUtils),
    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),
};

REG_JNI是系统定义的一个宏：

#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

以gRegJNI数组中的一项为例，REG_JNI(register_android_debug_JNITest) 展开REG_JNI后变为：

{ register_android_debug_JNITest, "register_android_debug_JNITest" }

所以当register_jni_procs（）中调用mProcess时，最终调用的是android_debug_JNITest类中的register_android_debug_JNITest：

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

到这里JNI注册就讲完了。

ZygoteInit初始化

在JNI注册完成后，让我们再回头继续看AndroidRuntime中start函数：

env->CallStaticVoidMethod(startClass, startMeth, strArray);//通过JNI调用main()方法

在start()中通过JNI调用ZygoteInit类的main()方法，这个main()方法即使从native世界到Java世界的入口。

@/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java

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

            registerZygoteSocket();//注册zygote用的socket
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preload();//初始化，主要进行framework中一些类和资源的预加载
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());

            // Finish profiling the zygote initialization.
            SamplingProfilerIntegration.writeZygoteSnapshot();//结束统计并生成结果文件

            // Do an initial gc to clean up after startup
            gc();//强制进行一次回收

            // 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();//启动system_server进程
            } else if (!argv[1].equals("")) {
                throw new RuntimeException(argv[0] + USAGE_STRING);
            }

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

            runSelectLoop();//变成守护进程，接收socket信息进行处理

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

简单总结一下ZygoteInit类中main()函数主要做了如下几件事：

1. 注册Zygote用的socket
2. 类和资源的预加载
3. 启动system_server进程
4. 进入一个死循环，等待接收和处理socket事件

接下来将对它们一一进行分析。

registerZygoteSocket()方法

registerZygoteSocket()方法的实现如下：

    /**
     * Registers a server socket for zygote command connections
     *
     * @throws RuntimeException when open fails
     */
    private static void registerZygoteSocket() {
        if (sServerSocket == null) {
            int fileDesc;
            try {
                String env = System.getenv(ANDROID_SOCKET_ENV);//从环境变量中获取socket的fd
                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);
            }
        }
    }

registerZygoteSocket（）方法比较简单，就是创建了一个服务端Socket。

类和资源的预加载

在Zygote中通过preload()方法完成类和资源的加载，它的实现如下：

    static void preload() {
        preloadClasses();//加载类
        preloadResources();//加载资源
        preloadOpenGL();//加载OpenGL
    }

先看一下preloadClasses()：

   /**
     * Performs Zygote process initialization. Loads and initializes
     * commonly used classes.
     *
     * Most classes only cause a few hundred bytes to be allocated, but
     * a few will allocate a dozen Kbytes (in one case, 500+K).
     */
    private static void preloadClasses() {
        final VMRuntime runtime = VMRuntime.getRuntime();

        InputStream is = ClassLoader.getSystemClassLoader().getResourceAsStream(
                PRELOADED_CLASSES);//加载preloaded-classes这个文件中定义的需要预加载的类
        if (is == null) {
            Log.e(TAG, "Couldn't find " + PRELOADED_CLASSES + ".");
        } else {
            Log.i(TAG, "Preloading classes...");
            long startTime = SystemClock.uptimeMillis();

            // Drop root perms while running static initializers.
            setEffectiveGroup(UNPRIVILEGED_GID);
            setEffectiveUser(UNPRIVILEGED_UID);

            // Alter the target heap utilization.  With explicit GCs this
            // is not likely to have any effect.
            float defaultUtilization = runtime.getTargetHeapUtilization();
            runtime.setTargetHeapUtilization(0.8f);

            // Start with a clean slate.
            System.gc();
            runtime.runFinalizationSync();
            Debug.startAllocCounting();

            try {
                BufferedReader br
                    = new BufferedReader(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);//以反射的方式加载类
                        if (Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {
                            if (false) {
                                Log.v(TAG,
                                    " GC at " + Debug.getGlobalAllocSize());
                            }
                            System.gc();
                            runtime.runFinalizationSync();
                            Debug.resetGlobalAllocSize();
                        }
                        count++;
                    } catch (ClassNotFoundException e) {
                        Log.w(TAG, "Class not found for preloading: " + line);
                    } catch (Throwable t) {
                        Log.e(TAG, "Error preloading " + line + ".", t);
                        if (t instanceof Error) {
                            throw (Error) t;
                        }
                        if (t instanceof RuntimeException) {
                            throw (RuntimeException) t;
                        }
                        throw new RuntimeException(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);
                // Restore default.
                runtime.setTargetHeapUtilization(defaultUtilization);

                // Fill in dex caches with classes, fields, and methods brought in by preloading.
                runtime.preloadDexCaches();

                Debug.stopAllocCounting();

                // Bring back root. We'll need it later.
                setEffectiveUser(ROOT_UID);
                setEffectiveGroup(ROOT_GID);
            }
        }
    }

preloadClasses（）的实现很简单，这里说一下preloaded-classes文件：

# Classes which are preloaded by com.android.internal.os.ZygoteInit.
# Automatically generated by frameworks/base/tools/preload/WritePreloadedClassFile.java.
# MIN_LOAD_TIME_MICROS=1250
# MIN_PROCESSES=10
android.R$styleable
android.accounts.Account
android.accounts.Account$1
android.accounts.AccountManager
android.accounts.AccountManager$12
android.accounts.AccountManager$13
android.accounts.AccountManager$6
android.accounts.AccountManager$AmsTask
android.accounts.AccountManager$AmsTask$1
android.accounts.AccountManager$AmsTask$Response

在Android4.4的源代码中有2782行，也就说这里在系统启动时需要预加载两千多个类，而这仅仅是源代码，在手机厂商的代码中，需要进行预加载的类的数量将会超过这个数。preloaded-classes文件时由WritePreloadedClassFile类生成的。WritePreloadedClassFile将某个类加入预加载文件preloaded-classes中的条件时：该类被不少于10个进程使用，并且家中该类好使超过1250微秒。WritePreloadedClassFile里面的实现非常简单，感兴趣的读者可以自行阅读。

    /**
     * Preload any class that take longer to load than MIN_LOAD_TIME_MICROS us.
     */
    static final int MIN_LOAD_TIME_MICROS = 1250;

    /**
     * Preload any class that was loaded by at least MIN_PROCESSES processes.
     */
    static final int MIN_PROCESSES = 10;

这里我简单估算了一下，在Android4.4中预加载这些类需要4秒钟作用，这对于系统启动来说是一个比较长的时间，因此在进行系统启动速度的优化时，这里可以作为一个优化大点。

在看preloadClass的代码时有些读者看的setEffectiveUser的几句代码不明白什么意思：

    private static void preloadClasses() {
    		......
            // Drop root perms while running static initializers.
            setEffectiveGroup(UNPRIVILEGED_GID);
            setEffectiveUser(UNPRIVILEGED_UID);
            ......
            } finally {
            	......
                // Bring back root. We'll need it later.
                setEffectiveUser(ROOT_UID);
                setEffectiveGroup(ROOT_GID);
            }
        }
    }

以setEffectiveUser为例看一下它的实现：

    /**
     * Sets effective user ID.
     */
    private static void setEffectiveUser(int uid) {
        int errno = setreuid(ROOT_UID, uid);
        if (errno != 0) {
            Log.e(TAG, "setreuid() failed. errno: " + errno);
        }
    }

    /**
     * The Linux syscall "setreuid()"
     * @param ruid real uid
     * @param euid effective uid
     * @return 0 on success, non-zero errno on fail
     */
    static native int setreuid(int ruid, int euid);

可以看出这里setEffectiveUser这几句的意思是：在类加载之前临时降低euid（真实用户ID）权限，加载完成后恢复。关于Linux各种userid的说明如下：

有效用户ID

有效用户ID（Effective UID，即EUID）与有效用户组ID（Effective Group ID，即EGID）在创建与访问文件的时候发挥作用；具体来说，创建文件时，系统内核将根据创建文件的进程的EUID与EGID设定文件的所有者/组属性，而在访问文件时，内核亦根据访问进程的EUID与EGID决定其能否访问文件。

真实用户ID

真实用户ID（Real UID,即RUID）与真实用户组ID（Real GID，即RGID）用于辨识进程的真正所有者，且会影响到进程发送信号的权限。没有超级用户权限的进程仅在其RUID与目标进程的RUID相匹配时才能向目标进程发送信号，例如在父子进程间，子进程从父进程处继承了认证信息，使得父子进程间可以互相发送信号。

暂存用户ID

暂存用户ID（Saved UID，即SUID）于以提升权限运行的进程暂时需要做一些不需特权的操作时使用，这种情况下进程会暂时将自己的有效用户ID从特权用户（常为root）对应的UID变为某个非特权用户对应的UID，而后将原有的特权用户UID复制为SUID暂存；之后当进程完成不需特权的操作后，进程使用SUID的值重置EUID以重新获得特权。在这里需要说明的是，无特权进程的EUID值只能设为与RUID、SUID与EUID（也即不改变）之一相同的值。

文件系统用户ID

文件系统用户ID（File System UID，即FSUID）在Linux中使用，且只用于对文件系统的访问权限控制，在没有明确设定的情况下与EUID相同（若FSUID为root的UID，则SUID、RUID与EUID必至少有一亦为root的UID），且EUID改变也会影响到FSUID。设立FSUID是为了允许程序（如NFS服务器）在不需获取向给定UID账户发送信号的情况下以给定UID的权限来限定自己的文件系统权限。

这段代码转自 http://zh.wikipedia.org/wiki/%E7%94%A8%E6%88%B7ID

那这里这样做的用意何在？我猜这里是为了保证预加载的类是所有的用户都是可用的。

预加载资源的代码如下：

   /**
     * Load in commonly used resources, so they can be shared across
     * processes.
     *
     * These tend to be a few Kbytes, but are frequently in the 20-40K
     * range, and occasionally even larger.
     */
    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);//预加载Drawable
                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);//预加载Color
                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();
        }
    }

preloadResources的加载过程又分为加载Drawable和加载Color。

除了加载类和资源，还会加载OpenGL的一些东西：

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

在创建socket和资源加载前后有这么两句：

            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();//启动性能统计

            registerZygoteSocket();//注册zygote用的socket
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preload();//初始化，主要进行framework中一些类和资源的预加载
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());

            // Finish profiling the zygote initialization.
            SamplingProfilerIntegration.writeZygoteSnapshot();//结束统计并生成结果文件

所以，这里SamplingProfilerIntegration统计的是创建socket和类及资源初始化的时间。

启动system_server

    /**
     * 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
        );
        /* Hardcoded command line to start the system server */
        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",
            "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(//以fork的方式创建system_server进程
                    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) {//pid==0说明在子进程中，父进程为Zygote
            handleSystemServerProcess(parsedArgs);
        }

        return true;
    }

这里前面的一大段代码主要是在为fork准备参数parsedArgs，然后Zygote会forkSystemServer来创建system_server，forkSystemServer()方法最终会调用Linux中的fork()。

runSelectLoop()方法

在创建system_server后，Zygote调用runSelectLoop（）进入到一个死循环中：

    /**
     * Runs the zygote process's select loop. Accepts new connections as
     * they happen, and reads commands from connections one spawn-request's
     * worth at a time.
     *
     * @throws MethodAndArgsCaller in a child process when a main() should
     * be executed.
     */
    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());//registerZygoteSocket中创建的socket的描述符
        peers.add(null);

        int loopCount = GC_LOOP_COUNT;
        while (true) {//死循环
            int index;//selectReadable方法监控的句柄的下标（fdArray中的下标）

            /*
             * Call gc() before we block in select().
             * It's work that has to be done anyway, and it's better
             * to avoid making every child do it.  It will also
             * madvise() any free memory as a side-effect.
             *
             * Don't call it every time, because walking the entire
             * heap is a lot of overhead to free a few hundred bytes.
             */
            if (loopCount <= 0) {//Zygote每循环GC_LOOP_COUNT（这里的值是10）次就会进行一次内存回收
                gc();
                loopCount = GC_LOOP_COUNT;
            } else {
                loopCount--;
            }


            try {
                fdArray = fds.toArray(fdArray);
                index = selectReadable(fdArray);//内部由select()实现，在没有客户端事件时会堵塞
            } catch (IOException ex) {
                throw new RuntimeException("Error in select()", ex);
            }

            if (index < 0) {
                throw new RuntimeException("Error in select()");
            } else if (index == 0) {//index==0表示selcet接收到的是Zygote的socket的事件
                ZygoteConnection newPeer = acceptCommandPeer();
                peers.add(newPeer);
                fds.add(newPeer.getFileDesciptor());
            } else {//调用ZygoteConnection对象的runOnce方法，ZygoteConnection是在index == 0时被添加到peers的
                boolean done;
                done = peers.get(index).runOnce();

                if (done) {
                    peers.remove(index);
                    fds.remove(index);
                }
            }
        }
    }

下面是selcetReadable方法的代码：

    /**
     * Invokes select() on the provider array of file descriptors (selecting
     * for readability only). Array elements of null are ignored.
     *
     * @param fds non-null; array of readable file descriptors
     * @return index of descriptor that is now readable or -1 for empty array.
     * @throws IOException if an error occurs
     */
    static native int selectReadable(FileDescriptor[] fds) throws IOException;

selectReadable的native实现在com_android_internal_os_ZygoteInit.cpp中。
Zygote接收到socket客户端的链接后会将其（客户端Socket）保存到一个ZygoteConnection对象中，然后保存到peers

    /**
     * Waits for and accepts a single command connection. Throws
     * RuntimeException on failure.
     */
    private static ZygoteConnection acceptCommandPeer() {
        try {
            return new ZygoteConnection(sServerSocket.accept());
        } catch (IOException ex) {
            throw new RuntimeException(
                    "IOException during accept()", ex);
        }
    }

最后，客户端的请求会有ZygoteConnection的runOnce来处理。