Android系統啟動-zygote篇

/frameworks/base/cmds/app_process/App_main.cpp （內含AppRuntime類）
/frameworks/base/core/jni/AndroidRuntime.cpp
/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/frameworks/base/core/java/com/android/internal/os/Zygote.java
/frameworks/base/core/java/android/net/LocalServerSocket.java

App_main.main
    AndroidRuntime.start
        startVm
        startReg
        ZygoteInit.main
            registerZygoteSocket
            preload
            startSystemServer
            runSelectLoop

int main(int argc, char* const argv[])
{
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));

    //忽略第一個參數
    argc--;
    argv++;

    int i;
    for (i = 0; i < argc; i++) {
        if (argv[i][0] != '-') {
            break;
        }
        if (argv[i][1] == '-' && argv[i][2] == 0) {
            ++i;
            break;
        }
        runtime.addOption(strdup(argv[i]));
    }
    //參數解析
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    String8 niceName;
    String8 className;
    ++i;
    while (i < argc) {
        const char* arg = argv[i++];
        if (strcmp(arg, "--zygote") == 0) {
            zygote = true;
            niceName = ZYGOTE_NICE_NAME; //對于64位系統nice_name為zygote64,32位系統為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.setTo(arg + 12);
        } else if (strncmp(arg, "--", 2) != 0) {
            className.setTo(arg);
            break;
        } else {
            --i;
            break;
        }
    }
    Vector<String8> args;
    if (!className.isEmpty()) {
        // 運行application或tool程序
        args.add(application ? String8("application") : String8("tool"));
        runtime.setClassNameAndArgs(className, argc - i, argv + i);
    } else {
        //進入zygote模式，創建 /data/dalvik-cache路徑
        maybeCreateDalvikCache();
        if (startSystemServer) {
            args.add(String8("start-system-server"));
        }
        char prop[PROP_VALUE_MAX];
        if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) {
            return 11;
        }
        String8 abiFlag("--abi-list=");
        abiFlag.append(prop);
        args.add(abiFlag);

        for (; i < argc; ++i) {
            args.add(String8(argv[i]));
        }
    }

    //設置進程名
    if (!niceName.isEmpty()) {
        runtime.setArgv0(niceName.string());
        set_process_name(niceName.string());
    }
    if (zygote) {
        // 啟動AppRuntime 【見小節3】
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } else if (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
    } else {
        //沒有指定類名或zygote，參數錯誤
        return 10;
    }
}

 app_process [可選參數] 命令所在路徑 啟動的類名 [可選參數]

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
    static const String8 startSystemServer("start-system-server");

    for (size_t i = 0; i < options.size(); ++i) {
        if (options[i] == startSystemServer) {
           const int LOG_BOOT_PROGRESS_START = 3000;
        }
    }
    const char* rootDir = getenv("ANDROID_ROOT");
    if (rootDir == NULL) {
        rootDir = "/system";
        if (!hasDir("/system")) {
            return;
        }
        setenv("ANDROID_ROOT", rootDir, 1);
    }
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    // 虛擬機創建【見小節3.1】
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);
    // JNI方法注冊【見小節3.2】
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }

    jclass stringClass;
    jobjectArray strArray;
    jstring classNameStr;

    //等價 strArray= new String[options.size() + 1];
    stringClass = env->FindClass("java/lang/String");
    strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);

    //等價 strArray[0] = "com.android.internal.os.ZygoteInit"
    classNameStr = env->NewStringUTF(className);
    env->SetObjectArrayElement(strArray, 0, classNameStr);

    //等價 strArray[1] = "start-system-server"；
    //   strArray[2] = "--abi-list=xxx"；其中xxx為系統響應的cpu架構類型，比如arm64-v8a.
    for (size_t i = 0; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        env->SetObjectArrayElement(strArray, i + 1, optionsStr);
    }

    //將"com.android.internal.os.ZygoteInit"轉換為"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 {
            // 調用ZygoteInit.main()方法【見小節4.0】
            env->CallStaticVoidMethod(startClass, startMeth, strArray);
        }
    }
    free(slashClassName); //釋放相應對象的內存空間
    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");
}

int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
{
    // JNI檢測功能，用于native層調用jni函數時進行常規檢測，比較弱字符串格式是否符合要求，資源是否正確釋放。該功能一般用于早期系統調試或手機Eng版，對于User版往往不會開啟，引用該功能比較消耗系統CPU資源，降低系統性能。
    bool checkJni = false;
    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;
        }
    }
    if (checkJni) {
        addOption("-Xcheck:jni");
    }

    //虛擬機產生的trace文件，主要用于分析系統問題，路徑默認為/data/anr/traces.txt
    parseRuntimeOption("dalvik.vm.stack-trace-file", stackTraceFileBuf, "-Xstacktracefile:");

    //對于不同的軟硬件環境，這些參數往往需要調整、優化，從而使系統達到最佳性能
    parseRuntimeOption("dalvik.vm.heapstartsize", heapstartsizeOptsBuf, "-Xms", "4m");
    parseRuntimeOption("dalvik.vm.heapsize", heapsizeOptsBuf, "-Xmx", "16m");
    parseRuntimeOption("dalvik.vm.heapgrowthlimit", heapgrowthlimitOptsBuf, "-XX:HeapGrowthLimit=");
    parseRuntimeOption("dalvik.vm.heapminfree", heapminfreeOptsBuf, "-XX:HeapMinFree=");
    parseRuntimeOption("dalvik.vm.heapmaxfree", heapmaxfreeOptsBuf, "-XX:HeapMaxFree=");
    parseRuntimeOption("dalvik.vm.heaptargetutilization",
                       heaptargetutilizationOptsBuf, "-XX:HeapTargetUtilization=");

    //preloaded-classes文件內容是由WritePreloadedClassFile.java生成的，在ZygoteInit類中會預加載工作將其中的classes提前加載到內存，以提高系統性能
    if (!hasFile("/system/etc/preloaded-classes")) {
        return -1;
    }

    //創建虛擬機
    if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
        ALOGE("JNI_CreateJavaVM failed\n");
        return -1;
    }
}

int AndroidRuntime::startReg(JNIEnv* env)
{
    //設置線程創建方法為javaCreateThreadEtc
    androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);

    env->PushLocalFrame(200);
    //進程NI方法的注冊
    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL);
        return -1;
    }
    env->PopLocalFrame(NULL);
    return 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) { 【見下文】
            return -1;
        }
    }
    return 0;
}

static const RegJNIRec gRegJNI[] = {
    REG_JNI(register_com_android_internal_os_RuntimeInit),
    REG_JNI(register_android_os_Binder)，
    ...
}；

#define REG_JNI(name)      { name }
struct RegJNIRec {
    int (*mProc)(JNIEnv*);
};

int register_com_android_internal_os_RuntimeInit(JNIEnv* env)
{
    return jniRegisterNativeMethods(env, "com/android/internal/os/RuntimeInit",
        gMethods, NELEM(gMethods));
}

//gMethods：java層方法名與jni層的方法的一一映射關系
static JNINativeMethod gMethods[] = {
    { "nativeFinishInit", "()V",
        (void*) com_android_internal_os_RuntimeInit_nativeFinishInit },
    { "nativeZygoteInit", "()V",
        (void*) com_android_internal_os_RuntimeInit_nativeZygoteInit },
    { "nativeSetExitWithoutCleanup", "(Z)V",
        (void*) com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup },
};

public static void main(String argv[]) {
    try {
        RuntimeInit.enableDdms(); //開啟DDMS功能
        SamplingProfilerIntegration.start();
        boolean startSystemServer = false;
        String socketName = "zygote";
        String abiList = null;
        for (int i = 1; i < argv.length; i++) {
            if ("start-system-server".equals(argv[i])) {
                startSystemServer = true;
            } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                abiList = argv[i].substring(ABI_LIST_ARG.length());
            } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                socketName = argv[i].substring(SOCKET_NAME_ARG.length());
            } else {
                throw new RuntimeException("Unknown command line argument: " + argv[i]);
            }
        }
        if (abiList == null) {
            throw new RuntimeException("No ABI list supplied.");
        }
        registerZygoteSocket(socketName); //為Zygote注冊socket【見小節4.1】
        preload(); // 預加載類和資源【見小節4.2】
        SamplingProfilerIntegration.writeZygoteSnapshot();
        gcAndFinalize(); //GC操作
        if (startSystemServer) {
            startSystemServer(abiList, socketName);//啟動system_server【見小節4.3】
        }
        runSelectLoop(abiList); //進入循環模式【見小節4.4】
        closeServerSocket();
    } catch (MethodAndArgsCaller caller) {
        caller.run(); //啟動system_server中會講到。
    } catch (RuntimeException ex) {
        closeServerSocket();
        throw ex;
    }
}

private static void registerZygoteSocket(String socketName) {
    if (sServerSocket == null) {
        int fileDesc;
        final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
        try {
            String env = System.getenv(fullSocketName);
            fileDesc = Integer.parseInt(env);
        } catch (RuntimeException ex) {
            throw new RuntimeException(fullSocketName + " unset or invalid", ex);
        }

        try {
            FileDescriptor fd = new FileDescriptor();
            fd.setInt$(fileDesc); //設置文件描述符
            sServerSocket = new LocalServerSocket(fd); //創建Socket的本地服務端
        } catch (IOException ex) {
            throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);
        }
    }
}

static void preload() {
    //預加載位于/system/etc/preloaded-classes文件中的類
    preloadClasses(); 

    //預加載資源，包含drawable和color資源
    preloadResources();

    //預加載OpenGL
    preloadOpenGL();

    //通過System.loadLibrary()方法，
    //預加載"android","compiler_rt","jnigraphics"這3個共享庫
    preloadSharedLibraries();

    //預加載  文本連接符資源
    preloadTextResources();

    //僅用于zygote進程，用于內存共享的進程
    WebViewFactory.prepareWebViewInZygote();
}

private static boolean startSystemServer(String abiList, String socketName)
        throws MethodAndArgsCaller, RuntimeException {
    long capabilities = posixCapabilitiesAsBits(
        OsConstants.CAP_BLOCK_SUSPEND,
        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
    );
    //參數準備
    String args[] = {
        "--setuid=1000",
        "--setgid=1000",
        "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007",
        "--capabilities=" + capabilities + "," + capabilities,
        "--nice-name=system_server",
        "--runtime-args",
        "com.android.server.SystemServer",
    };

    ZygoteConnection.Arguments parsedArgs = null;
    int pid;
    try {
        //用于解析參數，生成目標格式
        parsedArgs = new ZygoteConnection.Arguments(args);
        ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
        ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

        // fork子進程，用于運行system_server
        pid = Zygote.forkSystemServer(
                parsedArgs.uid, parsedArgs.gid,
                parsedArgs.gids,
                parsedArgs.debugFlags,
                null,
                parsedArgs.permittedCapabilities,
                parsedArgs.effectiveCapabilities);
    } catch (IllegalArgumentException ex) {
        throw new RuntimeException(ex);
    }

    //進入子進程system_server
    if (pid == 0) {
        if (hasSecondZygote(abiList)) {
            waitForSecondaryZygote(socketName);
        }
        // 完成system_server進程剩余的工作
        handleSystemServerProcess(parsedArgs);
    }
    return true;
}

private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
    ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
    ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
    //sServerSocket是socket通信中的服務端，即zygote進程
    fds.add(sServerSocket.getFileDescriptor());
    peers.add(null);

    while (true) {
        StructPollfd[] pollFds = new StructPollfd[fds.size()];
        for (int i = 0; i < pollFds.length; ++i) {
            pollFds[i] = new StructPollfd();
            pollFds[i].fd = fds.get(i);
            pollFds[i].events = (short) POLLIN;
        }
        try {
            Os.poll(pollFds, -1);
        } catch (ErrnoException ex) {
            throw new RuntimeException("poll failed", ex);
        }
        for (int i = pollFds.length - 1; i >= 0; --i) {
            //采用I/O多路復用機制，當客戶端發出連接請求或者數據處理請求時，跳過continue，執行后面的代碼
            if ((pollFds[i].revents & POLLIN) == 0) {
                continue;
            }
            if (i == 0) {
                //創建客戶端連接
                ZygoteConnection newPeer = acceptCommandPeer(abiList);
                peers.add(newPeer);
                fds.add(newPeer.getFileDesciptor());
            } else {
                //處理客戶端數據事務
                boolean done = peers.get(i).runOnce();
                if (done) {
                    peers.remove(i);
                    fds.remove(i);
                }
            }
        }
    }
}