android源碼分析-Zygote

時間 2020-05-05

原文原文鏈接

android也是基於linux的系統，所以全部的進程都是從init進程開始的（直接或間接的從init進程fock出來的）。Zygote是受精卵進程，也是系統啓動過程當中由init進程建立的，具體的看下啓動腳本/system/core/rootdir/init.zygote64.rc：java

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server
class main
priority -20
user root
group root readproc
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart audioserver
onrestart restart cameraserver
onrestart restart media
onrestart restart netd
onrestart restart wificond
writepid /dev/cpuset/foreground/tasks

能夠看出，要執行的進程是/system/bin/app_process64。代碼在/frameworks/base/cmds/app_process/app_main.cpp。入口函數是main:linux

......
// 建立AppRuntime
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
......
while (i < argc) {
    const char* arg = argv[i++];
    if (strcmp(arg, "--zygote") == 0) {
        // 確認是zygote進程
        zygote = true;
        niceName = ZYGOTE_NICE_NAME;
    } 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;
    }
}
......
if (zygote) {
    // 執行zygote進程
    runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
} else if (className) {
    runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
} else {
    fprintf(stderr, "Error: no class name or --zygote supplied.\n");
    app_usage();
    LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
}
......

若是忽略掉參數這些細節，剩下的就是AppRuntime的創建和調用AppRuntime的start方法了，啓動的是com.android.internal.os.ZygoteInit。
看下AppRuntime：android

class AppRuntime : public AndroidRuntime
{
public:
    AppRuntime(char* argBlockStart, const size_t argBlockLength)
        : AndroidRuntime(argBlockStart, argBlockLength)
        , mClass(NULL)
    {
    }
    ......
};

構造函數中調用了基類的構造方法，基類在/frameworks/base/core/jni/AndroidRuntime.cpp：ios

AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
        mExitWithoutCleanup(false),
        mArgBlockStart(argBlockStart),
        mArgBlockLength(argBlockLength)
{
    SkGraphics::Init();
    // There is also a global font cache, but its budget is specified by
    // SK_DEFAULT_FONT_CACHE_COUNT_LIMIT and SK_DEFAULT_FONT_CACHE_LIMIT.

    // Pre-allocate enough space to hold a fair number of options.
    mOptions.setCapacity(20);

    assert(gCurRuntime == NULL);        // one per process
    gCurRuntime = this;
}

保留了本身做爲全局gCurRuntime。
直接看start方法：web

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
    ......
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    // 啓動虛擬機
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    // 回調虛擬機的建立
    onVmCreated(env);

    /*
     * Register android functions.
     */
     // 註冊函數
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
    ......
    jclass stringClass;
    jobjectArray strArray;
    jstring classNameStr;

    // 得到一個string的對象的引用
    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    // 建立一個String數組對象
    strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    // 設置第一個string數組的第一個元素是classNameStr，在這裏就是ZygoteInit的全名
    env->SetObjectArrayElement(strArray, 0, classNameStr);
    // 設置其餘參數
    for (size_t i = 0; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        assert(optionsStr != NULL);
        env->SetObjectArrayElement(strArray, i + 1, optionsStr);
    }

    /*
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
     // 轉換類中間的.爲/，這裏是轉換格式
    char* slashClassName = toSlashClassName(className);
    // 從jni環境中找到這個類
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        // 調用找到的類的main方法，這裏就是調用ZygoteInit的main方法
        jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
            "([Ljava/lang/String;)V");
        if (startMeth == NULL) {
            ALOGE("JavaVM unable to find main() in '%s'\n", className);
            /* keep going */
        } else {
            env->CallStaticVoidMethod(startClass, startMeth, strArray);

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

關鍵部分已經給出了註釋。總結一下：
1.啓動虛擬機startVM；
2.經過startReg註冊jni方法；
3.調用ZygoteInit類的main方法；
startVm基本上就是爲這個進程創建一個Dalvik虛擬機環境，爲當前線程初始化一個jni環境。startReg基本上是註冊一大堆的jni方法，以供後面調用。不是本文重點，所以這裏再也不累述。數組

下面咱們要關注的是ZygoteInit類的main方法了。
/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java：網絡

public static void main(String argv[]) {
    ZygoteServer zygoteServer = new ZygoteServer();
    ......
    zygoteServer.registerServerSocket(socketName);
    ......
    preload();
    ......
    if (startSystemServer) {
        startSystemServer(abiList, socketName, zygoteServer);
    }
    ......
    zygoteServer.runSelectLoop(abiList);
    ......
}

1.建立ZygoteServer（能夠看出是個cs架構的東西）;
2.註冊socket（使用socket進行通信方式）；
3.預加載；
4.啓動SystemServer；
5.運行select循環體；
裏面涉及到ZygoteHooks的運轉，爲了避免影響總體，暫時作個標記，後面再閱讀。
這裏能夠看到Zygote基本上是個cs架構的狀況，而且經過socket進行這種架構的通信。先來看看預加載過程：架構

static void preload() {
        Log.d(TAG, "begin preload");
        Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "BeginIcuCachePinning");
        beginIcuCachePinning();
        Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
        Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadClasses");
        //預加載位於framework/base/preload-classes文件中的類
        preloadClasses();
        Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
        Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadResources");
        // 預加載資源
        preloadResources();
        Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
        Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadOpenGL");
        // 預加載資源
        preloadOpenGL();
        Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
        //經過System.loadLibrary()方法，預加載"android","compiler_rt","jnigraphics"這3個共享庫
        preloadSharedLibraries();
        //預加載文本鏈接符資源
        preloadTextResources();
        // webview的初始化
        // Ask the WebViewFactory to do any initialization that must run in the zygote process,
        // for memory sharing purposes.
        WebViewFactory.prepareWebViewInZygote();
        endIcuCachePinning();
        warmUpJcaProviders();
        Log.d(TAG, "end preload");
    }

看到了吧，都是android自己的一些資源的初始化過程，就是在這裏完成的。
下面再看下startSystemServer：app

private static boolean startSystemServer(String abiList, String socketName, ZygoteServer zygoteServer)
            throws Zygote.MethodAndArgsCaller, RuntimeException {
        long capabilities = posixCapabilitiesAsBits(
            OsConstants.CAP_IPC_LOCK,
            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,
            OsConstants.CAP_WAKE_ALARM
        );
        /* Containers run without this capability, so avoid setting it in that case */
        if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
            capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
        }
        /* Hardcoded command line to start the system server */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007,3009,3010",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "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) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }

            zygoteServer.closeServerSocket();
            handleSystemServerProcess(parsedArgs);
        }

        return true;
    }

其實主要的就是Zygote.forkSystemServer這句話，前面的都是參數的配置。再向下看一層/frameworks/base/core/java/com/android/internal/os/Zygote.java：socket

public static int forkSystemServer(int uid, int gid, int[] gids, int debugFlags,
            int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {
        VM_HOOKS.preFork();
        int pid = nativeForkSystemServer(
                uid, gid, gids, debugFlags, rlimits, permittedCapabilities, effectiveCapabilities);
        // Enable tracing as soon as we enter the system_server.
        if (pid == 0) {
            Trace.setTracingEnabled(true);
        }
        VM_HOOKS.postForkCommon();
        return pid;
    }

根據傳遞進來的uid,gid等調用函數nativeForkSystemServer，最終會在/frameworks/base/core/jni/com_android_internal_os_Zygote.cpp下的ForkAndSpecializeCommon中調用fork函數，那麼實際上就能夠知道，就是在c層fork分裂出一個進程來做爲SystemServer。

如今咱們回來看java層的ZygoteInit.java，繼續看看與socket相關的部分，首先是registerServerSocket:

void registerServerSocket(String socketName) {
        if (mServerSocket == 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);
                mServerSocket = new LocalServerSocket(fd);
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }

這裏設置了文件描述符，而後建立了LocalServerSocket賦值給了mServerSocket。/frameworks/base/core/java/android/net/LocalServerSocket.java：

public LocalServerSocket(FileDescriptor fd) throws IOException
    {
        impl = new LocalSocketImpl(fd);
        impl.listen(LISTEN_BACKLOG);
        localAddress = impl.getSockAddress();
    }

new出LocalSocketImpl後，直接就開始listen了。下面暫時不用特別看了吧，就是一個走的正常的網絡socket了。這裏應該就能夠證實是以socket的方法進行的通信。而後再來看看runSelectLoop：

void runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller {
        ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
        ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();

        fds.add(mServerSocket.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) {
                if ((pollFds[i].revents & POLLIN) == 0) {
                    continue;
                }
                if (i == 0) {
                    ZygoteConnection newPeer = acceptCommandPeer(abiList);
                    peers.add(newPeer);
                    fds.add(newPeer.getFileDesciptor());
                } else {
                    boolean done = peers.get(i).runOnce(this);
                    if (done) {
                        peers.remove(i);
                        fds.remove(i);
                    }
                }
            }
        }
    }

進入一個死循環，每次都將全部要觀察的fd創建成數組，而後調用Os.poll(pollFds, -1)阻塞等待fd的變化。後面一個for循環是當fd有變化（即有客戶端鏈接，也就是說有其餘進程想要與ZygoteServer通信），此時調用ZygoteConnection的runOnce方法。這個方法若是簡單看下的話，最終是要調用Zygote.forkAndSpecialize分裂出進程來的，也就是說這個方法是一旦有鏈接創建後就表示有app啓動了，此時就要fork分裂出新的進程來，代碼暫時就不貼了。

至此爲止，Zygote的過程基本分析完畢。總結一下：
1.系統啓動，經過init進程會啓動Zygote進程。確切的將是經過runtime調用了ZygoteInit，這個初始化過程；
2.Zygote是cs架構的，基於socket通信機制的，在ZygoteInit過程當中會啓動ZygoteServer，爲了等待接收socket的通信來進行啓動app進程的處理；
3.分裂出SystemServer進程，負責啓動系統的一些關鍵服務。包括3類（廣播類、核心類、其餘類）；

最後附圖一張便於理解：