Android源码之SurfaceFlinger的启动(一)

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page1
在Android系统中, 显示系统在底层是通过SurfaceFlinger服务来完成的, 因此从今天开始, 我们从SurfaceFlinger服务作为入口来分析一下Android显示系统.
SurfaceFlinger服务是在System进程中, 而System进程是由Zygote进程启动的, 并且是以Java层的SystemServer类的静态成员函数main为入口函数的。
因此，接下来我们就从SystemServer类(定义在SystemServer.java文件中)的静态成员函数main开始，分析SurfaceFlinger服务的启动过程:
1 public static void main(String[] args) {
    2    if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
    3        // If a device's clock is before 1970 (before 0), a lot of
    4        // APIs crash dealing with negative numbers, notably
    5        // java.io.File#setLastModified, so instead we fake it and
    6        // hope that time from cell towers or NTP fixes it
    7        // shortly.
    8        Slog.w(TAG, "System clock is before 1970; setting to 1970.");
    9        SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
    10    }

    11    if (SamplingProfilerIntegration.isEnabled()) {
    12        SamplingProfilerIntegration.start();
    13        timer = new Timer();
    14        timer.schedule(new TimerTask() {
    15            @Override
    16            public void run() {
    17                SamplingProfilerIntegration.writeSnapshot("system_server", null);
    18            }
    19        }, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);
    20    }

        // Mmmmmm... more memory!
    21    dalvik.system.VMRuntime.getRuntime().clearGrowthLimit();

        // The system server has to run all of the time, so it needs to be
        // as efficient as possible with its memory usage.
    22    VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);

    23    System.loadLibrary("android_servers");
    24    init1(args);
    25 }
第24行(SystemServer.main)会调用init1函数来进行初始化操作, init1函数是个native函数来初始化C++层的服务, 会导致com_android_server_SystemServer.cpp的android_server_SystemServer_init1函数的调用, 关于com_android_server_SystemServer.cpp的android_server_SystemServer_init1函数的详细分析可以参考page2文件.

page2
1static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
2{
3 system_init();
4}
第3行(com_android_server_SystemServer->android_server_SystemServer_init1)会调用system_init函数, system_init函数是在system_init.cpp文件中定义的, system_init函数的定义如下:
1extern "C" status_t system_init()
2{
3 ALOGI("Entered system_init()");
4 sp proc(ProcessState::self());
5 sp sm = defaultServiceManager();
6 ALOGI("ServiceManager: %p\n", sm.get());
7 sp grim = new GrimReaper();
8 sm->asBinder()->linkToDeath(grim, grim.get(), 0);

9 char propBuf[PROPERTY_VALUE_MAX];
10 property_get("system_init.startsurfaceflinger", propBuf, "1");
11 if (strcmp(propBuf, "1") == 0) {
12 // Start the SurfaceFlinger
13 SurfaceFlinger::instantiate();
14 }

15 property_get("system_init.startsensorservice", propBuf, "1");
16 if (strcmp(propBuf, "1") == 0) {
17 // Start the sensor service
18 SensorService::instantiate();
19 }

// And now start the Android runtime.  We have to do this bit
// of nastiness because the Android runtime initialization requires
// some of the core system services to already be started.
// All other servers should just start the Android runtime at
// the beginning of their processes's main(), before calling
// the init function.
20 ALOGI("System server: starting Android runtime.\n");
21 AndroidRuntime* runtime = AndroidRuntime::getRuntime();

22 ALOGI("System server: starting Android services.\n");
23 JNIEnv* env = runtime->getJNIEnv();
24 if (env == NULL) {
25 return UNKNOWN_ERROR;
26 }
27 jclass clazz = env->FindClass("com/android/server/SystemServer");
28 if (clazz == NULL) {
29 return UNKNOWN_ERROR;
30 }
31 jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
32 if (methodId == NULL) {
33 return UNKNOWN_ERROR;
34 }
35 env->CallStaticVoidMethod(clazz, methodId);

36 ALOGI("System server: entering thread pool.\n");
37 ProcessState::self()->startThreadPool();
38 IPCThreadState::self()->joinThreadPool();
39 ALOGI("System server: exiting thread pool.\n");

40 return NO_ERROR;
41 }
第9-14行(system_init->system_init)如果发现系统属性中设置了启动SurfaceFlinger服务, 第13行(system_init->system_init)会调用SurfaceFlinger的instantiate函数来启动SurfaceFlinger服务, 关于SurfaceFlinger的instantiate的详细分析可以参考page3文件.
第37-38行(system_init->system_init)是启动Binder的线程池, 从此开始, SystemServer进程就可以接受Binder的进程间通信了.
page3
我们从SurfaceFlinger的instantiate函数作为入口来分析SurfaceFinger服务的启动.
我们先来看一下SurfaceFlinger类的继承体系:
class SurfaceFlinger : public BinderService,
                           public BnSurfaceComposer,
                           private IBinder::DeathRecipient,
                           private Thread,
                           private HWComposer::EventHandler

    我靠!SurfaceFlinger继承了5个父类.
    不管了, 先分析instantiate函数, instantiate函数是从BinderService继承而来的, 我们先来看一下BinderService类的声明:
    template class BinderService
    BinderService接受一个模板参数.
    BinderService的instantiate函数定义如下:
    static void instantiate() {
        publish();
    }

    instantiate函数只是调用了publish函数, publish函数的定义如下:
    1    static status_t publish(bool allowIsolated = false) {
    2        sp sm(defaultServiceManager());
    3        return sm->addService(String16(SERVICE::getServiceName()), new SERVICE(), allowIsolated);
    4    }
    第2行(BinderService->publish)获得ServiceManager.
    第3行(BinderService->publish)会new一个SERVICE实例, 并调用ServiceManager的addService函数来交给ServiceManager管理, 这是Binder机制的基础.
    因此这里会new一个SurfaceFlinger对象, 关于SurfaceFlinger的构造函数的分析可以参考page4文件.
    因为ServiceManager的addService函数的参数是个SP类型的, 因此这里会导致SurfaceFlinger的onFirstRef函数的调用, onFirstRef函数的定义如下:
    1void SurfaceFlinger::onFirstRef()
    2{
    3    mEventQueue.init(this);
    4
    5    run("SurfaceFlinger", PRIORITY_URGENT_DISPLAY);
    6
    7    // Wait for the main thread to be done with its initialization
    8    mReadyToRunBarrier.wait();
    9}
    第3行(SurfaceFlinger->onFirstRef)成员变量mEventQueue是一个MessageQueue类型的, 因此这里会调用MessageQueue的init函数. 关于MessageQueue的init函数的实现可以参考page5文件.
    第5行(SurfaceFlinger->onFirstRef)会调用run函数, run函数是从Thread类继承下来的. 关于Thread的run函数的详细分析可以参考page6文件.
page4
我们分析一下SurfaceFlinger的构造函数的实现:
    1 SurfaceFlinger::SurfaceFlinger()
    2     :   BnSurfaceComposer(), Thread(false),
    3         mTransactionFlags(0),
    4         mTransactionPending(false),
    5         mAnimTransactionPending(false),
    6         mLayersRemoved(false),
    7         mRepaintEverything(0),
    8         mBootTime(systemTime()),
    9         mVisibleRegionsDirty(false),
    10         mHwWorkListDirty(false),
    11         mDebugRegion(0),
    12         mDebugDDMS(0),
    13         mDebugDisableHWC(0),
    14         mDebugDisableTransformHint(0),
    15         mDebugInSwapBuffers(0),
    16         mLastSwapBufferTime(0),
    17         mDebugInTransaction(0),
    18         mLastTransactionTime(0),
    19         mBootFinished(false)
    20 {
    21     ALOGI("SurfaceFlinger is starting");
    22
    23     // debugging stuff...
    24     char value[PROPERTY_VALUE_MAX];
    25
    26     property_get("debug.sf.showupdates", value, "0");
    27     mDebugRegion = atoi(value);
    28
    29     property_get("debug.sf.ddms", value, "0");
    30     mDebugDDMS = atoi(value);
    31     if (mDebugDDMS) {
    32         if (!startDdmConnection()) {
    33             // start failed, and DDMS debugging not enabled
    34             mDebugDDMS = 0;
    35         }
    36     }
    37     ALOGI_IF(mDebugRegion, "showupdates enabled");
    38     ALOGI_IF(mDebugDDMS, "DDMS debugging enabled");
    39 }

    可以看到, SurfaceFlinger的构造函数除了初始化成员变量之外, 没有什么额外的逻辑.
page5
我们分析一下MessageQueue的init函数的实现, 如下所示:
1void MessageQueue::init(const sp& flinger)
    2{
    3    mFlinger = flinger;
    4    mLooper = new Looper(true);
    5    mHandler = new Handler(*this);
    6}
    第3行(MessageQueue->init)会将SurfaceFlinger对象保存到MessageQueue里.
    第4行(MessageQueue->init)会new一个Looper对象, 用于分发消息.
    第5行(MessageQueue->init)会用本MessageQueue去初始化一个Handler对象, 注意, Handler是在MessageQueue类里定义的一个内部类, 定义如下:

    class Handler : public MessageHandler {
            enum {
                eventMaskInvalidate = 0x1,
                eventMaskRefresh    = 0x2
            };
            MessageQueue& mQueue;
            int32_t mEventMask;
        public:
            Handler(MessageQueue& queue) : mQueue(queue), mEventMask(0) { }
            virtual void handleMessage(const Message& message);
            void dispatchRefresh();
            void dispatchInvalidate();
    };