从零开始怎么写android native service
Android service对于从事android开发的人都不是一个陌生的东西,很多人可能会觉得服务很简单。服务是简单,因为复杂的别人做了,所以才会觉得简单。我们先梳理一下服务的分类,首先有本地服务跟系统服务的区分,而在APP里写的服务大多就成为Java服务或者应用服务。
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做APP的人写个应用服务相对来说是最简单的,因为extends了一个service后几个简单的接口就可以跑起来了,写完这种服务可能也只是对服务一知半解,因为值钱的service类Google的人已经帮你写好了,这是你的福气为你带来了便利,当然也可能会麻痹你:),但是做APP的人会有能解决问题是首要任务了,有时间还是对它了解更清楚点比较好,在此不再讨论这个。
做设备做系统的人,经常可能会去写系统服务,也就是framework下面的服务,systemserver里面注册的服务,写这种服务一般来说比较少,只有做设备系统的才会这样干,才有机会有完成的系统代码,可以在里面*遨游,笔者三年前写过一个,可以看看【自己动手从零开始写一个完整的android Service】http://blog.csdn.net/edsam49/article/details/8163639
那剩下的一个是本地服务,也就是native service,这种服务我们了解的系统里面多媒体、audio system都是写成了本地服务,这样写的好处就是运行的效率更高一点,因为C/C++先天性就比JAVA的运行效率要高一点。笔者就是由于长期主要从事的都是底层开发的,我们有时有这么一种需求,又要运行效率高,又要好移植,主要是考虑推广写东西给广大客户,那么我就写一个本地服务,这样是最独立的了,效率也最高了,那一个本地服务到底怎么写呢?大多数的人写过的服务以java服务居多,真正写本地服务的不多,本地服务相对来说又是更复杂一点的。因此决定从零开始自己动手写一个本地service,下面就大概描述一下过程。
本地服务有四大块,服务接口(IService),服务代理(也就是BpService),服务stub(也就是BnService),服务实体(Service);下面笔者的实例就以demoNativeService来开启,力求简单,里面就写了两个接口;
首先定义好服务接口IdemoNativeService,IdemoNativeService服务接口的父类是IInterface,在里面主要是要声明一下接口,在DECLARE_META_INTERFACE(demoNativeService),代码如下:
class IdemoNativeService : public IInterface { public: enum { CONNECT = IBinder::FIRST_CALL_TRANSACTION, PRINTSTRING_CMD, }; public: DECLARE_META_INTERFACE(demoNativeService); virtual status_t connect(int pid,int previewhw,int intf,int fmt,int chan) = 0; virtual status_t printString(const char *str) = 0; };
当然定义好了IdemoNativeService的头文件,就需要去实操了,先来搞定BpdemoNativeService,它的父类是BpInterface<IdemoNativeService>,这里面主要是涉及数据的跨进程用到的parcel,读啊,写啊,按套路来,也不难,也有AIDL工具可以使用,帮你转出来,再稍微修改一下就可以了,里面有一个很重要的remote,这个和remote就是幕后功臣啊,它保存了服务实例的对象啊,它是来之BpRefBase的一个成员,生成服务的时候,会得到赋值,定义完了以后,很重要的一个程序就是要IMPLEMENT_META_INTERFACE(demoNativeService,"android.hardware.IdemoNativeService");这个宏是非常重要的,跟前面那个DECLARE是对应的,前面声明,后面实现,当然我们带的参数跟的名字是必须一致的,这样才能正常沟通嘛!
class BpdemoNativeService: public BpInterface<IdemoNativeService> { public: BpdemoNativeService(const sp<IBinder>& impl) : BpInterface<IdemoNativeService>(impl) { } virtual status_t connect(int pid,int previewhw,int intf,int fmt,int chan) { Parcel data, reply; data.writeInterfaceToken(IdemoNativeService::getInterfaceDescriptor()); data.writeInt32(pid); data.writeInt32(previewhw); data.writeInt32(intf); data.writeInt32(fmt); data.writeInt32(chan); remote()->transact(IdemoNativeService::CONNECT, data, &reply); return reply.readInt32(); } virtual status_t printString(const char *str) { Parcel data, reply; data.writeInterfaceToken(IdemoNativeService::getInterfaceDescriptor()); data.writeCString(str); remote()->transact(IdemoNativeService::PRINTSTRING_CMD, data, &reply); return reply.readInt32(); } }; IMPLEMENT_META_INTERFACE(demoNativeService, "android.hardware.IdemoNativeService");//android.hardware.IdemoNativeService ds.demonativeservice
接着需要写服务stub了,BndemoNativeService的父类是BnInterface<IdemoNativeService>,有没有发现BndemoNativeService跟BpdemoNativeService,都会基于接口类IdemoNativeService,这样沟通起来的接口就唯一了,就具备了对话的可能;
class BndemoNativeService: public BnInterface<IdemoNativeService> { public: virtual status_t onTransact( uint32_t code,const Parcel& data,Parcel* reply,uint32_t flags = 0); }; status_t BndemoNativeService::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) { switch(code) { /*case CONNECT: { CHECK_INTERFACE(IdemoNativeService, data, reply); int pid = data.readInt32(); int previewhw = data.readInt32(); int intf = data.readInt32(); int fmt = data.readInt32(); int chan = data.readInt32(); reply->writeInt32(connect(pid,previewhw,intf,fmt,chan)); return NO_ERROR; }break; case PRINTSTRING_CMD: { CHECK_INTERFACE(IdemoNativeService, data, reply); const char *str; str = data.readCString(); reply->writeInt32(printString(str)); return NO_ERROR; }break;*/ default: return BBinder::onTransact(code, data, reply, flags); } }
到这就轮到了大块头service实体demoNativeService了,demoNativeService是基于BndemoNativeService,在demoNativeService里面定义了一个instantiate()接口用于添加service到servicemanager里面去,注意demoNativeService()跟析构函数~demoNativeService()需要写成private的,免得别人可以new出对象来。在里面重写了onTransact,一旦BpdemoNativeService有风吹草动,就会联动到BndemoNativeService,因为服务实体重写了onTransact,所以实际就会先执行到demoNativeService::onTransact这里来,在这里面处理不了,可以再转给BpdemoNativeService的onTransact或者直接到BBinder的onTransact;
void demoNativeService::instantiate() { android::defaultServiceManager()->addService( IdemoNativeService::descriptor, new demoNativeService()); } demoNativeService::demoNativeService() { ALOGE("demoNativeService created"); mOpened = 1; } demoNativeService::~demoNativeService() { ALOGE("demoNativeService destroyed"); } status_t demoNativeService::connect(int pid,int previewhw,int intf,int fmt,int chan){ ALOGD("demoNativeService connect:%d, %d, %d, %d, %d", pid, previewhw, intf, fmt, chan); return 88; } status_t demoNativeService::printString(const char *str){ ALOGD("demoNativeService printString:%s", str); return 66; } #if 1 status_t demoNativeService::onTransact(uint32_t code, const android::Parcel &data, android::Parcel *reply, uint32_t flags) { ALOGD("OnTransact(%u,%u)", code, flags); switch(code) { case CONNECT: { CHECK_INTERFACE(IdemoNativeService, data, reply); int pid = data.readInt32(); int previewhw = data.readInt32(); int intf = data.readInt32(); int fmt = data.readInt32(); int chan = data.readInt32(); ALOGD("CONNECT: %d, %d, %d, %d, %d\n", pid,previewhw,intf,fmt,chan); reply->writeInt32(connect(pid,previewhw,intf,fmt,chan)); return NO_ERROR; }break; case PRINTSTRING_CMD: { CHECK_INTERFACE(IdemoNativeService, data, reply); const char *str; str = data.readCString(); ALOGD("PrintString: %s\n", str); ALOGD("printString: %s\n", str); reply->writeInt32(printString(str)); return NO_ERROR; } break; default: return BndemoNativeService::onTransact(code, data, reply, flags); } return NO_ERROR; } #endif
写完了服务,那我们就再写一个可执行文件来生成一下,里面startThreadPool生成线程池,然后再调用joinThreadPool来监听变化;
sp<ProcessState> proc(ProcessState::self()); sp<IServiceManager> sm = defaultServiceManager(); // ALOGI("ServiceManager: %p", sm.get()); demoNativeService::instantiate(); ProcessState::self()->startThreadPool(); IPCThreadState::self()->joinThreadPool();
写到这,可以说服务已经可以跑起来了,那我们怎么验证呢,最快的办法还是写一个可执行文件去测一下它的接口,看通没通就知道了;
int ret= -1; int pid = IPCThreadState::self()->getCallingPid(); ALOGI("demoNativeService client is now starting, pid=%d", pid); android::sp<android::IServiceManager> sm = android::defaultServiceManager(); android::sp<android::IBinder> binder; android::sp<IdemoNativeService> shw; do { binder = sm->getService(android::String16("ds.demonativeservice")); if (binder != 0) break; ALOGW("IdemoNativeService not published, waiting..."); usleep(500000); } while(true); ALOGI("IdemoNativeService client is now trying"); shw = android::interface_cast<IdemoNativeService>(binder); ret = shw->printString("Good man desheng"); ALOGI("demoNativeService client printString, ret=%d", ret); ret = shw->connect(pid,1, 2, 3, 4); ALOGI("demoNativeService client connect, ret=%d", ret);
下面就是笔者测试的打印,如下:
# dem demoNativeServiceclient demoNativeServiceserver # demoNativeServiceserver & [2] 2332 # --------- beginning of /dev/log/main 02-19 17:10:57.890 E/HelloWorldService( 2332): demoNativeService created # # dem demoNativeServiceclient demoNativeServiceserver # demoNativeServiceclient 02-19 17:11:02.520 I/demoNativeService/Service( 2334): demoNativeService client is now starting, pid=2334 02-19 17:11:02.520 I/demoNativeService/Service( 2334): IdemoNativeService client is now trying 02-19 17:11:02.520 D/HelloWorldService( 2332): OnTransact(2,16) 02-19 17:11:02.520 D/HelloWorldService( 2332): PrintString: Good man desheng 02-19 17:11:02.520 D/HelloWorldService( 2332): printString: Good man desheng 02-19 17:11:02.520 D/HelloWorldService( 2332): demoNativeService printString:Good man desheng 02-19 17:11:02.520 I/demoNativeService/Service( 2334): demoNativeService client printString, ret=66 02-19 17:11:02.520 D/HelloWorldService( 2332): OnTransact(1,16) 02-19 17:11:02.520 D/HelloWorldService( 2332): CONNECT: 2334, 1, 2, 3, 4 02-19 17:11:02.520 D/HelloWorldService( 2332): demoNativeService connect:2334, 1, 2, 3, 4 02-19 17:11:02.520 I/demoNativeService/Service( 2334): demoNativeService client connect, ret=88 02-19 17:11:02.520 I/demoNativeService/Service( 2334): Hello client is now exiting # # 02-19 17:11:07.540 D/InitAlarmsService( 2259): Clearing and rescheduling alarms. # service list Found 78 services: 0 ds.demonativeservice: [android.hardware.IdemoNativeService] 1 phone: [com.android.internal.telephony.ITelephony] 2 iphonesubinfo: [com.android.internal.telephony.IPhoneSubInfo] 3 simphonebook: [com.android.internal.telephony.IIccPhoneBook] 4 isms: [com.android.internal.telephony.ISms] 5 jeavoxmiddleware: [android.jeavox.IMiddleWareService]
写到这,如果要给应用调用的话,还需要写Client,JNI,JNI及以上在此不再讨论了,我们就简易来看看client怎么处理吧,其实有点类似上面那个可执行文件的写法,这里可能就是有一个对象的概念,可以保持,大概如下:
demoNativeServiceClient::demoNativeServiceClient() { sp<IServiceManager> sm = defaultServiceManager(); sp<IBinder> binder = sm->getService(String16("ds.demonativeservice")); mdemoNativeService = interface_cast<IdemoNativeService>(binder); } demoNativeServiceClient::~demoNativeServiceClient() { mdemoNativeService = NULL; } int32_t demoNativeServiceClient::connect(int previewhw,int intf,int fmt,int chan) { return mdemoNativeService->connect(getCallingPid(),previewhw,intf,fmt,chan); } int32_t demoNativeServiceClient::printString(const char *str) { return mdemoNativeService->printString(str); }
罗哩罗嗦写了这么多,请大家拍砖,轻拍一下:)