如何在Android中添加系统服务
发布网友
发布时间：2022-04-23 04:48
共1个回答
热心网友
时间：2022-04-11 20:41
① 首先提供接口文件ISurfaceComposer.h
//frameworks\native\include\gui\ISurfaceComposer.h
//首先是接口，c++实现是虚函数
class ISurfaceComposer: public IInterface {
public:
DECLARE_META_INTERFACE(SurfaceComposer);
// flags for setTransactionState()
enum {
eSynchronous = 0x01,
eAnimation = 0x02,
};
enum {
eDisplayIdMain = 0,
};
/* create connection with surface flinger, requires
* ACCESS_SURFACE_FLINGER permission
*/
virtual sp<ISurfaceComposerClient> createConnection() = 0;
}
② 建立BnSurfaceComposer
建立BnSurfaceComposer，需要重写BBinder的onTransact函数。
class BnSurfaceComposer: public BnInterface<ISurfaceComposer> {
public:
enum {
// Note: BOOT_FINISHED must remain this value, it is called from
// Java by ActivityManagerService.
BOOT_FINISHED = IBinder::FIRST_CALL_TRANSACTION,
CREATE_CONNECTION,
CREATE_GRAPHIC_BUFFER_ALLOC,
CREATE_DISPLAY_EVENT_CONNECTION,
CREATE_DISPLAY,
DESTROY_DISPLAY,
GET_BUILT_IN_DISPLAY,
SET_TRANSACTION_STATE,
AUTHENTICATE_SURFACE,
BLANK,
UNBLANK,
GET_DISPLAY_INFO,
CONNECT_DISPLAY,
CAPTURE_SCREEN,
};
virtual status_t onTransact(uint32_t code, const Parcel& data,
Parcel* reply, uint32_t flags = 0);
};
③ Bpxxx的实现
frameworks\native\libs\gui\ISurfaceComposer.cpp中，
//Bp实现，代理端
class BpSurfaceComposer : public BpInterface<ISurfaceComposer>
{
public:
BpSurfaceComposer(const sp<IBinder>& impl)
: BpInterface<ISurfaceComposer>(impl)
{
}
//代理接口
virtual sp<ISurfaceComposerClient> createConnection()
{
uint32_t n;
Parcel data, reply;
data.writeInterfaceToken(ISurfaceComposer::getInterfaceDescriptor());
remote()->transact(BnSurfaceComposer::CREATE_CONNECTION, data, &reply);
return interface_cast<ISurfaceComposerClient>(reply.readStrongBinder());
}
}
④ Bnxxx的实现
//Bn端，即server端
status_t BnSurfaceComposer::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case CREATE_CONNECTION: {
CHECK_INTERFACE(ISurfaceComposer, data, reply);
//createConnection就是server端的实现函数
sp<IBinder> b = createConnection()->asBinder();
reply->writeStrongBinder(b);
return NO_ERROR;
}
default: {
return BBinder::onTransact(code, data, reply, flags);
}
}
// should be unreachable
return NO_ERROR;
}
⑤ 注册service
通过上面几步已经完成了service的建立，我们需要将service注册到service manager中。
class SurfaceFlinger : public BnSurfaceComposer,
//在frameworks\native\services\surfaceflinger\main_surfaceflinger.cpp中，
// publish surface flinger
sp<IServiceManager> sm(defaultServiceManager());
sm->addService(String16(SurfaceFlinger::getServiceName()), flinger, false);
⑥ 使用service
//首先获取代理端BpSurfaceComposer
sp<ISurfaceComposer> composer(ComposerService::getComposerService());
//直接调用代理BpSurfaceComposer的接口
sp<IGraphicBufferAlloc> alloc(composer->createGraphicBufferAlloc());
其中getComposerService()的实现为，
/*static*/ sp<ISurfaceComposer> ComposerService::getComposerService() {
ComposerService& instance = ComposerService::getInstance();
Mutex::Autolock _l(instance.mLock);
if (instance.mComposerService == NULL) {
ComposerService::getInstance().connectLocked();
assert(instance.mComposerService != NULL);
ALOGD("ComposerService reconnected");
}
return instance.mComposerService;
}
void ComposerService::connectLocked() {
const String16 name("SurfaceFlinger");
//获取service，返回的mComposerService是BpSurfaceComposer，有了Bp就能直接调用代理接口了
while (getService(name, &mComposerService) != NO_ERROR) {
usleep(250000);
}
assert(mComposerService != NULL);
// Create the death listener.
class DeathObserver : public IBinder::DeathRecipient {
ComposerService& mComposerService;
virtual void binderDied(const wp<IBinder>& who) {
ALOGW("ComposerService remote (surfaceflinger) died [%p]",
who.unsafe_get());
mComposerService.composerServiceDied();
}
public:
DeathObserver(ComposerService& mgr) : mComposerService(mgr) { }
};
mDeathObserver = new DeathObserver(*const_cast<ComposerService*>(this));
mComposerService->asBinder()->linkToDeath(mDeathObserver);
}
java添加service
Android为了方便开发人员，提供了AIDL工具，简化了编写service的难度。下面以添加TestService这个服务为例，
① 编写AIDL文件
package android.app;
interface ITestService {
boolean enableWifi(boolean enabled);
}
TestService的AIDL文件提供了一个接口，enableWifi()。
② 创建TestService服务
TestService 服务需要继承ITestService.Stub类，这个类就是通过AIDL工具对①中的AIDL文件处理后产生的，
class TestService extends ITestService.Stub {
//实现接口
public boolean enableWifi(boolean enabled)
{
......
}
}
③ Context.java中添加service名字字符串
// Context.java中添加service名字字符串
public static final String TEST_SERVICE = "my_test";
④ 向ServiceManager中注册service
java中大部分的系统service都是在SystemServer中去向service manager注册的，
//ServiceManager注册service
// 在SystemServer.java中，模仿其他向ServiceManager添加service的方法
try {
TestService myService = new TestService(context);
ServiceManager.addService(Context.TEST_SERVICE, myService);
} catch (Throwable e) {
reportWtf("register my test service fail", e);
}
⑤创建服务对应的Manager
对于每一个service而言，通常会有一个相关的Manager。 Managers提供API给app使用，成为SDK的一部分，是apps和remote service的中间桥梁。Manager中的接口和Service中的接口必须一一对应。
public class TestServiceManager{
private final ITestService mService;
private final Context mContext;
//构造函数中传入的service，其实就是BpTestService
TestServiceManager(Context context,ITestService service) {
mContext = context;
mService = service;
}
public boolean enableWifi(boolean enabled) {
try {
return mService.enableWifi(enabled);
} catch (RemoteException ex) {
}
return false;
}
}
到目前为止，我们只是完成了Service的注册，但是还没有使用，该如何使用？
⑥ contextImpl中注册Manager
一旦我们实现了service和对应的Manager，需要有一种方法在app中调用他们。前面说过，Manager会成为SDK的一部分，供我们调用，那么Manager和Service是如何联系起来的？首先需要将我们的service和mangager注册到execution context，即contextImpl中，
registerService(TEST_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
IBinder b = ServiceManager.getService(TEST_SERVICE);
//asInterface(BpBinder)后就是BpTestService
ITestService service = ITestService.Stub.asInterface(b);
//创建TestServiceManager，第二个参数为BpBpTestService
return new TestServiceManager(ctx.getOuterContext(), service);
}});
registerService的第二个参数是一个ServiceFetcher对象，这里直接在调用时，新建了一个ServiceFetcher类，重写了createService方法。
ContextImpl.java中的registerService()方法，其核心就是把servicename和ServiceFetcher对象放到一个Hash的键值对中。
private static void registerService(String serviceName, ServiceFetcher fetcher) {
if (!(fetcher instanceof StaticServiceFetcher)) {
fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;
}
SYSTEM_SERVICE_MAP.put(serviceName, fetcher);
}
⑦ app如何使用service
那么app是如何调用的呢?
import android.app.TestServiceManager;
import android.content.Context;
TestServiceManager mTestServiceManager;
mTestServiceManager=(TestServiceManager)context.getSystemService(Context.TEST_SERVICE);
然后直接调用TestServiceManager中的方法即可，其中的奥秘需要分析下getSystemService函数。
ContextImpl.java中，
@Override
public Object getSystemService(String name) {
ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
//从ServiceFetcher 中获取service
return fetcher == null ? null : fetcher.getService(this);
}
getService()函数的核心就是上面重写的createService()函数，返回的就是TestServiceManager对象，所以通过context.getSystemService返回的是Manager对象。
public Object getService(ContextImpl ctx) {
ArrayList<Object> cache = ctx.mServiceCache;
Object service;
synchronized (cache) {
if (cache.size() == 0) {
// Initialize the cache vector on first access.
// At this point sNextPerContextServiceCacheIndex
// is the number of potential services that are
// cached per-Context.
for (int i = 0; i < sNextPerContextServiceCacheIndex; i++) {
cache.add(null);
}
} else {
service = cache.get(mContextCacheIndex);
if (service != null) {
return service;
}
}
//调用重载的createService函数，返回的就是TestServiceManager对象
service = createService(ctx);
cache.set(mContextCacheIndex, service);
return service;
}
}
