android—如何新增一個系統服務
阿新 • • 發佈：2019-02-05
前面分析過binder的實現流程，binder的實現包含了java和c++兩部分，所以service的實現也包括java和c++兩種，那麼應該如何去自己實現並新增一個service？
c++中新增service
下面以SurfaceComposer這個service為例，
① 首先提供介面檔案ISurfaceComposer.h
//frameworks\native\include\gui\ISurfaceComposer.h
//首先是介面，c++實現是虛擬函式
class ISurfaceComposer: public IInterface {
public:
DECLARE_META_INTERFACE ② 建立BnSurfaceComposer
建立BnSurfaceComposer，需要重寫BBinder的onTransact函式。
class BnSurfaceComposer: public BnInterface<ISurfaceComposer> {
public:
enum {
// Note: BOOT_FINISHED must remain this value, it is called from ③ Bpxxx的實現
frameworks\native\libs\gui\ISurfaceComposer.cpp中，
//Bp實現，代理端
class BpSurfaceComposer : public BpInterface<ISurfaceComposer>
{
public:
BpSurfaceComposer(const sp<IBinder>& impl)
: BpInterface<ISurfaceComposer>(impl)
{
}
//代理介面
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的實現
//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端的實現函式
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 註冊service
通過上面幾步已經完成了service的建立，我們需要將service註冊到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
//首先獲取代理端BpSurfaceComposer
sp<ISurfaceComposer> composer(ComposerService::getComposerService());
//直接呼叫代理BpSurfaceComposer的介面
sp<IGraphicBufferAlloc> alloc(composer->createGraphicBufferAlloc());其中getComposerService()的實現為，
/*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");
//獲取service，返回的mComposerService是BpSurfaceComposer，有了Bp就能直接呼叫代理介面了
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新增service
Android為了方便開發人員，提供了AIDL工具，簡化了編寫service的難度。下面以新增TestService這個服務為例，
① 編寫AIDL檔案
package android.app;
interface ITestService {
boolean enableWifi(boolean enabled);
}TestService的AIDL檔案提供了一個介面，enableWifi()。
② 建立TestService服務
TestService 服務需要繼承ITestService.Stub類，這個類就是通過AIDL工具對①中的AIDL檔案處理後產生的，
class TestService extends ITestService.Stub {
//實現介面
public boolean enableWifi(boolean enabled)
{
......
}
}③ Context.java中新增service名字字串
// Context.java中新增service名字字串
public static final String TEST_SERVICE = "my_test";④ 向ServiceManager中註冊service
java中大部分的系統service都是在SystemServer中去向service manager註冊的，
//ServiceManager註冊service
// 在SystemServer.java中，模仿其他向ServiceManager新增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建立服務對應的Manager
對於每一個service而言，通常會有一個相關的Manager。 Managers提供API給app使用，成為SDK的一部分，是apps和remote service的中間橋樑。Manager中的介面和Service中的介面必須一一對應。
public class TestServiceManager{
private final ITestService mService;
private final Context mContext;
//建構函式中傳入的service，其實就是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;
}
}到目前為止，我們只是完成了Service的註冊，但是還沒有使用，該如何使用？
⑥ contextImpl中註冊Manager
一旦我們實現了service和對應的Manager，需要有一種方法在app中呼叫他們。前面說過，Manager會成為SDK的一部分，供我們呼叫，那麼Manager和Service是如何聯絡起來的？首先需要將我們的service和mangager註冊到execution context，即contextImpl中，
registerService(TEST_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
IBinder b = ServiceManager.getService(TEST_SERVICE);
//asInterface(BpBinder)後就是BpTestService
ITestService service = ITestService.Stub.asInterface(b);
//建立TestServiceManager，第二個引數為BpBpTestService
return new TestServiceManager(ctx.getOuterContext(), service);
}});registerService的第二個引數是一個ServiceFetcher物件，這裡直接在呼叫時，新建了一個ServiceFetcher類，重寫了createService方法。
ContextImpl.java中的registerService()方法，其核心就是把servicename和ServiceFetcher物件放到一個Hash的鍵值對中。
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
那麼app是如何呼叫的呢?
import android.app.TestServiceManager;
import android.content.Context;
TestServiceManager mTestServiceManager;
mTestServiceManager=(TestServiceManager)context.getSystemService(Context.TEST_SERVICE);然後直接呼叫TestServiceManager中的方法即可，其中的奧祕需要分析下getSystemService函式。
ContextImpl.java中，
@Override
public Object getSystemService(String name) {
ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
//從ServiceFetcher 中獲取service
return fetcher == null ? null : fetcher.getService(this);
}getService()函式的核心就是上面重寫的createService()函式，返回的就是TestServiceManager物件，所以通過context.getSystemService返回的是Manager物件。
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;
}
}
//呼叫過載的createService函式，返回的就是TestServiceManager物件
service = createService(ctx);
cache.set(mContextCacheIndex, service);
return service;
}
}