Binder C++/JAVA implementation

Keywords: Java C++ Android

Understand the principle of Binder mechanism:

It is an inter process communication mode, CS architecture, user space and kernel space



The message is handled by a subclass of BnInterface

Send message:

 status_t  transact(uint32_t code,const Parcel& data,Parcel* reply,uint32_t flags = 0);

Process message:

status_t onTransact(uint32_t code, const Parcel& data,Parcel* reply, uint32_t flags)


The p in BpInterface and BpBinder means proxy, which means intermediate calls and proxies and will not specifically implement functions
The n in BnInterface and BBinder means that native is the final implementation
The BpBinder class is inherited by the client. Of course, if it is not available, it can also be omitted
If a class wants to use the smart pointer in android, it must inherit or indirectly inherit the RefBase class, such as client

Inheritance relationship

    Define an interface class IDemo, which inherits IInterface
    Define a BpDemoService class that inherits bpinterface < idemo >
    Define a BnDemoService class and inherit bninterface < idemo >
    Define a DemoService class that inherits BnDemoService and is used to implement specific functions

Use case:

First define an interface class IDemo, then define a proxy class BpDemoService, a native interface BnDemoService, and then define an implementation class DemoService that inherits BnDemoService to implement specific functions. Finally, create a client side. Call sequence: first obtain the object of servicemanager, then create a binder to load the service and call the interface_cast() gets the object of the service proxy end, and finally calls the interface.

Define an interface class IDemo,inherit IInterface,There is only one function:
class IDemo :public IInterface
        virtual int getAge();

Define a BpDemoService Class, inheritance BpInterface
class BpDemoService: public BnInterface<IDemo>
        BpDemoService(const sp<IBinder>& impl):BpInterface<IDemo> (impl){};
        virtual BOOL getAge()
			Parcel data, reply;
		    remote()->transact(CUSTOM_CODE, data, &reply);//send message
			return (BOOL)reply.readInt32();
Define a BnDemoService Class, inheritance BnInterface
class BnDemoService: public BnInterface<IDemo>
        virtual status_t onTransact(uint32_t code, const Parcel& data,Parcel* reply, uint32_t flags= 0 );
//Accept message processing
status_t BnDemoService::onTransact(uint32_t code, const Parcel& data,Parcel* reply, uint32_t flags)
			case CUSTOM_CODE:
					int res = getAge();
					return NO_ERROR;
	return BBinder::onTransact(code, data, reply, flags);
Define a Service Class, inheritance BnDemoService

Used to implement functions. Finally, remember to call IMPLEMENT_META_INTERFACE Macro to register

class DemoService : public BnDemoService,public BinderService<DemoService>
        int getAge();
IMPLEMENT_META_INTERFACE(DemoService, "demo.service");

Call order

  • When calling on the client side, first get the ServiceManager
  • Then create a binder to load the service
  • Using interface_cast() to get the corresponding proxy
  • Call the functions implemented in the service through the proxy
sp < IServiceManager > sm = defaultServiceManager();
sp < IBinder > binder;
binder = sm->getService(String16("demo.service"));
sp<IDemo> m_pDemoService;
m_pDemoService = interface_cast<IDemo> (binder);

interface_cast() function

The function is to forcibly convert the Binder obtained from getService into a subclass of iiinterface, that is, the classes that define their own interfaces

Remote() - > transact() function

The location is invoked in the proxy class, which is used to send messages and data to onTransact functions remotely, similar to handler.sendMessage().

onTransact() function

The position of this function is in the implementation class (implementing BnInterface), which is equivalent to the handleMessage function in the handler




C++ Binder mechanism learning_ Empty realm - CSDN blog

Posted by simonoc on Fri, 17 Sep 2021 06:57:46 -0700