Android进程间通信（五）——Native层获取servicemanager服务流程

Android进程间通信（四）——系统服务注册到servicemanager源码分析中包含了ServerManager服务获取servicemanager并注册系统服务的流程。这个流程中包含了Java层获取servicemanager服务的流程，常见的还有native层获取servicemanager，与Java层有所不同，本篇文章着重介绍native层获取servicemanager的流程。native层获取servicemanager的方法为IServiceManager.cpp的defaultServiceManager方法，源码github地址

sp defaultServiceManager()
{
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;
    
    {
        AutoMutex _l(gDefaultServiceManagerLock);
        while (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast(
                ProcessState::self()->getContextObject(NULL));
            if (gDefaultServiceManager == NULL)
                sleep(1);
        }
    }
    
    return gDefaultServiceManager;
}

ProcessState::self()->getContextObject(NULL)这个之前我们已经介绍过，返回的是一个BpBinder(0)，注意这个方法是个单例，用while循环是防止servicemanager还没有注册完成。
1.ProcessState::self()->getContextObject(NULL)

• ProcessState::self()
  a.打开binder驱动
  b.设置最大线程数15
  c.mmap设置共享内存

2.getContextObject

• 创建一个BpBinder对象

3.interface_cast

template
inline sp interface_cast(const sp& obj)
{
    return INTERFACE::asInterface(obj);
}

IServiceManager替换INTERFACE，也就是调用IServiceManager.asInterface，但我们在IServiceManager中式找不到这个方法的，其实这个asInterface式通过define定义的,实际我们就是把IServiceManager替换宏定义中的INTERFACE

#define DECLARE_META_INTERFACE(INTERFACE)                               \
    static const android::String16 descriptor;                          \
    static android::sp asInterface(                       \
            const android::sp& obj);                  \
    virtual const android::String16& getInterfaceDescriptor() const;    \
    I##INTERFACE();                                                     \
    virtual ~I##INTERFACE();                                            \


#define IMPLEMENT_META_INTERFACE(INTERFACE, NAME)                       \
    const android::String16 I##INTERFACE::descriptor(NAME);             \
    const android::String16&                                            \
            I##INTERFACE::getInterfaceDescriptor() const {              \
        return I##INTERFACE::descriptor;                                \
    }                                                                   \
    android::sp I##INTERFACE::asInterface(                \
            const android::sp& obj)                   \
    {                                                                   \
        android::sp intr;                                 \
        if (obj != NULL) {                                              \
            intr = static_cast(                          \
                obj->queryLocalInterface(                               \
                        I##INTERFACE::descriptor).get());               \
            if (intr == NULL) {                                         \
                intr = new Bp##INTERFACE(obj);                          \
            }                                                           \
        }                                                               \
        return intr;                                                    \
    }                                                                   \
    I##INTERFACE::I##INTERFACE() { }                                    \
    I##INTERFACE::~I##INTERFACE() { }                                   \

带入展开后,其实这个只是声明，类似Java中的接口

 static const android::String16 descriptor;                          \
   static android::sp asInterface(                       \
           const android::sp& obj);                  \
   virtual const android::String16& getInterfaceDescriptor() const;    \
  IServiceManager();                                                     \
   virtual ~IServiceManager();                                            \

实现代码为

 const android::String16 IServiceManager::descriptor(NAME);             \
    const android::String16&                                            \
           IServiceManager::getInterfaceDescriptor() const {              \
        return IServiceManager::descriptor;                                \
    }                                                                   \
    android::sp IServiceManager::asInterface(                \
            const android::sp& obj)                   \
    {                                                                   \
        android::sp intr;                                 \
        if (obj != NULL) {                                              \
            intr = static_cast(                          \
                obj->queryLocalInterface(                               \
                        IServiceManager::descriptor).get());               \
            if (intr == NULL) {                                         \
                intr = new BpServiceManager(obj);                          \
            }                                                           \
        }                                                               \
        return intr;                                                    \
    }                                                                   \
   IServiceManager::IServiceManager() { }                                    \
 IServiceManager::~IServiceManagerE() { }                                   \

所以 asInterface 的返回为 new BpServiceManager(BpBinder(0))

class BpServiceManager : public BpInterface
{
public:
    BpServiceManager(const sp& impl)
        : BpInterface(impl)
    {
    }
    virtual status_t addService(const String16& name, const sp& service,
            bool allowIsolated)
    {
        Parcel data, reply;
        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
        data.writeString16(name);
        data.writeStrongBinder(service);
        data.writeInt32(allowIsolated ? 1 : 0);
        status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);//实际还是会调用BpBinder
        return err == NO_ERROR ? reply.readExceptionCode() : err;
    }
......

总结：Nativce层和Java层作为客户端获取servicemanager流程有所不同，但最终都是对BpBinder的封装