主要用到TLS，首次进入gHaveTLS为false，锁保护说明此函数很多其他函数在调用。
通过if (pthread_key_create(&gTLS, threadDestructor) != 0)，中threadDestructor(void *st)
调用IPCThreadState::IPCThreadState()创建IPCThreadState对象，并将对象的索引设置为gTLS。再次进入后通过gTLS获取到IPCThreadState对象。

 IPCThreadState* IPCThreadState::self()

 {

     if (gHaveTLS) {

 restart:

         const pthread_key_t k = gTLS;

         IPCThreadState* st = (IPCThreadState*)pthread_getspecific(k);

         if (st) return st;

         return new IPCThreadState;

     }

     if (gShutdown) return NULL;

     pthread_mutex_lock(&gTLSMutex);

     if (!gHaveTLS) {

         if (pthread_key_create(&gTLS, threadDestructor) != ) {

             pthread_mutex_unlock(&gTLSMutex);

             return NULL;

         }

         gHaveTLS = true;

     }

     pthread_mutex_unlock(&gTLSMutex);

     goto restart;

 }

 IPCThreadState::IPCThreadState()

     : mProcess(ProcessState::self()),

       mMyThreadId(androidGetTid()),

       mStrictModePolicy(),

       mLastTransactionBinderFlags()

 {

     pthread_setspecific(gTLS, this);

     clearCaller();

     mIn.setDataCapacity();

     mOut.setDataCapacity();

 }