Code example for Semaphore

Methods: tryAcquire

                        // This method runs in the service's main thread (and there's no way 
                        // to move it to a child thread, since it needs to return an answer 
                        // to the GfxEngine), and when the keyguard blocks here, Android has 
                        // problems. So it's better to add a timeout to the block waiting. 
                        if (!mEventBufferSemaphore.tryAcquire(10, TimeUnit.SECONDS)) {
                            // The GfxEngine is not currently expecting this exception and it will 
                            // try to use the null pointer. We should probably fix this in the GfxEngine, 
                            // but a short term solution is to return "" instead of null. 
                            return ""; 
            } catch (InterruptedException e) {
                        // return null; 
                        return ""; 
            /* remove the next event from the queue and return it */ 
            if (mEventBuffer.isEmpty()) {
                return ""; 
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