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POSourceDataLine.start() nulls the object
 

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  1. POSourceDataLine.start() nulls the object
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    <p>Here's the code that has the symptoms:</p> <pre><code> /* * To change this template, choose Tools | Templates * and open the template in the editor. */ package com.zove.xuggleraudio; import com.xuggle.xuggler.IAudioSamples; import com.xuggle.xuggler.IContainer; import com.xuggle.xuggler.IStream; import com.xuggle.xuggler.IStreamCoder; import com.xuggle.xuggler.ICodec; import com.xuggle.xuggler.IPacket; import javax.sound.sampled.AudioFormat; import javax.sound.sampled.AudioSystem; import javax.sound.sampled.DataLine; import javax.sound.sampled.Line; import javax.sound.sampled.LineUnavailableException; import javax.sound.sampled.SourceDataLine; /** * Class that represents an audio player with the expected * controls (start, stop, pause, resume). * @author Mciavilli */ public class Audio { //The name of the file to be played private final String filename; //Our connection to the mixer private final SourceDataLine mLine; //The index of the audio stream inside the file private final int audioId; //Xuggler media container private final IContainer container; //The stream decoder private final IStreamCoder streamCoder; /* * Constructor that takes a String argument */ public Audio(String filename) { this.filename = filename; //Create Xuggler container object this.container = IContainer.make(); //Open the container if(container.open(filename, IContainer.Type.READ, null) &lt; 0) throw new IllegalArgumentException("Invalid file name: " + this.filename); //find the audio stream within contained streams this.audioId = getAudioId(container); //get the audio stream IStream stream = container.getStream(audioId); //get the stream decoder this.streamCoder = stream.getStreamCoder(); //open the stream decoder if (this.streamCoder.open() &lt; 0) throw new RuntimeException("could not open audio decoder for container: " + filename); //Get a pipe to the sound mixer this.mLine = readySoundSystem(streamCoder); } private int getAudioId(IContainer container) { //see how many streams are there int numStreams = container.getNumStreams(); int audioId = -1; for(int i = 0; i &lt; numStreams ; i++) { IStream stream = container.getStream(i); IStreamCoder streamCoder = stream.getStreamCoder(); if(streamCoder.getCodecType() == ICodec.Type.CODEC_TYPE_AUDIO) audioId = i; break; }//end for statement //No audio stream found if(audioId == -1) throw new RuntimeException("Failed to find an audio stream in:" + this.filename); return audioId; }//end method getAudioId private SourceDataLine readySoundSystem(IStreamCoder aAudioCoder) { AudioFormat audioFormat = new AudioFormat(aAudioCoder.getSampleRate(), (int)IAudioSamples.findSampleBitDepth(aAudioCoder.getSampleFormat()), aAudioCoder.getChannels(), true, /* xuggler defaults to signed 16 bit samples */ false); DataLine.Info info = new DataLine.Info(SourceDataLine.class, audioFormat); try { SourceDataLine mLine = (SourceDataLine) AudioSystem.getLine(info); /** * if that succeeded, try opening the line. */ mLine.open(audioFormat); /** * And if that succeed, start the line. */ mLine.start(); } catch (LineUnavailableException e) { throw new RuntimeException("could not open audio line"); } return mLine; }//end method readySoundSystem /* * starts playing the file. * returns true if successful. * should be called only once per Audio object */ public boolean start() { if(!mLine.isActive()) { IPacket packet = IPacket.make(); while(container.readNextPacket(packet) &gt;= 0) { /* * Now we have a packet, let's see if it belongs to our audio stream */ if (packet.getStreamIndex() == this.audioId) { /* * We allocate a set of samples with the same number of channels as the * coder tells us is in this buffer. * * We also pass in a buffer size (1024 in our example), although Xuggler * will probably allocate more space than just the 1024 (it's not important why). */ IAudioSamples samples = IAudioSamples.make(1024, this.streamCoder.getChannels()); /* * A packet can actually contain multiple sets of samples (or frames of samples * in audio-decoding speak). So, we may need to call decode audio multiple * times at different offsets in the packet's data. We capture that here. */ int offset = 0; /* * Keep going until we've processed all data */ while(offset &lt; packet.getSize()) { int bytesDecoded = this.streamCoder.decodeAudio(samples, packet, offset); if (bytesDecoded &lt; 0) throw new RuntimeException("got error decoding audio in: " + filename); offset += bytesDecoded; /* * Some decoder will consume data in a packet, but will not be able to construct * a full set of samples yet. Therefore you should always check if you * got a complete set of samples from the decoder */ if (samples.isComplete()) { playSound(samples); } }//end inner while block }//end inner if block else { /* * This packet isn't part of our audio stream, so we just silently drop it. */ do {} while(false); }//end else block }//end outer while block //success! return true; }//end outer if block //The sound is already playing return false; }//end method start private void playSound(IAudioSamples aSamples) { /** * We're just going to dump all the samples into the line. */ byte[] rawBytes = aSamples.getData().getByteArray(0, aSamples.getSize()); this.mLine.write(rawBytes, 0, aSamples.getSize()); }//end method playJavaSound /* * stops the playback * returns true if suucessful */ public boolean stop() { if(mLine.isActive()) { this.mLine.stop(); return true; } return false; } public static void main(String args[]) throws InterruptedException { if(args.length != 1) throw new IllegalArgumentException("illegal arguments passed"); Audio audio = new Audio(args[0]); audio.start(); Thread.sleep(10 * 1000); audio.stop(); } }//end class Audio </code></pre> <p>The line that causes the problem is line 104:</p> <pre><code>mLine.start(); </code></pre> <p>When i checked the debugger, the mLine object (a SourceDataLine object) is fine until this line is executed, which causes mLine to equal 'null'.</p> <p>I think this problem is the same as <a href="https://stackoverflow.com/questions/7396364/null-pointer-exception-while-writing-to-sourcedataline">here</a>.</p> <p>I also tried to use Clip instead of SourceDataLine which ended up having the same issue.</p> <p>What's interesting is that this problem doesn't happen in the <a href="http://xuggle.googlecode.com/svn/trunk/java/xuggle-xuggler/src/com/xuggle/xuggler/demos/DecodeAndPlayAudio.java" rel="nofollow noreferrer">original Xuggler program</a>, and calling start() didn't have that bad effect.</p>
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    1. COThe fact that you can invoke `start()` means that `mLine` isn't null at that point. Are you sure you are reading the debugger correctly? What happens if you run the code normally (i.e. what problem do you see?)
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    2. COHey joe yuxuan here
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    3. COThe object is null after the call to start().which causes NullPointerException at "if(!mLine.isActive())".
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