java - 停止线程不适用于函数 setRunning(false)

Question

我有一个使用 Canvas 画线的项目。我的项目(下载 here )将创建一个随机数并使用 Canvas 将它们连接到线上。效果很好。但问题是，当我的应用程序运行并单击 HOME 按钮时，我的应用程序将崩溃，例如

FATAL EXCEPTION: Thread-10001
java.lang.NullPointerException
at com.example.waveplot.WaveformView.PlotPoints(WaveformView.java:78)
at com.example.waveplot.WaveformPlotThread.run(WaveformPlotThread.java:35)

如何解决？太感谢了。让我们解释一下我的应用程序。我的应用程序包括三个类:

WaveformView.java: class to draw the line in thread with random number.
MainActivity.java: main calss and create the random number and call WaveformView class
WaveformPlotThread: create a thread and manager canvas

这是我的代码MainActivity.java

        private Runnable Timer_Tick = new Runnable() {
        public void run() {
            numRandom=genNum(0,200)-100;// from -100 to 100
            if(dataIndex1<MAX_SAMPLES) 
                dataY[dataIndex1++] = numRandom;
            else
            {
                dataY[MAX_SAMPLES-1] = numRandom;
               for (int i = 0; i <  WIDTH/10 - 1; i++)
                   //Shift data
                   dataY[i]=dataY[i+1];

            }
            mWaveform.set_data(dataY);
        }
     };

WaveformView.java

public class WaveformView extends SurfaceView implements SurfaceHolder.Callback{

    // plot area size
    private final static int WIDTH = 660;
    private static int[] dataX = new int[WIDTH];
    private static int[] dataY = new int[WIDTH];
    private WaveformPlotThread plot_thread; 
    private Paint dataY_color = new Paint();
    private int  index = 0;

    public WaveformView(Context context, AttributeSet attrs){
        super(context, attrs);

        getHolder().addCallback(this);
        plot_thread = new WaveformPlotThread(getHolder(), this);        
        dataY_color.setColor(Color.GREEN);
        dataY_color.setStrokeWidth(3);

    }   
    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height){

    }   
    @Override
    public void surfaceCreated(SurfaceHolder holder){
        plot_thread.setRunning(true);
        plot_thread.start();
        //plot_thread.startThread();
    }   
    @Override
    public void surfaceDestroyed(SurfaceHolder holder){
        Log.d("D","STOOOOOOOOOOOOOOOOOOP");

        boolean retry = true;
        plot_thread.setRunning(false);
        while (retry){
            try{
                plot_thread.join();
                retry = false;
            }catch(InterruptedException e){

            }
        }
    }

    @Override
    public void onDraw(Canvas canvas){
        PlotPoints(canvas);

    }

    public void set_data(int[] data1){      
        plot_thread.setRunning(false);
        if(index<WIDTH/10-1)
            index++;
        for(int x=0; x<WIDTH/10-1; x++){
            if(x<(data1.length)){
            dataX[x+1]=(x+1)*WIDTH/66;
            dataY[x] = data1[x];
            }
            else{
                dataY[x] = 0;
            }
        }
        plot_thread.setRunning(true);
    }

    public void PlotPoints(Canvas canvas){
        if(canvas == null) { return ; }
        // clear screen
        canvas.drawColor(Color.rgb(20, 20, 20));
        // plot data
        for(int x=0; x<index-1; x++){           
            canvas.drawLine(dataX[x], dataY[x], dataX[x+1], dataY[x+1], dataY_color);
        }

    }

}

波形图线程

public class WaveformPlotThread extends Thread {
    private SurfaceHolder holder;
    private WaveformView plot_area;
   // private volatile Thread runner;
    private volatile boolean _run=true;

    public WaveformPlotThread(SurfaceHolder surfaceHolder, WaveformView view){
        holder = surfaceHolder;
        plot_area = view;
    }
    public void setRunning(boolean run){
        _run = run;
    }
    @Override
    public void run(){
        Canvas c;
        while(_run){
        //while(Thread.currentThread() == runner){
            c = null;
            try{
                c = holder.lockCanvas(null);
                synchronized (holder) {
                    plot_area.PlotPoints(c);
                }
            }finally{
                if(c!=null){
                    holder.unlockCanvasAndPost(c);
                }
            }
        }
    }
}

这是日志文件

 D/D(9747): STOOOOOOOOOOOOOOOOOOP
 W/SurfaceView(9747): CHECK surface infomation creating=false formatChanged=false sizeChanged=false visible=false visibleChanged=true surfaceChanged=true realSizeChanged=false redrawNeeded=false left=false top=false
 W/System.err(9747): java.lang.IllegalThreadStateException: Thread already started.
 W/System.err(9747):    at java.lang.Thread.start(Thread.java:1045)
 W/System.err(9747):    at com.example.waveplot.WaveformView.surfaceCreated(WaveformView.java:41)
 W/System.err(9747):    at android.view.SurfaceView.updateWindow(SurfaceView.java:609)
 W/System.err(9747):    at android.view.SurfaceView.onWindowVisibilityChanged(SurfaceView.java:235)
 W/System.err(9747):    at android.view.View.dispatchWindowVisibilityChanged(View.java:7625)
 W/System.err(9747):    at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1346)
 W/System.err(9747):    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1121)
 W/System.err(9747):    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4598)
 W/System.err(9747):    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
 W/System.err(9747):    at android.view.Choreographer.doCallbacks(Choreographer.java:555)
 W/System.err(9747):    at android.view.Choreographer.doFrame(Choreographer.java:525)
 W/System.err(9747):    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
 W/System.err(9747):    at android.os.Handler.handleCallback(Handler.java:615)
 W/System.err(9747):    at android.os.Handler.dispatchMessage(Handler.java:92)
 W/System.err(9747):    at android.os.Looper.loop(Looper.java:137)
 W/System.err(9747):    at android.app.ActivityThread.main(ActivityThread.java:4950)
 W/System.err(9747):    at java.lang.reflect.Method.invokeNative(Native Method)
 W/System.err(9747):    at java.lang.reflect.Method.invoke(Method.java:511)
 W/System.err(9747):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:997)
 W/System.err(9747):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:764)
 W/System.err(9747):    at dalvik.system.NativeStart.main(Native Method)

解决方案:根据Veritas的建议，我将创建新线程的函数放入SurfaceCreated中

@Override
public void surfaceCreated(SurfaceHolder holder){
    plot_thread = new WaveformPlotThread(getHolder(), this);
    plot_thread.setRunning(true);
    plot_thread.start();

}   
public WaveformView(Context context, AttributeSet attrs){
    super(context, attrs);

    getHolder().addCallback(this);
    //plot_thread = new WaveformPlotThread(getHolder(), this);      
    dataY_color.setColor(Color.GREEN);
    dataY_color.setStrokeWidth(3);

}

Answer 1

四件事

1. 检查 PlotPoints 方法中的 canvas 是否为 null，

   if(canvas == null) { 返回 ;//默默返回 }

   可能在表面上被破坏的表面固定器返回给您一个空 Canvas

2. 正如isalgueiro建议的那样，将_run设置为 volatile /

3. 将 _run 的默认值设置为 true，因为如果您在某个地方启动该线程而没有将 _run 设置为 true，则该线程很可能在您可以对其进行任何建设性操作之前就结束了。默认值 false 在您的情况下没有任何意义。

4. 不要扩展线程，而是实现可运行的。

最后两点更多的是最佳实践..

--编辑--暂时在surfaceCreated中创建一个新线程然后启动它，因为当_run设置为false时退出while循环的逻辑将使线程完成其run方法，并且线程将死亡。该错误是自指示的，一旦线程已经启动，您就无法启动该线程。您可以通过更正 _run = false/true 的逻辑来重用线程，或者通过使用 Executor ThreadPool 来采用更好的解决方案，请参阅 http://docs.oracle.com/javase/tutorial/essential/concurrency/pools.html 。暂时您可以在 surfacecreated 方法而不是构造函数中创建一个新线程