java停止线程未使用函数setRunning（false）

2 周，6 日 Questions & Answers 505

我有一个项目，用画布画一条线。我的项目（下载here）将创建一个随机数，并使用canvas在线连接它们。它工作得很好。但问题是，当我的应用程序运行时，我单击HOME按钮，然后我的应用程序将崩溃，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)

如何修复它？非常感谢你。让我解释一下我的申请。我的申请包括三类：

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

这是我的密码主要活动。爪哇

        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);
        }
     };

波形视图。爪哇

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);
        }

    }

}

波形多线程

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);
                }
            }
        }
    }
}

这是一个日志文件

 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 安卓.view.SurfaceView.updateWindow(SurfaceView.java:609)
 W/System.err(9747):    at 安卓.view.SurfaceView.onWindowVisibilityChanged(SurfaceView.java:235)
 W/System.err(9747):    at 安卓.view.View.dispatchWindowVisibilityChanged(View.java:7625)
 W/System.err(9747):    at 安卓.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at 安卓.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at 安卓.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at 安卓.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at 安卓.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at 安卓.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:1047)
 W/System.err(9747):    at 安卓.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1346)
 W/System.err(9747):    at 安卓.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1121)
 W/System.err(9747):    at 安卓.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4598)
 W/System.err(9747):    at 安卓.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
 W/System.err(9747):    at 安卓.view.Choreographer.doCallbacks(Choreographer.java:555)
 W/System.err(9747):    at 安卓.view.Choreographer.doFrame(Choreographer.java:525)
 W/System.err(9747):    at 安卓.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
 W/System.err(9747):    at 安卓.os.Handler.handleCallback(Handler.java:615)
 W/System.err(9747):    at 安卓.os.Handler.dispatchMessage(Handler.java:92)
 W/System.err(9747):    at 安卓.os.Looper.loop(Looper.java:137)
 W/System.err(9747):    at 安卓.app.ActivityThread.main(ActivityThread.java:4950)
 W/System.err(9747):    at java.lang.reflect.Method.invokeNative(Native Method)
 W/System.err(9747):    at java.lang.reflect.Method.invoke(Method.java:511)
 W/System.err(9747):    at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:997)
 W/System.err(9747):    at com.安卓.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到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);

}

# 3 楼答案

我遇到了同样的问题。我不明白为什么调用setRunning(false)时Thread不会停止，但我通过在另一个if(canvas != null)语句中包装plot_area.PlotPoints()成功绕过了NullPointerException。因此WaveformPlotThread中的run()方法如下所示：

@Override
public void run(){
    Canvas c;
    while(_run){
    //while(Thread.currentThread() == runner){
        c = null;
        try{
            c = holder.lockCanvas(null);
            synchronized (holder) {
                if(c!=null) {
                    plot_area.PlotPoints(c);
                }
            }
        }finally{
            if(c!=null){
                holder.unlockCanvasAndPost(c);
            }
        }
    }
}

另外，作为旁注，@Override public void onDraw(Canvas canvas)是不必要的，并且不建议重写onDraw()或draw()。整个方法可以删除

共 (3) 个答案

# 1 楼答案
四件事
1. 检查PlotPoints方法中的canvas是否为null
  
  如果（画布==null）{ 返回；//悄悄地回来 }
  
  可能在surface销毁的surfaceholder将返回一个空画布
2. 正如isalgueiro所建议的那样，让您以易失性的方式运行/
3. 将_run的默认值设置为true，因为如果在某个地方启动此线程而没有将_run设置为true，则很可能在您可以对其执行任何有建设性的操作之前线程就结束了。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runnable
最后两点是更多的最佳实践

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运行方法，线程将死亡。该错误自指示线程一旦启动就无法启动。要么通过更正_run=false/true的逻辑重用线程，要么通过使用Executor ThreadPool采用更好的解决方案请参见http://docs.oracle.com/javase/tutorial/essential/concurrency/pools.html。目前，您可以使用surfacecreated方法而不是构造函数创建新线程

# 2 楼答案

使_run易失性，因此读写是原子的。见http://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html

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

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java停止线程未使用函数setRunning（false）

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