视图上的空指针

Question

最近，为了将手机与无线传感器网络集成在一起，我修改了一个类，将其用作activity和listener类，后者从串行端口读取数据，并将一些读数绘制到一个简单的基于画布的视图上。我最初尝试使用更普通的消息处理程序和更新和失效的线程来构建线程画布，但是，存储我需要绘制的读数的类也在侦听USB端口的线程上工作。我当时不能让它工作，我决定我可能不需要不断更新画布，因为我只需要用新的读数更新它。

因此activity类如下所示。

public class Oscilloscope extends Activity implements MessageListener 

Graph graph;
boolean guard=false;
MoteIF mote;
Data data;
String comm;
static String writeLog;
public Vector <Integer>  arrayvals = new Vector<Integer>();


public Oscilloscope(String comm, String writelog)
{
    this.comm = comm;
    this.writeLog = writelog;
}

public Oscilloscope()
{

}


public void onCreate(Bundle b)
{
    super.onCreate(b);
    graph = new Graph(this);
    this.setContentView(graph);
    if(guard==false)
    {
        create("dummy:1",null).run();
    }

}
synchronized public void messageReceived(int dest_addr, Message msg) {
    if (msg instanceof OscilloscopeMsg) {
        OscilloscopeMsg omsg = (OscilloscopeMsg)msg;



        periodUpdate(omsg.get_version(), omsg.get_interval());
        data.update(omsg.get_id(), omsg.get_count(), omsg.get_readings());

        List temp1 = Arrays.asList(omsg.get_readings());
        Vector<Integer> temp= new Vector<Integer>(temp1);

        this.arrayvals = temp;
        Paint p = new Paint();
        p.setColor(Color.BLUE);

        this.graph.update();
        this.graph.invalidate();

    }
}
public Oscilloscope create(String a, String b)
{
    guard = true;
    Oscilloscope os = new Oscilloscope(a,b);
    os.guard = true;
    return os;
}

不知道为什么我不能让代码括号在那里正常工作，我道歉。类中还有其他方法，但类本身的行为就像它应该减去视图部分一样，特别注意消息接收方法，在该方法中，读数作为int数组读入。我的问题是，一旦调用了一次onDraw方法，然后调用了messagereceived，图形对象看起来就像是null，这显然会在我试图调用update时抛出一个null指针。我可能看不到什么很明显的东西。图形代码非常简单，如下所示：

public class Graph extends View 
    Paint paint= new Paint();
    public float x;
    public float y;
    Vector<CoOrdinate> coVec = new Vector<CoOrdinate>();

    public Graph(Context context) {
        super(context);
    }

    public void onDraw(Canvas canvas)
    {


            Log.v("Line 28 Graph.java","Y: "+y+"X: "+x);
            canvas.drawCircle(x, y, 2, paint);
            Log.v(this.toString(),"onDraw in the graph");

    }

    public void update()
    {
        Log.v("Line 31 of Graph.java","");
        x+=100;
        y+=100;
        paint.setColor(Color.GREEN);
    }
}

感谢您的帮助。谢谢。

02-11 19:03:00.933: DEBUG/AndroidRuntime(301): >>>>>>>>>>>>>> AndroidRuntime START <<<<<<<<<<<<<<
02-11 19:03:00.933: DEBUG/AndroidRuntime(301): CheckJNI is ON
02-11 19:03:02.012: DEBUG/AndroidRuntime(301): --- registering native functions ---
02-11 19:03:03.942: INFO/ActivityManager(60): Starting activity: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=graphAndroid.graphs/.Oscilloscope }
02-11 19:03:04.062: DEBUG/AndroidRuntime(301): Shutting down VM
02-11 19:03:04.082: DEBUG/jdwp(301): adbd disconnected
02-11 19:03:04.122: INFO/AndroidRuntime(301): NOTE: attach of thread 'Binder Thread #3' failed
02-11 19:03:04.263: INFO/ActivityManager(60): Start proc graphAndroid.graphs for activity graphAndroid.graphs/.Oscilloscope: pid=308 uid=10040 gids={1015}
02-11 19:03:05.263: INFO/ARMAssembler(60): generated scanline__00000077:03545404_00000004_00000000 [ 47 ipp] (67 ins) at [0x361990:0x361a9c] in 6676000 ns
02-11 19:03:05.493: INFO/System.out(308): Creating dummy source (dummy:1)
02-11 19:03:05.493: INFO/System.out(308): No Motes:1
02-11 19:03:05.503: VERBOSE/Line 129 Oscilloscope.java(308): Made it to here
02-11 19:03:05.503: VERBOSE/Line 135 Oscilloscope.java(308): Made it here
02-11 19:03:05.513: VERBOSE/Line 137 Oscilloscope.java(308): Made it here
02-11 19:03:05.513: VERBOSE/Line 114 MoteIF.java(308): Third constructor
02-11 19:03:05.513: VERBOSE/Line 121 MoteIF.java(308): Made it here
02-11 19:03:05.513: VERBOSE/Line 123 MoteIF.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 127 MoteIF.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 130 MoteIF.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 69 PhoenixSource.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 71 PhoenixSource.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 74 PhoenixSource.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 133 MoteIF.java(308): Made it here
02-11 19:03:05.523: VERBOSE/Line 138 MoteIF.java(308): Made it here
02-11 19:03:05.533: VERBOSE/Line 140 MoteIF.java(308): Made it here
02-11 19:03:05.543: VERBOSE/Line 142 MoteIF.java(308): Made it here
02-11 19:03:05.543: VERBOSE/Line 138 Oscilloscope.java(308): Made it here
02-11 19:03:05.543: VERBOSE/Line 140 Oscilloscope.java(308): Made it to here
02-11 19:03:05.783: VERBOSE/Line 28 Graph.java(308): Y: 0.0X: 0.0
02-11 19:03:05.793: VERBOSE/graphAndroid.graphs.Graph@43e3deb8(308): onDraw in the graph
02-11 19:03:05.823: INFO/ActivityManager(60): Displayed activity graphAndroid.graphs/.Oscilloscope: 1679 ms (total 1679 ms)
02-11 19:03:07.543: INFO/System.out(308): SimulatedOscilloscopePacketSource moteId is 0
02-11 19:03:07.543: VERBOSE/1(308): A test, line 424 OscilloscopeMsg class
02-11 19:03:07.543: WARN/dalvikvm(308): threadid=7: thread exiting with uncaught exception (group=0x4001d800)
02-11 19:03:07.563: ERROR/AndroidRuntime(308): FATAL EXCEPTION: Thread-8
02-11 19:03:07.563: ERROR/AndroidRuntime(308): java.lang.NullPointerException
02-11 19:03:07.563: ERROR/AndroidRuntime(308):     at graphAndroid.graphs.Oscilloscope.messageReceived(Oscilloscope.java:108)
02-11 19:03:07.563: ERROR/AndroidRuntime(308):     at net.tinyos.message.Receiver.packetReceived(Receiver.java:210)
02-11 19:03:07.563: ERROR/AndroidRuntime(308):     at net.tinyos.packet.PhoenixSource.dispatch(PhoenixSource.java:165)
02-11 19:03:07.563: ERROR/AndroidRuntime(308):     at net.tinyos.packet.PhoenixSource.packetDipatchLoop(PhoenixSource.java:157)
02-11 19:03:07.563: ERROR/AndroidRuntime(308):     at net.tinyos.packet.PhoenixSource.run(PhoenixSource.java:174)
02-11 19:03:07.613: WARN/ActivityManager(60):   Force finishing activity graphAndroid.graphs/.Oscilloscope
02-11 19:03:07.932: WARN/IInputConnectionWrapper(129): showStatusIcon on inactive InputConnection
02-11 19:03:08.463: INFO/ARMAssembler(60): generated scanline__00000077:03515104_00000000_00000000 [ 33 ipp] (47 ins) at [0x36b288:0x36b344] in 746000 ns

Answer 1

由于graph是在onCreate中初始化的，因此新线程中的graph字段将为null，因为只有活动才会调用onCreate。我认为这是导致崩溃的直接原因。

我可能会将这个类重写为两个类(一个活动，一个线程)，以避免混淆；但如果您想保持相同的通用结构，我建议您为Oscilloscope添加第二个构造函数，该构造函数将图形和任何其他共享成员作为参数。在create中调用此构造函数，以确保传入共享字段。(然后在必要的地方进行同步时要非常小心...)

图形编辑:进一步检查：对象被传递到onCreate中的setContentView中。这意味着你的不能试图在另一个线程中使用它。研究一下如何使用MessageQueues和Handlers来回发到UI线程，在那里您可以安全地更新图形。

顺便说一句，您似乎没有在任何地方保存对新线程的引用。我认为这意味着新的线程可以在任何时候被垃圾回收。

Answer 2

当您的活动被创建时，您创建了它的另一个实例(在create(string，string)中)，但是在那个新实例上永远不会调用onCreate。因此，现在，当在新实例上调用messageReceived时，graph对象从未被实例化，您会得到一个空指针异常。