检查串口连接外设时遇到的NullPointerException

记录于2021-07-30
前不久接到旧项目的维护，其中涉及到了外设连接（平板连接到佳博厂商的称重机），需要实时返回数据到平板上显示。问题就在于数据返回的时快时慢，只能一个一个点去排查了。
打开AS，连接平板，看看日志，这个异常刷都刷不完：

		...
java.lang.NullPointerException: Attempt to invoke virtual method 'int com.gprinter.io.PortManager.readData(byte[])' on a null object reference
        at com.hycashier.devicelibrary.helper.BluetoothHelper$4.run(BluetoothHelper.java:198)
java.lang.NullPointerException: Attempt to invoke virtual method 'int com.gprinter.io.PortManager.readData(byte[])' on a null object reference
        at com.hycashier.devicelibrary.helper.BluetoothHelper$4.run(BluetoothHelper.java:198)
java.lang.NullPointerException: Attempt to invoke virtual method 'int com.gprinter.io.PortManager.readData(byte[])' on a null object reference
        at com.hycashier.devicelibrary.helper.BluetoothHelper$4.run(BluetoothHelper.java:198)
        ...

这一块的代码是这样的：

/**
     * 接收数据的方法
     */
    public void startReceiveData(){
    
    
        receiveFlag = true;
        /*创建子线程接收串口数据
         */
        receiveThread = new Thread(){
    
    
            @Override
            public void run() {
    
    
                while (receiveFlag){
    
    
                    try {
    
    
                        serialData = "0".getBytes();

                        byte[] readData = new byte[1024];
                        int size = portManager.readData(readData);
                        byte[] realReadData = new byte[size];
                        System.arraycopy(readData,0,realReadData,0,size);
                        if (size>0&&receiveFlag) {
    
    
                            serialData = realReadData;
                        }
                        getMainHandler().sendEmptyMessage(receiveFlagWhat);
                        Thread.sleep(intervalTime);
                    } catch (Exception e){
    
    
                        e.printStackTrace();
                    }
                }
            }
        };
        //启动接收线程
        receiveThread.start();
    }

通过Debug发现，portManager为空了；而且只有不断的打开新的Activity，就会再次去这里拿实时数据，然后CPU飙升…


也就是说，每一次都会来这个方法中new 一个子线程，并且无休止的取数据（而且这平板性能简直是5-6年前的配置）；那么只能试试非空判断了

/**
     * 接收数据的方法
     */
    public void startReceiveData(){
    
    
        receiveFlag = true;
        /*创建子线程接收串口数据
         */
        receiveThread = new Thread(){
    
    
            @Override
            public void run() {
    
    
                while (receiveFlag && portManager!=null){
    
    
                    try {
    
    
                        serialData = "0".getBytes();

                        byte[] readData = new byte[1024];
                        int size = portManager.readData(readData);
                        byte[] realReadData = new byte[size];
                        System.arraycopy(readData,0,realReadData,0,size);
                        if (size>0&&receiveFlag) {
    
    
                            serialData = realReadData;
                        }
                        getMainHandler().sendEmptyMessage(receiveFlagWhat);
                        Thread.sleep(intervalTime);
                    } catch (Exception e){
    
    
                        e.printStackTrace();
                    }
                }
            }
        };
        //启动接收线程
        receiveThread.start();
    }

亲测成功，至少cpu降下来了