nullpointerexception在java中使用串行端口时如何避免null指针异常？

Question

所以，我正在制作一个程序，通过发送命令与传感器进行通信。但是，当我尝试单击GUI中的按钮时，会出现两个错误：nullpointer异常和portinuse异常，这会阻止按钮工作。我该如何解决这个问题

SimpleRead类：

import java.awt.event.ActionListener;
import java.io.*;
import java.util.*; 
import javax.comm.*;
import javax.swing.*;



public class SimpleRead  {
    static Enumeration portList;
    static CommPortIdentifier portId;
    static  PrintStream os;
    static BufferedReader is;
    static java.util.Timer t = new java.util.Timer();
    static TimersTask tt = new TimersTask();

    public static void main(String[] args) {

        GUI GUI = new GUI();
        JFrame window = new JFrame("DI-100"); 
        window.setSize(700, 300); 
        window.setVisible(true); 
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.add(GUI);

        Enumeration portList = CommPortIdentifier.getPortIdentifiers();
        String wantedPortName;
        CommPortIdentifier portId = (CommPortIdentifier) portList.nextElement();
        if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {
            ArrayList<String> serialports = new ArrayList<String>();
            serialports.add(portId.getName());
            String[] ports = new String[serialports.size()];
            ports = serialports.toArray(ports); 
            GUI.jComboBox2 = new JComboBox(ports);
            GUI.jComboBox2.addActionListener(GUI.jComboBox2);
            wantedPortName = (String) GUI.jComboBox2.getSelectedItem();


            while (portList.hasMoreElements()) { 
                if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL &&
                  portId.getName().equals(wantedPortName)) {
                    CommPortIdentifier pid = portId;
                    SerialPort port = null;


                    try {
                        port = (SerialPort) portId.open("OpenPort", 1000);

                    } catch(PortInUseException e) {
                        System.err.println("Port already in use: " + e);
                    }   



                    try {
                        port.setSerialPortParams(9600,SerialPort.DATABITS_8,SerialPort.STOPBITS_1,SerialPort.PARITY_NONE);
                    }
                    catch (UnsupportedCommOperationException e) {
                        System.out.println("you suck");
                    }



                    try {
                        os = new PrintStream(port.getOutputStream(), true, "ISO-8859-1");
                    }
                    catch (IOException e) {
                        System.err.println("Can't open input stream: write-only");
                    }



                    try {
                        is = new BufferedReader(new InputStreamReader(port.getInputStream()));
                    } catch (IOException e) {
                        System.err.println("Can't open input stream: write-only");
                    }



                    try{
                        os = new PrintStream(port.getOutputStream(), true);
                    } catch (IOException e){   
                    }
                }
                else {
                }
            }
        }
    }
}

TimerTask类：

import java.io.IOException;
import java.util.*;

public class TimersTask extends TimerTask{
    @Override
    public void run() {
        SimpleRead.os.print("W");
        try {           
            String returnvalue = SimpleRead.is.readLine(); 
            System.out.println(returnvalue);
            GUI.jTextField1.setText(returnvalue); 
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {    
            }
            //is.readLine(); // Second read will remove the extra line feed that AT generates as 
        } catch (IOException e){    
        }
    }
}

Answer 1

好的，我想这就是正在发生的事情。您试图打开端口：port = (SerialPort) portId.open("OpenPort", 1000);，但无论出于何种原因，它都会失败。然后继续使用它：port.setSerialPortParams(9600,SerialPort.DATABITS_8,SerialPort.STOPBITS_1,SerialPort.PARITY_NONE);，只是此时它为空，所以程序会爆炸

SimpleRead类：

import java.awt.event.ActionListener;
import java.io.*;
import java.util.*; 
import javax.comm.*;
import javax.swing.*;

public class SimpleRead  {
    static Enumeration portList;
    static CommPortIdentifier portId;
    static PrintStream os;
    static BufferedReader is;
    static java.util.Timer t = new java.util.Timer();
    static TimersTask tt = new TimersTask();

    public static void main(String[] args) {

        GUI GUI = new GUI();
        JFrame window = new JFrame("DI-100"); 
        window.setSize(700, 300); 
        window.setVisible(true); 
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.add(GUI);

        Enumeration portList = CommPortIdentifier.getPortIdentifiers();
        String wantedPortName;
        CommPortIdentifier portId = (CommPortIdentifier) portList.nextElement();
        if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {
            ArrayList<String> serialports = new ArrayList<String>();
            serialports.add(portId.getName());
            String[] ports = new String[serialports.size()];
            ports = serialports.toArray(ports); 
            GUI.jComboBox2 = new JComboBox(ports);
            GUI.jComboBox2.addActionListener(GUI.jComboBox2);
            wantedPortName = (String) GUI.jComboBox2.getSelectedItem();


            while (portList.hasMoreElements()) { 
                if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL &&
                  portId.getName().equals(wantedPortName)) {
                    CommPortIdentifier pid = portId;
                    SerialPort port = null;


                    try {
                        port = (SerialPort) portId.open("OpenPort", 1000);
                        port.setSerialPortParams(9600,SerialPort.DATABITS_8,SerialPort.STOPBITS_1,SerialPort.PARITY_NONE);
                        os = new PrintStream(port.getOutputStream(), true, "ISO-8859-1");
                        is = new BufferedReader(new InputStreamReader(port.getInputStream()));
                        os = new PrintStream(port.getOutputStream(), true); // you had this line in your original code.  Is this really necessary, since you have a similar line 2 lines before it?
                    } catch(PortInUseException e) {
                        System.err.println("Port already in use: " + e);
                    } catch (UnsupportedCommOperationException e) {
                        System.out.println("you suck");
                    } catch (IOException e) {
                        System.err.println("Can't open input or output stream");
                    }
                }
                else {
                }
            }
        }
        try {
            os.close();
        } catch (Exception ex) {
            //already closed
        }
        try {
            is.close();
        } catch (Exception ex) {
            //already closed
        }
        // close the port here
    }
}

TimersTask类：

import java.io.IOException;
import java.util.*;

public class TimersTask extends TimerTask{
    @Override
    public void run() {
        SimpleRead.os.print("W");
        try {           
            String returnvalue = SimpleRead.is.readLine(); 
            System.out.println(returnvalue);
            GUI.jTextField1.setText(returnvalue); 
            Thread.sleep(1000);
            //is.readLine(); // Second read will remove the extra line feed that AT generates as 
        } catch (IOException e){    
        } catch (InterruptedException e) {    
        }
    }
}

这解决了你的一个问题，还有很多你不知道的问题。try/catch的工作方式是，当某个代码失败时，块中的其余代码不会运行，因为它取决于失败的代码。相反，您的代码只需在失败时一个接一个地抛出异常即可

根本的问题是PortInUseException，这是不言自明的。其他东西已经在使用该端口。检查你的代码（在这里和其他地方），确保你没有试图打开端口两次（比如，如果你多次点击按钮？），如果这还不能解决问题，你的电脑上可能有另一个程序正在抓取这个端口

“记住，孩子们，用完后一定要关闭端口！”