我正在用 Java 开发一个简单的服务器，它应该能够跨计算机传输文件。我在 Protocol.class 的第 77 行收到一个 NullPointerException 。这是堆栈:
java.lang.NullPointerException
at Protocol.processInput(Protocol.java:77)
at Server.main(Server.java:41)
为什么会发生这种情况？第 77 行没有 null 引用!
客户端.java:
import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.ArrayList;
import javax.swing.JFileChooser;
import javax.swing.JOptionPane;
import javax.swing.filechooser.FileNameExtensionFilter;
public class Client {
private static boolean filein = false;
private static ArrayList<String> fln = new ArrayList<>();
public static void main(String[] args) throws IOException {
if (args.length != 2) {
System.err.println(
"Usage: java Client <host name> <port number>");
System.exit(1);
}
String hostName = args[0];
int portNumber = Integer.parseInt(args[1]);
try (
Socket kkSocket = new Socket(hostName, portNumber);
PrintWriter out = new PrintWriter(kkSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(kkSocket.getInputStream()));
) {
BufferedReader stdIn =
new BufferedReader(new InputStreamReader(System.in));
String fromServer;
String fromUser = null;
while ((fromServer = in.readLine()) != null) {
System.out.println("Server: " + fromServer);
if (fromServer.equals("@file")) { filein = true;
fromUser = "";}
else if(fromServer.equals("@end@")) {
filein = false;
JFileChooser chooser = new JFileChooser();
int returnVal = chooser.showSaveDialog(null);
if(returnVal == JFileChooser.APPROVE_OPTION) {
String fname = chooser.getSelectedFile().getAbsolutePath();
File f = new File(fname);
f.createNewFile();
PrintWriter p = new PrintWriter(f);
for(int i = 0; i < fln.size(); i++) {
p.println(fln.get(i));
}
p.close();
JOptionPane.showMessageDialog(null, "File saved!");
}
}
else if (filein == true) {
fln.add(fromServer);
System.out.println(fln.get(fln.size() - 1));
}
if (fromServer.equals("Bye."))
break;
if (!filein) fromUser = stdIn.readLine();
else if (filein) fromUser = "@contintueFileRun";
if (fromUser != null) {
System.out.println("Client: " + fromUser);
out.println(fromUser);
}
}
} catch (UnknownHostException e) {
System.err.println("Don't know about host " + hostName);
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection to " +
hostName);
System.exit(1);
}
}
}
服务器.java:
import java.io.*;
import java.net.*;
import static java.lang.System.out;
/**
* Title: FTP Server
* @author Galen Nare
* @version 1.0
*/
public class Server {
public static void main(String[] args) throws IOException {
out.println("Starting server!");
if (args.length != 1) {
System.err.println("Usage: java Server <port number>");
System.exit(1);
}
int portNumber = Integer.parseInt(args[0]);
try (
ServerSocket serverSocket = new ServerSocket(portNumber);
Socket clientSocket = serverSocket.accept();
PrintWriter out =
new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(clientSocket.getInputStream()));
) {
String inputLine, outputLine;
// Initiate conversation with client
Protocol kkp = new Protocol();
outputLine = kkp.processInput("");
out.println(outputLine);
while ((inputLine = in.readLine()) != null) {
outputLine = kkp.processInput(inputLine);
out.println(outputLine);
if (outputLine.equals("Bye."))
break;
}
} catch (IOException e) {
System.out.println("Exception caught when trying to listen on port "
+ portNumber + " or listening for a connection");
System.out.println(e.getMessage());
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
最后，Protocol.java:
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;
public class Protocol {
enum ServerState {
STARTING,
WAITING
}
ArrayList<String> lns;
boolean fileout = false;
int i = 0;
private ServerState state = ServerState.STARTING;
public String processInput(String theInput) throws Exception {
String theOutput = "";
if (state == ServerState.STARTING) {
theOutput = "Hello, Client!";
state = ServerState.WAITING;
}
if (!theInput.equals("")) {
if(theInput.length() > 10 && theInput.startsWith("e")) {
if (theInput.substring(0,11).equalsIgnoreCase("executecmd ")) {
theOutput = theInput.substring(11);
System.out.println(theOutput);
try {
@SuppressWarnings("unused")
Process child = Runtime.getRuntime().exec(theInput.substring(11));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
theOutput = "Executed " + theInput.substring(11) + ".";
}
} else if (theInput.equalsIgnoreCase("stop")) {
theOutput = "Stopping Server!";
System.exit(0);
} else if (theInput.equalsIgnoreCase("executecmd")) {
theOutput = "Usage: executecmd <command [-options]>";
} else if (theInput.equalsIgnoreCase("getfile")) {
theOutput = "Usage: getfile <file>";
} else if(theInput.length() > 7 && theInput.startsWith("g")) {
System.out.println("in");
if (theInput.substring(0,8).equalsIgnoreCase("getfile ")) {
theOutput = theInput.substring(8);
File f = new File(theInput.substring(8));
Scanner scan = new Scanner(f);
ArrayList<String> lns = new ArrayList<>();
while(scan.hasNext()) {
lns.add(scan.nextLine());
}
for (int i=0; i < lns.size(); i++) {
System.out.println(lns.get(i));
}
scan.close();
lns.add("@end@");
theOutput = "@file";
fileout = true;
}
} else if (fileout && i < lns.size()) {
theOutput = lns.get(i);
i++;
} else if (fileout && i == lns.size()) {
i = 0;
fileout = false;
} else {
theOutput = "That is not a command!";
}
}
System.out.print(theOutput);
return theOutput;
}
}
提前致谢!
最佳答案
您永远不会在 Protocol 中初始化 lns，因此它始终是一个 null 引用。您只需将声明更改为:
private List<String> lns = new ArrayList<String>();
(出于习惯，我已将其设为私有(private)并将类型更改为 List...)
您还应该考虑给它一个更易读的名称 - 它是否意味着代表线条？如果是这样，请将其称为lines!
(接下来，考虑为什么您无法自己诊断此问题。您是否在调试器中单步执行了此操作？为什么您认为第 77 行没有空引用？您在添加方面采取了哪些诊断步骤额外的日志记录等？使用此类错误作为学习经验非常重要，可以使 future 的问题更容易处理。)
关于java - 向客户端发送文件时出现棘手的 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21443892/