输入和输出无法解析为类型（Java套接字）

Question

I really don't know what the problem is. I am learning Java and I am following Oracle's Docs on networking. I simply did a copy and paste for their example and get this error. Could someone be kind enough to help? Thank you :)

Here's the code from Oracle:

import java.net.Socket;
import java.net.ServerSocket;

public class EchoServer {

    public static void main(String[] args) throws Exception {

        // create socket
        int port = 4444;
        ServerSocket serverSocket = new ServerSocket(port);
        System.err.println("Started server on port " + port);

        // repeatedly wait for connections, and process
        while (true) {

            // a "blocking" call which waits until a connection is requested
            Socket clientSocket = serverSocket.accept();
            System.err.println("Accepted connection from client");

            // open up IO streams
            In  in  = new In (clientSocket);
            Out out = new Out(clientSocket);

            // waits for data and reads it in until connection dies
            // readLine() blocks until the server receives a new line from client
            String s;
            while ((s = in.readLine()) != null) {
                out.println(s);
            }

            // close IO streams, then socket
            System.err.println("Closing connection with client");
            out.close();
            in.close();
            clientSocket.close();
        }
    }
}

Answer 1

It seems you copied it wrong.

I was able to run it on the first try.

echo: 
hi mom
echo: hi mom
how are you?
echo: how are you?

I took what they had and remixed it a bit to make it simpler (less code, and it does the same thing). You need JDK 7, but you can drop these in an IDE. Hit run on Server first. Then hit run on client. Then click the output area of the IDE and start typing in the client. Or you can run them from the command line.

package com.examples;
import java.net.*;
import java.io.*;

public class EchoServer {
  public static void main(String... args) throws IOException {

    int port;

    if ( args.length != 1 ) {
        System.out.println("listening to port 9999");
        port = 9999;
    } else {
        port = Integer.parseInt ( args[ 0 ] );

    }

    try (
            ServerSocket serverSocket =
                    new ServerSocket(port);
            Socket clientSocket = serverSocket.accept();
            PrintWriter out =
                    new PrintWriter(clientSocket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                    new InputStreamReader(clientSocket.getInputStream()));
    ) {
        String inputLine;
        while ((inputLine = in.readLine()) != null) {
            out.println(inputLine);
        }
    }
  }
}

The above uses the new JDK 7 try with auto close syntax. You want this.

Then here is the remixed client.

package com.examples;

import java.io.*;
import java.net.*;

public class EchoClient {
  public static void main ( String... args ) throws IOException {

    String host;
    int port;

    if ( args.length != 2 ) {
        System.out.println("binding to port localhost:9999");
        host = "localhost";
        port = 9999;
    } else {
        host = args[ 0 ];
        port = Integer.parseInt ( args[ 1 ] );
    }

    try (
            Socket echoSocket = new Socket ( host, port );
            PrintWriter out =
                    new PrintWriter ( echoSocket.getOutputStream (), true );
            BufferedReader in =
                    new BufferedReader (
                            new InputStreamReader ( echoSocket.getInputStream () ) );
            BufferedReader stdIn =
                    new BufferedReader (
                            new InputStreamReader ( System.in ) )
    ) {
        System.out.println("Type in some text please.");
        String userInput;
        while ( ( userInput = stdIn.readLine () ) != null ) {
            out.println ( userInput );
            System.out.println ( "echo: " + in.readLine () );
        }
    }
  }
}

The server is as follows:

We grab the port number from the args passed to main or just set them to 9999 if they are not passed on command line.

    int port;

    if ( args.length != 1 ) {
        System.out.println("listening to port 9999");
        port = 9999;
    } else {
        port = Integer.parseInt ( args[ 0 ] );

    }

Then in the try statement parens we open up our server socket streak gak

            ServerSocket serverSocket =
                    new ServerSocket(port);
            Socket clientSocket = serverSocket.accept();
            PrintWriter out =
                    new PrintWriter(clientSocket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                    new InputStreamReader(clientSocket.getInputStream())); 

If the above does not make sense, then you have to learn more about streams. Go to the tutorial on input/output (read and writing files), then come back to this.

Basically it is wrapping the output stream of the server socket in PrintWriter and wrapping the input stream in a BufferedReader. The Java I/O API is a bit daunting if you have not worked with it before so I suggest starting with the I/O tutorial a bit.

Here is the original:
EchoClient
EchoServer

Based on where you got messed up, start reading about IO-Streams , then IO-Character Streams .

Then come back to this after a few hours of background.

Answer 2

In and Out are not defined types. Thus you get the error. Change it to another defined type and it should be fine.

Perhaps you were trying to do something like this?

PrintWriter out =
                new PrintWriter(clientSocket.getOutputStream(), true);                   
BufferedReader in = new BufferedReader(
                new InputStreamReader(clientSocket.getInputStream()));

Answer 3

From the EchoServer.java example, I assume that In and Out should be PrintWriter and BufferedReader , respectively (or some other output and input stream objects, as In and Out are meaningless unless you wrote the objects yourself):

PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);                  
BufferedReader in = new BufferedReader(new InputStreamReader
                                      (clientSocket.getInputStream()));

If you haven't already, check out the Reading from and Writing to a Socket Tutorial .

Answer 4

What Oracle doc are you looking at? (link please)

Lesson: All About Sockets looks like what you should be reading. It has the following EchoServer :

import java.net.*;
import java.io.*;

public class EchoServer {
public static void main(String[] args) throws IOException {

    if (args.length != 1) {
        System.err.println("Usage: java EchoServer <port number>");
        System.exit(1);
    }

    int portNumber = Integer.parseInt(args[0]);

    try (
        ServerSocket serverSocket =
            new ServerSocket(Integer.parseInt(args[0]));
        Socket clientSocket = serverSocket.accept();    
        PrintWriter out =
            new PrintWriter(clientSocket.getOutputStream(), true);                  
        BufferedReader in = new BufferedReader(
            new InputStreamReader(clientSocket.getInputStream()));
    ) {
        String inputLine;
        while ((inputLine = in.readLine()) != null) {
            out.println(inputLine);
        }
    } 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());
    }
}
}

Answer 5

I improved the example a bit. EchoServer no longer terminates after first connection (it was annoying me). I tried to use System.console(), but it would not work in my IDE so I canned it.

Notice the use of Scanner (in the EchoClient) which cut down the code quite a bit and made the example much more readable.

Client:

        try (
                Socket echoSocket = new Socket ( host, port );
                PrintWriter socketOut =
                        new PrintWriter ( echoSocket.getOutputStream (), true );
                Scanner socketIn =  new Scanner( echoSocket.getInputStream () );
                Scanner console = new Scanner(System.in);
        ) {
            System.out.println("Type in some text please.");
            while ( console.hasNextLine () ) {
                String userInput = console.nextLine ();
                socketOut.println ( userInput );
                System.out.println ( "echo: " + socketIn.nextLine () );
            }
        }

Here is a better example and I think it also answers the original question better.

Full server with while true loop:

package com.examples;
import java.net.*;
import java.io.*;

public class EchoServer {
    public static void main(String... args) throws IOException {

        int port;

        if ( args.length != 1 ) {
            System.out.println("listening to port 9999");
            port = 9999;
        } else {
            port = Integer.parseInt ( args[ 0 ] );

        }

        while(true) {
            try (
                    ServerSocket serverSocket =
                            new ServerSocket(port);
                    Socket clientSocket = serverSocket.accept();
                    PrintWriter out =
                            new PrintWriter(clientSocket.getOutputStream(), true);
                    BufferedReader in = new BufferedReader(
                            new InputStreamReader(clientSocket.getInputStream()));
            ) {
                String inputLine;
                while ((inputLine = in.readLine()) != null) {
                    out.println(inputLine);
                }
            }
        }
    }
}

Notice how the main method works even if the end user (a developer learning Sockets) is running in an IDE and passing args are not convenient. When I write examples, I spend a lot of time trying to simplify and amplify the topic I am covering with as little clutter as possible.

Here is the client once again, notice how the code is much shorter using Scanner than BufferredReader and it is more logical (you don't have to check for null, if there are lines process them).

Full Client

package com.examples;

import java.io.*;
import java.net.*;
import java.util.Scanner;


public class EchoClient {
    public static void main ( String... args ) throws IOException {

        String host;
        int port;

        if ( args.length != 2 ) {
            System.out.println("binding to port localhost:9999");
            host = "localhost";
            port = 9999;
        } else {
            host = args[ 0 ];
            port = Integer.parseInt ( args[ 1 ] );

        }



        try (
                Socket echoSocket = new Socket ( host, port );
                PrintWriter socketOut =
                        new PrintWriter ( echoSocket.getOutputStream (), true );
                Scanner socketIn =  new Scanner( echoSocket.getInputStream () );
                Scanner console = new Scanner(System.in);
        ) {
            System.out.println("Type in some text please.");
            while ( console.hasNextLine () ) {
                String userInput = console.nextLine ();
                socketOut.println ( userInput );
                System.out.println ( "echo: " + socketIn.nextLine () );
            }
        }


    }

}

I also renamed the variables in Client so it is more clear what they are doing. Now that I write this I realize I could do the same for the EchoServer.

I might add a set of socket utils to Boon .