浏览器无法连接到本地主机

Question

I'm trying to setup a simple web server in java, and I can read the web browser's request to my machine, but the web browser can't seem to read my reply. You can duplicate my issue by running the code below and attempting to connect to http://localhost:8080 .

The browser doesn't acknowledge any response, and just keeps waiting forever. Does anyone know what is causing this? I suspect it's related to the output stream but I don't know what's wrong.

I'm running a slightly modified version of the code in this tutorial: https://javarevisited.blogspot.com/2015/06/how-to-create-http-server-in-java-serversocket-example.html

EDIT: I just tested the connection with curl localhost:8080 and it worked fine. No clue why the web browser has any issues. You're welcome to download and test this code on your own machine.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;

public class SimpleHttpServer {
    public static void main(String args[]) throws IOException {
        Socket client = null;
        try {
            ServerSocket server = new ServerSocket(8080);
            BufferedReader reader;

            System.out.println("Listening for connection on port 8080 ....");
            client = server.accept();
            System.out.println("Got connection.");

            while (true) { 
                reader = new BufferedReader(new InputStreamReader(client.getInputStream()));

                String temp = reader.readLine();
                while ( temp != null && !temp.isEmpty() ) {
                    System.out.println("<"+temp+">");
                    temp = reader.readLine();
                }

                Date today = new Date();
                String httpResponse = "HTTP/1.1 200 OK\r\n\r\n" + today;
                client.getOutputStream().write(httpResponse.getBytes());
                System.out.println("Sent: "+httpResponse);

                Thread.sleep(1000);
            }
        } catch (Exception e) {
            client.getOutputStream().close();
            client.getInputStream().close();
            client.close();
        }
    }
}

Answer 1

Move these two lines inside while(true) :

client = server.accept();
System.out.println("Got connection.");

also after writing to output stream client.getOutputStream().write(httpResponse.getBytes()); close the stream:

client.getOutputStream().close();

Answer 2

The reason your browser is hanging is because it doesn't know how many bytes it is going to receive.

According to the HTTP/1.1 RFC

Applications SHOULD use [ Content-Length ] to indicate the transfer-length of the message-body, unless this is prohibited by [the type of message you are sending].

Include Content-Length in your HTTP response and it should work:

Date today = new Date();
String body = today.toString();
String httpResponse = "HTTP/1.1 200 OK\r\n"
                    + "Content-Length: " + body.getBytes().length
                    + "\r\n\r\n"
                    + body;

This is useful to know if you want to keep the connection open for further communication, rather than closing it to indicate the end of a message and having to reconnect for further communication. I also prefer Content-Length because it is more clear, whereas a closed connection could've happened from external circumstances.

Answer 3

You are missing required headers and you're not flushing the output.

Since you seem to want to keep the connection alive for multiple requests, you need to send header:

Connection: keep-alive

You also need to tell the browser what you're sending, and how much, so it knows when the response is complete:

Content-Type: text/plain
Content-Length: 999

Working code:

// ... existing code ...

Date today = new Date();
String content = today.toString();
String httpResponse = "HTTP/1.1 200 OK\r\n" +
                      "Content-Type: text/plain\r\n" +
                      "Content-Length: " + content.length() + "\r\n" +
                      "Connection: keep-alive\r\n" +
                      "\r\n" +
                      content;
client.getOutputStream().write(httpResponse.getBytes());
client.getOutputStream().flush();
System.out.println("Sent: "+httpResponse);

// ... existing code ...

Answer 4

The client isn't sure the server isn't going to send more data. You need to close the stream on the server to let the client know the response is complete.

The following code works: the while loop is not needed, and you can just send the response, as long as you end by closing the output stream to the client.

EDIT: Others have pointed out that you could, of course, specify a Content-Length header, and so forth. You could also accept multiple connections, and so forth. But I think it's clear that your example was a toy example, and so this is the best toy answer -- the simplest way to get something like your Hello, World example working without additional background information.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;

public class SimpleHttpServer {
    public static void main(String args[]) throws IOException {
        Socket client = null;
        try {
            ServerSocket server = new ServerSocket(8080);
            BufferedReader reader;

            System.out.println("Listening for connection on port 8080 ....");
            client = server.accept();
            System.out.println("Got connection.");

            reader = new BufferedReader(new InputStreamReader(client.getInputStream()));

            String temp = reader.readLine();
            while ( temp != null && !temp.isEmpty() ) {
                System.out.println("<"+temp+">");
                temp = reader.readLine();
            }

            Date today = new Date();
            String httpResponse = "HTTP/1.1 200 OK\r\n\r\n" + today;
            client.getOutputStream().write(httpResponse.getBytes());
            System.out.println("Sent: "+httpResponse);
            client.getOutputStream().close();
        } catch (Exception e) {
            client.getOutputStream().close();
            client.getInputStream().close();
            client.close();
        }
    }
}