Building a Multi-Threaded Web Server

In this lab we will develop a Web server in two steps. In the end, you will have built a multithreaded Web server that is capable of processing multiple simultaneous service requests in parallel. You should be able to demonstrate that your Web server is capable of delivering your home page to a Web browser.

We are going to implement version 1.0 of HTTP, as defined in RFC 1945, where separate HTTP requests are sent for each component of the Web page. The server will be able to handle multiple simultaneous service requests in parallel. This means that the Web server is multithreaded. In the main thread, the server listens to a fixed port. When it receives a TCP connection request, it sets up a TCP connection through another port and services the request in a separate thread. To simplify this programming task, we will develop the code in two stages. In the first stage, you will write a multithreaded server that simply displays the contents of the HTTP request message that it receives. After this program is running properly, you will add the code required to generate an appropriate response.

As you are developing the code, you can test your server from a Web browser. But remember that you are not serving through the standard port 80, so you need to specify the port number within the URL that you give to your browser. For example, if your machine's name is host.someschool.edu, your server is listening to port 6789, and you want to retrieve the file index.html, then you would specify the following URL within the browser:

http://host.someschool.edu:6789/index.html

If you omit ":6789", the browser will assume port 80 which most likely will not have a server listening on it. When the server encounters an error, it sends a response message with the appropriate HTML source so that the error information is displayed in the browser window.

Web Server in Java: Part B

Instead of simply terminating the thread after displaying the browser's HTTP request message, we will analyze the request and send an appropriate response. We are going to ignore the information in the header lines, and use only the file name contained in the request line. In fact, we are going to assume that the request line always specifies the GET method, and ignore the fact that the client may be sending some other type of request, such as HEAD or POST.

We extract the file name from the request line with the aid of the StringTokenizer class. First, we create a StringTokenizer object that contains the string of characters from the request line. Second, we skip over the method specification, which we have assumed to be "GET". Third, we extract the file name.

// Extract the filename from the request line.

StringTokenizer tokens = new StringTokenizer(requestLine);

tokens.nextToken(); // skip over the method, which should be "GET"

String fileName = tokens.nextToken();

// Prepend a "." so that file request is within the current directory.

fileName = "." + fileName;

Because the browser precedes the filename with a slash, we prefix a dot so that the resulting pathname starts within the current directory.

Now that we have the file name, we can open the file as the first step in sending it to the client. If the file does not exist, the FileInputStream() constructor will throw the FileNotFoundException. Instead of throwing this possible exception and terminating the thread, we will use a try/catch construction to set the boolean variable fileExists to false. Later in the code, we will use this flag to construct an error response message, rather than try to send a nonexistent file.

// Open the requested file.

FileInputStream fis = null;

boolean fileExists = true;

try {

fis = new FileInputStream(fileName);

} catch (FileNotFoundException e) {

fileExists = false;

}

There are three parts to the response message: the status line, the response headers, and the entity body. The status line and response headers are terminated by the character sequence CRLF. We are going to respond with a status line, which we store in the variable statusLine, and a single response header, which we store in the variable contentTypeLine. In the case of a request for a nonexistent file, we return 404 Not Found in the status line of the response message and include an error message in the form of an HTML document in the entity body.

// Construct the response message.

String statusLine = null;

String contentTypeLine = null;

String entityBody = null;

if (fileExists) {

statusLine = ?;

contentTypeLine = "Contenttype: " +

contentType( fileName ) + CRLF;

} else {

statusLine = ?;

contentTypeLine = ?;

entityBody = "

" +

"

Not Found

" +

"

Not Found

";

}

When the file exists, we need to determine the file's MIME type and send the appropriate MIMEtype

specifier. We make this determination in a separate private method called contentType(), which returns a string that we can include in the content type line that we are constructing.

Now we can send the status line and our single header line to the browser by writing into the socket's output stream.

// Send the status line.

os.writeBytes(statusLine);

// Send the content type line.

os.writeBytes(?);

// Send a blank line to indicate the end of the header lines.

os.writeBytes(CRLF);

Now that the status line and header line with delimiting CRLF have been placed into the output stream on their way to the browser, it is time to do the same with the entity body. If the requested file exists, we call a separate method to send the file. If the requested file does not exist, we send the HTMLencoded error message that we have prepared.

// Send the entity body.

if (fileExists) {

sendBytes(fis, os);

fis.close();

} else {

os.writeBytes(?);

}

After sending the entity body, the work in this thread has finished, so we close the streams and socket before terminating.

We still need to code the two methods that we have referenced in the above code, namely, the method that determines the MIME type, contentType(), and the method that writes the requested file onto the socket's output stream. Let's first take a look at the code for sending the file to the client.

private static void sendBytes(FileInputStream fis, OutputStream os)

throws Exception

{

// Construct a 1K buffer to hold bytes on their way to the socket.

byte[] buffer = new byte[1024];

int bytes = 0;

// Copy requested file into the socket's output stream.

while((bytes = fis.read(buffer)) != 1 ) {

os.write(buffer, 0, bytes);

}

}

Both read() and write() throw exceptions. Instead of catching these exceptions and handling them in our code, we throw them to be handled by the calling method.

The variable, buffer, is our intermediate storage space for bytes on their way from the file to the output stream. When we read the bytes from the FileInputStream, we check to see if read() returns minus one, indicating that the end of the file has been reached. If the end of the file has not been reached, read() returns the number of bytes that have been placed into buffer. We use the write() method of the OutputStream class to place these bytes into the output stream, passing to it the name of the byte array, buffer, the starting point in the array, 0, and the number of bytes in the array to write, bytes.

The final piece of code needed to complete the Web server is a method that will examine the extension of a file name and return a string that represents it's MIME type. If the file extension is unknown, we return the type application/octetstream.

private static String contentType(String fileName)

{

if(fileName.endsWith(".htm") || fileName.endsWith(".html")) {

return "text/html";

}

if(?) {

?;

}

if(?) {

?;

}

return "application/octetstream";

}

There is a lot missing from this method. For instance, nothing is returned for GIF or JPEG files. You may want to add the missing file types yourself, so that the components of your home page are sent with the content type correctly specified in the content type header line. For GIFs the MIME type is image/gif and for JPEGs it is image/jpeg.

This completes the code for the second phase of development of your Web server. Try running the server from the directory where your home page is located, and try viewing your home page files with a browser. Remember to include a port specifier in the URL of your home page, so that your browser doesn't try to connect to the default port 80. When you connect to the running web server with the browser, examine the GET message requests that the web server receives from the browser.