epubjs

<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Sockets</title><link rel="stylesheet" href="core.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"/></head><body><div class="sect1" title="Sockets"><div class="titlepage"><div><div><h1 class="title"><a id="learnjava3-CHP-13-SECT-1"/>Sockets</h1></div></div></div><p>Sockets are a low-level programming interface for networked communications. They send streams of data between applications that may or may not be on the same host. Sockets originated in BSD Unix and are, in some programming languages, hairy, complicated things with lots of small parts that can break off and endanger little children. The reason for this is that most socket APIs can be used with almost any kind of underlying network protocol. Since the protocols that transport data across the network can have radically different features, the socket interface can be quite complex.<sup>[<a id="learnjava3-CHP-13-FN-1" href="#ftn.learnjava3-CHP-13-FN-1" class="footnote">37</a>]</sup></p><p>The <a id="I_indexterm13_id764911" class="indexterm"/><code class="literal">java.net</code> package supports a simplified, object-oriented socket interface that makes network communications considerably easier. If you’ve done network programming using sockets in other languages, you should be pleasantly surprised at how simple things can be when objects encapsulate the gory details. If this is the first time you’ve come across sockets, you’ll find that talking to another application over the network can be as simple as reading a file or getting user input. Most forms of I/O in Java, including most network I/O, use the stream classes described in <a class="xref" href="ch12.html" title="Chapter 12. Input/Output Facilities">Chapter 12</a>. Streams provide a unified I/O interface so that reading or writing across the Internet is similar to reading or writing on the local system. In addition to the stream-oriented interfaces, the Java networking APIs can work with the Java NIO buffer-oriented API for highly scalable applications. We’ll see both in this chapter.</p><p>Java provides sockets to support three distinct classes of underlying protocols: <code class="literal">Socket</code>s, <a id="I_indexterm13_id764947" class="indexterm"/><code class="literal">DatagramSocket</code>s, and <a id="I_indexterm13_id764958" class="indexterm"/><code class="literal">MulticastSocket</code>s. In this first section, we look at Java’s basic <code class="literal">Socket</code> class, which uses a <a id="I_indexterm13_id764975" class="indexterm"/><span class="emphasis"><em>connection-oriented</em></span> and <span class="emphasis"><em>reliable</em></span> protocol. A connection-oriented protocol provides the equivalent of a telephone conversation. After establishing a connection, two applications can send streams of data back and forth and the connection stays in place even when no one is talking. Because the protocol is reliable, it also ensures that no data is lost (resending data as necessary) and that whatever you send always arrives in the order in which you sent it.</p><p>In the next section, we look at the <code class="literal">DatagramSocket</code> class, which uses a <a id="I_indexterm13_id764999" class="indexterm"/><span class="emphasis"><em>connectionless</em></span>, <span class="emphasis"><em>unreliable</em></span> protocol. A connectionless protocol is like the postal service. Applications can send short messages to each other, but no end-to-end connection is set up in advance and no attempt is made to keep the messages in order. It’s not even guaranteed that the messages will arrive at all. A <a id="I_indexterm13_id765016" class="indexterm"/><code class="literal">MulticastSocket</code> is a variation of a <code class="literal">DatagramSocket</code> that performs multicasting—simultaneously sending data to multiple recipients. Working with multicast sockets is very much like working with datagram sockets. However, because multicasting is currently not widely supported across the Internet, we do not cover it here.</p><p>In theory, just about any protocol can be used underneath the socket layer (old-schoolers will remember things like Novell’s IPX, Apple’s AppleTalk, etc.). But in practice, there’s only one important protocol family used on the Internet, and only one protocol family that Java supports: the <a id="I_indexterm13_id765045" class="indexterm"/><a id="I_indexterm13_id765050" class="indexterm"/>Internet Protocol (IP). The <a id="I_indexterm13_id765057" class="indexterm"/><a id="I_indexterm13_id765062" class="indexterm"/><a id="I_indexterm13_id765068" class="indexterm"/><code class="literal">Socket</code> class speaks TCP, the connection-oriented flavor of IP, and the <a id="I_indexterm13_id765080" class="indexterm"/><a id="I_indexterm13_id765085" class="indexterm"/><a id="I_indexterm13_id765091" class="indexterm"/><code class="literal">DatagramSocket</code> class speaks UDP, the connectionless kind.</p><div class="sect2" title="Clients and Servers"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-13-SECT-1.1"/>Clients and Servers</h2></div></div></div><p>When writing network applications, it’s common to talk about clients and servers. The distinction is increasingly vague, but the side that initiates the conversation is usually considered the <span class="emphasis"><em>client</em></span>. The side that accepts the request is usually the <span class="emphasis"><em>server</em></span>. In the case where two peer applications use sockets to talk, the distinction is less important, but for simplicity we’ll use this definition.</p><p>For our purposes, the most important difference between a client and a server is that a client can create a socket to initiate a conversation with a server application at any time, while a server must be prepared in advance to listen for incoming conversations. The <code class="literal">java.net.Socket</code> class represents one side of an individual socket connection on both the client and server. In addition, the server uses the <a id="I_indexterm13_id765135" class="indexterm"/><code class="literal">java.net.ServerSocket</code> class to listen for new connections from clients. In most cases, an application acting as a server creates a <code class="literal">ServerSocket</code> object and waits, blocked in a call to its <a id="I_indexterm13_id765153" class="indexterm"/><code class="literal">accept()</code> method, until a connection arrives. When it arrives, the <code class="literal">accept()</code> method creates a <code class="literal">Socket</code> object that the server uses to communicate with the client. A server may carry on conversations with multiple clients at once; in this case, there is still only a single <code class="literal">ServerSocket</code>, but the server has multiple <code class="literal">Socket</code> objects—one associated with each client, as shown in <a class="xref" href="ch13s01.html#learnjava3-CHP-13-FIG-2" title="Figure 13-2. Clients and servers, Sockets and ServerSockets">Figure 13-2</a>.</p><p>At the socket level, a client needs two pieces of information to locate and connect to a server on the Internet: a <span class="emphasis"><em>hostname</em></span> (used to find the host computer’s network address) and a <span class="emphasis"><em>port number</em></span>. The port number is an identifier that differentiates between multiple clients or servers on the same host. A server application listens on a prearranged port while waiting for connections. Clients use the port number assigned to the service they want to access. If you think of the host computers as hotels and the applications as guests, the ports are like the guests’ room numbers. For one person to call another, he or she must know the other party’s hotel name and room number.</p><div class="figure"><a id="learnjava3-CHP-13-FIG-2"/><div class="figure-contents"><div class="mediaobject"><a id="I_13_tt835"/><img src="httpatomoreillycomsourceoreillyimages1707645.png" alt="Clients and servers, Sockets and ServerSockets"/></div></div><p class="title">Figure 13-2. Clients and servers, Sockets and ServerSockets</p></div><div class="sect3" title="Clients"><div class="titlepage"><div><div><h3 class="title"><a id="learnjava3-CHP-13-SECT-1.1.1"/>Clients</h3></div></div></div><p><a id="idx10760" class="indexterm"/> <a id="idx10780" class="indexterm"/> <a id="idx10805" class="indexterm"/>A client application opens a connection to a server by constructing a <a id="I_indexterm13_id765277" class="indexterm"/><code class="literal">Socket</code> that specifies the hostname and port number of the desired server:</p><a id="I_13_tt836"/><pre class="programlisting"> <code class="k">try</code> <code class="o">{</code> <code class="n">Socket</code> <code class="n">sock</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Socket</code><code class="o">(</code><code class="s">"wupost.wustl.edu"</code><code class="o">,</code> <code class="mi">25</code><code class="o">);</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code> <code class="n">UnknownHostException</code> <code class="n">e</code> <code class="o">)</code> <code class="o">{</code> <code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="s">"Can't find host."</code><code class="o">);</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code> <code class="n">IOException</code> <code class="n">e</code> <code class="o">)</code> <code class="o">{</code> <code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="s">"Error connecting to host."</code><code class="o">);</code> <code class="o">}</code></pre><p>This code fragment attempts to connect a <code class="literal">Socket</code> to port 25 (the SMTP mail service) of the host <span class="emphasis"><em>wupost.wustl.edu</em></span>. The client handles the possibility that the hostname can’t be resolved (<code class="literal">UnknownHostException</code>) and that it might not be able to connect to it (<code class="literal">IOException</code>). In the preceding case, Java used DNS, the standard Domain Name Service, to resolve the hostname to an IP address for us. The constructor can also accept a string containing the host’s raw IP address:</p><a id="I_13_tt837"/><pre class="programlisting"> <code class="n">Socket</code> <code class="n">sock</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Socket</code><code class="o">(</code><code class="s">"22.66.89.167"</code><code class="o">,</code> <code class="mi">25</code><code class="o">);</code></pre><p>After a connection is made, input and output streams can be retrieved with the <code class="literal">Socket getInputStream()</code> and <a id="I_indexterm13_id765344" class="indexterm"/><code class="literal">getOutputStream()</code> methods. The following (rather arbitrary) code sends and receives some data with the streams:</p><a id="I_13_tt838"/><pre class="programlisting"> <code class="k">try</code> <code class="o">{</code> <code class="n">Socket</code> <code class="n">server</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Socket</code><code class="o">(</code><code class="s">"foo.bar.com"</code><code class="o">,</code> <code class="mi">1234</code><code class="o">);</code> <code class="n">InputStream</code> <code class="n">in</code> <code class="o">=</code> <code class="n">server</code><code class="o">.</code><code class="na">getInputStream</code><code class="o">();</code> <code class="n">OutputStream</code> <code class="n">out</code> <code class="o">=</code> <code class="n">server</code><code class="o">.</code><code class="na">getOutputStream</code><code class="o">();</code> <code class="c1">// write a byte</code> <code class="n">out</code><code class="o">.</code><code class="na">write</code><code class="o">(</code><code class="mi">42</code><code class="o">);</code> <code class="c1">// write a newline or carriage return delimited string</code> <code class="n">PrintWriter</code> <code class="n">pout</code> <code class="o">=</code> <code class="k">new</code> <code class="n">PrintWriter</code><code class="o">(</code> <code class="n">out</code><code class="o">,</code> <code class="kc">true</code> <code class="o">);</code> <code class="n">pout</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="s">"Hello!"</code><code class="o">);</code> <code class="c1">// read a byte</code> <code class="kt">byte</code> <code class="n">back</code> <code class="o">=</code> <code class="o">(</code><code class="kt">byte</code><code class="o">)</code><code class="n">in</code><code class="o">.</code><code class="na">read</code><code class="o">();</code> <code class="c1">// read a newline or carriage return delimited string</code> <code class="n">BufferedReader</code> <code class="n">bin</code> <code class="o">=</code> <code class="k">new</code> <code class="nf">BufferedReader</code><code class="o">(</code> <code class="k">new</code> <code class="n">InputStreamReader</code><code class="o">(</code> <code class="n">in</code> <code class="o">)</code> <code class="o">);</code> <code class="n">String</code> <code class="n">response</code> <code class="o">=</code> <code class="n">bin</code><code class="o">.</code><code class="na">readLine</code><code class="o">();</code> <code class="c1">// send a serialized Java object</code> <code class="n">ObjectOutputStream</code> <code class="n">oout</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ObjectOutputStream</code><code class="o">(</code> <code class="n">out</code> <code class="o">);</code> <code class="n">oout</code><code class="o">.</code><code class="na">writeObject</code><code class="o">(</code> <code class="k">new</code> <code class="n">java</code><code class="o">.</code><code class="na">util</code><code class="o">.</code><code class="na">Date</code><code class="o">()</code> <code class="o">);</code> <code class="n">oout</code><code class="o">.</code><code class="na">flush</code><code class="o">();</code> <code class="err"> </code> <code class="n">server</code><code class="o">.</code><code class="na">close</code><code class="o">();</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code><code class="n">IOException</code> <code class="n">e</code> <code class="o">)</code> <code class="o">{</code> <code class="o">...</code> <code class="o">}</code></pre><p>In this exchange, the client first creates a <code class="literal">Socket</code> for communicating with the server. The <code class="literal">Socket</code> constructor specifies the server’s hostname (<span class="emphasis"><em>foo.bar.com</em></span>) and a prearranged port number (1234). Once the connection is established, the client writes a single byte to the server using the <code class="literal">OutputStream</code>’s <code class="literal">write()</code> method. To send a string of text more easily, it then wraps a <code class="literal">PrintWriter</code> around the <code class="literal">OutputStream</code>. Next, it performs the complementary operations: reading a byte back from the server using <code class="literal">InputStream</code>’s <code class="literal">read()</code> method and then creating a <code class="literal">BufferedReader</code> from which to get a full string of text. Finally, we do something really funky and send a serialized Java object to the server, using an <code class="literal">ObjectOutputStream</code>. (We’ll talk in depth about sending serialized objects later in this chapter.) The client then terminates the connection with the <code class="literal">close()</code> method. All these operations have the potential to generate <a id="I_indexterm13_id765453" class="indexterm"/><code class="literal">IOException</code>s; our application will deal with these using the <code class="literal">catch</code> clause.<a id="I_indexterm13_id765469" class="indexterm"/><a id="I_indexterm13_id765476" class="indexterm"/><a id="I_indexterm13_id765483" class="indexterm"/></p></div><div class="sect3" title="Servers"><div class="titlepage"><div><div><h3 class="title"><a id="learnjava3-CHP-13-SECT-1.1.2"/>Servers</h3></div></div></div><p><a id="idx10785" class="indexterm"/> <a id="idx10803" class="indexterm"/> <a id="idx10808" class="indexterm"/>After a connection is established, a server application uses the same kind of <code class="literal">Socket</code> object for its side of the communications. However, to accept a connection from a client, it must first create a <a id="I_indexterm13_id765542" class="indexterm"/><code class="literal">ServerSocket</code>, bound to the correct port. Let’s recreate the previous conversation from the server’s point of view:</p><a id="I_13_tt839"/><pre class="programlisting"> <code class="c1">// Meanwhile, on foo.bar.com...</code> <code class="k">try</code> <code class="o">{</code> <code class="n">ServerSocket</code> <code class="n">listener</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ServerSocket</code><code class="o">(</code> <code class="mi">1234</code> <code class="o">);</code> <code class="k">while</code> <code class="o">(</code> <code class="o">!</code><code class="n">finished</code> <code class="o">)</code> <code class="o">{</code> <code class="n">Socket</code> <code class="n">client</code> <code class="o">=</code> <code class="n">listener</code><code class="o">.</code><code class="na">accept</code><code class="o">();</code> <code class="c1">// wait for connection</code> <code class="n">InputStream</code> <code class="n">in</code> <code class="o">=</code> <code class="n">client</code><code class="o">.</code><code class="na">getInputStream</code><code class="o">();</code> <code class="n">OutputStream</code> <code class="n">out</code> <code class="o">=</code> <code class="n">client</code><code class="o">.</code><code class="na">getOutputStream</code><code class="o">();</code> <code class="c1">// read a byte</code> <code class="kt">byte</code> <code class="n">someByte</code> <code class="o">=</code> <code class="o">(</code><code class="kt">byte</code><code class="o">)</code><code class="n">in</code><code class="o">.</code><code class="na">read</code><code class="o">();</code> <code class="c1">// read a newline or carriage-return-delimited string</code> <code class="n">BufferedReader</code> <code class="n">bin</code> <code class="o">=</code> <code class="k">new</code> <code class="nf">BufferedReader</code><code class="o">(</code> <code class="k">new</code> <code class="n">InputStreamReader</code><code class="o">(</code> <code class="n">in</code> <code class="o">)</code> <code class="o">);</code> <code class="n">String</code> <code class="n">someString</code> <code class="o">=</code> <code class="n">bin</code><code class="o">.</code><code class="na">readLine</code><code class="o">();</code> <code class="c1">// write a byte</code> <code class="n">out</code><code class="o">.</code><code class="na">write</code><code class="o">(</code><code class="mi">43</code><code class="o">);</code> <code class="c1">// say goodbye</code> <code class="n">PrintWriter</code> <code class="n">pout</code> <code class="o">=</code> <code class="k">new</code> <code class="n">PrintWriter</code><code class="o">(</code> <code class="n">out</code><code class="o">,</code> <code class="kc">true</code> <code class="o">);</code> <code class="n">pout</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="s">"Goodbye!"</code><code class="o">);</code> <code class="err"> </code> <code class="c1">// read a serialized Java object</code> <code class="n">ObjectInputStream</code> <code class="n">oin</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ObjectInputStream</code><code class="o">(</code> <code class="n">in</code> <code class="o">);</code> <code class="n">Date</code> <code class="n">date</code> <code class="o">=</code> <code class="o">(</code><code class="n">Date</code><code class="o">)</code><code class="n">oin</code><code class="o">.</code><code class="na">readObject</code><code class="o">();</code> <code class="n">client</code><code class="o">.</code><code class="na">close</code><code class="o">();</code> <code class="o">}</code> <code class="n">listener</code><code class="o">.</code><code class="na">close</code><code class="o">();</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code><code class="n">IOException</code> <code class="n">e</code> <code class="o">)</code> <code class="o">{</code> <code class="o">...</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code><code class="n">ClassNotFoundException</code> <code class="n">e2</code> <code class="o">)</code> <code class="o">{</code> <code class="o">...</code> <code class="o">}</code></pre><p>First, our server creates a <code class="literal">ServerSocket</code> attached to port 1234. On some systems, there are rules about which ports an application can use. Port numbers below 1024 are usually reserved for system processes and standard, <span class="emphasis"><em>well-known</em></span> services, so we pick a port number outside of this range. The <code class="literal">ServerSocket</code> is created only once; thereafter, we can accept as many connections as arrive.</p><p>Next, we enter a loop, waiting for the <a id="I_indexterm13_id765603" class="indexterm"/><code class="literal">accept()</code> method of the <code class="literal">ServerSocket</code> to return an active <code class="literal">Socket</code> connection from a client. When a connection has been established, we perform the server side of our dialog, then close the connection and return to the top of the loop to wait for another connection. Finally, when the server application wants to stop listening for connections altogether, it calls the <code class="literal">close()</code> method of the <code class="literal">ServerSocket</code>.</p><p>This server is single-threaded; it handles one connection at a time, not calling <code class="literal">accept()</code> to listen for a new connection until it’s finished with the current connection. A more realistic server would have a loop that accepts connections concurrently and passes them off to their own threads for processing. There is a lot to be said about implementing multithreaded servers. Later in this chapter, we’ll create a tiny web server that starts a new thread for each connection and also a slightly more complex web server that uses the NIO package to handle many connections with a small number of threads.<a id="I_indexterm13_id765652" class="indexterm"/><a id="I_indexterm13_id765659" class="indexterm"/><a id="I_indexterm13_id765666" class="indexterm"/></p></div><div class="sect3" title="Sockets and security"><div class="titlepage"><div><div><h3 class="title"><a id="learnjava3-CHP-13-SECT-1.1.3"/>Sockets and security</h3></div></div></div><p><a id="idx10784" class="indexterm"/> <a id="idx10802" class="indexterm"/> <a id="I_indexterm13_id765708" class="indexterm"/>The previous examples presuppose that the client has permission to connect to the server and that the server is allowed to listen on the specified socket. If you’re writing a general, standalone application, this is normally the case (and you can probably skip this section). However, untrusted applications (such as Java applets in a web browser) run under the auspices of a security policy that can impose arbitrary restrictions on what hosts they may or may not talk to and whether or not they can listen for connections.</p><p>For example, the security policy imposed on applets by most browsers allow untrusted applets to open socket connections only to the host that served them—that is, they can talk back only to the server from which their class files were retrieved. Untrusted applets are generally not allowed to open server sockets for incoming connections themselves. This doesn’t mean that an untrusted applet can’t cooperate with its server to communicate with anyone, anywhere. The applet’s server could run a proxy that lets the applet communicate indirectly with anyone it likes. What this security policy prevents is malicious applets poking around inside corporate firewalls, making connections to trusted services. It places the burden of security on the originating server, not the client machine. Restricting access to the originating server limits the usefulness of Trojan applications that do annoying things from the client side. (You probably won’t let your proxy spam people, because you’ll be blamed.)</p><p>If you are going to run your own application under a security manager, you should be aware that the default security manager dissallows all network access. So in order to make network connections, you would have to modify your policy file to grant the appropriate permissions to your code (see <a class="xref" href="ch03.html" title="Chapter 3. Tools of the Trade">Chapter 3</a> for details). The following policy file fragment sets the socket permissions to allow connections to or from any host on any nonprivileged port:</p><a id="I_13_tt840"/><pre class="programlisting"> <code class="n">grant</code> <code class="o">{</code> <code class="n">permission</code> <code class="n">java</code><code class="o">.</code><code class="na">net</code><code class="o">.</code><code class="na">SocketPermission</code> <code class="s">"*:1024-"</code><code class="o">,</code> <code class="s">"listen,accept,connect"</code><code class="o">;</code> <code class="o">};</code></pre><p>When starting the Java interpreter, you can install the security manager and use this file (call it <span class="emphasis"><em>mysecurity.policy</em></span>):<a id="I_indexterm13_id765779" class="indexterm"/><a id="I_indexterm13_id765786" class="indexterm"/></p><a id="I_13_tt841"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">java</code> <code class="o">-</code><code class="n">Djava</code><code class="o">.</code><code class="na">security</code><code class="o">.</code><code class="na">manager</code> <code class="err">\</code></code></strong><strong class="userinput"><code><code class="o">-</code><code class="n">Djava</code><code class="o">.</code><code class="na">security</code><code class="o">.</code><code class="na">policy</code><code class="o">=</code><code class="n">mysecurity</code><code class="o">.</code><code class="na">policy</code> <code class="n">MyApplication</code></code></strong></pre></div></div><div class="sect2" title="author="pat” timestamp="20120926T110720-0500” comment="one of those sections I hate to get rid of but is less relevant in terms of the example... should probably find a more modern example...”The DateAtHost Client"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-13-SECT-1.2"/>The DateAtHost Client</h2></div></div></div><p><a id="idx10765" class="indexterm"/> <a id="idx10781" class="indexterm"/> <a id="idx10806" class="indexterm"/>In the past, many networked computers ran a simple time service that dispensed their clock’s local time on a well-known port. This was a precursor of NTP, the more general Network Time Protocol. The next example, <code class="literal">DateAtHost</code>, includes a subclass of <code class="literal">java.util.Date</code> that fetches the time from a remote host instead of initializing itself from the local clock. (See <a class="xref" href="ch11.html" title="Chapter 11. Core Utilities">Chapter 11</a> for a complete discussion of the <code class="literal">Date</code> class.)</p><p><code class="literal">DateAtHost</code> connects to the time service (port 37) and reads four bytes representing the time on the remote host. These four bytes have a peculiar specification that we decode to get the time. Here’s the code:</p><a id="I_13_tt842"/><pre class="programlisting"> <code class="c1">//file: DateAtHost.java</code> <code class="kn">import</code> <code class="nn">java.net.Socket</code><code class="o">;</code> <code class="kn">import</code> <code class="nn">java.io.*</code><code class="o">;</code> <code class="err"> </code> <code class="kd">public</code> <code class="kd">class</code> <code class="nc">DateAtHost</code> <code class="kd">extends</code> <code class="n">java</code><code class="o">.</code><code class="na">util</code><code class="o">.</code><code class="na">Date</code> <code class="o">{</code> <code class="kd">static</code> <code class="kt">int</code> <code class="n">timePort</code> <code class="o">=</code> <code class="mi">37</code><code class="o">;</code> <code class="c1">// seconds from start of 20th century to Jan 1, 1970 00:00 GMT</code> <code class="kd">static</code> <code class="kd">final</code> <code class="kt">long</code> <code class="n">offset</code> <code class="o">=</code> <code class="mi">2208988800L</code><code class="o">;</code> <code class="kd">public</code> <code class="nf">DateAtHost</code><code class="o">(</code> <code class="n">String</code> <code class="n">host</code> <code class="o">)</code> <code class="kd">throws</code> <code class="n">IOException</code> <code class="o">{</code> <code class="k">this</code><code class="o">(</code> <code class="n">host</code><code class="o">,</code> <code class="n">timePort</code> <code class="o">);</code> <code class="o">}</code> <code class="err"> </code> <code class="kd">public</code> <code class="nf">DateAtHost</code><code class="o">(</code> <code class="n">String</code> <code class="n">host</code><code class="o">,</code> <code class="kt">int</code> <code class="n">port</code> <code class="o">)</code> <code class="kd">throws</code> <code class="n">IOException</code> <code class="o">{</code> <code class="n">Socket</code> <code class="n">server</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Socket</code><code class="o">(</code> <code class="n">host</code><code class="o">,</code> <code class="n">port</code> <code class="o">);</code> <code class="n">DataInputStream</code> <code class="n">din</code> <code class="o">=</code> <code class="k">new</code> <code class="nf">DataInputStream</code><code class="o">(</code> <code class="n">server</code><code class="o">.</code><code class="na">getInputStream</code><code class="o">()</code> <code class="o">);</code> <code class="kt">int</code> <code class="n">time</code> <code class="o">=</code> <code class="n">din</code><code class="o">.</code><code class="na">readInt</code><code class="o">();</code> <code class="n">server</code><code class="o">.</code><code class="na">close</code><code class="o">();</code> <code class="n">setTime</code><code class="o">(</code> <code class="o">(((</code><code class="mi">1L</code> <code class="o"><<</code> <code class="mi">32</code><code class="o">)</code> <code class="o">+</code> <code class="n">time</code><code class="o">)</code> <code class="o">-</code> <code class="n">offset</code><code class="o">)</code> <code class="o">*</code> <code class="mi">1000</code> <code class="o">);</code> <code class="o">}</code> <code class="o">}</code></pre><p>That’s all there is to it. It’s not very long, even with a few frills. We have supplied two possible constructors for <code class="literal">DateAtHost</code>. Normally we’d expect to use the first, which simply takes the name of the remote host as an argument. The second constructor specifies the hostname and the port number of the remote time service. (If the time service were running on a nonstandard port, we would use the second constructor to specify the alternate port number.) This second constructor does the work of making the connection and setting the time. The first constructor simply invokes the second (using the <code class="literal">this()</code> construct) with the default port as an argument. Supplying simplified constructors that invoke their siblings with default arguments is a common and useful pattern in Java; that is the main reason we’ve shown it here.</p><p>The second constructor opens a socket to the specified port on the remote host. It creates a <a id="I_indexterm13_id765947" class="indexterm"/><code class="literal">DataInputStream</code> to wrap the input stream and then reads a four-byte integer using the <a id="I_indexterm13_id765959" class="indexterm"/><code class="literal">readInt()</code> method. It’s no coincidence that the bytes are in the right order. Java’s <code class="literal">DataInputStream</code> and <code class="literal">DataOutputStream</code> classes work with the bytes of integer types in <span class="emphasis"><em>network byte order</em></span> (most significant to least significant). The time protocol (and other standard network protocols that deal with binary data) also uses the network byte order, so we don’t need to call any conversion routines. Explicit data conversions would probably be necessary if we were using a nonstandard protocol, especially when talking to a non-Java client or server. In that case, we’d have to read byte by byte and do some rearranging to get our four-byte value. After reading the data, we’re finished with the socket, so we close it, terminating the connection to the server. Finally, the constructor initializes the rest of the object by calling <code class="literal">Date</code>’s <a id="I_indexterm13_id765999" class="indexterm"/><code class="literal">setTime()</code> method with the calculated time value.</p><p>The four bytes of the time value are interpreted as an integer representing the number of seconds since the beginning of the 20th century. <code class="literal">DateAtHost</code> converts this to Java’s notion of absolute time—the count of milliseconds since January 1, 1970 (an arbitrary date standardized by C and Unix). The conversion first creates a <code class="literal">long</code> value, which is the unsigned equivalent of the integer <code class="literal">time</code>. It subtracts an offset to make the time relative to the epoch (January 1, 1970) rather than the century, and multiplies by 1,000 to convert to milliseconds. The converted time is used to initialize the object.</p><p>The <code class="literal">DateAtHost</code> class can work with a time retrieved from a remote host almost as easily as <code class="literal">Date</code> is used with the time on the local host. The only additional overhead is dealing with the possible <a id="I_indexterm13_id766050" class="indexterm"/><code class="literal">IOException</code> that can be thrown by the <code class="literal">DateAtHost</code> constructor:</p><a id="I_13_tt843"/><pre class="programlisting"> <code class="k">try</code> <code class="o">{</code> <code class="n">Date</code> <code class="n">d</code> <code class="o">=</code> <code class="k">new</code> <code class="n">DateAtHost</code><code class="o">(</code> <code class="s">"someserver.net"</code> <code class="o">);</code> <code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code> <code class="s">"The time over there is: "</code> <code class="o">+</code> <code class="n">d</code> <code class="o">);</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code> <code class="n">IOException</code> <code class="n">e</code> <code class="o">)</code> <code class="o">{</code> <code class="o">...</code> <code class="o">}</code></pre><p>This example fetches the time at the host <span class="emphasis"><em>someserver.net</em></span> and prints its value.<a id="I_indexterm13_id766082" class="indexterm"/><a id="I_indexterm13_id766089" class="indexterm"/><a id="I_indexterm13_id766096" class="indexterm"/></p></div><div class="sect2" title="The TinyHttpd Server"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-13-SECT-1.3"/>The TinyHttpd Server</h2></div></div></div><p><a id="idx10786" class="indexterm"/> <a id="idx10809" class="indexterm"/> <a id="idx10810" class="indexterm"/>Have you ever wanted to write your very own web server? Well, you’re in luck. In this section, we’re going to build <code class="literal">TinyHttpd</code>, a minimal but functional web server. <code class="literal">TinyHttpd</code> listens on a specified port and services simple HTTP <code class="literal">GET</code> requests. <code class="literal">GET</code> requests are simple text commands that look something like this:</p><a id="I_13_tt844"/><pre class="programlisting"> <code class="n">GET</code> <code class="o">/</code><em class="replaceable"><code><code class="n">path</code></code></em><code class="o">/</code><em class="replaceable"><code><code class="n">filename</code></code></em> <code class="o">[</code> <em class="replaceable"><code><code class="n">optional</code> <code class="n">stuff</code></code></em> <code class="o">]</code></pre><p>Your web browser sends one or more of these requests for each document it retrieves from a web server. Upon reading a request, our server attempts to open the specified file and send its contents. If that document contains references to images or other items to be displayed inline, the browser follows up with additional <code class="literal">GET</code> requests. For best performance, <code class="literal">TinyHttpd</code> services each request in its own thread. Therefore, <code class="literal">TinyHttpd</code> can service several requests concurrently.</p><p>This example works, but it’s a bit oversimplified. First, it implements a very old subset of the HTTP protocol, so some browsers may turn their nose up at it. (I tested this in Safari on my Mac at the time of this writing and it worked well enough for this example’s purposes.) Also remember that file pathnames are still somewhat architecture-dependent in Java. This example should work as it is on most systems, but would require some enhancement to be more robust. It’s possible to write slightly more elaborate code that uses the environmental information provided by Java to tailor itself to the local system. (<a class="xref" href="ch12.html" title="Chapter 12. Input/Output Facilities">Chapter 12</a> gives some hints about how.)</p><div class="warning" title="Warning"><h3 class="title"><a id="learnjava3-CHP-13-NOTE-5"/>Warning</h3><p>Unless you have a firewall or other security in place, the next example serves files from your host without protection. Don’t try this at work!</p></div><p>Now, without further ado, here’s <code class="literal">TinyHttpd</code>:</p><a id="I_13_tt845"/><pre class="programlisting"> <code class="c1">//file: TinyHttpd.java</code> <code class="kn">import</code> <code class="nn">java.net.*</code><code class="o">;</code> <code class="kn">import</code> <code class="nn">java.io.*</code><code class="o">;</code> <code class="kn">import</code> <code class="nn">java.util.regex.*</code><code class="o">;</code> <code class="kn">import</code> <code class="nn">java.util.concurrent.*</code><code class="o">;</code> <code class="err"> </code> <code class="kd">public</code> <code class="kd">class</code> <code class="nc">TinyHttpd</code> <code class="o">{</code> <code class="kd">public</code> <code class="kd">static</code> <code class="kt">void</code> <code class="nf">main</code><code class="o">(</code> <code class="n">String</code> <code class="n">argv</code><code class="o">[]</code> <code class="o">)</code> <code class="kd">throws</code> <code class="n">IOException</code> <code class="o">{</code> <code class="n">Executor</code> <code class="n">executor</code> <code class="o">=</code> <code class="n">Executors</code><code class="o">.</code><code class="na">newFixedThreadPool</code><code class="o">(</code><code class="mi">3</code><code class="o">);</code> <code class="n">ServerSocket</code> <code class="n">ss</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ServerSocket</code><code class="o">(</code> <code class="n">Integer</code><code class="o">.</code><code class="na">parseInt</code><code class="o">(</code><code class="n">argv</code><code class="o">[</code><code class="mi">0</code><code class="o">])</code> <code class="o">);</code> <code class="k">while</code> <code class="o">(</code> <code class="kc">true</code> <code class="o">)</code> <code class="n">executor</code><code class="o">.</code><code class="na">execute</code><code class="o">(</code> <code class="k">new</code> <code class="n">TinyHttpdConnection</code><code class="o">(</code> <code class="n">ss</code><code class="o">.</code><code class="na">accept</code><code class="o">()</code> <code class="o">)</code> <code class="o">);</code> <code class="o">}</code> <code class="o">}</code> <code class="kd">class</code> <code class="nc">TinyHttpdConnection</code> <code class="kd">implements</code> <code class="n">Runnable</code> <code class="o">{</code> <code class="n">Socket</code> <code class="n">client</code><code class="o">;</code> <code class="n">TinyHttpdConnection</code> <code class="o">(</code> <code class="n">Socket</code> <code class="n">client</code> <code class="o">)</code> <code class="kd">throws</code> <code class="n">SocketException</code> <code class="o">{</code> <code class="k">this</code><code class="o">.</code><code class="na">client</code> <code class="o">=</code> <code class="n">client</code><code class="o">;</code> <code class="o">}</code> <code class="kd">public</code> <code class="kt">void</code> <code class="nf">run</code><code class="o">()</code> <code class="o">{</code> <code class="k">try</code> <code class="o">{</code> <code class="n">BufferedReader</code> <code class="n">in</code> <code class="o">=</code> <code class="k">new</code> <code class="n">BufferedReader</code><code class="o">(</code> <code class="k">new</code> <code class="nf">InputStreamReader</code><code class="o">(</code><code class="n">client</code><code class="o">.</code><code class="na">getInputStream</code><code class="o">(),</code> <code class="s">"8859_1"</code> <code class="o">)</code> <code class="o">);</code> <code class="n">OutputStream</code> <code class="n">out</code> <code class="o">=</code> <code class="n">client</code><code class="o">.</code><code class="na">getOutputStream</code><code class="o">();</code> <code class="n">PrintWriter</code> <code class="n">pout</code> <code class="o">=</code> <code class="k">new</code> <code class="n">PrintWriter</code><code class="o">(</code> <code class="k">new</code> <code class="nf">OutputStreamWriter</code><code class="o">(</code><code class="n">out</code><code class="o">,</code> <code class="s">"8859_1"</code><code class="o">),</code> <code class="kc">true</code> <code class="o">);</code> <code class="n">String</code> <code class="n">request</code> <code class="o">=</code> <code class="n">in</code><code class="o">.</code><code class="na">readLine</code><code class="o">();</code> <code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code> <code class="s">"Request: "</code><code class="o">+</code><code class="n">request</code><code class="o">);</code> <code class="n">Matcher</code> <code class="n">get</code> <code class="o">=</code> <code class="n">Pattern</code><code class="o">.</code><code class="na">compile</code><code class="o">(</code><code class="s">"GET /?(\\S*).*"</code><code class="o">).</code><code class="na">matcher</code><code class="o">(</code> <code class="n">request</code> <code class="o">);</code> <code class="k">if</code> <code class="o">(</code> <code class="n">get</code><code class="o">.</code><code class="na">matches</code><code class="o">()</code> <code class="o">)</code> <code class="o">{</code> <code class="n">request</code> <code class="o">=</code> <code class="n">get</code><code class="o">.</code><code class="na">group</code><code class="o">(</code><code class="mi">1</code><code class="o">);</code> <code class="k">if</code> <code class="o">(</code> <code class="n">request</code><code class="o">.</code><code class="na">endsWith</code><code class="o">(</code><code class="s">"/"</code><code class="o">)</code> <code class="o">||</code> <code class="n">request</code><code class="o">.</code><code class="na">equals</code><code class="o">(</code><code class="s">""</code><code class="o">)</code> <code class="o">)</code> <code class="n">request</code> <code class="o">=</code> <code class="n">request</code> <code class="o">+</code> <code class="s">"index.html"</code><code class="o">;</code> <code class="k">try</code> <code class="o">{</code> <code class="n">FileInputStream</code> <code class="n">fis</code> <code class="o">=</code> <code class="k">new</code> <code class="n">FileInputStream</code> <code class="o">(</code> <code class="n">request</code> <code class="o">);</code> <code class="kt">byte</code> <code class="o">[]</code> <code class="n">data</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">byte</code> <code class="o">[</code> <code class="mi">64</code><code class="o">*</code><code class="mi">1024</code> <code class="o">];</code> <code class="k">for</code><code class="o">(</code><code class="kt">int</code> <code class="n">read</code><code class="o">;</code> <code class="o">(</code><code class="n">read</code> <code class="o">=</code> <code class="n">fis</code><code class="o">.</code><code class="na">read</code><code class="o">(</code> <code class="n">data</code> <code class="o">))</code> <code class="o">></code> <code class="o">-</code><code class="mi">1</code><code class="o">;</code> <code class="o">)</code> <code class="n">out</code><code class="o">.</code><code class="na">write</code><code class="o">(</code> <code class="n">data</code><code class="o">,</code> <code class="mi">0</code><code class="o">,</code> <code class="n">read</code> <code class="o">);</code> <code class="n">out</code><code class="o">.</code><code class="na">flush</code><code class="o">();</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code> <code class="n">FileNotFoundException</code> <code class="n">e</code> <code class="o">)</code> <code class="o">{</code> <code class="n">pout</code><code class="o">.</code><code class="na">println</code><code class="o">(</code> <code class="s">"404 Object Not Found"</code> <code class="o">);</code> <code class="o">}</code> <code class="o">}</code> <code class="k">else</code> <code class="n">pout</code><code class="o">.</code><code class="na">println</code><code class="o">(</code> <code class="s">"400 Bad Request"</code> <code class="o">);</code> <code class="n">client</code><code class="o">.</code><code class="na">close</code><code class="o">();</code> <code class="o">}</code> <code class="k">catch</code> <code class="o">(</code> <code class="n">IOException</code> <code class="n">e</code> <code class="o">)</code> <code clas