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class="sidebar-link">FileUpload to AWS</a></li><li><a href="/java/appdynamics-and-tibco-businessworks-instrumentation-for-easy-integration.html" class="sidebar-link">AppDynamics and TIBCO BusinessWorks Instrumentation for Easy Integration</a></li><li><a href="/java/the-classpath.html" class="sidebar-link">The Classpath</a></li><li><a href="/java/resources-on-classpath.html" class="sidebar-link">Resources (on classpath)</a></li><li><a href="/java/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="java-sockets"><a href="#java-sockets" class="header-anchor">#</a> Java Sockets</h1> <p>Sockets are a low-level network interface that helps in creating a connection between two program mainly clients which may or may not be running on the same machine.</p> <p>Socket Programming is one of the most widely used networking concepts.</p> <h2 id="a-simple-tcp-echo-back-server"><a href="#a-simple-tcp-echo-back-server" class="header-anchor">#</a> A simple TCP echo back server</h2> <p>Our TCP echo back server will be a separate thread.
It's simple as its a start. It will just echo back whatever you send it but in capitalised form.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">CAPECHOServer</span> <span class="token keyword">extends</span> <span class="token class-name">Thread</span><span class="token punctuation">{</span>
<span class="token comment">// This class implements server sockets. A server socket waits for requests to come </span>
<span class="token comment">// in over the network only when it is allowed through the local firewall</span>
<span class="token class-name">ServerSocket</span> serverSocket<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">CAPECHOServer</span><span class="token punctuation">(</span><span class="token keyword">int</span> port<span class="token punctuation">,</span> <span class="token keyword">int</span> timeout<span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token comment">// Create a new Server on specified port.</span>
serverSocket <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ServerSocket</span><span class="token punctuation">(</span>port<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// SoTimeout is basiacally the socket timeout.</span>
<span class="token comment">// timeout is the time until socket timeout in milliseconds</span>
serverSocket<span class="token punctuation">.</span><span class="token function">setSoTimeout</span><span class="token punctuation">(</span>timeout<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOServer</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token comment">// We want the server to continuously accept connections</span>
<span class="token keyword">while</span><span class="token punctuation">(</span><span class="token operator">!</span><span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">interrupted</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token punctuation">}</span>
<span class="token comment">// Close the server once done.</span>
serverSocket<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOServer</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Now to accept connections. Let's update the run method.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">while</span><span class="token punctuation">(</span><span class="token operator">!</span><span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">interrupted</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token comment">// Log with the port number and machine ip</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>INFO<span class="token punctuation">,</span> <span class="token string">"Listening for Clients at {0} on {1}"</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span>serverSocket<span class="token punctuation">.</span><span class="token function">getLocalPort</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">InetAddress</span><span class="token punctuation">.</span><span class="token function">getLocalHost</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getHostAddress</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Socket</span> client <span class="token operator">=</span> serverSocket<span class="token punctuation">.</span><span class="token function">accept</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Accept client conncetion</span>
<span class="token comment">// Now get DataInputStream and DataOutputStreams</span>
<span class="token class-name">DataInputStream</span> istream <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">DataInputStream</span><span class="token punctuation">(</span>client<span class="token punctuation">.</span><span class="token function">getInputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// From client's input stream</span>
<span class="token class-name">DataOutputStream</span> ostream <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">DataOutputStream</span><span class="token punctuation">(</span>client<span class="token punctuation">.</span><span class="token function">getOutputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Important Note</span>
<span class="token comment">/*
The server's input is the client's output
The client's input is the server's output
*/</span>
<span class="token comment">// Send a welcome message</span>
ostream<span class="token punctuation">.</span><span class="token function">writeUTF</span><span class="token punctuation">(</span><span class="token string">"Welcome!"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Close the connection</span>
istream<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
ostream<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
client<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOServer</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// Close the server once done</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
serverSocket<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOServer</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Now if you can open telnet and try connecting
You'll see a Welcome message.</p> <p>You must connect with the port you specified and IP Adress.</p> <p>You should see a result similar to this:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Welcome</span><span class="token operator">!</span>
<span class="token class-name">Connection</span> <span class="token keyword">to</span> <span class="token namespace">host</span> lost<span class="token punctuation">.</span>
</code></pre></div><p>Well, the connection was lost because we terminated it.
Sometimes we would have to program our own TCP client.
In this case, we need a client to request input from the user and send it across the network, receive the capitalised input.</p> <p>If the server sends data first, then the client must read the data first.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">CAPECHOClient</span> <span class="token keyword">extends</span> <span class="token class-name">Thread</span><span class="token punctuation">{</span>
<span class="token class-name">Socket</span> server<span class="token punctuation">;</span>
<span class="token class-name">Scanner</span> key<span class="token punctuation">;</span> <span class="token comment">// Scanner for input</span>
<span class="token keyword">public</span> <span class="token class-name">CAPECHOClient</span><span class="token punctuation">(</span><span class="token class-name">String</span> ip<span class="token punctuation">,</span> <span class="token keyword">int</span> port<span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
server <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Socket</span><span class="token punctuation">(</span>ip<span class="token punctuation">,</span> port<span class="token punctuation">)</span><span class="token punctuation">;</span>
key <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Scanner</span><span class="token punctuation">(</span><span class="token class-name">System</span><span class="token punctuation">.</span>in<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOClient</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token class-name">DataInputStream</span> istream <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">DataOutputStream</span> ostream <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
istream <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">DataInputStream</span><span class="token punctuation">(</span>server<span class="token punctuation">.</span><span class="token function">getInputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Familiar lines</span>
ostream <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">DataOutputStream</span><span class="token punctuation">(</span>server<span class="token punctuation">.</span><span class="token function">getOutputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>istream<span class="token punctuation">.</span><span class="token function">readUTF</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Print what the server sends</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">">"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> tosend <span class="token operator">=</span> key<span class="token punctuation">.</span><span class="token function">nextLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
ostream<span class="token punctuation">.</span><span class="token function">writeUTF</span><span class="token punctuation">(</span>tosend<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Send whatever the user typed to the server</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>istream<span class="token punctuation">.</span><span class="token function">readUTF</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Finally read what the server sends before exiting.</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOClient</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
istream<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
ostream<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
server<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Logger</span><span class="token punctuation">.</span><span class="token function">getLogger</span><span class="token punctuation">(</span><span class="token class-name">CAPECHOClient</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token class-name">Level</span><span class="token punctuation">.</span>SEVERE<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Now update the server</p> <div class="language-java extra-class"><pre class="language-java"><code>ostream<span class="token punctuation">.</span><span class="token function">writeUTF</span><span class="token punctuation">(</span><span class="token string">"Welcome!"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> inString <span class="token operator">=</span> istream<span class="token punctuation">.</span><span class="token function">readUTF</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Read what the user sent</span>
<span class="token class-name">String</span> outString <span class="token operator">=</span> inString<span class="token punctuation">.</span><span class="token function">toUpperCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Change it to caps</span>
ostream<span class="token punctuation">.</span><span class="token function">writeUTF</span><span class="token punctuation">(</span>outString<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Close the connection</span>
istream<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>And now run the server and client,
You should have an output similar to this</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Welcome</span><span class="token operator">!</span>
<span class="token operator">></span>
</code></pre></div><h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p>There are two types of Internet Protocol Traffic -<br></p> <ol><li>TCP - Transmission Control Protocol</li> <li>UDP - User Datagram Protocol</li></ol> <p>TCP is a connection-oriented protocol.<br>
UDP is a connectionless protocol.</p> <p>TCP is suited for applications that require high reliability, and transmission time is relatively less critical.</p> <p>UDP is suitable for applications that need fast, efficient transmission, such as games. UDP's stateless nature is also useful for servers that answer small queries from huge numbers of clients.</p> <p>In simpler words -<br>
Use TCP when you cannot afford to loose data and when time to send and receive data doesn't matter.
Use UDP when you cannot afford to loose time and when loss of data doesn't matter.</p> <p>There is an absolute guarantee that the data transferred remains intact and arrives in the same order in which it was sent in case of TCP.<br>
whereas there is no guarantee that the messages or packets sent would reach at all in UDP.</p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/java/java-sockets.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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