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href="/java/java-performance-tuning.html" class="sidebar-link">Java Performance Tuning</a></li><li><a href="/java/benchmarks.html" class="sidebar-link">Benchmarks</a></li><li><a href="/java/fileupload-to-aws.html" 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="completablefuture"><a href="#completablefuture" class="header-anchor">#</a> CompletableFuture</h1> <p>CompletableFuture is a class added to Java SE 8 which implements the Future interface from Java SE 5. In addition to supporting the Future interface it adds many methods that allow asynchronous callback when the future is completed.</p> <h2 id="simple-example-of-completablefuture"><a href="#simple-example-of-completablefuture" class="header-anchor">#</a> Simple Example of CompletableFuture</h2> <p>In the example below, the <code>calculateShippingPrice</code> method calculates shipping cost, which takes some processing time. In a real world example, this would e.g. be contacting another server which returns the price based on the weight of the product and the shipping method.</p> <p>By modeling this in an async way via <code>CompletableFuture</code>, we can continue different work in the method (i.e. calculating packaging costs).</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> price <span class="token operator">=</span> <span class="token number">15</span><span class="token punctuation">;</span> <span class="token comment">// Let's keep it simple and work with whole number prices here</span>
<span class="token keyword">int</span> weightInGrams <span class="token operator">=</span> <span class="token number">900</span><span class="token punctuation">;</span>
<span class="token function">calculateShippingPrice</span><span class="token punctuation">(</span>weightInGrams<span class="token punctuation">)</span> <span class="token comment">// Here, we get the future</span>
<span class="token punctuation">.</span><span class="token function">thenAccept</span><span class="token punctuation">(</span>shippingPrice <span class="token operator">-></span> <span class="token punctuation">{</span> <span class="token comment">// And then immediately work on it!</span>
<span class="token comment">// This fluent style is very useful for keeping it concise</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><span class="token string">"Your total price is: "</span> <span class="token operator">+</span> <span class="token punctuation">(</span>price <span class="token operator">+</span> shippingPrice<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>
<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><span class="token string">"Please stand by. We are calculating your total price."</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">CompletableFuture</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> <span class="token function">calculateShippingPrice</span><span class="token punctuation">(</span><span class="token keyword">int</span> weightInGrams<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token class-name">CompletableFuture</span><span class="token punctuation">.</span><span class="token function">supplyAsync</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
<span class="token comment">// supplyAsync is a factory method that turns a given </span>
<span class="token comment">// Supplier<U> into a CompletableFuture<U></span>
<span class="token comment">// Let's just say each 200 grams is a new dollar on your shipping costs</span>
<span class="token keyword">int</span> shippingCosts <span class="token operator">=</span> weightInGrams <span class="token operator">/</span> <span class="token number">200</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">2000L</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Now let's simulate some waiting time...</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">/* We can safely ignore that */</span> <span class="token punctuation">}</span>
<span class="token keyword">return</span> shippingCosts<span class="token punctuation">;</span> <span class="token comment">// And send the costs back!</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="convert-blocking-method-to-asynchonous"><a href="#convert-blocking-method-to-asynchonous" class="header-anchor">#</a> Convert blocking method to asynchonous</h2> <p>The following method will take a second or two depending on your connection to retrieve a web page and count the text length. Whatever thread calls it will block
for that period of time. Also it rethrows an exception which is useful later on.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">long</span> <span class="token function">blockingGetWebPageLength</span><span class="token punctuation">(</span><span class="token class-name">String</span> urlString<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">(</span><span class="token class-name">BufferedReader</span> br <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">InputStreamReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token function">URL</span><span class="token punctuation">(</span>urlString<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">openConnection</span><span class="token punctuation">(</span><span class="token punctuation">)</span><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 punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">StringBuilder</span> sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> line<span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>line <span class="token operator">=</span> br<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">length</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 keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</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>This converts it to a method that will return immediately by moving the blocking method call to another thread. By default the supplyAsync method will run the supplier on the common pool. For a blocking method this is probably not a good choice since one might exhaust the threads in that pool which is why I added the optional service parameter.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">private</span> <span class="token class-name">ExecutorService</span> service <span class="token operator">=</span> <span class="token class-name">Executors</span><span class="token punctuation">.</span><span class="token function">newCachedThreadPool</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">public</span> <span class="token class-name">CompletableFuture</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Long</span><span class="token punctuation">></span></span> <span class="token function">asyncGetWebPageLength</span><span class="token punctuation">(</span><span class="token class-name">String</span> url<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token class-name">CompletableFuture</span><span class="token punctuation">.</span><span class="token function">supplyAsync</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token function">blockingGetWebPageLength</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">,</span> service<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>To use the function in an asynchronous fashion one should use on of the methods that accepts a lamda to be called with the result of the of the supplier when it completes such as thenAccept. Also it is important to use exceptionally or handle method to log any exceptions that might have happened.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">asyncGetWebPageLength</span><span class="token punctuation">(</span><span class="token string">"https://stackoverflow.com/"</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">thenAccept</span><span class="token punctuation">(</span>l <span class="token operator">-></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><span class="token string">"Stack Overflow returned "</span> <span class="token operator">+</span> l<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">exceptionally</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">Throwable</span> throwable<span class="token punctuation">)</span> <span class="token operator">-></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 string">"myclass"</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 string">""</span><span class="token punctuation">,</span> throwable<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">null</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></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/java/completablefuture.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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