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.68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">Codegen</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">Overview</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>Overview</h1>
<p>ActiveJ Codegen is an efficient Java code generator with a streamlined and simple API. It is built on top of ObjectWeb ASM and has zero-overhead performance. ActiveJ Codegen is widely used in <a href="/serializer">ActiveJ Serializer</a>, making it <a href="https://github.com/activej/jvm-serializers" target="_blank" rel="noopener noreferrer">the fastest JVM-based serializer in the world</a>).
ActiveJ Codegen is one of the ActiveJ technologies, but has minimal third-party dependencies and can be used as a stand-alone component.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="why-activej-codegen">Why ActiveJ Codegen?<a href="#why-activej-codegen" class="hash-link" aria-label="Direct link to Why ActiveJ Codegen?" title="Direct link to Why ActiveJ Codegen?">​</a></h2>
<ul>
<li>Utilizes ASM library for code generation and manipulation</li>
<li>Provides a concise Expressions API that provides a Lisp-like DSL for describing new classes</li>
<li>Supports caching of generated classes</li>
<li>Suitable for dynamic class generation based on runtime parameters (e.g. query processing)</li>
<li>Supports relational algebra operations for individual items: aggregate functions, predicates, ordering, group-by, etc.</li>
<li>Features automatic type inference</li>
<li>Zero-overhead performance</li>
<li>Built-in in-memory caching as well as the ability to use a persistent bytecode cache</li>
<li>Support for Records (heterogeneous data containers)</li>
</ul>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="code-generation">Code generation<a href="#code-generation" class="hash-link" aria-label="Direct link to Code generation" title="Direct link to Code generation">​</a></h2>
<p>Suppose we want to create a class that implements the <code>Person</code> interface.</p>
<div class="language-java codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#F8F8F2;--prism-background-color:#282A36"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-java codeBlock_bY9V thin-scrollbar" style="color:#F8F8F2;background-color:#282A36"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#F8F8F2"><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">public</span><span class="token plain"> </span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">interface</span><span class="token plain"> </span><span class="token class-name">Greeter</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(248, 248, 242)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">    </span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">void</span><span class="token plain"> </span><span class="token function" style="color:rgb(80, 250, 123)">sayHello</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain"></span><span class="token punctuation" style="color:rgb(248, 248, 242)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>A generated class should simply output &quot;Hello World&quot; message when the <code>sayHello()</code> method is called.</p>
<div class="language-java codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#F8F8F2;--prism-background-color:#282A36"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-java codeBlock_bY9V thin-scrollbar" style="color:#F8F8F2;background-color:#282A36"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#F8F8F2"><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">public</span><span class="token plain"> </span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">class</span><span class="token plain"> </span><span class="token class-name">WorldGreeter</span><span class="token plain"> </span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">implements</span><span class="token plain"> </span><span class="token class-name">Greeter</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(248, 248, 242)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">	</span><span class="token annotation punctuation" style="color:rgb(248, 248, 242)">@Override</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">	</span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">public</span><span class="token plain"> </span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">void</span><span class="token plain"> </span><span class="token function" style="color:rgb(80, 250, 123)">sayHello</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(248, 248, 242)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">		</span><span class="token class-name">System</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token plain">out</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">println</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token string" style="color:rgb(255, 121, 198)">&quot;Hello World&quot;</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">	</span><span class="token punctuation" style="color:rgb(248, 248, 242)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain"></span><span class="token punctuation" style="color:rgb(248, 248, 242)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>To generate such a class programmatically we need use the <a href="https://github.com/activej/activej/blob/v6.0-beta2/core-codegen/src/main/java/io/activej/codegen/ClassGenerator.java" target="_blank"><code>ClassGenerator</code></a> class
to define how a class should be created. We also need to use the <a href="https://github.com/activej/activej/blob/v6.0-beta2/core-codegen/src/main/java/io/activej/codegen/DefiningClassLoader.java" target="_blank"><code>DefiningClassLoader</code></a>
to actually define a new class.</p>
<div class="language-java codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#F8F8F2;--prism-background-color:#282A36"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-java codeBlock_bY9V thin-scrollbar" style="color:#F8F8F2;background-color:#282A36"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#F8F8F2"><span class="token class-name">DefiningClassLoader</span><span class="token plain"> classLoader </span><span class="token operator">=</span><span class="token plain"> </span><span class="token class-name">DefiningClassLoader</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">create</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain"></span><span class="token class-name">Class</span><span class="token generics punctuation" style="color:rgb(248, 248, 242)">&lt;</span><span class="token generics class-name">Greeter</span><span class="token generics punctuation" style="color:rgb(248, 248, 242)">&gt;</span><span class="token plain"> greeterClass </span><span class="token operator">=</span><span class="token plain"> </span><span class="token class-name">ClassGenerator</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">builder</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token class-name">Greeter</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">class</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">	</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">withMethod</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token string" style="color:rgb(255, 121, 198)">&quot;sayHello&quot;</span><span class="token punctuation" style="color:rgb(248, 248, 242)">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">		</span><span class="token function" style="color:rgb(80, 250, 123)">call</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token function" style="color:rgb(80, 250, 123)">staticField</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token class-name">System</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token keyword" style="color:rgb(189, 147, 249);font-style:italic">class</span><span class="token punctuation" style="color:rgb(248, 248, 242)">,</span><span class="token plain"> </span><span class="token string" style="color:rgb(255, 121, 198)">&quot;out&quot;</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">,</span><span class="token plain"> </span><span class="token string" style="color:rgb(255, 121, 198)">&quot;println&quot;</span><span class="token punctuation" style="color:rgb(248, 248, 242)">,</span><span class="token plain"> </span><span class="token function" style="color:rgb(80, 250, 123)">value</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token string" style="color:rgb(255, 121, 198)">&quot;Hello world&quot;</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">    </span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">build</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">	</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">generateClass</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token plain">classLoader</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>Notice the <code>call(staticField(System.class, &quot;out&quot;), &quot;println&quot;, value(&quot;Hello world&quot;))</code> expression.</p>
<p>This is how Expressions DSL looks like.
First, we access static field <code>out</code> of <code>System</code> class. Then we call <code>println()</code> method on it, passing value <code>&quot;Hello world&quot;</code> as an argument.
This is equivalent to a <code>System.out.println(&quot;Hello world)</code></p>
<p>When a new instance of this class is created and the <code>sayHello()</code> method is called, the output is <em>&quot;Hello world&quot;</em> as expected.</p>
<div class="language-java codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#F8F8F2;--prism-background-color:#282A36"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-java codeBlock_bY9V thin-scrollbar" style="color:#F8F8F2;background-color:#282A36"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#F8F8F2"><span class="token class-name">Greeter</span><span class="token plain"> greeter </span><span class="token operator">=</span><span class="token plain"> greeterClass</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">getDeclaredConstructor</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">newInstance</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#F8F8F2"><span class="token plain">greeter</span><span class="token punctuation" style="color:rgb(248, 248, 242)">.</span><span class="token function" style="color:rgb(80, 250, 123)">sayHello</span><span class="token punctuation" style="color:rgb(248, 248, 242)">(</span><span class="token punctuation" style="color:rgb(248, 248, 242)">)</span><span class="token punctuation" style="color:rgb(248, 248, 242)">;</span><span class="token plain"> </span><span class="token comment" style="color:rgb(98, 114, 164)">// prints &quot;Hello world&quot; to stdout</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>This is merely a hint of what ActiveJ Codegen is capable of. For more information, please refer to examples or internal documentation.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="add-activej-codegen-to-your-project">Add ActiveJ Codegen to your project<a href="#add-activej-codegen-to-your-project" class="hash-link" aria-label="Direct link to Add ActiveJ Codegen to your project" title="Direct link to Add ActiveJ Codegen to your project">​</a></h2>
<p>You can add ActiveJ Codegen to your project by importing its Maven repository.
These docs cover the most recent release of ActiveJ Codegen <strong>v6.0-beta2</strong> (see on <a href="https://github.com/activej/activej/tree/v6.0-beta2/core-codegen" target="_blank">Github</a> or <a href="https://mvnrepository.com/artifact/io.activej/activej-codegen" target="_blank" rel="noopener noreferrer">Maven</a>).</p></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/inject/benchmarks"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Benchmarks</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/codegen/examples"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Examples</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#why-activej-codegen" class="table-of-contents__link toc-highlight">Why ActiveJ Codegen?</a></li><li><a href="#code-generation" class="table-of-contents__link toc-highlight">Code generation</a></li><li><a href="#add-activej-codegen-to-your-project" class="table-of-contents__link toc-highlight">Add ActiveJ Codegen to your project</a></li></ul></div></div></div></div></main></div></div></div><footer class="footer"><div class="container container-fluid"><div class="footer__bottom text--center"><div class="footer__copyright">Copyright © 2024 ActiveJ LLC. All Rights Reserved</div></div></div></footer></div>
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