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</div><div id='content' class='page-1'><div class='row'><div class='row-fluid'><div class='col-lg-3'><ul class='nav-sidebar'><li><a href='./'>Blog index</a></li><li class='active'><a href='#doc'>JVM Hello World with Groovy</a></li><li><a href='#_proguardcore' class='anchor-link'>ProGuardCORE</a></li><li><a href='#_asm' class='anchor-link'>ASM</a></li><li><a href='#_byte_buddy' class='anchor-link'>Byte Buddy</a></li><li><a href='#_using_groovy_asts' class='anchor-link'>Using Groovy ASTs</a></li><li><a href='#_further_information' class='anchor-link'>Further information</a></li></ul></div><div class='col-lg-8 col-lg-pull-0'><a name='doc'></a><h1>JVM Hello World with Groovy</h1><p><span>Author: <i>Paul King</i></span><br/><span>Published: 2022-12-22 02:24PM</span></p><hr/><div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>For those that haven&#8217;t seen it yet, the <a href="https://www.javaadvent.com/">JVM Advent</a> <a href="https://twitter.com/JavaAdvent">folks</a> posted a great <a href="https://www.javaadvent.com/2022/12/groovy-and-data-science.html">Groovy and Data Science blog post</a>
several days ago as part of the 2022 JVM Advent series. If you have an interest
in Data Science, we recommend you check that out before continuing with this post.</p>
</div>
<div class="paragraph">
<p><a href="https://www.javaadvent.com/2022/12/jvm-hello-world.html">Today&#8217;s post</a>
in the JVM Advent series is looking at the world of bytecode libraries
on the JVM. Let&#8217;s look at creating the same hello world example from that
post using <a href="http://groovy-lang.org/index.html">Groovy</a> with the
<a href="https://github.com/Guardsquare/proguard-core">ProGuardCORE</a>,
<a href="https://asm.ow2.io/">ASM</a>, and <a href="https://bytebuddy.net/">Byte Buddy</a> libraries.</p>
</div>
<div class="paragraph">
<p>First, we highly recommend you read the previously mentioned <a href="https://www.javaadvent.com/2022/12/jvm-hello-world.html">JVM Advent post</a> first for more background.
After all, it&#8217;s easy to create a simple hello-world class file example directly
as a Java source file (as that post shows) or as a Groovy source file like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">println 'Hello world'</code></pre>
</div>
</div>
<div class="paragraph">
<p>The examples shown in this blog post illustrate how you could create the equivalent
class file using libraries which let you manipulate the generated bytecode directly.
It&#8217;s a bit of a deep dive if you want to know more about JVM internals and can also
be handy for numerous use cases like building tools or modifying Java classes on
the fly. I suggest you read the websites for those libraries if you want further
details or additional motivation.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_proguardcore">ProGuardCORE</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The <a href="https://github.com/Guardsquare/proguard-core">ProGuardCORE</a>
library lets you read, analyze, modify, and write Java class files.
Here&#8217;s how we could use it to write a hello-world class file:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var name = 'HelloProGuardCORE'
var superclass = 'java/lang/Object'
var classBuilder = new ClassBuilder(CLASS_VERSION_1_8, PUBLIC, name, superclass).tap {
addMethod(PUBLIC | STATIC, 'main', '([Ljava/lang/String;)V', 100, builder -&gt;
builder
.getstatic('java/lang/System', 'out', 'Ljava/io/PrintStream;')
.ldc("Hello from $name")
.invokevirtual('java/io/PrintStream', 'println', '(Ljava/lang/String;)V')
.return_()
)
}
new File("${name}.class").withDataOutputStream { dos -&gt;
classBuilder.programClass.accept(new ProgramClassWriter(dos))
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>This is essentially the <em>"Groovified"</em> version of the example in the JVM Advent blog post. We are using the libraries <code>ClassBuilder</code> class, adding a method, then adding four bytecode statements as the body of the method. If you haven&#8217;t seen method and type descriptor syntax before, a few parts might seem a little strange, but you possibly won&#8217;t be surprised that it seems to be referencing a <code>System.out.println</code> call and passing it a constant String.</p>
</div>
<div class="paragraph">
<p>When we run this script, a <code>HelloProGuardCORE</code> class file is produced. We can invoke that class file in the normal way:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="shell">$ java HelloProGuardCORE
Hello from HelloProGuardCORE</code></pre>
</div>
</div>
<div class="paragraph">
<p>We encourage you to read the JVM Advent post or the library documentation
if you want more details.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_asm">ASM</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://asm.ow2.io/">ASM</a> is an all-purpose Java bytecode manipulation and analysis framework.
In fact, it&#8217;s the one that Groovy uses in its parser and some of its tools.
Here is how to use it to generate more or less the same class as the previous example:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var name = 'HelloASM'
var superclass = 'java/lang/Object'
var cw = new ClassWriter(0)
cw.visit(V1_8, ACC_PUBLIC + ACC_SUPER, name, null, superclass, null)
cw.visitMethod(ACC_PUBLIC + ACC_STATIC, 'main', '([Ljava/lang/String;)V', null, null).with {
visitCode()
visitFieldInsn(GETSTATIC, 'java/lang/System', 'out', 'Ljava/io/PrintStream;')
visitLdcInsn('Hello from ' + name)
visitMethodInsn(INVOKEVIRTUAL, 'java/io/PrintStream', 'println', '(Ljava/lang/String;)V', false)
visitInsn(RETURN)
visitMaxs(3, 3)
visitEnd()
}
cw.visitEnd()
new File("${name}.class").withDataOutputStream { dos -&gt;
dos.write(cw.toByteArray())
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>After running this script, a <code>HelloASM</code> class file is produced,
and here is the output when running that class file:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="shell">$ java HelloASM
Hello from HelloASM</code></pre>
</div>
</div>
<div class="paragraph">
<p>Parts of the code should look familiar to the previous example.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_byte_buddy">Byte Buddy</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://bytebuddy.net/">Byte Buddy</a> is a code generation and manipulation library
for creating and modifying Java classes. Its strengths lie in its ability to
create and modify classes dynamically. So, its power is perhaps not needed for
our simple example. A nice aspect of this library however, is that it hides
some of the low-level details like type and method descriptors behind its
fluent API. Here is our example:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var name = 'HelloByteBuddy'
new ByteBuddy()
.subclass(Object)
.name(name)
.defineMethod('main', Void.TYPE, PUBLIC | STATIC)
.withParameter(String[])
.intercept(MethodCall.invoke(
PrintStream.getMethod('println', String))
.onField(System.getField('out'))
.with('Hello from ' + name))
.make()
.saveIn('.' as File)</code></pre>
</div>
</div>
<div class="paragraph">
<p>Like the other scripts, this also produces a class file which we can
invoke as shown here:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="shell">$ java HelloByteBuddy
Hello from HelloByteBuddy</code></pre>
</div>
</div>
<div class="paragraph">
<p>That wraps up our examples using the three libraries, but we have one
more fun alternative to cover!</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_using_groovy_asts">Using Groovy ASTs</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Groovy is a very extensible language. It provides among other things,
a compile-time metaprogramming mechanism called AST Transforms
(Abstract Syntax Tree Transformations). This mechanism uses annotations
to indicate to the compiler that special processing is required during
compilation. A now somewhat outdated AST transform, <code>@Bytecode</code>,
experimented with allowing you to write bytecode instructions directly
in your Groovy code. Let&#8217;s look at using that AST transform here:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">@CompileStatic @POJO
class HelloAST {
@Bytecode
static void main(args) {
getstatic 'java/lang/System.out', 'Ljava/io/PrintStream;'
ldc 'Hello from HelloAST'
invokevirtual 'java/io/PrintStream.println', '(Ljava/lang/String;)V'
return
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>We are writing directly the instructions that the Java or Groovy compiler
(with static compilation enabled) would produce. For this example, we don&#8217;t run
the script to produce the class file, we just compile it using the Groovy compiler.</p>
</div>
<div class="paragraph">
<p>We definitely don&#8217;t recommend relying on the <code>@Bytecode</code> AST transform for any
production code, but it can be fun to play with. We&#8217;ve also used the <code>@CompileStatic</code>
and <code>@POJO</code> AST transforms to tell the compiler that we aren&#8217;t using any Groovy
dynamic features, so that it should write Java-like code whenever possible and
avoid calling the Groovy runtime.</p>
</div>
<div class="paragraph">
<p>We can examine the bytecode using javap and indeed it has bytecode similar to
that produced by the other libraries:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="ruby">public static void main(java.lang.String...);
descriptor: ([Ljava/lang/String;)V
flags: (0x0089) ACC_PUBLIC, ACC_STATIC, ACC_VARARGS
Code:
stack=2, locals=1, args_size=1
0: getstatic #21 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #23 // String Hello from HelloAST
5: invokevirtual #29 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args Ljava/lang/Object;</code></pre>
</div>
</div>
<div class="paragraph">
<p>Because the code is not calling the Groovy runtime,
we can invoke it directly without the Groovy jar:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="shell">$ java HelloAST
Hello from HelloAST</code></pre>
</div>
</div>
<div class="paragraph">
<p>That wraps up our little tour of bytecode libraries.
I hope you have learnt some additional JVM details!</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_further_information">Further information</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p><a href="https://github.com/paulk-asert/bytecode-fun">Github repo</a> of the above examples</p>
</li>
<li>
<p>A <a href="https://openjdk.org/jeps/8280389">draft JEP</a> for what might become an official API for parsing, generating, and transforming Java class files</p>
</li>
<li>
<p>Social media: <a href="https://twitter.com/ApacheGroovy">@ApacheGroovy</a> <a href="https://fosstodon.org/@ApacheGroovy">@<a href="mailto:ApacheGroovy@fosstodon.org">ApacheGroovy@fosstodon.org</a></a></p>
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