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<h1 class="post-title-main">Transformation with Templates</h1>
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<div class="post-content">
<p>Spoon provides developers a way of writing code transformations called
<strong>code templates</strong>. Those templates are statically type-checked, in
order to ensure statically that the generated code will be correct.</p>
<p>A Spoon template is a regular Java class that taken as input by the Spoon templating engine to perform a transformation.
This is summarized in Figure below. A Spoon template can be seen as a
higher-order program, which takes program elements as arguments, and returns a
transformed program. Like any function, a template can be used in different
contexts and give different results, depending on its parameters.</p>
<p><img src="/images/template-overview.svg" alt="Overview of Spoon's Templating System" /></p>
<h3 id="definition-of-templates">Definition of templates</h3>
<p>Class <code class="language-plaintext highlighter-rouge">CheckBoundTemplate</code> below defines a Spoon template.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CheckBoundTemplate</span> <span class="kd">extends</span> <span class="nc">StatementTemplate</span> <span class="o">{</span>
<span class="nc">TemplateParameter</span><span class="o"><</span><span class="nc">Collection</span><span class="o"><?>></span> <span class="n">_col_</span><span class="o">;</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">statement</span><span class="o">()</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">_col_</span><span class="o">.</span><span class="na">S</span><span class="o">().</span><span class="na">size</span><span class="o">()</span> <span class="o">></span> <span class="mi">10</span><span class="o">)</span>
<span class="k">throw</span> <span class="k">new</span> <span class="nf">OutOfBoundException</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p>This template specifies a
statement (in method <code class="language-plaintext highlighter-rouge">statement</code>) that is a precondition to check that a list
is smaller than a certain size. This piece of code will be injected at the
beginning of all methods dealing with size-bounded lists. This template has
one single template parameter called <code class="language-plaintext highlighter-rouge">_col_</code>, typed by <code class="language-plaintext highlighter-rouge">TemplateParameter</code> (<a href="http://spoon.gforge.inria.fr/mvnsites/spoon-core/apidocs/spoon/template/TemplateParameter.html">javadoc</a>).
In this case, the template parameter is meant to be an expression (<code class="language-plaintext highlighter-rouge">CtExpression</code>)
that returns a Collection (see constructor, line 3). All meta-model classes,
incl. <code class="language-plaintext highlighter-rouge">CtExpression</code> (<a href="http://spoon.gforge.inria.fr/mvnsites/spoon-core/apidocs/spoon/reflect/code/CtExpression.html">javadoc</a>),
implement interface <code class="language-plaintext highlighter-rouge">TemplateParameter</code>. A template parameter has a special method
(named <code class="language-plaintext highlighter-rouge">S</code>, for Substitution) that is used as a marker to indicate the places where
a template parameter substitution should occur. For a <code class="language-plaintext highlighter-rouge">CtExpression</code>, method <code class="language-plaintext highlighter-rouge">S()</code>
returns the return type of the expression.</p>
<p>A method <code class="language-plaintext highlighter-rouge">S()</code> is never executed, its only goal is to get the template statically
checked. Instead of being executed, the template source code is taken as input by the
templating engine which is described above. Consequently, the template source is
well-typed, compiles, but the binary code of the template is thrown away.</p>
<h3 id="template-instantiation">Template Instantiation</h3>
<p>In order to be correctly substituted, the template parameters need
to be bound to actual values. This is done during template instantiation.</p>
<p>The code at the end of this page shows how to use the check-bound
of template, <code class="language-plaintext highlighter-rouge">CheckBoundTemplate</code>, presented in the previous section.
One first instantiates a template, then one sets the template parameters,
and finally, one calls the template engine. In last line, the bound check
is injected at the beginning of a method body.</p>
<p>Since the template is given the first method parameter which is in the
scope of the insertion location, the generated code is guaranteed to compile.
The Java compiler ensures that the template compiles with a given scope, the
developer is responsible for checking that the scope where she uses
template-generated code is consistent with the template scope.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// creating a template instance</span>
<span class="nc">Template</span> <span class="n">t</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">CheckBoundTemplate</span><span class="o">();</span>
<span class="n">t</span><span class="o">.</span><span class="na">_col_</span> <span class="o">=</span> <span class="n">createVariableAccess</span><span class="o">(</span><span class="n">method</span><span class="o">.</span><span class="na">getParameters</span><span class="o">().</span><span class="na">get</span><span class="o">(</span><span class="mi">0</span><span class="o">));</span>
<span class="c1">// getting the final AST</span>
<span class="nc">CtStatement</span> <span class="n">injectedCode</span> <span class="o">=</span> <span class="n">t</span><span class="o">.</span><span class="na">apply</span><span class="o">();</span>
<span class="c1">// adds the bound check at the beginning of a method</span>
<span class="n">method</span><span class="o">.</span><span class="na">getBody</span><span class="o">().</span><span class="na">insertBegin</span><span class="o">(</span><span class="n">injectedCode</span><span class="o">);</span>
</code></pre></div></div>
<h3 id="kinds-of-templating">Kinds of templating</h3>
<p>There are different kinds of templating.</p>
<h4 id="subclassing-statementtemplate">Subclassing <code class="language-plaintext highlighter-rouge">StatementTemplate</code></h4>
<p>Using method <code class="language-plaintext highlighter-rouge">apply()</code> enables to get a new statement (see example <code class="language-plaintext highlighter-rouge">CheckBoundTemplate</code> above)</p>
<h4 id="subclassing-blocktemplate">Subclassing <code class="language-plaintext highlighter-rouge">BlockTemplate</code></h4>
<p>Using method <code class="language-plaintext highlighter-rouge">apply()</code> enables to get a new block.</p>
<h4 id="subclassing-expressiontemplate">Subclassing <code class="language-plaintext highlighter-rouge">ExpressionTemplate</code></h4>
<p>Using method <code class="language-plaintext highlighter-rouge">apply()</code> enables to get a new expression. The core template method must be called <code class="language-plaintext highlighter-rouge">expression</code> and only contain a return with the expression to be templated.</p>
<h4 id="subclassing-extensiontemplate">Subclassing <code class="language-plaintext highlighter-rouge">ExtensionTemplate</code></h4>
<p>Using method <code class="language-plaintext highlighter-rouge">apply()</code> enables to get a new class where all possible templating in all methods. In addition, the following class level transformations are made:</p>
<p>1) methods and field of the templates are injected in the target class</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">ATemplate1</span> <span class="kd">extends</span> <span class="nc">ExtensionTemplate</span> <span class="o">{</span>
<span class="kt">int</span> <span class="n">i</span><span class="o">;</span>
<span class="kt">void</span> <span class="nf">foo</span><span class="o">()</span> <span class="o">{};</span>
<span class="o">}</span>
<span class="c1">// inject `i` and `foo` in aCtClass</span>
<span class="nc">Substitution</span><span class="o">.</span><span class="na">insertAll</span><span class="o">(</span><span class="n">aCtClass</span><span class="o">,</span> <span class="k">new</span> <span class="nc">ATemplate1</span><span class="o">());</span>
</code></pre></div></div>
<p>2) parametrized superinterfaces are injected in the target class</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">class</span> <span class="nc">ATemplate2</span> <span class="kd">extends</span> <span class="nc">ExtensionTemplate</span> <span class="kd">implements</span> <span class="nc">Serializable</span><span class="o">,</span> <span class="no">A</span><span class="o">,</span> <span class="no">B</span> <span class="o">{</span>
<span class="c1">// interface templates supports TypeReference</span>
<span class="nd">@Parameter</span>
<span class="nc">Class</span> <span class="no">A</span><span class="o">;</span>
<span class="o">}</span>
<span class="nc">Template</span> <span class="n">t</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ATemplate2</span><span class="o">();</span>
<span class="n">t</span><span class="o">.</span><span class="na">A</span> <span class="o">=</span> <span class="nc">Comparable</span><span class="o">.</span><span class="na">class</span>
<span class="nc">Substitution</span><span class="o">.</span><span class="na">insertAll</span><span class="o">(</span><span class="n">aCtClass</span><span class="o">,</span> <span class="n">t</span><span class="o">);</span>
<span class="c1">// aCtClass now implements Serializable and Serializable</span>
</code></pre></div></div>
<p>3) method parameters are replaced</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">class</span> <span class="nc">ATemplate3</span> <span class="kd">extends</span> <span class="nc">ExtensionTemplate</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">methodWithTemplatedParameters</span><span class="o">(</span><span class="nc">Object</span> <span class="n">params</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">// code</span>
<span class="o">}</span>
<span class="nd">@Parameter</span>
<span class="kd">public</span> <span class="nc">List</span><span class="o"><</span><span class="nc">CtParameter</span><span class="o">></span> <span class="n">params</span><span class="o">;</span>
<span class="o">}</span>
<span class="nc">Template</span> <span class="n">t</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ATemplate3</span><span class="o">();</span>
<span class="n">t</span><span class="o">.</span><span class="na">params</span> <span class="o">=</span> <span class="o">...</span>
<span class="nc">Substitution</span><span class="o">.</span><span class="na">insertAll</span><span class="o">(</span><span class="n">aCtClass</span><span class="o">,</span> <span class="n">t</span><span class="o">);</span>
<span class="c1">// aCtClass contains methodmethodWithTemplatedParameters with specific parameters</span>
</code></pre></div></div>
<h3 id="template-parameters">Template parameters</h3>
<h4 id="ast-elements">AST elements</h4>
<p>All meta-model elements can be used as template parameter.
There are two ways of defining such a template parameter.</p>
<p>1) Using a subtype of <code class="language-plaintext highlighter-rouge">TemplateParameter</code></p>
<p>The following template uses a block as template parameter.
This template type-checks, and can be used as input by the substitution
engine to wrap a method body into a try/catch block. The substitution engine
contains various methods that implement different substitution scenarios.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">TryCatchOutOfBoundTemplate</span> <span class="kd">extends</span> <span class="nc">BlockTemplate</span> <span class="o">{</span>
<span class="c1">// CTBlock is a subtype of TemplateParameter as most metamodel elements</span>
<span class="nc">CtBlock</span> <span class="n">_body_</span><span class="o">;</span> <span class="c1">// the body to surround</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">block</span><span class="o">()</span> <span class="o">{</span>
<span class="k">try</span> <span class="o">{</span>
<span class="n">_body_</span><span class="o">.</span><span class="na">S</span><span class="o">();</span>
<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="nc">OutOfBoundException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
<span class="n">e</span><span class="o">.</span><span class="na">printStackTrace</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p>One can also type the field directly with <code class="language-plaintext highlighter-rouge">TemplateParameter</code>:</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">TryCatchOutOfBoundTemplate</span> <span class="kd">extends</span> <span class="nc">BlockTemplate</span> <span class="o">{</span>
<span class="nc">TemplateParameter</span><span class="o"><</span><span class="nc">Void</span><span class="o">></span> <span class="n">_body_</span><span class="o">;</span> <span class="c1">// the body to surround</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">block</span><span class="o">()</span> <span class="o">{</span>
<span class="k">try</span> <span class="o">{</span>
<span class="n">_body_</span><span class="o">.</span><span class="na">S</span><span class="o">();</span>
<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="nc">OutOfBoundException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
<span class="n">e</span><span class="o">.</span><span class="na">printStackTrace</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p>2) Using annotation <code class="language-plaintext highlighter-rouge">@Parameter</code></p>
<p>Fields annotated with <code class="language-plaintext highlighter-rouge">@Parameter</code> are template parameters.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nd">@Parameter</span>
<span class="nc">CtInvocation</span> <span class="n">invocation</span><span class="o">;</span>
</code></pre></div></div>
<p>and then all <code class="language-plaintext highlighter-rouge">invocation.S()</code> will be replaced by the actual invocation.</p>
<h4 id="literal-template-parameters">Literal template Parameters</h4>
<p>For literals, Spoon provides developers with <em>literal template parameters</em>. When the parameter is known to
be a literal (primitive types, <code class="language-plaintext highlighter-rouge">Class</code> or a one-dimensional array of
these types), a template parameter, annotated with <code class="language-plaintext highlighter-rouge">@Parameter</code> enables one to have concise template code.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// with literal template parameter</span>
<span class="nd">@Parameter</span>
<span class="kt">int</span> <span class="n">val</span><span class="o">;</span>
<span class="o">...</span>
<span class="n">val</span> <span class="o">=</span> <span class="mi">5</span><span class="o">;</span>
<span class="o">...</span>
<span class="k">if</span> <span class="o">(</span><span class="n">list</span><span class="o">.</span><span class="na">size</span><span class="o">()></span><span class="n">val</span><span class="o">)</span> <span class="o">{...}</span>
</code></pre></div></div>
<p>String parameters are not working like other primitive type parameters, since we’re using String parameters only to rename elements of the code like fields and methods.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// with String template parameter, which is used to substitute method name. </span>
<span class="nd">@Parameter</span>
<span class="nc">String</span> <span class="n">methodName</span><span class="o">;</span>
<span class="o">...</span>
<span class="n">methodName</span> <span class="o">=</span> <span class="s">"generatedMethod"</span><span class="o">;</span>
<span class="o">...</span>
<span class="kt">void</span> <span class="nf">methodName</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">//a body of generated method</span>
<span class="o">}</span>
</code></pre></div></div>
<p>To use a parameter with a type String like other primitive types, use <code class="language-plaintext highlighter-rouge">CtLiteral<String></code>.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// with CtLiteral<String> template parameter, which is used to substitute String literal </span>
<span class="nd">@Parameter</span>
<span class="nc">CtLiteral</span><span class="o"><</span><span class="nc">String</span><span class="o">></span> <span class="n">val</span><span class="o">;</span>
<span class="o">...</span>
<span class="n">val</span> <span class="o">=</span> <span class="n">factory</span><span class="o">.</span><span class="na">Code</span><span class="o">().</span><span class="na">createLiteral</span><span class="o">(</span><span class="s">"Some string"</span><span class="o">);</span>
<span class="o">...</span>
<span class="nc">String</span> <span class="nf">someMethod</span><span class="o">()</span> <span class="o">{</span>
<span class="k">return</span> <span class="n">val</span><span class="o">.</span><span class="na">S</span><span class="o">();</span> <span class="c1">//is substituted as return "Some string";</span>
<span class="o">}</span>
</code></pre></div></div>
<p>or String literal can be optionally generated like this</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// with CtLiteral<String> template parameter, which is used to substitute String literal </span>
<span class="nd">@Parameter</span>
<span class="nc">String</span> <span class="n">val</span><span class="o">;</span>
<span class="o">...</span>
<span class="n">val</span> <span class="o">=</span> <span class="s">"Some string"</span><span class="o">;</span>
<span class="o">...</span>
<span class="nc">String</span> <span class="nf">someMethod</span><span class="o">()</span> <span class="o">{</span>
<span class="k">return</span> <span class="s">"val"</span><span class="o">;</span> <span class="c1">//is substituted as return "Some string";</span>
<span class="o">}</span>
</code></pre></div></div>
<h4 id="inlining-foreach-expressions">Inlining foreach expressions</h4>
<p>Foreach expressions can be inlined. They have to be declared as follows:</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nd">@Parameter</span>
<span class="nc">CtExpression</span><span class="o">[]</span> <span class="n">intValues</span><span class="o">;</span>
<span class="o">...</span>
<span class="n">template</span><span class="o">.</span><span class="na">intValues</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">CtExpression</span><span class="o">[</span><span class="mi">2</span><span class="o">];</span>
<span class="n">template</span><span class="o">.</span><span class="na">intValues</span><span class="o">[</span><span class="mi">0</span><span class="o">]</span> <span class="o">=</span> <span class="n">factory</span><span class="o">.</span><span class="na">Code</span><span class="o">().</span><span class="na">createLiteral</span><span class="o">(</span><span class="mi">0</span><span class="o">);</span>
<span class="n">template</span><span class="o">.</span><span class="na">intValues</span><span class="o">[</span><span class="mi">1</span><span class="o">]</span> <span class="o">=</span> <span class="n">factory</span><span class="o">.</span><span class="na">Code</span><span class="o">().</span><span class="na">createLiteral</span><span class="o">(</span><span class="mi">1</span><span class="o">);</span>
</code></pre></div></div>
<p>and then,</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">for</span><span class="o">(</span><span class="nc">Object</span> <span class="n">x</span> <span class="o">:</span> <span class="n">intValues</span><span class="o">)</span> <span class="o">{</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="n">x</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></div></div>
<p>is transformed into:</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">{</span>
<span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="mi">0</span><span class="o">);</span>
<span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="mi">1</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></div></div>
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