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| <article class="container"> |
| <section class="col-md-12"> |
| <a class="edit-on-gh" href="https://github.com/apache/struts-site/edit/master/source/tag-developers/tag-syntax.md" title="Edit this page on GitHub">Edit on GitHub</a> |
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| <a href="index" title="back to Tag Developers Guide"><< back to Tag Developers Guide</a> |
| |
| <h1 class="no_toc" id="tag-syntax">Tag Syntax</h1> |
| |
| <ul id="markdown-toc"> |
| <li><a href="#creating-a-dynamic-input-field" id="markdown-toc-creating-a-dynamic-input-field">Creating a dynamic input field</a> <ul> |
| <li><a href="#using-an-expression-to-set-the-label" id="markdown-toc-using-an-expression-to-set-the-label">Using an expression to set the label</a></li> |
| </ul> |
| </li> |
| <li><a href="#non-string-attributes" id="markdown-toc-non-string-attributes">Non-String Attributes</a> <ul> |
| <li><a href="#evaluating-booleans" id="markdown-toc-evaluating-booleans">Evaluating booleans</a></li> |
| <li><a href="#evaluating-booleans-verbose" id="markdown-toc-evaluating-booleans-verbose">Evaluating booleans (verbose)</a></li> |
| <li><a href="#evaluating-booleans-with-property" id="markdown-toc-evaluating-booleans-with-property">Evaluating booleans (with property)</a></li> |
| <li><a href="#evaluating-booleans-verbose-with-property" id="markdown-toc-evaluating-booleans-verbose-with-property">Evaluating booleans (verbose with property)</a></li> |
| </ul> |
| </li> |
| <li><a href="#value-is-an-object" id="markdown-toc-value-is-an-object">value is an Object!</a></li> |
| <li><a href="#probably-wrong" id="markdown-toc-probably-wrong">Probably wrong!</a></li> |
| <li><a href="#passing-a-literal-value-the-right-way" id="markdown-toc-passing-a-literal-value-the-right-way">Passing a literal value the right way</a></li> |
| <li><a href="#expression-language-notations" id="markdown-toc-expression-language-notations">Expression Language Notations</a></li> |
| <li><a href="#disallowed-property-names" id="markdown-toc-disallowed-property-names">Disallowed property names</a></li> |
| </ul> |
| |
| <p>The tags are designed to display dynamic data. To create a input field that displays the property “postalCode”, |
| we’d pass the String “postalCode” to the textfield tag.</p> |
| |
| <h2 id="creating-a-dynamic-input-field">Creating a dynamic input field</h2> |
| |
| <pre><code class="language-jsp"><s:textfield name="postalCode"/> |
| </code></pre> |
| |
| <p>If there is a “postalCode” property on the value stack, its value will be set to the input field. When the field is |
| submitted back to the framework, the value of the control will be set back to the “postalCode” property.</p> |
| |
| <p>Sometimes, we want to pass the dynamic data to a tag. For example, we might want to display a label with the input |
| field, and we might want to obtain the label from the application’s messages resources. Accordingly, the framework will |
| parse expressions found in the tag attributes, so that we can merge dynamic data into the tag attributes at runtime. |
| The expression escape sequence is <code class="highlighter-rouge">%{ ... }</code>. Any text embedded in the escape sequence is evalulated as an expression.</p> |
| |
| <h3 id="using-an-expression-to-set-the-label">Using an expression to set the label</h3> |
| |
| <pre><code class="language-jsp"><s:textfield key="postalCode.label" name="postalCode"/> |
| </code></pre> |
| |
| <p>The expression language (<a href="ognl">OGNL</a>) lets us call methods and evaluate properties. The method <code class="highlighter-rouge">getText</code> is provided |
| by <code class="highlighter-rouge">ActionSupport</code>, which is the base class for most Actions. Since the Action is on the stack, we can call any of its |
| methods from an expression, including <code class="highlighter-rouge">getText</code>.</p> |
| |
| <h2 id="non-string-attributes">Non-String Attributes</h2> |
| |
| <p>The HTTP protocol is text-based, but some tags have non-String attribute types, like <code class="highlighter-rouge">bool</code> or <code class="highlighter-rouge">int</code>. To make using |
| non-String attributes intuitative, the framework evaulates <strong>all</strong> non-String attributes as an expression. |
| In this case, you do not need to use the escape notation. (But, if you do anyway , the framework will just strip it off.)</p> |
| |
| <h3 id="evaluating-booleans">Evaluating booleans</h3> |
| |
| <pre><code class="language-jsp"><s:select key="state.label" name="state" multiple="true"/> |
| </code></pre> |
| |
| <p>Since the attribute <code class="highlighter-rouge">multiple</code> maps to a boolean property, the framework does not interpret the value as a String. |
| The value is evaluated as an expression and automtically converted to a boolean.</p> |
| |
| <p>Since it’s easy to forget which attributes are String and which are non-String, you can still use the escape notation.</p> |
| |
| <h3 id="evaluating-booleans-verbose">Evaluating booleans (verbose)</h3> |
| |
| <pre><code class="language-jsp"><s:select key="state.label" name="state" multiple="%{true}"/> |
| </code></pre> |
| |
| <h3 id="evaluating-booleans-with-property">Evaluating booleans (with property)</h3> |
| |
| <pre><code class="language-jsp"><s:select key="state.label" name="state" multiple="allowMultiple"/> |
| </code></pre> |
| |
| <h3 id="evaluating-booleans-verbose-with-property">Evaluating booleans (verbose with property)</h3> |
| |
| <pre><code class="language-jsp"><s:select key="state.label" name="state" multiple="%{allowMultiple}"/> |
| </code></pre> |
| |
| <h2 id="value-is-an-object">value is an Object!</h2> |
| |
| <p>Most often, the <code class="highlighter-rouge">value</code> attribute is set automatically, since <code class="highlighter-rouge">name</code> attribute usually tells the framework which |
| property to call to set the <code class="highlighter-rouge">value</code>. But, if there is a reason to set the <code class="highlighter-rouge">value</code> directly, be advised that <code class="highlighter-rouge">value</code> |
| <strong>is an Object <em>NOT</em> a String</strong>.</p> |
| |
| <blockquote> |
| <p>NOTE: Since <code class="highlighter-rouge">value</code> is not a String, whatever is passed to <code class="highlighter-rouge">value</code> is evaluated as an expression - <strong>NOT</strong> a String literal.</p> |
| </blockquote> |
| |
| <h2 id="probably-wrong">Probably wrong!</h2> |
| |
| <pre><code class="language-jsp"><s:textfield key="state.label" name="state" value="ca"/> |
| </code></pre> |
| |
| <p>If a <code class="highlighter-rouge">textfield</code> is passed the value attribute <code class="highlighter-rouge">ca</code>, the framework will look for a property named <code class="highlighter-rouge">getCa</code>. Generally, |
| this is not what we mean. What we mean to do is pass a literal String. In the expression language, literals are placed |
| within quotes</p> |
| |
| <h2 id="passing-a-literal-value-the-right-way">Passing a literal value the right way</h2> |
| |
| <pre><code class="language-jsp"><s:textfield key="state.label" name="state" value="%{'ca'}" /> |
| </code></pre> |
| |
| <p>Another approach would be to use the idiom <code class="highlighter-rouge">value="'ca'"</code>, but, in this case, using the expression notation is recommended.</p> |
| |
| <p>Boiled down, the tag attributes are evaluated using three rules.</p> |
| |
| <ol> |
| <li>All <em>String</em> attribute types are <em>parsed</em> for the <code class="highlighter-rouge">%{ ... }</code> notation.</li> |
| <li>All <em>non-String</em> attribute types are <strong>not</strong> parsed, but evaluated directly as an expression</li> |
| <li>The exception to rule #2 is that if the <em>non-String</em> attribute uses the escape notion <code class="highlighter-rouge">%{}</code>, the notation is ignored |
| as redundant, and the content evaluated.</li> |
| </ol> |
| |
| <p>Please remember about <em>altSyntax</em> option that can change when value is evaluated as an expression - <a href="alt-syntax">Alt Syntax</a></p> |
| |
| <h2 id="expression-language-notations">Expression Language Notations</h2> |
| |
| <ul> |
| <li>A JavaBean object in a standard context in Freemarker, Velocity, or JSTL EL (Not OGNL). |
| <pre><code class="language-jsp">Username: ${user.username} |
| </code></pre> |
| </li> |
| <li>A username property on the Value Stack. |
| <pre><code class="language-jsp"><s:textfield name="username"/> |
| </code></pre> |
| </li> |
| <li>Another way to refer to a property placed on the Value Stack. |
| <pre><code class="language-jsp"><s:url var="es" action="Hello"> |
| <s:param name="request_locale">es</s:param> |
| </s:url> |
| <s:a href="%{es}">Espanol</s:a> |
| </code></pre> |
| </li> |
| <li>A static Map, as in <code class="highlighter-rouge">put("username","trillian")</code>. |
| <pre><code class="language-jsp"><s:property value="#session.user.username" /> |
| <s:select label="FooBar" name="foo" list="#{'username':'trillian', 'username':'zaphod'}" /> |
| </code></pre> |
| </li> |
| </ul> |
| |
| <h2 id="disallowed-property-names">Disallowed property names</h2> |
| |
| <p>The following names of property are disallowed:</p> |
| |
| <ul> |
| <li>parameters</li> |
| <li>application</li> |
| <li>session</li> |
| <li>struts</li> |
| <li>request</li> |
| <li>servletRequest</li> |
| <li>servletResponse</li> |
| </ul> |
| |
| <p>The below code will not work:</p> |
| |
| <pre><code class="language-jsp"><s:iterator value="parameters"/> |
| </code></pre> |
| |
| <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">MyAction</span> <span class="o">{</span> |
| |
| <span class="kd">private</span> <span class="n">String</span><span class="o">[]</span> <span class="n">parameters</span><span class="o">;</span> |
| |
| <span class="kd">public</span> <span class="n">String</span><span class="o">[]</span> <span class="nf">getParameters</span><span class="o">()</span> <span class="o">{</span> |
| <span class="k">return</span> <span class="n">parameters</span><span class="o">;</span> |
| <span class="o">}</span> |
| |
| <span class="o">}</span> |
| |
| </code></pre></div></div> |
| |
| </section> |
| </article> |
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