tutorial/cdi-basic006.html at gh-pages · javaee/tutorial · GitHub

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<!DOCTYPE html>

<head>

<title>Using Qualifiers</title>

</head>

<body>

<tr>

<b>Java Platform, Enterprise Edition (Java EE) 8</b><br />

<b>The Java EE Tutorial</b>

</td>

</tr>

</table>

<hr />

</colgroup>

<tr>

<span style="position:relative;top:-2px;">Previous</span>

</a>

</td>

</a>

</td>

<span style="position:relative;top:-2px;">Contents</span>

</a>

</td>

</tr>

</table>

</div>

<h2 id="_using_qualifiers">Using Qualifiers</h2>

<p>You can use qualifiers to provide various implementations of a

particular bean type. A qualifier is an annotation that you apply to a

bean. A qualifier type is a Java annotation defined as

<code>@Target({METHOD, FIELD, PARAMETER, TYPE})</code> and <code>@Retention(RUNTIME)</code>.</p>

</div>

<p>For example, you could declare an <code>@Informal</code> qualifier type and apply

it to another class that extends the <code>Greeting</code> class. To declare this

qualifier type, use the following code:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">package greetings;

import static java.lang.annotation.ElementType.FIELD;

import static java.lang.annotation.ElementType.METHOD;

import static java.lang.annotation.ElementType.PARAMETER;

import static java.lang.annotation.ElementType.TYPE;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Target;

import javax.inject.Qualifier;

@Qualifier

@Retention(RUNTIME)

@Target({TYPE, METHOD, FIELD, PARAMETER})

public @interface Informal {}</code></pre>

</div>

<p>You can then define a bean class that extends the <code>Greeting</code> class and

uses this qualifier:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">package greetings;

@Informal

public class InformalGreeting extends Greeting {

public String greet(String name) {

return "Hi, " + name + "!";

}

}</code></pre>

</div>

<p>Both implementations of the bean can now be used in the application.</p>

</div>

<p>If you define a bean with no qualifier, then the bean automatically has the

qualifier <code>@Default</code>. The unannotated <code>Greeting</code> class could be declared

as follows:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">package greetings;

import javax.enterprise.inject.Default;

@Default

public class Greeting {

public String greet(String name) {

return "Hello, " + name + ".";

}

}</code></pre>

</div>

<hr />

</colgroup>

<tr>

<span style="position:relative;top:-2px;">Previous</span>

</a>

</td>

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<span style="position:relative;top:-2px;">Contents</span>

</a>

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</span>

</body>

</html>

tutorial/cdi-basic006.html at gh-pages · javaee/tutorial · GitHub

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