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<!DOCTYPE html>

<head>

<title>Getting Started</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="_getting_started">Getting Started</h2>

<p>Contexts and Dependency Injection (CDI) enables your objects to have

their dependencies provided to them automatically, instead of creating

them or receiving them as parameters. CDI also manages the lifecycle of

those dependencies for you.</p>

</div>

<p>For example, consider the following servlet:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">@WebServlet("/cdiservlet")

public class NewServlet extends HttpServlet {

private Message message;

@Override

public void init() {

message = new MessageB();

}

@Override

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws IOException {

response.getWriter().write(message.get());

}

}</code></pre>

</div>

<p>This servlet needs an instance of an object that implements the

<code>Message</code> interface:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">public interface Message {

public String get();

}</code></pre>

</div>

<p>The servlet creates itself an instance of the following object:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">public class MessageB implements Message {

public MessageB() { }

@Override

public String get() {

return "message B";

}

}</code></pre>

</div>

<p>Using CDI, this servlet can declare its dependency on a <code>Message</code>

instance and have it injected automatically by the CDI runtime. The new

servlet code is the following:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">@WebServlet("/cdiservlet")

public class NewServlet extends HttpServlet {

@Inject private Message message;

@Override

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws IOException {

response.getWriter().write(message.get());

}

}</code></pre>

</div>

<p>The CDI runtime looks for classes that implement the <code>Message</code>

interface, finds the <code>MessageB</code> class, creates a new instance of it, and

injects it into the servlet at runtime. To manage the lifecycle of the

new instance, the CDI runtime needs to know what the scope of the

instance should be. In this example, the servlet only needs the instance

to process an HTTP request; the instance can then be garbage collected.

This is specified using the <code>javax.enterprise.context.RequestScoped</code>

annotation:</p>

</div>

<pre class="prettyprint highlight"><code class="language-oac_no_warn" data-lang="oac_no_warn">@RequestScoped

public class MessageB implements Message { ... }</code></pre>

</div>

<p>For more information on scopes, see <a href="cdi-basic008.html#GJBBK">Using

Scopes</a>.</p>

</div>

<p>The <code>MessageB</code> class is a CDI bean. CDI beans are classes that CDI can

instantiate, manage, and inject automatically to satisfy the

dependencies of other objects. Almost any Java class can be managed and

injected by CDI. For more information on beans, see

<a href="cdi-basic003.html#GJEBJ">About Beans</a>. A JAR or WAR file that

contains a CDI bean is a bean archive. For more information on packaging

bean archives, see <a href="cdi-basic013.html#GJBNZ">Configuring a CDI

Application</a> in this chapter and <a href="cdi-adv001.html#CACDCFDE">Packaging

CDI Applications</a> in <a href="cdi-adv.html#GJEHI">Chapter 27, "Contexts and

Dependency Injection for Java EE: Advanced Topics"</a>.</p>

</div>

<p>In this example, <code>MessageB</code> is the only class that implements the

<code>Message</code> interface. If an application has more than one implementation

of an interface, CDI provides mechanisms that you can use to select

which implementation to inject. For more information, see

<a href="cdi-basic006.html#GJBCK">Using Qualifiers</a> in this chapter and

<a href="cdi-adv002.html#GJSDF">Using Alternatives in CDI Applications</a> in

<a href="cdi-adv.html#GJEHI">Chapter 27, "Contexts and Dependency Injection

for Java EE: Advanced Topics"</a>.</p>

</div>

<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>

</a>

</span>

</body>

</html>

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