<?xml version="1.0" encoding="UTF-8"?>

<section xml:id="beans-java"

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<title>Java-based container configuration</title>

<section xml:id="beans-java-basic-concepts">

<title>Basic concepts: <literal>@Bean</literal> and <literal>@Configuration</literal></title>

<sidebar>

<title>Full @Configuration vs 'lite' @Beans mode?</title>

<para>When <interfacename>@Bean</interfacename> methods are declared within

classes that are <emphasis>not</emphasis> annotated with

<interfacename>@Configuration</interfacename> they are referred to as being

processed in a 'lite' mode. For example, bean methods declared in a

<interfacename>@Component</interfacename> or even in a <emphasis>plain old

class</emphasis> will be considered 'lite'.</para>

<para>Unlike full <interfacename>@Configuration</interfacename>, lite

<interfacename>@Bean</interfacename> methods cannot easily declare inter-bean

dependencies. Usually one <interfacename>@Bean</interfacename> method should not

invoke another <interfacename>@Bean</interfacename> method when operating in

'lite' mode.</para>

<para>Only using <interfacename>@Bean</interfacename> methods within

<interfacename>@Configuration</interfacename> classes is a recommended approach

of ensuring that 'full' mode is always used. This will prevent the same

<interfacename>@Bean</interfacename> method from accidentally being invoked

multiple times and helps to reduce subtle bugs that can be hard to track down

when operating in 'lite' mode.</para>

</sidebar>

<para>The central artifacts in Spring's new Java-configuration support are

<interfacename>@Configuration</interfacename>-annotated classes and

<interfacename>@Bean</interfacename>-annotated methods.</para>

<para>The <interfacename>@Bean</interfacename> annotation is used to indicate that a

method instantiates, configures and initializes a new object to be managed by

the Spring IoC container. For those familiar with Spring's

<literal>&lt;beans/&gt;</literal> XML configuration the <literal>@Bean</literal>

annotation plays the same role as the <literal>&lt;bean/&gt;</literal>

element. You can use <interfacename>@Bean</interfacename> annotated methods with

any Spring <interfacename>@Component</interfacename>, however, they are most

often used with <interfacename>@Configuration</interfacename> beans.</para>

<para>Annotating a class with <interfacename>@Configuration</interfacename>

indicates that its primary purpose is as a source of bean definitions. Furthermore,

<interfacename>@Configuration</interfacename> classes allow inter-bean

dependencies to be defined by simply calling other <interfacename>@Bean</interfacename>

methods in the same class. The simplest possible

<interfacename>@Configuration</interfacename> class would read as follows:

<programlisting language="java">@Configuration

public class AppConfig {

@Bean

public MyService myService() {

return new MyServiceImpl();

}

}</programlisting></para>

<para>The <literal>AppConfig</literal> class above would be equivalent to the

following Spring <literal>&lt;beans/&gt;</literal> XML:

<programlisting language="xml">&lt;beans&gt;

&lt;bean id="myService" class="com.acme.services.MyServiceImpl"/&gt;

&lt;/beans&gt;</programlisting>

The <interfacename>@Bean</interfacename> and <interfacename>@Configuration</interfacename>

annotations will be discussed in depth in the sections below. First, however, we'll

cover the various ways of creating a spring container using Java-based

configuration.</para>

</section>

<section xml:id="beans-java-instantiating-container">

<title>Instantiating the Spring container using

<literal>AnnotationConfigApplicationContext</literal></title>

<para>The sections below document Spring's

<literal>AnnotationConfigApplicationContext</literal>, new in Spring 3.0.

This versatile <literal>ApplicationContext</literal> implementation is

capable of accepting not only <literal>@Configuration</literal> classes as

input, but also plain <literal>@Component</literal> classes and classes

annotated with JSR-330 metadata.</para>

<para>When <literal>@Configuration</literal> classes are provided as input,

the <literal>@Configuration</literal> class itself is registered as a bean

definition, and all declared <literal>@Bean</literal> methods within the

class are also registered as bean definitions.</para>

<para>When <literal>@Component</literal> and JSR-330 classes are provided,

they are registered as bean definitions, and it is assumed that DI

metadata such as <literal>@Autowired</literal> or

<literal>@Inject</literal> are used within those classes where

necessary.</para>

<section xml:id="beans-java-instantiating-container-contstructor">

<title>Simple construction</title>

<para>In much the same way that Spring XML files are used as input when

instantiating a <literal>ClassPathXmlApplicationContext</literal>,

<literal>@Configuration</literal> classes may be used as input when

instantiating an <literal>AnnotationConfigApplicationContext</literal>.

This allows for completely XML-free usage of the Spring container:

<programlisting language="java">public static void main(String[] args) {

ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);

MyService myService = ctx.getBean(MyService.class);

myService.doStuff();

}</programlisting>

As mentioned above,

<literal>AnnotationConfigApplicationContext</literal> is not limited to

working only with <literal>@Configuration</literal> classes. Any

<literal>@Component</literal> or JSR-330 annotated class may be supplied

as input to the constructor. For example:

<programlisting language="java">public static void main(String[] args) {

ApplicationContext ctx = new AnnotationConfigApplicationContext(MyServiceImpl.class, Dependency1.class, Dependency2.class);

MyService myService = ctx.getBean(MyService.class);

myService.doStuff();

}</programlisting>

The above assumes that <literal>MyServiceImpl</literal>,

<literal>Dependency1</literal> and <literal>Dependency2</literal> use

Spring dependency injection annotations such as

<literal>@Autowired</literal>.</para>

</section>

<section xml:id="beans-java-instantiating-container-register">

<title>Building the container programmatically using

<literal>register(Class&lt;?&gt;...)</literal></title>

<para>An <literal>AnnotationConfigApplicationContext</literal> may be

instantiated using a no-arg constructor and then configured using the

<literal>register()</literal> method. This approach is particularly

useful when programmatically building an

<literal>AnnotationConfigApplicationContext</literal>.

<programlisting language="java">public static void main(String[] args) {

AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();

ctx.register(AppConfig.class, OtherConfig.class);

ctx.register(AdditionalConfig.class);

ctx.refresh();

MyService myService = ctx.getBean(MyService.class);

myService.doStuff();

}</programlisting></para>

</section>

<section xml:id="beans-java-instantiating-container-scan">

<title>Enabling component scanning with

<literal>scan(String...)</literal></title>

<para>Experienced Spring users will be familiar with the following

commonly-used XML declaration from Spring's <literal>context:</literal>

namespace

<programlisting language="xml">&lt;beans&gt;

&lt;context:component-scan base-package="com.acme"/&gt;

&lt;/beans&gt;</programlisting>

In the example above, the <literal>com.acme</literal> package will be

scanned, looking for any <literal>@Component</literal>-annotated

classes, and those classes will be registered as Spring bean definitions

within the container.

<literal>AnnotationConfigApplicationContext</literal> exposes the

<literal>scan(String...)</literal> method to allow for the same

component-scanning

functionality:<programlisting language="java">public static void main(String[] args) {

AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();

ctx.scan("com.acme");

ctx.refresh();

MyService myService = ctx.getBean(MyService.class);

}</programlisting></para>

<note>

<para>Remember that <literal>@Configuration</literal> classes are

meta-annotated with <literal>@Component</literal>, so they are

candidates for component-scanning! In the example above, assuming that

<literal>AppConfig</literal> is declared within the

<literal>com.acme</literal> package (or any package underneath), it

will be picked up during the call to <literal>scan()</literal>, and

upon <literal>refresh()</literal> all its <literal>@Bean</literal>

methods will be processed and registered as bean definitions within

the container.</para>

</note>

</section>

<section xml:id="beans-java-instantiating-container-web">

<title>Support for web applications with

<literal>AnnotationConfigWebApplicationContext</literal></title>

<para>A <literal>WebApplicationContext</literal> variant of

<literal>AnnotationConfigApplicationContext</literal> is available with

<literal>AnnotationConfigWebApplicationContext</literal>. This

implementation may be used when configuring the Spring

<literal>ContextLoaderListener</literal> servlet listener, Spring MVC

<literal>DispatcherServlet</literal>, etc. What follows is a

<literal>web.xml</literal> snippet that configures a typical Spring MVC

web application. Note the use of the <literal>contextClass</literal>

context-param and init-param:

<programlisting language="xml">

&lt;web-app&gt;

&lt;!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext

instead of the default XmlWebApplicationContext --&gt;

&lt;context-param&gt;

&lt;param-name&gt;contextClass&lt;/param-name&gt;

&lt;param-value&gt;

org.springframework.web.context.support.AnnotationConfigWebApplicationContext

&lt;/param-value&gt;

&lt;/context-param&gt;

&lt;!-- Configuration locations must consist of one or more comma- or space-delimited

fully-qualified @Configuration classes. Fully-qualified packages may also be

specified for component-scanning --&gt;

&lt;context-param&gt;

&lt;param-name&gt;contextConfigLocation&lt;/param-name&gt;

&lt;param-value&gt;com.acme.AppConfig&lt;/param-value&gt;

&lt;/context-param&gt;

&lt;!-- Bootstrap the root application context as usual using ContextLoaderListener --&gt;

&lt;listener&gt;

&lt;listener-class&gt;org.springframework.web.context.ContextLoaderListener&lt;/listener-class&gt;

&lt;/listener&gt;

&lt;!-- Declare a Spring MVC DispatcherServlet as usual --&gt;

&lt;servlet&gt;

&lt;servlet-name&gt;dispatcher&lt;/servlet-name&gt;

&lt;servlet-class&gt;org.springframework.web.servlet.DispatcherServlet&lt;/servlet-class&gt;

&lt;!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext

instead of the default XmlWebApplicationContext --&gt;

&lt;init-param&gt;

&lt;param-name&gt;contextClass&lt;/param-name&gt;

&lt;param-value&gt;

org.springframework.web.context.support.AnnotationConfigWebApplicationContext

&lt;/param-value&gt;

&lt;/init-param&gt;

&lt;!-- Again, config locations must consist of one or more comma- or space-delimited

and fully-qualified @Configuration classes --&gt;

&lt;init-param&gt;

&lt;param-name&gt;contextConfigLocation&lt;/param-name&gt;

&lt;param-value&gt;com.acme.web.MvcConfig&lt;/param-value&gt;

&lt;/init-param&gt;

&lt;/servlet&gt;

&lt;!-- map all requests for /app/* to the dispatcher servlet --&gt;

&lt;servlet-mapping&gt;

&lt;servlet-name&gt;dispatcher&lt;/servlet-name&gt;

&lt;url-pattern&gt;/app/*&lt;/url-pattern&gt;

&lt;/servlet-mapping&gt;

&lt;/web-app&gt;</programlisting></para>

</section>

</section>

<section xml:id="beans-java-bean-annotation">

<title>Using the <interfacename>@Bean</interfacename> annotation</title>

<para><interfacename>@Bean</interfacename> is a method-level annotation and

a direct analog of the XML <code>&lt;bean/&gt;</code> element. The

annotation supports some of the attributes offered by

<code>&lt;bean/&gt;</code>, such as: <code><link

linkend="beans-factory-lifecycle-initializingbean"

>init-method</link></code>, <code><link

linkend="beans-factory-lifecycle-disposablebean"

>destroy-method</link></code>, <code><link

linkend="beans-factory-autowire">autowiring</link></code> and

<code>name</code>.</para>

<para>You can use the <interfacename>@Bean</interfacename> annotation in a

<interfacename>@Configuration</interfacename>-annotated or in a

<interfacename>@Component</interfacename>-annotated class.</para>

<section xml:id="beans-java-declaring-a-bean">

<title>Declaring a bean</title>

<para>To declare a bean, simply annotate a method with the

<interfacename>@Bean</interfacename> annotation. You use this method to

register a bean definition within an <code>ApplicationContext</code> of

the type specified as the method's return value. By default, the bean

name will be the same as the method name. The following is a simple

example of a <interfacename>@Bean</interfacename> method declaration:

<programlisting language="java">@Configuration

public class AppConfig {

@Bean

public TransferService transferService() {

return new TransferServiceImpl();

}

}</programlisting></para>

<para>The preceding configuration is exactly equivalent to the following

Spring XML:

<programlisting language="xml">&lt;beans&gt;

&lt;bean id="transferService" class="com.acme.TransferServiceImpl"/&gt;

&lt;/beans&gt; </programlisting></para>

<para>Both declarations make a bean named <code>transferService</code>

available in the <code>ApplicationContext</code>, bound to an object

instance of type <code>TransferServiceImpl</code>:

<programlisting>

transferService -&gt; com.acme.TransferServiceImpl

</programlisting></para>

</section>

<section xml:id="beans-java-lifecycle-callbacks">

<title>Receiving lifecycle callbacks</title>

<para>Any classes defined with the

<literal>@Bean</literal> annotation support

the regular lifecycle callbacks and can use the

<literal>@PostConstruct</literal> and <literal>@PreDestroy</literal>

annotations from JSR-250, see <link

linkend="beans-postconstruct-and-predestroy-annotations">JSR-250

annotations</link> for further details.</para>

<para>The regular Spring <link linkend="beans-factory-nature"

>lifecycle</link> callbacks are fully supported as well. If a bean

implements <code>InitializingBean</code>, <code>DisposableBean</code>,

or <code>Lifecycle</code>, their respective methods are called by the

container.</para>

<para>The standard set of <code>*Aware</code> interfaces such as

<code><link linkend="beans-beanfactory">BeanFactoryAware</link></code>,

<code><link linkend="beans-factory-aware">BeanNameAware</link></code>,

<code><link linkend="context-functionality-messagesource"

>MessageSourceAware</link></code>, <code><link

linkend="beans-factory-aware">ApplicationContextAware</link></code>, and

so on are also fully supported.</para>

<para>The <interfacename>@Bean</interfacename> annotation supports

specifying arbitrary initialization and destruction callback methods,

much like Spring XML's <code>init-method</code> and

<code>destroy-method</code> attributes on the <code>bean</code> element:

<programlisting language="java">public class Foo {

public void init() {

// initialization logic

}

}

public class Bar {

public void cleanup() {

// destruction logic

}

}

@Configuration

public class AppConfig {

@Bean(initMethod = "init")

public Foo foo() {

return new Foo();

}

@Bean(destroyMethod = "cleanup")

public Bar bar() {

return new Bar();

}

}

</programlisting></para>

<para>Of course, in the case of <code>Foo</code> above, it would be

equally as valid to call the <code>init()</code> method directly during

construction:

<programlisting language="java">@Configuration

public class AppConfig {

@Bean

public Foo foo() {

Foo foo = new Foo();

foo.init();

return foo;

}

// ...

} </programlisting></para>

<tip>

<para>When you work directly in Java, you can do anything you like with

your objects and do not always need to rely on the container

lifecycle!</para>

</tip>

</section>

<section xml:id="beans-java-specifying-bean-scope">

<title>Specifying bean scope</title>

<section xml:id="beans-java-available-scopes">

<title>Using the <interfacename>@Scope</interfacename>

annotation</title>

<!-- MLP: Beverly, did not apply your edit as it changed meaning -->

<para>You can specify that your beans defined with the

<interfacename>@Bean</interfacename> annotation should have a specific

scope. You can use any of the standard scopes specified in the <link

linkend="beans-factory-scopes">Bean Scopes</link> section.</para>

<para>The default scope is <literal>singleton</literal>, but you can

override this with the <interfacename>@Scope</interfacename>

annotation:

<programlisting language="java">@Configuration

public class MyConfiguration {

@Bean

<emphasis role="bold">@Scope("prototype")</emphasis>

public Encryptor encryptor() {

// ...

}

}</programlisting></para>

</section>

<section xml:id="beans-java-scoped-proxy">

<title><code>@Scope and scoped-proxy</code></title>

<para>Spring offers a convenient way of working with scoped dependencies

through <link linkend="beans-factory-scopes-other-injection">scoped

proxies</link>. The easiest way to create such a proxy when using the

XML configuration is the <code>&lt;aop:scoped-proxy/&gt;</code>

element. Configuring your beans in Java with a @Scope annotation

offers equivalent support with the proxyMode attribute. The default is

no proxy (<varname>ScopedProxyMode.NO</varname>), but you can specify

<classname>ScopedProxyMode.TARGET_CLASS</classname> or

<classname>ScopedProxyMode.INTERFACES</classname>.</para>

<para>If you port the scoped proxy example from the XML reference

documentation (see preceding link) to our

<interfacename>@Bean</interfacename> using Java, it would look like

the following:

<programlisting language="java">// an HTTP Session-scoped bean exposed as a proxy

@Bean

<emphasis role="bold">@Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS)</emphasis>

public UserPreferences userPreferences() {

return new UserPreferences();

}

@Bean

public Service userService() {

UserService service = new SimpleUserService();

// a reference to the proxied userPreferences bean

service.setUserPreferences(userPreferences());

return service;

} </programlisting></para>

</section>

</section>

<section xml:id="beans-java-customizing-bean-naming">

<title>Customizing bean naming</title>

<para>By default, configuration classes use a

<interfacename>@Bean</interfacename> method's name as the name of the

resulting bean. This functionality can be overridden, however, with the

<code>name</code> attribute.

<programlisting language="java">@Configuration

public class AppConfig {

@Bean(name = "myFoo")

public Foo foo() {

return new Foo();

}

} </programlisting></para>

</section>

<section xml:id="beans-java-bean-aliasing">

<title>Bean aliasing</title>

<para>As discussed in <xref linkend="beans-beanname"/>, it is sometimes

desirable to give a single bean multiple names, otherwise known as

<emphasis>bean aliasing</emphasis>. The <literal>name</literal>

attribute of the <literal>@Bean</literal> annotation accepts a String

array for this purpose.

<programlisting language="java">@Configuration

public class AppConfig {

@Bean(name = { "dataSource", "subsystemA-dataSource", "subsystemB-dataSource" })

public DataSource dataSource() {

// instantiate, configure and return DataSource bean...

}

} </programlisting></para>

</section>

</section>

<section xml:id="beans-java-configuration-annotation">

<title>Using the <interfacename>@Configuration</interfacename> annotation</title>

<para><interfacename>@Configuration</interfacename> is a class-level annotation

indicating that an object is a source of bean definitions.

<interfacename>@Configuration</interfacename> classes declare beans via

public <interfacename>@Bean</interfacename> annotated methods. Calls to

<interfacename>@Bean</interfacename> methods on

<interfacename>@Configuration</interfacename> classes can also be used to

define inter-bean dependencies. See <xref linkend="beans-java-basic-concepts"/> for

a general introduction.</para>

<section xml:id="beans-java-injecting-dependencies">

<title>Injecting inter-bean dependencies</title>

<para>When <interfacename>@Bean</interfacename>s have dependencies on one

another, expressing that dependency is as simple as having one bean

method call another:

<programlisting language="java">@Configuration

public class AppConfig {

@Bean

public Foo foo() {

return new Foo(bar());

}

@Bean

public Bar bar() {

return new Bar();

}

} </programlisting></para>

<para>In the example above, the <code>foo</code> bean receives a reference

to <code> bar</code> via constructor injection.</para>

<note>

<para>This method of declaring inter-bean dependencies only works when

the <interfacename>@Bean</interfacename> method is declared within a

<interfacename>@Configuration</interfacename> class. You cannot declare

inter-bean dependencies using plain <interfacename>@Component</interfacename>

classes.</para>

</note>

</section>

<section xml:id="beans-java-method-injection">

<title>Lookup method injection</title>

<para>As noted earlier, <link linkend="beans-factory-method-injection"

>lookup method injection</link> is an advanced feature that you should

use rarely. It is useful in cases where a singleton-scoped bean has a

dependency on a prototype-scoped bean. Using Java for this type of

configuration provides a natural means for implementing this pattern.

<programlisting language="java">public abstract class CommandManager {

public Object process(Object commandState) {

// grab a new instance of the appropriate Command interface

Command command = createCommand();

// set the state on the (hopefully brand new) Command instance

command.setState(commandState);

return command.execute();

}

// okay... but where is the implementation of this method?

protected abstract Command createCommand();

} </programlisting></para>

<para>Using Java-configuration support , you can create a subclass of

<code>CommandManager</code> where the abstract

<code>createCommand()</code> method is overridden in such a way that

it looks up a new (prototype) command object:

<programlisting language="java">@Bean

@Scope("prototype")

public AsyncCommand asyncCommand() {

AsyncCommand command = new AsyncCommand();

// inject dependencies here as required

return command;

}

@Bean

public CommandManager commandManager() {

// return new anonymous implementation of CommandManager with command() overridden

// to return a new prototype Command object

return new CommandManager() {

protected Command createCommand() {

return asyncCommand();

}

}

} </programlisting></para>

</section>

<section xml:id="beans-java-further-information-java-config">

<title>Further information about how Java-based configuration works

internally</title>

<para>The following example shows a <literal>@Bean</literal> annotated

method being called twice:</para>

<programlisting language="java">

@Configuration

public class AppConfig {

@Bean

public ClientService clientService1() {

ClientServiceImpl clientService = new ClientServiceImpl();

clientService.setClientDao(clientDao());

return clientService;

}

@Bean

public ClientService clientService2() {

ClientServiceImpl clientService = new ClientServiceImpl();

clientService.setClientDao(clientDao());

return clientService;

}

@Bean

public ClientDao clientDao() {

return new ClientDaoImpl();

}

}

</programlisting>

<para> <methodname>clientDao()</methodname> has been called once in

<methodname>clientService1()</methodname> and once in

<methodname>clientService2()</methodname>. Since this method creates a new

instance of <classname>ClientDaoImpl</classname> and returns it, you would

normally expect having 2 instances (one for each service). That definitely

would be problematic: in Spring, instantiated beans have a

<literal>singleton</literal> scope by default. This is where the magic

comes in: All <literal>@Configuration</literal> classes are subclassed at

startup-time with <literal>CGLIB</literal>. In the subclass, the child

method checks the container first for any cached (scoped) beans before it

calls the parent method and creates a new instance. Note that as of Spring

3.2, it is no longer necessary to add CGLIB to your classpath because

CGLIB classes have been repackaged under org.springframework and included

directly within the spring-core JAR.</para>

<note>

<para> The behavior could be different according to the scope of your

bean. We are talking about singletons here. </para>

</note>

<note>

<para> There are a few restrictions due to the fact that CGLIB dynamically

adds features at startup-time: <itemizedlist>

<listitem>

<para>Configuration classes should not be final</para>

</listitem>

<listitem>

<para>They should have a constructor with no arguments</para>

</listitem>

</itemizedlist> </para>

</note>

</section>

</section>

<section xml:id="beans-java-composing-configuration-classes">

<title>Composing Java-based configurations</title>

<section xml:id="beans-java-using-import">

<title>Using the <literal>@Import</literal> annotation</title>

<para>Much as the <literal>&lt;import/&gt;</literal> element is used

within Spring XML files to aid in modularizing configurations, the

<literal>@Import</literal> annotation allows for loading

<literal>@Bean</literal> definitions from another configuration

class:<programlisting language="java">@Configuration

public class ConfigA {

public @Bean A a() { return new A(); }

}

@Configuration

@Import(ConfigA.class)

public class ConfigB {

public @Bean B b() { return new B(); }

}</programlisting>

Now, rather than needing to specify both

<literal>ConfigA.class</literal> and <literal>ConfigB.class</literal>

when instantiating the context, only <literal>ConfigB</literal> needs to

be supplied

explicitly:<programlisting language="java">public static void main(String[] args) {

ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);

// now both beans A and B will be available...

A a = ctx.getBean(A.class);

B b = ctx.getBean(B.class);

}</programlisting>

This approach simplifies container instantiation, as only one class

needs to be dealt with, rather than requiring the developer to remember

a potentially large number of <literal>@Configuration</literal> classes

during construction.</para>

<section xml:id="beans-java-injecting-imported-beans">

<title>Injecting dependencies on imported <literal>@Bean</literal>

definitions</title>

<para>The example above works, but is simplistic. In most practical

scenarios, beans will have dependencies on one another across

configuration classes. When using XML, this is not an issue, per se,

because there is no compiler involved, and one can simply declare

<literal>ref="someBean"</literal> and trust that Spring will work it

out during container initialization. Of course, when using

<literal>@Configuration</literal> classes, the Java compiler places

constraints on the configuration model, in that references to other

beans must be valid Java syntax.</para>

<para>Fortunately, solving this problem is simple. Remember that

<literal>@Configuration</literal> classes are ultimately just another

bean in the container - this means that they can take advantage of

<literal>@Autowired</literal> injection metadata just like any other

bean!</para>

<para>Let's consider a more real-world scenario with several

<literal>@Configuration</literal> classes, each depending on beans

declared in the

others:<programlisting language="java">@Configuration

public class ServiceConfig {

private @Autowired AccountRepository accountRepository;

public @Bean TransferService transferService() {

return new TransferServiceImpl(accountRepository);

}

}

@Configuration

public class RepositoryConfig {

private @Autowired DataSource dataSource;

public @Bean AccountRepository accountRepository() {

return new JdbcAccountRepository(dataSource);

}

}

@Configuration

@Import({ServiceConfig.class, RepositoryConfig.class})

public class SystemTestConfig {

public @Bean DataSource dataSource() { /* return new DataSource */ }

}

public static void main(String[] args) {

ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);

// everything wires up across configuration classes...

TransferService transferService = ctx.getBean(TransferService.class);

transferService.transfer(100.00, "A123", "C456");

}</programlisting></para>

<section xml:id="beans-java-injecting-imported-beans-fq">

<title>Fully-qualifying imported beans for ease of navigation</title>

<para>In the scenario above, using <literal>@Autowired</literal> works

well and provides the desired modularity, but determining exactly

where the autowired bean definitions are declared is still somewhat

ambiguous. For example, as a developer looking at

<literal>ServiceConfig</literal>, how do you know exactly where the

<literal>@Autowired AccountRepository</literal> bean is declared?

It's not explicit in the code, and this may be just fine. Remember

that the <link xl:href="http://www.springsource.com/products/sts"

>SpringSource Tool Suite</link> provides tooling that can render

graphs showing how everything is wired up - that may be all you

need. Also, your Java IDE can easily find all declarations and uses

of the <literal>AccountRepository</literal> type, and will quickly

show you the location of <literal>@Bean</literal> methods that

return that type.</para>

<para>In cases where this ambiguity is not acceptable and you wish to

have direct navigation from within your IDE from one

<literal>@Configuration</literal> class to another, consider

autowiring the configuration classes themselves:

<programlisting language="java">@Configuration

public class ServiceConfig {

private @Autowired RepositoryConfig repositoryConfig;

public @Bean TransferService transferService() {

// navigate 'through' the config class to the @Bean method!

return new TransferServiceImpl(repositoryConfig.accountRepository());

}

}</programlisting>

In the situation above, it is completely explicit where

<literal>AccountRepository</literal> is defined. However,

<literal>ServiceConfig</literal> is now tightly coupled to

<literal>RepositoryConfig</literal>; that's the tradeoff. This tight

coupling can be somewhat mitigated by using interface-based or

abstract class-based <literal>@Configuration</literal> classes.

Consider the following:

<programlisting language="java">@Configuration

public class ServiceConfig {

private @Autowired RepositoryConfig repositoryConfig;

public @Bean TransferService transferService() {

return new TransferServiceImpl(repositoryConfig.accountRepository());

}

}

@Configuration

public interface RepositoryConfig {

@Bean AccountRepository accountRepository();

}

@Configuration

public class DefaultRepositoryConfig implements RepositoryConfig {

public @Bean AccountRepository accountRepository() {

return new JdbcAccountRepository(...);

}

}

@Configuration

@Import({ServiceConfig.class, DefaultRepositoryConfig.class}) // import the concrete config!

public class SystemTestConfig {

public @Bean DataSource dataSource() { /* return DataSource */ }

}

public static void main(String[] args) {

ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);

TransferService transferService = ctx.getBean(TransferService.class);

transferService.transfer(100.00, "A123", "C456");

}</programlisting>

Now <literal>ServiceConfig</literal> is loosely coupled with respect

to the concrete <literal>DefaultRepositoryConfig</literal>, and

built-in IDE tooling is still useful: it will be easy for the

developer to get a type hierarchy of

<literal>RepositoryConfig</literal> implementations. In this way,

navigating <literal>@Configuration</literal> classes and their

dependencies becomes no different than the usual process of

navigating interface-based code.</para>

</section>

</section>

</section>

<section xml:id="beans-java-combining">

<title>Combining Java and XML configuration</title>

<para>Spring's <literal>@Configuration</literal> class support does not

aim to be a 100% complete replacement for Spring XML. Some facilities

such as Spring XML namespaces remain an ideal way to configure the

container. In cases where XML is convenient or necessary, you have a

choice: either instantiate the container in an "XML-centric" way using,

for example, <literal>ClassPathXmlApplicationContext</literal>, or in a

"Java-centric" fashion using

<literal>AnnotationConfigApplicationContext</literal> and the

<literal>@ImportResource</literal> annotation to import XML as

needed.</para>

<section xml:id="beans-java-combining-xml-centric">

<title>XML-centric use of <literal>@Configuration</literal>

classes</title>

<para>It may be preferable to bootstrap the Spring container from XML

and include <literal>@Configuration</literal> classes in an ad-hoc

fashion. For example, in a large existing codebase that uses Spring

XML, it will be easier to create <literal>@Configuration</literal>

classes on an as-needed basis and include them from the existing XML

files. Below you'll find the options for using

<literal>@Configuration</literal> classes in this kind of

"XML-centric" situation.</para>

<section xml:id="beans-java-combining-xml-centric-declare-as-bean">

<title>Declaring <literal>@Configuration</literal> classes as plain

Spring <literal>&lt;bean/&gt;</literal> elements</title>

<para>Remember that <literal>@Configuration</literal> classes are

ultimately just bean definitions in the container. In this example,

we create a <literal>@Configuration</literal> class named

<literal>AppConfig</literal> and include it within

<literal>system-test-config.xml</literal> as a

<literal>&lt;bean/&gt;</literal>definition. Because

<literal>&lt;context:annotation-config/&gt;</literal> is switched

on, the container will recognize the

<literal>@Configuration</literal> annotation, and process the

<literal>@Bean</literal> methods declared in

<literal>AppConfig</literal>

properly.<programlisting language="java">@Configuration

public class AppConfig {

private @Autowired DataSource dataSource;

public @Bean AccountRepository accountRepository() {

return new JdbcAccountRepository(dataSource);

}

public @Bean TransferService transferService() {

return new TransferService(accountRepository());

}

}</programlisting>

<programlisting language="xml"><lineannotation role="listingtitle">system-test-config.xml</lineannotation>

&lt;beans&gt;

&lt;!-- enable processing of annotations such as @Autowired and @Configuration --&gt;

&lt;context:annotation-config/&gt;

&lt;context:property-placeholder location="classpath:/com/acme/jdbc.properties"/&gt;

&lt;bean class="com.acme.AppConfig"/&gt;

&lt;bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"&gt;

&lt;property name="url" value="${jdbc.url}"/&gt;

&lt;property name="username" value="${jdbc.username}"/&gt;

&lt;property name="password" value="${jdbc.password}"/&gt;

&lt;/bean&gt;

&lt;/beans&gt;</programlisting>

<programlisting><lineannotation role="listingtitle">jdbc.properties</lineannotation>

jdbc.url=jdbc:hsqldb:hsql://localhost/xdb

jdbc.username=sa

jdbc.password=</programlisting>

<programlisting language="java">public static void main(String[] args) {

ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml");

TransferService transferService = ctx.getBean(TransferService.class);

// ...

}</programlisting></para>

<note>

<para>In <literal>system-test-config.xml</literal> above, the

<literal>AppConfig&lt;bean/&gt;</literal> does not declare an

<literal>id</literal> element. While it would be acceptable to do

so, it is unnecessary given that no other bean will ever refer to

it, and it is unlikely that it will be explicitly fetched from the

container by name. Likewise with the <literal>DataSource</literal>

bean - it is only ever autowired by type, so an explicit bean id

is not strictly required.</para>

</note>

</section>

<section xml:id="beans-java-combining-xml-centric-component-scan">

<title>Using <literal>&lt;context:component-scan/&gt;</literal> to

pick up <literal>@Configuration</literal> classes</title>

<para>Because <literal>@Configuration</literal> is meta-annotated with

<literal>@Component</literal>,

<literal>@Configuration</literal>-annotated classes are

automatically candidates for component scanning. Using the same

scenario as above, we can redefine

<literal>system-test-config.xml</literal> to take advantage of

component-scanning. Note that in this case, we don't need to

explicitly declare

<literal>&lt;context:annotation-config/&gt;</literal>, because

<literal>&lt;context:component-scan/&gt;</literal> enables all the

same

functionality.<programlisting language="xml"><lineannotation role="listingtitle">system-test-config.xml</lineannotation>

&lt;beans&gt;

&lt;!-- picks up and registers AppConfig as a bean definition --&gt;

&lt;context:component-scan base-package="com.acme"/&gt;

&lt;context:property-placeholder location="classpath:/com/acme/jdbc.properties"/&gt;

&lt;bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"&gt;

&lt;property name="url" value="${jdbc.url}"/&gt;

&lt;property name="username" value="${jdbc.username}"/&gt;

&lt;property name="password" value="${jdbc.password}"/&gt;

&lt;/bean&gt;

&lt;/beans&gt;</programlisting></para>

</section>

</section>

<section xml:id="beans-java-combining-java-centric">

<title><literal>@Configuration</literal> class-centric use of XML with

<literal>@ImportResource</literal></title>

<para>In applications where <literal>@Configuration</literal> classes

are the primary mechanism for configuring the container, it will still

likely be necessary to use at least some XML. In these scenarios,

simply use <literal>@ImportResource</literal> and define only as much

XML as is needed. Doing so achieves a "Java-centric" approach to

configuring the container and keeps XML to a bare minimum.

<programlisting language="java">@Configuration

@ImportResource("classpath:/com/acme/properties-config.xml")

public class AppConfig {

private @Value("${jdbc.url}") String url;

private @Value("${jdbc.username}") String username;

private @Value("${jdbc.password}") String password;

public @Bean DataSource dataSource() {

return new DriverManagerDataSource(url, username, password);

}

}</programlisting>

<programlisting language="xml"><lineannotation role="listingtitle">properties-config.xml</lineannotation>

&lt;beans&gt;

&lt;context:property-placeholder location="classpath:/com/acme/jdbc.properties"/&gt;

&lt;/beans&gt;</programlisting>

<programlisting><lineannotation role="listingtitle">jdbc.properties</lineannotation>

jdbc.url=jdbc:hsqldb:hsql://localhost/xdb

jdbc.username=sa

jdbc.password=</programlisting>

<programlisting language="java">public static void main(String[] args) {

ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);

TransferService transferService = ctx.getBean(TransferService.class);

// ...

}</programlisting></para>

</section>

</section>

</section>

</section>