Spring 4.0新功能：@Conditional注解详细介绍

<bean id="flag">
 <constructor-arg value="#{systemProperties['system.propery.flag'] ?: false }" />
</bean>
<bean id="bean">
 <property name="property" value="#{ flag ? 'yes' : 'no' }"/>
</bean>

<!-- default configuration - will be loaded if no profile is specified -->
<!-- This will only work if it's put at the end of the configuration file -->
<!-- so no bean definitions after that -->
<beans profile="default">
  <import resource="classpath:default.xml" />
</beans>
<!-- some other profile -->
<beans profile="otherProfile">
 <import resource="classpath:other-profile.xml" />
</beans>

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE, ElementType.METHOD)
public @interface Conditional{
 Class <!--?extends Condition-->[] value();
}

public interface Condition{
/** Determine if the condition matches.
* @param context the condition context
* @param metadata meta-data of the {@link AnnotationMetadata class} or
* {@link Method method} being checked.
* @return {@code true} if the condition matches and the component can be registered
* or {@code false} to veto registration.
*/
boolean matches(ConditionContext context, AnnotatedTypeMedata metadata);
}

public class SystemPropertyCondition implements Condition {
 @Override
 public boolean matches(ConditionContext context, AnnotatedTypeMetadata   metadata) {
  return (System.getProperty("flag") != null);
 }
}

class SystemPropertyAbsentCondition implements Condition {
 @Override
 public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
  return (System.getProperty("flag") == null);
 }
}

@Bean
@Conditional(SystemPropertyCondition.class)
public SampleService service1() {
 return new SampleServiceImpl1();
}

@Bean
@Conditional(SystemPropertyAbsentCondition.class)
public SampleService service2() {
 return new SampleServiceImpl2();
}