有没有办法可以像有条件地声明一个Spring bean:
<bean class="path.to.the.class.MyClass" if="${1+2=3}" />
这将是有用的,而不必使用配置文件。 我没有考虑具体的用例,但它来找我。
答案 0 :(得分:27)
您可以使用Spring 4 中的 @Conditional 或来自Spring 引导的 @ConditionalOnProperty 。
1.使用Spring4(仅),
如果您不使用spring 启动,则此可能过度。
首先,创建一个Condition类,ConditionContext可以在其中访问Environment:
public class MyCondition implements Condition {
@Override
public boolean matches(ConditionContext context,
AnnotatedTypeMetadata metadata) {
Environment env = context.getEnvironment();
return null != env
&& "true".equals(env.getProperty("server.host"));
}
}
然后注释你的bean:
@Bean
@Conditional(MyCondition.class)
public ObservationWebSocketClient observationWebSocketClient(){
//return bean
}
@ConditionalOnProperty(name="server.host", havingValue="localhost")
在你的abcd.properties文件中,
server.host=localhost
答案 1 :(得分:5)
我有一个片段用于这样的事情。它检查在注释中设置的属性的值,因此您可以使用
之类的内容@ConditionalOnProperty(value="usenew", on=false, propertiesBeanName="myprops")
@Service("service")
public class oldService implements ServiceFunction{
// some old implementation of the service function.
}
它甚至允许您定义具有相同名称的不同bean:
@ConditionalOnProperty(value="usenew", on=true, propertiesBeanName="myprops")
@Service("service")
public class newService implements ServiceFunction{
// some new implementation of the service function.
}
这两个可以同时声明,允许你有一个"service"
命名bean,具有不同的实现,具体取决于属性是打开还是关闭......
它自己的代码片段:
/**
* Components annotated with ConditionalOnProperty will be registered in the spring context depending on the value of a
* property defined in the propertiesBeanName properties Bean.
*/
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {
/**
* The name of the property. If not found, it will evaluate to false.
*/
String value();
/**
* if the properties value should be true (default) or false
*/
boolean on() default true;
/**
* Name of the bean containing the properties.
*/
String propertiesBeanName();
}
/**
* Condition that matches on the value of a property.
*
* @see ConditionalOnProperty
*/
class OnPropertyCondition implements ConfigurationCondition {
private static final Logger LOG = LoggerFactory.getLogger(OnPropertyCondition.class);
@Override
public boolean matches(final ConditionContext context, final AnnotatedTypeMetadata metadata) {
final Map attributes = metadata.getAnnotationAttributes(ConditionalOnProperty.class.getName());
final String propertyName = (String) attributes.get("value");
final String propertiesBeanName = (String) attributes.get("propertiesBeanName");
final boolean propertyDesiredValue = (boolean) attributes.get("on");
// for some reason, we cannot use the environment here, hence we get the actual properties bean instead.
Properties props = context.getBeanFactory().getBean(propertiesBeanName, Properties.class);
final boolean propValue = parseBoolean(props.getProperty(propertyName, Boolean.toString(false)));
LOG.info("Property '{}' resolved to {}, desired: {}", new Object[] { propertyName, propValue, "" + propertyDesiredValue });
return propValue == propertyDesiredValue;
}
/**
* Set the registration to REGISTER, else it is handled during the parsing of the configuration file
* and we have no guarantee that the properties bean is loaded/exposed yet
*/
@Override
public ConfigurationPhase getConfigurationPhase() {
return ConfigurationPhase.REGISTER_BEAN;
}
}