我们可以有条件地申报春豆吗？

Question

有没有办法我们可以有条件地声明一个Spring bean:

<bean class="path.to.the.class.MyClass" if="${1+2=3}" />

它将是有用的,而不必使用配置文件.我没有考虑具体的用例,但它来到我身边.

Answer 1

您可以使用@Conditional从春季4或@ConditionalOnProperty从春天启动.

• 1.使用Spring4(仅限),

  如果你不使用弹簧靴,这可能是矫枉过正.

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
}

• 2.使用Spring Boot:

@ConditionalOnProperty(name="server.host", havingValue="localhost")

在你的abcd.properties文件中,

server.host=localhost

Answer 2

我有这样一个片段.它检查在注释中设置的属性的值,因此您可以使用类似的东西

@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"具有不同实现的命名bean,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;
    }
}