我們可以有條件地申報春豆嗎？

Question

有沒有我們可以聲明一個Spring bean的方式有條件地想：

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

這將是有益的，而不必使用配置文件。 我沒有具體的使用案例，但它找到了我。

Answer 1

您可以從春引導使用@Conditional從春天或@ConditionalOnProperty。

• 1.使用Spring4（僅），如果你使用的是春天啓動不

  ，這可能矯枉過正。

首先，創造條件類，其中ConditionContext訪問環境：

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")); 
    } 
} 

然後註釋你的bean：

@Bean 
@Conditional(MyCondition.class) 
public ObservationWebSocketClient observationWebSocketClient(){ 
    //return bean 
} 

• 2，採用彈簧靴子：

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

而在你abcd.properties文件，

server.host=localhost 

Answer 2

我有這樣的事情的一個片段。它檢查這是在註釋設置屬性的值，所以你可以使用的東西像

@ConditionalOnProperty(value="usenew", on=false, propertiesBeanName="myprops") 
@Service("service") 
public class oldService implements ServiceFunction{ 
    // some old implementation of the service function. 
} 

它甚至可以讓你用相同的名字定義不同的豆類：

@ConditionalOnProperty(value="usenew", on=true, propertiesBeanName="myprops") 
@Service("service") 
public class newService implements ServiceFunction{ 
    // some new implementation of the service function. 
} 

這兩個可以同時聲明，讓您有這取決於屬性是否是開啓或關閉不同的實現一個名爲"service" ...豆

的片斷爲它本身：

/** 
* 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; 
    } 
}