spring4引入新注解@Conditional，可以使用在Bean注解的类和方法上。

示例：

@Service("loginService")@Conditional(value = LoginServiceCondition.class)public class MockLoginServiceImpl implements LoginService{	private static Logger logger = LoggerFactory.getLogger(MockLoginServiceImpl.class);	@Override	public BaseResult validateUserLogin(User user) {		logger.info("[MOCK数据]");		BaseResult result = new BaseResult();		result.setSuccess(Boolean.FALSE);		return result;	}	@Override	public BaseResult regeditUser(User user) {		return null;	}}

如上 @Conditional(value = LoginServiceCondition.class)，就可以通过LoginServiceCondition这个类进行判断是否加载这个bean。

@Retention(RetentionPolicy.RUNTIME)@Target({java.lang.annotation.ElementType.TYPE, java.lang.annotation.ElementType.METHOD})public @interface Conditional{  public abstract Class
    [] value();}

通过spring源码可以看出可以支持多个类进行判断。

public class LoginServiceCondition implements Condition{	@SuppressWarnings("unused")	@Override	public boolean matches(ConditionContext context, AnnotatedTypeMetadata anno) {		//获取环境变量		Environment env = context.getEnvironment();		//检查bean定义		BeanDefinitionRegistry beanDefinitionRegistry = context.getRegistry();		//检查bean是否存在		ConfigurableListableBeanFactory configurableListableBeanFactory = context.getBeanFactory();		//获取ResourceLoader所加载的资源		ResourceLoader resourceLoader =context.getResourceLoader();		//检查ClassLoader加载并检查类是否存在		ClassLoader classLoader = context.getClassLoader();				//判断bean是否还有其他注解		boolean isHaveOtherAnno = anno.isAnnotated("service");		//获取特定类型注解的属性		Map
    
      map1 = anno.getAnnotationAttributes("name");		Map
     
       map2 = anno.getAnnotationAttributes("name", false);		MultiValueMap
      
        map3 = anno.getAllAnnotationAttributes("name");		MultiValueMap
       
         map4 = anno.getAllAnnotationAttributes("name", false);		return !env.containsProperty("mock");	}	}
       
      
     
    

可以发现，注解中的value配置的为这个类，实现了org.springframework.context.annotation.Condition这个接口，该接口中只有一个方法matches，通过这个方法返回boolean的值判断@conditional是否生效。

注：@Profile的实现也是通过Conditional注解实现，源码如下：

@Retention(RetentionPolicy.RUNTIME)@Target({java.lang.annotation.ElementType.TYPE, java.lang.annotation.ElementType.METHOD})@Documented@Conditional({ProfileCondition.class})public @interface Profile{  public abstract String[] value();}/** 激活profile的判断类  **/class ProfileCondition implements Condition {	public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {		if (context.getEnvironment() != null) {			MultiValueMap attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());			if (attrs != null) {				for (Iterator localIterator = ((List) attrs.get("value")).iterator(); localIterator.hasNext();) {					Object value = localIterator.next();					if (context.getEnvironment().acceptsProfiles((String[]) (String[]) value)) {						return true;					}				}				return false;			}		}		return true;	}}