@Configuration
@ConditionalOnClass(JAXRSServerFactoryBean.class)
@ConditionalOnExpression("‘${cxf.jaxrs.component-scan}‘==‘true‘ && ‘${cxf.jaxrs.classes-scan}‘!=‘true‘")
@Import(SpringComponentScanServer.class)
protected static class JaxRsComponentConfiguration {
}
/**
* Configuration annotation for a conditional element that depends on the value of a SpEL
* expression.
*
* @author Dave Syer
*/
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnExpressionCondition.class)
public @interface ConditionalOnExpression {
/**
* The SpEL expression to evaluate. Expression should return {@code true} if the
* condition passes or {@code false} if it fails.
* @return the SpEL expression
*/
String value() default "true";
}
/*
* Copyright 2012-2016 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.boot.autoconfigure.condition;
import org.springframework.beans.factory.config.BeanExpressionContext;
import org.springframework.beans.factory.config.BeanExpressionResolver;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.context.expression.StandardBeanExpressionResolver;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.core.type.AnnotatedTypeMetadata;
/**
* A Condition that evaluates a SpEL expression.
*
* @author Dave Syer
* @see ConditionalOnExpression
*/
@Order(Ordered.LOWEST_PRECEDENCE - 20)
class OnExpressionCondition extends SpringBootCondition {
@Override
public ConditionOutcome getMatchOutcome(ConditionContext context,
AnnotatedTypeMetadata metadata) {
String expression = (String) metadata
.getAnnotationAttributes(ConditionalOnExpression.class.getName())
.get("value");
expression = wrapIfNecessary(expression);
String rawExpression = expression;
expression = context.getEnvironment().resolvePlaceholders(expression);
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
BeanExpressionResolver resolver = (beanFactory != null)
? beanFactory.getBeanExpressionResolver() : null;
BeanExpressionContext expressionContext = (beanFactory != null)
? new BeanExpressionContext(beanFactory, null) : null;
if (resolver == null) {
resolver = new StandardBeanExpressionResolver();
}
boolean result = (Boolean) resolver.evaluate(expression, expressionContext);
return new ConditionOutcome(result, ConditionMessage
.forCondition(ConditionalOnExpression.class, "(" + rawExpression + ")")
.resultedIn(result));
}
/**
* Allow user to provide bare expression with no ‘#{}‘ wrapper.
* @param expression source expression
* @return wrapped expression
*/
private String wrapIfNecessary(String expression) {
if (!expression.startsWith("#{")) {
return "#{" + expression + "}";
}
return expression;
}
}
了解注解表达式式怎么计算的。
原文：http://www.cnblogs.com/hudechun/p/7041312.html