@Value 春季启动测试中的“无法解析占位符”

我想对Spring-boot进行Junit测试，如下所示：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = {ApplicationTest.class})
public class TestOnSpring {
    @Value("${app.name}")
    private String appName;

    @Test
    public void testValue(){
        System.out.println(appName);
    }
}

和 ApplicationTest.java像这样

@ComponentScan("org.nerve.jiepu")
@EnableAutoConfiguration()
public class ApplicationTest {

    public static void main(String[] args) {
        SpringApplication.run(ApplicationTest.class, args);
    }
}

和我的POM像这样：

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.3.0.BUILD-SNAPSHOT</version>
    </parent>

当我运行测试时，我得到了下面的错误信息

Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'app.name' in string value "${app.name}"
    at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:174)
    at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:126)
    at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:204)
    at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:178)
    at org.springframework.context.support.PropertySourcesPlaceholderConfigurer$2.resolveStringValue(PropertySourcesPlaceholderConfigurer.java:172)
    at org.springframework.beans.factory.support.AbstractBeanFactory.resolveEmbeddedValue(AbstractBeanFactory.java:807)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1027)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1014)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:543)
    ... 31 more

但是当我像普通的Java应用程序一样运行这个应用程序时

@SpringBootApplication
public class Application {

    public static void main(String[] args){
        SpringApplication.run(Application.class, args);
    }
}

它工作得很好！

这是怎么回事？我应该如何使用Spring-boot进行junit测试？多谢！