1. 复现问题

今天使用@SpringBootTest测试发送邮件，如下代码所示：

@SpringBootTest
class SuperJsonManagerApplicationTests {

	@Test
	void contextLoads() {
	}

	@Autowired
	private MailConfig mailConfig;

	@Test
	public void testSendMail() {
    	MailPo mailPo=new MailPo();
    	mailPo.setTo(new String[];
    	mailPo.setSubject("测试发送邮件");
    	mailPo.setText("这封邮件能否发送出去，验证码为: <code>1233</code>");
    	mailConfig.sendSimpleMail(mailPo);
    }
}

但启动时，报出如下错误：

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mailConfig': Injection of autowired dependencies failed; nested exception is java.lang.IllegalArgumentException: Could not resolve placeholder 'spring.mail.username' in value "${spring.mail.username}"
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:405)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1413)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:601)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:758)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:438)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:337)
	at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:123)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:99)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)
	... 67 more
Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'spring.mail.username' in value "${spring.mail.username}"
	at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:180)
	at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:126)
	at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:239)
	at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:210)
	at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.lambda$processProperties$0(PropertySourcesPlaceholderConfigurer.java:175)
	at org.springframework.beans.factory.support.AbstractBeanFactory.resolveEmbeddedValue(AbstractBeanFactory.java:936)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)
	... 83 more

即nested exception is java.lang.IllegalArgumentException: Could not resolve placeholder 'spring.mail.username' in value "${spring.mail.username}"该问题。

2. 分析问题

上述问题是无法解析值${spring.mail.username}中的占位符spring.mail.username。

2.1 检查yml配置

如下是在application-local.yml文件中配置邮件信息：

spring:
 #发送邮箱配置
  mail:
    # 邮件服务器地址
    host: ""
    # 你的邮箱地址
    username: "你的邮箱"
    # 授权码
    password: "你的授权码"
    # 编码格式
    default-encoding: utf-8
    # 协议
    protocol: smtp
    properties:
      mail:
        smtp:
          auth: true
          starttls:
            enable: true
            required: true

没有发现问题

2.2 检查依赖

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-mail</artifactId>
</dependency>

没有发现依赖存在问题

2.3 检查启动

yml配置文件和依赖都没有问题，启动项目看没有问题，如下图所示：

通过这种方式，能正常启动项目，只能说明@SpringBootTest注解有问题。

3. 解决问题

既然知道是@SpringBootTest注解有问题，导致无法读取yml中的配置文件，那么，就要修改@SpringBootTest注解，如下所示：

// 指定需要加载的配置文件，比如application-local.yml
@ActiveProfiles("local")
// 修改启动环境
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class SuperJsonManagerApplicationTests {

    @Test
    void contextLoads() {
    }

    @Autowired
    private MailConfig mailConfig;

    @Ignore
    public void testSendMail() {
        MailPo mailPo=new MailPo();
        mailPo.setTo(new String[]{"144xxx@"});
        mailPo.setSubject("测试发送邮件");
        mailPo.setText("这封邮件能否发送出去，验证码为: <code>1233</code>");
        mailConfig.sendSimpleMail(mailPo);
    }
}

重新测试发送邮件方法，此时可以正常收到邮件，如下图所示：

4. 问题总结

出现这种问题，一般是没有正确加载配置文件造成的，比如application.yml 或者application.properties文件，导致spring无法读取配置文件中的数据，而无法使用@Value注解初始化数据。