使用springboot配置多数据源时，碰到一个坑，搜了一下网上的解决方案，都是说其中一个数据源要加注解@Primary，才能启动，但是都没有讲清楚原因。
stackoverflow上看到类似一个说清楚了，记录一下。

springboot文档有：

Spring JDBC has a DataSource initializer feature. Spring Boot enables it by default and loads SQL from the standard locations schema.sql and data.sql (in the root of the classpath).

也就是springboot对数据源有一个初始化的机制，确保可以正常访问配置的数据源，单一数据源没有问题，如果配置了多个数据源，在初始化org.springframework.boot.autoconfigure.jdbc.DataSourceInitializer时有以下逻辑：

@PostConstruct
public void init() {
    if (!this.properties.isInitialize()) {
        logger.debug("Initialization disabled (not running DDL scripts)");
        return;
    }
    if (this.applicationContext.getBeanNamesForType(DataSource.class, false, false).length > 0) {
        ==> this.dataSource = this.applicationContext.getBean(DataSource.class);
    }
    if (this.dataSource == null) {
        logger.debug("No DataSource found so not initializing");
        return;
    }
    runSchemaScripts();
}

问题就出在applicationContext.getBean(DataSource.class)这一步，看方法的定义：

T getBean(Class requiredType) throws BeansException Return the bean instance that uniquely matches the given object type, if any. This method goes into ListableBeanFactory by-type lookup territory but may also be translated into a conventional by-name lookup based on the name of the given type. For more extensive retrieval operations across sets of beans, use ListableBeanFactory and/or BeanFactoryUtils.
Parameters: requiredType - type the bean must match; can be an interface or superclass. null is disallowed.
Returns:
an instance of the single bean matching the required type
Throws:
NoSuchBeanDefinitionException - if no bean of the given type was found ==> NoUniqueBeanDefinitionException - if more than one bean of the given type was found BeansException - if the bean could not be created

因此，如果有多个bean符合，会抛出异常，导致启动失败。
解决方法也很简单：

• 如果你需要初始化数据源这个功能，那么指定其中一个数据源@Primary
• 如果不需要这个功能，可以配置springboot不作数据源初始化检查，在application.properties配置spring.datasource.initialize=false

参考

https://stackoverflow.com/questions/44757388/why-qualifier-not-work/44776477?noredirect=1#comment76540296_44776477