springboot（2.0以上） --数据源切换时报错

在进行数据源切换时spring.datasource.type类型根据源码所给的默认值修改后依然报错

• 先看源码：标色部分 ，  就是springboot所给的数据源 ，  正常来说只要在配置文件中修改type值就可以  ，但是我发现不行 依然报错：

@ConditionalOnClass(org.apache.tomcat.jdbc.pool.DataSource.class)
    @ConditionalOnProperty(name = "spring.datasource.type", havingValue = "org.apache.tomcat.jdbc.pool.DataSource", matchIfMissing = true)
    static class Tomcat extends DataSourceConfiguration {

        @Bean
        @ConfigurationProperties(prefix = "spring.datasource.tomcat")
        public org.apache.tomcat.jdbc.pool.DataSource dataSource(
                DataSourceProperties properties) {
            org.apache.tomcat.jdbc.pool.DataSource dataSource = createDataSource(
                    properties, org.apache.tomcat.jdbc.pool.DataSource.class);
            DatabaseDriver databaseDriver = DatabaseDriver
                    .fromJdbcUrl(properties.determineUrl());
            String validationQuery = databaseDriver.getValidationQuery();
            if (validationQuery != null) {
                dataSource.setTestOnBorrow(true);
                dataSource.setValidationQuery(validationQuery);
            }
            return dataSource;
        }

    }

    /**
     * Hikari DataSource configuration.
     */
    @ConditionalOnClass(HikariDataSource.class)
    @ConditionalOnProperty(name = "spring.datasource.type", havingValue = "com.zaxxer.hikari.HikariDataSource", matchIfMissing = true)
    static class Hikari extends DataSourceConfiguration {

        @Bean
        @ConfigurationProperties(prefix = "spring.datasource.hikari")
        public HikariDataSource dataSource(DataSourceProperties properties) {
            HikariDataSource dataSource = createDataSource(properties,
                    HikariDataSource.class);
            if (StringUtils.hasText(properties.getName())) {
                dataSource.setPoolName(properties.getName());
            }
            return dataSource;
        }

    }

    /**
     * DBCP DataSource configuration.
     */
    @ConditionalOnClass(org.apache.commons.dbcp2.BasicDataSource.class)
    @ConditionalOnProperty(name = "spring.datasource.type", havingValue = "org.apache.commons.dbcp2.BasicDataSource", matchIfMissing = true)
    static class Dbcp2 extends DataSourceConfiguration {

        @Bean
        @ConfigurationProperties(prefix = "spring.datasource.dbcp2")
        public org.apache.commons.dbcp2.BasicDataSource dataSource(
                DataSourceProperties properties) {
            return createDataSource(properties,
                    org.apache.commons.dbcp2.BasicDataSource.class);
        }

    }

• 错误信息：这个错好多人给出的答案是让编辑器自动提示  而不自己写 就不会出现（显然不行的），value值是springboot源码上自带的，因此也不会错，但是为什么会报错呢？

Description:

Failed to bind properties under 'spring.datasource.type' to java.lang.Class<javax.sql.DataSource>:

    Property: spring.datasource.type
    Value: org.apache.commons.dbcp2.BasicDataSource
    Origin: class path resource [application.yml]:7:11
    Reason: No converter found capable of converting from type [java.lang.String] to type [java.lang.Class<javax.sql.DataSource>]

Action:

Update your application's configuration

• 原因：

在导入druid数据源时，也就是外部数据源，我们必须导入相关依赖，因此我尝试将springboot默认数据源的相关依赖也导入

• 引入（maven）：

<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-dbcp2 -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-dbcp2</artifactId>
    <version>2.2.0</version>
</dependency>

• 结果：看控制台输出---nice（解决）

• 还有就是springboot2.0.2.RELEASE版本默认的数据源并不是org.apache.tomcat.jdbc.pool.DataSource，而是：com.zaxxer.hikari.HikariDataSource

因此除了com.zaxxer.hikari.HikariDataSource不用额外引入依赖，其他的数据源都要引入依赖

• http://mvnrepository.com：在这个网址中输入要引入的依赖，粘贴到pom中就ok了