DataSourceProperties 有一系列的determineXXX方法，Spring的DataSourceAutoConfiguration会利用这个去检测类路径下的H2这种可以内嵌运行的数据库，所以在不提供spring.datasource.xxx属性时，DataSource也是会被自动配置的：

public String determineUrl() {
		if (StringUtils.hasText(this.url)) {
			return this.url;
		}
		String url = this.embeddedDatabaseConnection.getUrl(determineDatabaseName());
		if (!StringUtils.hasText(url)) {
			throw new DataSourceBeanCreationException(this.embeddedDatabaseConnection,
					this.environment, "url");
		}
		return url;
	}

	public String determineDriverClassName() {
		if (StringUtils.hasText(this.driverClassName)) {
			Assert.state(driverClassIsLoadable(),
					"Cannot load driver class: " + this.driverClassName);
			return this.driverClassName;
		}
		String driverClassName = null;

		if (StringUtils.hasText(this.url)) {
			driverClassName = DatabaseDriver.fromJdbcUrl(this.url).getDriverClassName();
		}

		if (!StringUtils.hasText(driverClassName)) {
			driverClassName = this.embeddedDatabaseConnection.getDriverClassName();
		}

		if (!StringUtils.hasText(driverClassName)) {
			throw new DataSourceBeanCreationException(this.embeddedDatabaseConnection,
					this.environment, "driver class");
		}
		return driverClassName;
	}

Druid好像没有利用这个，而是直接拿的属性，是不是应该也利用一下这个？