Springboot2.5.x版本之自动创建（H2/DERBY/HSQL）数据源源码分析-yellowcong

场景：当我们没有配置mysql，postgresql等数据源的时候，pom.xml里面引入了H2/DERBY/HSQL jar包，也没有配置连接，却有数据源创建的情况。
springboot启动的第一步
1.DataSourceAutoConfiguration 配置类启动
2.DataSourceProperties 配置类自动探测datasource的配置项
3.EmbeddedDatabaseConnection 做检测，自动生成数据库连接
4.DataSourceProperties.initializeDataSourceBuilder 创建数据源连接
5.DataSourceConfiguration 根据不同的数据源来创建连接池

1. 数据源配置启动类

找到 spring-boot-autoconfigure-2.5.x.jar 这个jar包，里面的META-INF/spring.factories 文件，有各个自动化配置的入口，数据库入口查找org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration 这个类

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
...
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,
...

2. DataSourceAutoConfiguration类核心代码

数据库自动数据库连接池配置的前提是，项目中没有别的模块创建了DataSource （ @ConditionalOnMissingBean({DataSource.class, XADataSource.class}) 这句代码就是做限制的）

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure.jdbc;


@Configuration(
    proxyBeanMethods = false
)
@ConditionalOnClass({DataSource.class, EmbeddedDatabaseType.class})
@ConditionalOnMissingBean(
    type = {"io.r2dbc.spi.ConnectionFactory"}
)
//导入DataSourceProperties 的配置 ，实际数据库参数都是在这个类做的
@EnableConfigurationProperties({DataSourceProperties.class})
@Import({DataSourcePoolMetadataProvidersConfiguration.class, InitializationSpecificCredentialsDataSourceInitializationConfiguration.class, SharedCredentialsDataSourceInitializationConfiguration.class})
public class DataSourceAutoConfiguration {
    public DataSourceAutoConfiguration() {
    }

    //.... 省略代码

   //当没有手动创建DataSource的情况，就会走默认的数据源连接池
   //目前支持的数据源连接池
   //Hikari，Tomcat，Dbcp2，OracleUcp，Generic，DataSourceJmxConfiguration 
    @Configuration(
        proxyBeanMethods = false
    )
    @Conditional({DataSourceAutoConfiguration.PooledDataSourceCondition.class})
    @ConditionalOnMissingBean({DataSource.class, XADataSource.class})
    @Import({Hikari.class, Tomcat.class, Dbcp2.class, OracleUcp.class, Generic.class, DataSourceJmxConfiguration.class})
    protected static class PooledDataSourceConfiguration {
        protected PooledDataSourceConfiguration() {
        }
    }

    @Configuration(
        proxyBeanMethods = false
    )
    @Conditional({DataSourceAutoConfiguration.EmbeddedDatabaseCondition.class})
    @ConditionalOnMissingBean({DataSource.class, XADataSource.class})
    @Import({EmbeddedDataSourceConfiguration.class})
    protected static class EmbeddedDatabaseConfiguration {
        protected EmbeddedDatabaseConfiguration() {
        }
    }
    //.... 省略代码
}

3. DataSourceProperties 数据源配置

注入配置的数据库连接池配置项，然后设定对应的EmbeddedDatabaseConnection ，其中initializeDataSourceBuilder的调用方是DataSourceConfiguration.createDataSource ,在最后创建数据库连接池的时候调用。

数据库探测规则：1.优先使用用户配置的（spring.datasource.url）url选项，2.如果有户没有配置，就会使用EmbeddedDatabaseConnection 来探测，默认会扫描jar（H2/DERBY/HSQL）包，取第一个为默认值。

@ConfigurationProperties(
    prefix = "spring.datasource"
)
public class DataSourceProperties implements BeanClassLoaderAware, InitializingBean {
    //.... 省略代码
    //设置 EmbeddedDatabaseConnection的检查类
    public void afterPropertiesSet() throws Exception {
        if (this.embeddedDatabaseConnection == null) {
            this.embeddedDatabaseConnection = EmbeddedDatabaseConnection.get(this.classLoader);
        }

    }

    // 创建连接池，调用了this.determineDriverClassName 探测数据库连接地址
    public DataSourceBuilder<?> initializeDataSourceBuilder() {
        return DataSourceBuilder.create(this.getClassLoader()).type(this.getType()).driverClassName(this.determineDriverClassName()).url(this.determineUrl()).username(this.determineUsername()).password(this.determinePassword());
    }

	//探测连接地址，默认会根据url进行分析探测，如果没有的情况，就会走EmbeddedDatabaseConnection 来获取内置数据库
    public String determineDriverClassName() {
        if (StringUtils.hasText(this.driverClassName)) {
            Assert.state(this.driverClassIsLoadable(), () -> {
                return "Cannot load driver class: " + this.driverClassName;
            });
            return this.driverClassName;
        } else {
            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 DataSourceProperties.DataSourceBeanCreationException("Failed to determine a suitable driver class", this, this.embeddedDatabaseConnection);
            } else {
                return driverClassName;
            }
        }
    }
    //.... 省略代码
}

4. EmbeddedDatabaseConnection 内置数据库探测

通过EmbeddedDatabaseConnection ，来探测数据库信息，会根据大家有没有（h2/derby/hsql）这些jar包，来确认是。

public enum EmbeddedDatabaseConnection {
    NONE((EmbeddedDatabaseType)null, (String)null, (String)null, (url) -> {
        return false;
    }),
    H2(EmbeddedDatabaseType.H2, DatabaseDriver.H2.getDriverClassName(), "jdbc:h2:mem:%s;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=FALSE", (url) -> {
        return url.contains(":h2:mem");
    }),
    DERBY(EmbeddedDatabaseType.DERBY, DatabaseDriver.DERBY.getDriverClassName(), "jdbc:derby:memory:%s;create=true", (url) -> {
        return true;
    }),
    /** @deprecated */
    @Deprecated
    HSQL(EmbeddedDatabaseType.HSQL, DatabaseDriver.HSQLDB.getDriverClassName(), "org.hsqldb.jdbcDriver", "jdbc:hsqldb:mem:%s", (url) -> {
        return url.contains(":hsqldb:mem:");
    }),
    HSQLDB(EmbeddedDatabaseType.HSQL, DatabaseDriver.HSQLDB.getDriverClassName(), "org.hsqldb.jdbcDriver", "jdbc:hsqldb:mem:%s", (url) -> {
        return url.contains(":hsqldb:mem:");
    });
	
	//查找H2, HSQLDB, DERBY 中的driverClass 第一个
    private static EmbeddedDatabaseConnection getEmbeddedDatabaseConnection(String driverClass) {
        return (EmbeddedDatabaseConnection)Stream.of(H2, HSQLDB, DERBY).filter((connection) -> {
            return connection.isDriverCompatible(driverClass);
        }).findFirst().orElse(NONE);
    }
    //.... 省略代码
 }   

5. DataSourceConfiguration 数据库连接池配置类

里面包含了多种数据库连接池Hikari，Tomcat，Dbcp2，OracleUcp，Generic，DataSourceJmxConfiguration ，以Hikari举例 说明。

上面DataSourceAutoConfiguration 类中的PooledDataSourceConfiguration 配置内部类，有个注解@Import({Hikari.class, Tomcat.class, Dbcp2.class, OracleUcp.class, Generic.class, DataSourceJmxConfiguration.class}) ， 就会使用DataSourceConfiguration 中的数据库连接池配置信息。

abstract class DataSourceConfiguration {
    //.... 省略代码
    //创建数据库连接池的核心方法
    protected static <T> T createDataSource(DataSourceProperties properties, Class<? extends DataSource> type) {
        return properties.initializeDataSourceBuilder().type(type).build();
    }
    
    @Configuration(
        proxyBeanMethods = false
    )
    @ConditionalOnClass({HikariDataSource.class})
    @ConditionalOnMissingBean({DataSource.class})
    @ConditionalOnProperty(
        name = {"spring.datasource.type"},
        havingValue = "com.zaxxer.hikari.HikariDataSource",
        matchIfMissing = true
    )
    static class Hikari {
        Hikari() {
        }

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

            return dataSource;
        }
    }
    //.... 省略代码    
}

附录

数据库测试连接配置

---
spring:
  profiles:
    active: db-h2
  #应用唯一标识
  application:
    name: BITSFLOW-CONSOLE
  datasource:
    type: com.zaxxer.hikari.HikariDataSource
    hikari:
      maximum-pool-size: 20
      connection-timeout: 300000
      max-life-time: 1800000
      idle-timeout: 0
      pool-name: "CONN-POOL"

---      
spring:
  h2:
    console:
      enabled: true
      path: /h2-console
      settings:
        trace: false
        web-allow-others: true
  config:
    activate:
      on-profile: db-h2
  datasource:
    url: jdbc:h2:mem:yc_test;MODE=MySql;DB_CLOSE_ON_EXIT=FALSE;DB_CLOSE_DELAY=-1
    username: sa
    password:
    driver-class-name: org.h2.Driver
  jpa:
    properties:
      hibernate:
        dialect: org.hibernate.dialect.H2Dialect
---
spring:
  config:
    activate:
      on-profile: db-postgresql
  datasource:
    url: jdbc:postgresql://127.0.0.1:5432/yc_test
    username: postgres
    password: demo
    driver-class-name: org.postgresql.Driver
  jpa:
    properties:
      hibernate:
        dialect: org.hibernate.dialect.PostgreSQLDialect

---
spring:
  config:
    activate:
      on-profile: db-mysql
  datasource:
    url: jdbc:mysql://127.0.0.1:3306/yc_test?useUnicode=true&characterEncoding=utf-8&useSSL=false&CreateDatabaseIfNotExists=true&allowMultiQueries=true
    username: root
    password: 'root'
    driver-class-name: com.mysql.cj.jdbc.Driver
  jpa:
    properties:
      hibernate:
        dialect: org.hibernate.dialect.MySQLDialect