spring boot 2.x連接數據庫可以不配置驅動的原因 源碼解析

• 爲什麼Springboot2.x 可以不配置連接數據庫的驅動，仍然可以成功連接數據庫。如下所示：

spring:
  datasource:
    url: jdbc:mysql://127.0.0.1:3306/jdbc?serverTimezone=GMT%2B8&useSSL=false
    username: 'root'
    password: '!QAZ2wsx'
#    driver-class-name: com.mysql.cj.jdbc.Driver

driver-class-name 註釋掉了，當然也可以不註釋

• DataSourceBuilder類是構建數據源的一個類（源碼放在最下邊了），裏面有一個方法。方法代碼如下：

 private void maybeGetDriverClassName() {
        if (!this.properties.containsKey("driverClassName") && this.properties.containsKey("url")) {
            String url = (String)this.properties.get("url");
            String driverClass = DatabaseDriver.fromJdbcUrl(url).getDriverClassName();
            this.properties.put("driverClassName", driverClass);
        }
    }

在這段代碼裏：
properties是一個map，裏面存入了關於dataSource 的屬性值
可以看到 如果配置了URL 而沒有配置 driver-class-name 會自動查找最可能的驅動類是什麼。
詳細內容還需要查看相關的類

• 完整的DataSourceBuilder類源碼

public final class DataSourceBuilder<T extends DataSource> {
    private static final String[] DATA_SOURCE_TYPE_NAMES = new String[]{"com.zaxxer.hikari.HikariDataSource", "org.apache.tomcat.jdbc.pool.DataSource", "org.apache.commons.dbcp2.BasicDataSource"};
    private Class<? extends DataSource> type;
    private ClassLoader classLoader;
    private Map<String, String> properties = new HashMap();

    public static DataSourceBuilder<?> create() {
        return new DataSourceBuilder((ClassLoader)null);
    }

    public static DataSourceBuilder<?> create(ClassLoader classLoader) {
        return new DataSourceBuilder(classLoader);
    }

    private DataSourceBuilder(ClassLoader classLoader) {
        this.classLoader = classLoader;
    }

    public T build() {
        Class<? extends DataSource> type = this.getType();
        DataSource result = (DataSource)BeanUtils.instantiateClass(type);
        this.maybeGetDriverClassName();
        this.bind(result);
        return result;
    }

    private void maybeGetDriverClassName() {
        if (!this.properties.containsKey("driverClassName") && this.properties.containsKey("url")) {
            String url = (String)this.properties.get("url");
            String driverClass = DatabaseDriver.fromJdbcUrl(url).getDriverClassName();
            this.properties.put("driverClassName", driverClass);
        }

    }

    private void bind(DataSource result) {
        ConfigurationPropertySource source = new MapConfigurationPropertySource(this.properties);
        ConfigurationPropertyNameAliases aliases = new ConfigurationPropertyNameAliases();
        aliases.addAliases("url", new String[]{"jdbc-url"});
        aliases.addAliases("username", new String[]{"user"});
        Binder binder = new Binder(new ConfigurationPropertySource[]{source.withAliases(aliases)});
        binder.bind(ConfigurationPropertyName.EMPTY, Bindable.ofInstance(result));
    }

    public <D extends DataSource> DataSourceBuilder<D> type(Class<D> type) {
        this.type = type;
        return this;
    }

    public DataSourceBuilder<T> url(String url) {
        this.properties.put("url", url);
        return this;
    }

    public DataSourceBuilder<T> driverClassName(String driverClassName) {
        this.properties.put("driverClassName", driverClassName);
        return this;
    }

    public DataSourceBuilder<T> username(String username) {
        this.properties.put("username", username);
        return this;
    }

    public DataSourceBuilder<T> password(String password) {
        this.properties.put("password", password);
        return this;
    }

    public static Class<? extends DataSource> findType(ClassLoader classLoader) {
        String[] var1 = DATA_SOURCE_TYPE_NAMES;
        int var2 = var1.length;
        int var3 = 0;

        while(var3 < var2) {
            String name = var1[var3];

            try {
                return ClassUtils.forName(name, classLoader);
            } catch (Exception var6) {
                ++var3;
            }
        }

        return null;
    }

    private Class<? extends DataSource> getType() {
        Class<? extends DataSource> type = this.type != null ? this.type : findType(this.classLoader);
        if (type != null) {
            return type;
        } else {
            throw new IllegalStateException("No supported DataSource type found");
        }
    }
}