Scala 最佳实践：特征继承与枚举

Question

我目前正在试验 Scala 并寻找最佳实践。我发现自己有两种相反的方法来解决一个问题。我想知道哪个更好，为什么，哪个更传统，如果你知道其他更好的方法。我觉得第二个更漂亮。

1.基于枚举的解决方案

import org.squeryl.internals.DatabaseAdapter
import org.squeryl.adapters.{H2Adapter, MySQLAdapter, PostgreSqlAdapter}
import java.sql.Driver

object DBType extends Enumeration {
  val MySql, PostgreSql, H2 = Value

  def fromUrl(url: String) = {
    url match {
      case u if u.startsWith("jdbc:mysql:") => Some(MySql)
      case u if u.startsWith("jdbc:postgresql:") => Some(PostgreSql)
      case u if u.startsWith("jdbc:h2:") => Some(H2)
      case _ => None
    }
  }
}

case class DBType(typ: DBType) {
  lazy val driver: Driver = {
    val name = typ match {
      case DBType.MySql => "com.mysql.jdbc.Driver"
      case DBType.PostgreSql => "org.postgresql.Driver"
      case DBType.H2 => "org.h2.Driver"
    }
    Class.forName(name).newInstance().asInstanceOf[Driver]
  }
  lazy val adapter: DatabaseAdapter = {
    typ match {
      case DBType.MySql => new MySQLAdapter
      case DBType.PostgreSql => new PostgreSqlAdapter
      case DBType.H2 => new H2Adapter
    }
  }
}

2。基于单例的解决方案

import org.squeryl.internals.DatabaseAdapter
import org.squeryl.adapters.{H2Adapter, MySQLAdapter, PostgreSqlAdapter}
import java.sql.Driver

trait DBType {
  def driver: Driver
  def adapter: DatabaseAdapter
}

object DBType {
  object MySql extends DBType {
    lazy val driver = Class.forName("com.mysql.jdbc.Driver").newInstance().asInstanceOf[Driver]
    lazy val adapter = new MySQLAdapter
  }

  object PostgreSql extends DBType {
    lazy val driver = Class.forName("org.postgresql.Driver").newInstance().asInstanceOf[Driver]
    lazy val adapter = new PostgreSqlAdapter
  }

  object H2 extends DBType {
    lazy val driver = Class.forName("org.h2.Driver").newInstance().asInstanceOf[Driver]
    lazy val adapter = new H2Adapter
  }

  def fromUrl(url: String) = {
    url match {
      case u if u.startsWith("jdbc:mysql:") => Some(MySql)
      case u if u.startsWith("jdbc:postgresql:") => Some(PostgreSql)
      case u if u.startsWith("jdbc:h2:") => Some(H2)
      case _ => None
    }
  }
}

Answer 1

如果你声明一个sealed trait DBType，你可以通过穷举检查对其进行模式匹配（即，如果你忘记了一个案例，Scala 会告诉你）。

无论如何，我不喜欢 Scala 的 Enumeration，而且我并不孤单。我从不使用它，如果枚举确实是最干净的解决方案，最好使用 Java 的枚举用 Java 编写它。

Answer 2

对于这种特殊情况，您实际上并不需要每种数据库类型的类；这只是数据。除非实际情况要复杂得多，否则我会使用基于映射和字符串解析的解决方案来最大程度地减少代码重复量：

case class DBRecord(url: String, driver: String, adapter: () => DatabaseAdapter) {}

class DBType(record: DBRecord) {
  lazy val driver = Class.forName(record.driver).newInstance().asInstanceOf[Driver]
  lazy val adapter = record.adapter()
}

object DBType {
  val knownDB = List(
    DBRecord("mysql", "com.mysql.jdbc.Driver", () => new MySQLAdapter),
    DBRecord("postgresql", "org.postgresql.Driver", () => new PostgreSqlAdapter),
    DBRecord("h2", "org.h2.Driver", () => new H2Adapter)
  )

  val urlLookup = knownDB.map(rec => rec.url -> rec).toMap

  def fromURL(url: String) = {
    val parts = url.split(':')
    if (parts.length < 3 || parts(0) != "jdbc") None
    else urlLookup.get(parts(1)).map(rec => new DBType(rec))
  }
}

Answer 3

我会选择单例变体，因为它允许更清晰的子类化。

此外，您可能需要执行特定于 db 的操作/覆盖，因为某些查询/子查询/操作符可能不同。

但我会尝试这样的：

import org.squeryl.internals.DatabaseAdapter
import org.squeryl.adapters.{H2Adapter, MySQLAdapter, PostgreSqlAdapter}
import java.sql.Driver

abstract class DBType(jdbcDriver: String) {
  lazy val driver = Class.forName(jdbcDriver).newInstance().asInstanceOf[Driver]
  def adapter: DatabaseAdapter
}


object DBType {
  object MySql extends DBType("com.mysql.jdbc.Driver") {
    lazy val adapter = new MySQLAdapter
  }

  object PostgreSql extends DBType("org.postgresql.Driver") {
    lazy val adapter = new PostgreSqlAdapter
  }

  object H2 extends DBType("org.h2.Driver") {
    lazy val adapter = new H2Adapter
  }

  def fromUrl(url: String) = {
    url match {
      case _ if url.startsWith("jdbc:mysql:") => Some(MySql(url))
      case _ if url.startsWith("jdbc:postgresql:") => Some(PostgreSql(url))
      case _ if url.startsWith("jdbc:h2:") => Some(H2(url))
      case _ => None
    }

}

如果这有帮助，请考虑对此 +1 :)