postgresql - 使用 Slick 执行并发查询时出现 NullPointerException

Question

我正在使用 Postgres 9.3 和 Slick 3.1.1 开发 Scala 应用程序。当多个查询同时执行时，我在 slick 驱动程序上得到 Null Pointer Exception。

这是我的简化代码。我正在创建多个参与者，它们将调用相同的方法从数据库中查询。

package com.app.repo

import java.sql.Timestamp

import akka.actor.{Actor, ActorSystem, Props}
import slick.driver.PostgresDriver
import slick.driver.PostgresDriver.api._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.FiniteDuration
import scala.util.{Failure, Success}

case class SampleData(id: Long, name: String, createDate: java.sql.Timestamp)

object Tables extends {
  val profile = PostgresDriver
} with Tables

trait Tables {
  val profile: PostgresDriver

  import profile.api._

  class SampleDataTable(_tableTag: Tag) extends Table[SampleData](_tableTag, Some("processing"), "SampleData") {
    def * = (id, name, createDate) <>(SampleData.tupled, SampleData.unapply)

    def ? = (Rep.Some(id), Rep.Some(name), Rep.Some(createDate)).shaped.<>({ r => import r._; _1.map(_ => SampleData.tupled((_1.get, _2.get, _3.get))) }, (_: Any) => throw new Exception("Inserting into ? projection not supported."))

    val id: Rep[Long] = column[Long]("SampleId", O.AutoInc, O.PrimaryKey)
    val name: Rep[String] = column[String]("Name")
    val createDate: Rep[java.sql.Timestamp] = column[java.sql.Timestamp]("CreateDate")
  }

  lazy val sampleDataTable = new TableQuery(tag => new SampleDataTable(tag))
}

class SampleQueryingActor(delay: FiniteDuration, duration: FiniteDuration) extends Actor {

  import scala.concurrent.duration._

  override def preStart() = {
    context.system.scheduler.schedule(0.second, duration, self, "tick")
  }

  override def receive: Receive = {
    case "tick" => {
      println("tick received.. ")
      //val range = 1 until 1000
      RepositoryImpl.reader.onComplete({
        case Success(r)  => println(s"got sum as ${r.getOrElse(0)}")
        case Failure(ex) => ex.printStackTrace()
      })

    }
  }
}

object DriverHelper {
  val user = "postgres"
  val url = "jdbc:postgresql://192.168.1.50:5432/MyDatabase"
  val password = "password"
  val jdbcDriver = "org.postgresql.Driver"
  val db: PostgresDriver.backend.DatabaseDef = Database.forURL(url, user = user, password = password, driver = jdbcDriver)
}

object RepositoryImpl {
  val db: PostgresDriver.backend.DatabaseDef = DriverHelper.db

  val now = new Timestamp(System.currentTimeMillis())

  def reader = {
    db.run(Tables.sampleDataTable.filter(_.createDate > now).map(_.id).sum.result)
  }

  def insertBatchRecords(list: List[SampleData]) = {
    db.run(Tables.sampleDataTable ++= list)
  }

}

object PGConnectionTester extends App {

  import scala.concurrent.duration._

  val sys = ActorSystem("sys")
  sys.actorOf(Props(classOf[SampleQueryingActor], 1.seconds, 10.seconds))
  sys.actorOf(Props(classOf[SampleQueryingActor], 1.seconds, 10.seconds))
  sys.actorOf(Props(classOf[SampleQueryingActor], 1.seconds, 10.seconds))
}

当我执行上面的代码时，出现如下错误:

java.lang.NullPointerException
    at slick.jdbc.DriverDataSource.getConnection(DriverDataSource.scala:98)
    at slick.jdbc.DataSourceJdbcDataSource.createConnection(JdbcDataSource.scala:64)
    at slick.jdbc.JdbcBackend$BaseSession.conn$lzycompute(JdbcBackend.scala:415)
    at slick.jdbc.JdbcBackend$BaseSession.conn(JdbcBackend.scala:414)
    at slick.jdbc.JdbcBackend$SessionDef$class.prepareStatement(JdbcBackend.scala:297)
    at slick.jdbc.JdbcBackend$BaseSession.prepareStatement(JdbcBackend.scala:407)
    at slick.jdbc.StatementInvoker.results(StatementInvoker.scala:33)
    at slick.jdbc.StatementInvoker.iteratorTo(StatementInvoker.scala:22)
    at slick.jdbc.Invoker$class.first(Invoker.scala:31)
    at slick.jdbc.StatementInvoker.first(StatementInvoker.scala:16)
    at slick.driver.JdbcActionComponent$QueryActionExtensionMethodsImpl$$anon$3.run(JdbcActionComponent.scala:228)
    at slick.driver.JdbcActionComponent$SimpleJdbcDriverAction.run(JdbcActionComponent.scala:32)
    at slick.driver.JdbcActionComponent$SimpleJdbcDriverAction.run(JdbcActionComponent.scala:29)
    at slick.backend.DatabaseComponent$DatabaseDef$$anon$2.liftedTree1$1(DatabaseComponent.scala:237)
    at slick.backend.DatabaseComponent$DatabaseDef$$anon$2.run(DatabaseComponent.scala:237)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

Actor 将每 10 秒调用一次相同的方法。但是，我只是第一次收到此错误。之后查询正确执行。我不明白为什么会这样。在这个示例案例中，只有一些简单的读取操作。但在我的实际情况中，由于查询失败，一些数据在没有正确处理的情况下丢失了。这个错误与连接池有关吗？

Answer 1

我想你已经找到了this问题。尝试对 db 使用 lazy val，这样它只初始化一次:

object DriverHelper {
  val user = "postgres"
  val url = "jdbc:postgresql://192.168.1.50:5432/MyDatabase"
  val password = "password"
  val jdbcDriver = "org.postgresql.Driver"
  lazy val db: PostgresDriver.backend.DatabaseDef = Database.forURL(url, user = user, password = password, driver = jdbcDriver)
}