Java 9+与Scala中Cleanable无法被System.gc()触发的原因及解决

问题场景 #

在Scala 2.13.12与Java 11环境下，使用java.lang.ref.Cleaner实现对象自动清理逻辑，测试中System.gc()无法触发清理动作，但改用已过时的finalize()方法时测试可正常通过。

测试代码 #

class CleanableSpike extends AnyFunSpec {

  import CleanableSpike._

  it("triggered by GC") {

    var v = Dummy(inc)
    v = null

    System.gc()

    Thread.sleep(1000)
    assert(count == 1)
  }
}

object CleanableSpike {

  val jvmCleaner = Cleaner.create()

  @transient var count = 0

  val inc = () => count += 1

  case class Dummy(fn: () => Unit) extends AutoCloseable {

    final private val cleanable = jvmCleaner.register(
      this,
      { () =>
        println("\ncleaned\n")
        fn()
      }
    )

    override def close(): Unit = cleanable.clean()
  }
}

测试报错信息 #

org.scalatest.exceptions.TestFailedException: 0 did not equal 1
    at org.scalatest.Assertions.newAssertionFailedException(Assertions.scala:472)
    at org.scalatest.Assertions.newAssertionFailedException$(Assertions.scala:471)
    at org.scalatest.Assertions$.newAssertionFailedException(Assertions.scala:1231)
    at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:1295)
    at com.tribbloids.spookystuff.lifespan.CleanableSpike.$anonfun$new$1(CleanableSpike.scala:19)

原因分析 #

核心问题是清理逻辑的lambda表达式意外捕获了Dummy实例的强引用，形成循环引用导致对象无法被GC回收：

• Scala中匿名lambda默认会捕获外部类的this（即当前Dummy实例），而Cleaner注册后返回的Cleanable对象会持有该lambda的引用。
• 最终形成循环强引用链：Dummy → Cleanable → lambda → Dummy，GC无法识别该链上的对象为可回收状态，因此不会触发清理动作。
• finalize()是对象自身方法，不存在额外强引用依赖，所以GC可以正常触发。

解决方法 #

方法一：用弱引用包裹清理逻辑依赖对象，避免捕获this #

将清理函数用WeakReference包裹，切断lambda对Dummy实例的强引用：

object CleanableSpike {

  val jvmCleaner = Cleaner.create()

  @transient var count = 0

  val inc = () => count += 1

  case class Dummy(fn: () => Unit) extends AutoCloseable {
    // 用弱引用持有清理函数，避免捕获外部类实例
    private val weakFn = new WeakReference(fn)
    
    final private val cleanable = jvmCleaner.register(
      this,
      new Runnable {
        override def run(): Unit = {
          println("\ncleaned\n")
          weakFn.get().foreach(_())
        }
      }
    )

    override def close(): Unit = cleanable.clean()
  }
}

方法二：将清理逻辑抽离为静态方法，彻底解耦与目标对象的引用 #

把清理逻辑封装成独立静态方法，确保清理动作不持有目标对象的强引用：

object CleanableSpike {

  val jvmCleaner = Cleaner.create()

  @transient var count = 0

  val inc = () => count += 1

  // 静态清理逻辑，不依赖Dummy实例
  private def createCleanupAction(fn: () => Unit): Runnable = {
    val weakFn = new WeakReference(fn)
    () => {
      println("\ncleaned\n")
      weakFn.get().foreach(_())
    }
  }

  case class Dummy(fn: () => Unit) extends AutoCloseable {

    final private val cleanable = jvmCleaner.register(
      this,
      createCleanupAction(fn)
    )

    override def close(): Unit = cleanable.clean()
  }
}

测试补充注意事项 #

• System.gc()仅为JVM提供垃圾回收建议，不保证立即执行，测试时可配合System.runFinalization()提升触发概率。
• 确保测试代码中没有其他地方持有Dummy实例的强引用，否则GC同样无法回收目标对象。

内容的提问来源于stack exchange，提问作者tribbloid