为什么 Scala ConcurrentMap.putIfAbsent 总是尝试放置？

Question

文档声明中的 ConcurrentMap 部分：

m putIfAbsent(k, v) Adds key/value binding k -> v unless k is already defined in m

但实际上，我发现 putIfAbsent 总是试图插入 v。解决这个问题的方法是使用 getOrElseUpdate，但这真的是一个错误，还是我遗漏了什么？

我的代码如下：

val instanceMap: ConcurrentMap[Address, (MongodExecutable, MongodProcess)] =
    (new JavaConcurrentMap[Address, (MongodExecutable, MongodProcess)]()).asScala

def start(host: String = DefaultAddress.host, port: Int = DefaultAddress.port): MongodProcess = {
  val addr = Address(host, port)
  instanceMap.putIfAbsent(addr, { // this block is ALWAYS executed
    val mongodExecutable = starter.prepare(mongodConfig(host, port))
    val mongod = mongodExecutable.start()
    logProcessInfo(mongod)

    (mongodExecutable, mongod)
  })

  instanceMap(addr)
    ._2
}

Answer 1

putIfAbsent 确实计算了 k, v 的值，因为值不是 function/lambda。

下面的例子

原并发图

scala> import java.util.concurrent.ConcurrentHashMap
import java.util.concurrent.ConcurrentHashMap

scala> new ConcurrentHashMap[String, String]()
res0: java.util.concurrent.ConcurrentHashMap[String,String] = {}

insert key1 - 插入成功returns null

scala> res0.putIfAbsent("127.0.0.1", "mongod-process1")
res1: String = null

scala> res0
res2: java.util.concurrent.ConcurrentHashMap[String,String] = {127.0.0.1=mongod-process1}

insert key2 - 插入成功returns null

scala> res0.putIfAbsent("127.0.0.2", "mongod-process2")
res3: String = null

scala> res0
res4: java.util.concurrent.ConcurrentHashMap[String,String] = {127.0.0.2=mongod-process2, 127.0.0.1=mongod-process1}

再次尝试插入key1 - 插入失败returnsvalue1

scala> res0.putIfAbsent("127.0.0.1", { println("evaluating mongod-process"); "mongod-process1-again" } )
evaluating mongod-process
res7: String = mongod-process1

最终地图

scala> res0
res8: java.util.concurrent.ConcurrentHashMap[String,String] = {127.0.0.2=mongod-process2, 127.0.0.1=mongod-process1}

此外，请注意 putIfAbsent 在开始时对键值进行空检查（ConcurrentHashMap.java:1011），因此无论如何都必须评估该值。

scala> res0.putIfAbsent("127.0.0.1", { println("evaluating mongod-process"); null } )
evaluating mongod-process
java.lang.NullPointerException
  at java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011)
  at java.util.concurrent.ConcurrentHashMap.putIfAbsent(ConcurrentHashMap.java:1535)
  ... 29 elided

为什么 Scala ConcurrentMap.putIfAbsent 总是尝试放置？

Why does Scala ConcurrentMap.putIfAbsent always try to put?

java

scala

concurrenthashmap

java.util.concurrent