Scala中的println与System.out.println [英] println vs System.out.println in Scala

问题描述

我一直认为 Predef.println 仅仅是 System.out.println 的快捷方式，但显然我弄错了，因为它似乎根本不使用 System.out 。为什么？以及如何在Scala中重定向下面的 System.out ？

I always thought that Predef.println was merely a shortcut for System.out.println, but apparently I am mistaken, since it doesn't seem to use System.out at all. Why is that so? And how can I do the "redirecting" of System.out below in Scala?

scala> val baos = new java.io.ByteArrayOutputStream
baos: java.io.ByteArrayOutputStream = 

scala> val ps = new java.io.PrintStream(baos)
ps: java.io.PrintStream = java.io.PrintStream@6c5ac4

scala> System.setOut(ps)

scala> println("hello")
hello

scala> new String(baos.toByteArray)
res2: java.lang.String = ""

scala> System.out.println("hello")

scala> new String(baos.toByteArray)
res7: java.lang.String = 
"hello
"

推荐答案

Predef.println 是 Console.println ，则可以使用 Console.setOut 或 Console.withOut 进行重定向。

Predef.println is shortcut for Console.println and you can use Console.setOut or Console.withOut for redirecting.

此外， Console.setOut 仅影响当前线程，而System.setOut
影响整个JVM 。另外，Scala 2.9 repl 会在其自己的线程中评估每一行，因此 Console.setOut 在那里不可用。

Also, Console.setOut only affects the current thread while System.setOut affects the whole JVM. Additionally Scala 2.9 repl evaluates each line in its own thread, so Console.setOut is not usable there.

scala> val baos = new java.io.ByteArrayOutputStream
baos: java.io.ByteArrayOutputStream = 

scala> Console.withOut(baos)(print("hello"))

scala> println(baos)
hello

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