为什么PrintStream“out”和InputStream“in”空值？ [英] Why are the PrintStream "out" and InputStream "in" null?

问题描述

出于好奇，我正在查看 docjar.com 上的一些Java API类的源代码。我看到了java.lang.System类，发现PrintStreamout（即System.out）有以下代码：

Out of curiosity I was looking through the source code for some of the Java API classes found at docjar.com. I saw the java.lang.System class and saw that the PrintStream "out" (i.e., System.out) had the following code:

public final static PrintStream out = null;

并在评论中说：

The "standard" output stream. This stream is already
open and ready to accept output data.

我的问题是，我知道最终的变量无法修改，所以为什么声明为null ？为什么在调用out方法时Java不会抛出NullPointerException？ System.in也是如此。

My queston is, I know final variables can't be modified, so why is it null when declared? Why doesn't Java throw a NullPointerException when a method for "out" is called? The same goes for System.in as well.

感谢您的任何澄清。

推荐答案

有一个java.lang.System #registerNatives（）本机方法。如果你在jvm源代码中查看它，它会从本机代码中分配sysin，sysout和syserr。在java 7之前，这些声明看起来像 public final static InputStream in = nullInputStream（）; 但是我猜他们只是将它改为null而javac中的一些hacks不仅仅是内联它。

There's a java.lang.System#registerNatives() native method. If you look at it in jvm source code, it assigns sysin, sysout, and syserr from native code. Before java 7 these declarations looked like public final static InputStream in = nullInputStream(); but I guess they changed it to be just null with some hacks in javac not to just inline it.

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