为什么 user.dir 系统属性在 Java 中有效？
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Why is the user.dir system property working in Java?
提问by Geo
Almost every article I read told me that you can't have chdirin Java. The accepted answer to this questionsays you can't do it in Java.
我读过的几乎每篇文章都告诉我，在 Java 中不能有chdir。这个问题的公认答案说你不能用 Java 来做。
However, here's some of the stuff I tried:
但是，这是我尝试过的一些东西：
geo@codebox:~$ java -version java version "1.6.0_14" Java(TM) SE Runtime Environment (build 1.6.0_14-b08) Java HotSpot(TM) Client VM (build 14.0-b16, mixed mode, sharing)
Here's a test class I'm using:
这是我正在使用的测试类：
import java.io.*;
public class Ch {
public static void main(String[] args) {
System.out.println(new File(".").getAbsolutePath());
System.setProperty("user.dir","/media");
System.out.println(new File(".").getAbsolutePath());
}
}
geo@codebox:~$ pwd /home/geo geo@codebox:~$ java Ch /home/geo/. /media/.
Please explain why this worked. Can I use this from now on and expect it to work the same way on all platforms?
请解释为什么这有效。我可以从现在开始使用它并期望它在所有平台上都以相同的方式工作吗？
回答by Jon Skeet
Just because new File(".")gives the desired answer doesn't mean it's doing what you want it to.
仅仅因为new File(".")给出了想要的答案并不意味着它正在做你想要的。
For example, try:
例如，尝试：
new FileOutputStream("foo.txt").close();
Where does that end up? On my Windows box, even though new File(".").getAbsolutePath()moves around based on user.dir, foo.txtis always created in the original working directory. It strikes me that setting user.dirsuch that new File(".")doesn'trefer to the current working directory is just asking for trouble.
最终会在哪里？在我的 Windows 机器上，即使new File(".").getAbsolutePath()基于 移动user.dir，foo.txt也总是在原始工作目录中创建。这让我感到设定user.dir，使得new File(".")不参考当前的工作目录是自找麻烦。
回答by 01es
Quote:
引用：
The user.dir property is set at VM startup to be the working directory. You should not change this property or set it on the command-line. If you do, then you will see some inconsistent behaviour as there places in the implementation that assumes that the user.dir is the working directory and that it doesn't change during the lifetime of the VM.
user.dir 属性在 VM 启动时设置为工作目录。您不应更改此属性或在命令行上设置它。如果你这样做了，那么你会看到一些不一致的行为，因为在假设 user.dir 是工作目录并且在 VM 的生命周期内它不会更改的实现中有一些地方。
The discussion is here
讨论在这里
回答by Laurence Gonsalves
File.getAbsoluteFile() is just looking at the user.dir system property, which is a copy of the process's working directory at VM startup.
File.getAbsoluteFile() 只是查看 user.dir 系统属性，它是 VM 启动时进程工作目录的副本。
A better test might be to check that the process's working directory is actually changing. How you can do this varies by platform, but on Linux you can to something like:
更好的测试可能是检查进程的工作目录是否确实在更改。如何执行此操作因平台而异，但在 Linux 上，您可以执行以下操作：
$ ls -l /proc/18037/cwd
lrwxrwxrwx 1 laurence laurence 0 2009-08-05 11:16 /proc/18037/cwd -> /home/laurence/
where "18037" is the pid of the process in question. If you do this I believe you'll find that the process's working directory doesn't actually change when you update user.dir.
其中“18037”是相关进程的pid。如果你这样做，我相信你会发现当你更新 user.dir 时，进程的工作目录实际上并没有改变。
