File / FileInputStream相对路径的奇怪行为

Question

I set java property user.dir to /home/alex/projects/poltava/rpgu/workingdir . Also I have file q.txt inside above folder.

Below is the code snippets and their return values (after =):

System.getProperty("user.dir") = /home/alex/projects/poltava/rpgu/workingdir
new File(".").getAbsolutePath() = /home/alex/projects/poltava/rpgu/workingdir/.
new File(".").exists() = true
new File("q.txt").getAbsolutePath() = /home/alex/projects/poltava/rpgu/workingdir/q.txt
new File("q.txt").exists() = false
new File(new File("q.txt").getAbsolutePath()).exists() = true
new FileInputStream("q.txt") = threw FileNotFoundException

So that as you can see file is realy exists in file system. When I try to get it with absolute path, everything is well. When I try to get it with relative path, it fails.

What do I wrong with relative path?

EDITED:

Small application that demonstrates the problem:

import java.io.File;

public class Test {
    public static void main(String[] args) {
        System.setProperty("user.dir", "/home/alex/projects/poltava/rpgu/workingdir");
        System.out.println(System.getProperty("user.dir"));
        System.out.println(new File("q.txt").exists());
        System.out.println(new File("q.txt").isFile());
        System.out.println(new File("q.txt").canRead());

        System.out.println(new File("q.txt").getAbsolutePath());
        System.out.println(new File(new File("q.txt").getAbsolutePath()).exists());
        System.out.println(new File(new File("q.txt").getAbsolutePath()).isFile());
        System.out.println(new File(new File("q.txt").getAbsolutePath()).canRead());

        try {
            new FileInputStream("q.txt");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Output:

/home/alex/projects/poltava/rpgu/workingdir
false
false
false
/home/alex/projects/poltava/rpgu/workingdir/q.txt
true
true
true
java.io.FileNotFoundException: q.txt (No such file or directory)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.<init>(FileInputStream.java:146)
    at java.io.FileInputStream.<init>(FileInputStream.java:101)
    at Test.main(Test.java:24)

EDITED 2:

I also tried another simple example:

File f = new File("q1.txt");
System.out.println(f.createNewFile());
System.out.println(f.getPath());
System.out.println(f.getAbsolutePath());

An output:

true
q1.txt
/home/alex/projects/poltava/rpgu/workingdir/q1.txt

In result file is created in directory from which I launch the application. Not in user.dir directory. And getAbsolutePath() returns incorrect file path.

Answer 1

I think it would serve you better to read the javadoc for File .

A little bit of explanation to help you get started:

For the constructor that you're using:

public File(String pathname)

Creates a new File instance by converting the given pathname string into an abstract pathname. If the given string is the empty string, then the result is the empty abstract pathname.

So essentially what you get is a File instance, with the abstract pathname " q.txt ".

What happens when you do a getAbsolutePath() on this. Again from the javadoc:

public String getAbsolutePath()

Returns the absolute pathname string of this abstract pathname. If this abstract pathname is already absolute, then the pathname string is simply returned as if by the getPath() method. If this abstract pathname is the empty abstract pathname then the pathname string of the current user directory, which is named by the system property user.dir , is returned. Otherwise this pathname is resolved in a system-dependent way. On UNIX systems, a relative pathname is made absolute by resolving it against the current user directory.

On Microsoft Windows systems, a relative pathname is made absolute by resolving it against the current directory of the drive named by the pathname, if any; if not, it is resolved against the current user directory.

Do you see what is happening? Especially, with respect to user.dir ?

More hints:

Now create another variable, eg

File newFile = new File(System.getProperty("user.dir"), "q.txt") .

Try the same operations on newFile . Try using getParent() on your previous one as well as on this one. You'll see the difference.

I hope this helps clarify a few points for you :)

Answer 2

It is because File(String str) will call normalize, but getAbsolutePath will also call resolve, there is where the user.dir is being used. Look at the following example.

System.setProperty("user.dir", "/home/alex/projects/poltava/rpgu/workingdir");

File fString = new File("Test.txt");
File fAbsolutePath = new File(fString.getAbsolutePath());

System.out.println(System.getProperty("user.dir"));
System.out.println(fString.getPath());
System.out.println(fAbsolutePath.getPath());

Here is the output:

/home/alex/projects/poltava/rpgu/workingdir
Test.txt
\home\alex\projects\poltava\rpgu\workingdir\Test.txt

You can see the src code here: new File(String str) http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/io/File.java#File.%3Cinit%3E%28java.io.File%2Cjava.lang.String%29

getAbsolutePath: http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/io/File.java#File.getAbsolutePath%28%29