Changing the current working directory in Java?

问题

How can I change the current working directory from within a Java program? Everything I\'ve been able to find about the issue claims that you simply can\'t do it, but I can\'t believe that that\'s really the case.

I have a piece of code that opens a file using a hard-coded relative file path from the directory it\'s normally started in, and I just want to be able to use that code from within a different Java program without having to start it from within a particular directory. It seems like you should just be able to call System.setProperty( \"user.dir\", \"/path/to/dir\" ), but as far as I can figure out, calling that line just silently fails and does nothing.

I would understand if Java didn\'t allow you to do this, if it weren\'t for the fact that it allows you to get the current working directory, and even allows you to open files using relative file paths....

回答1:

There is no reliable way to do this in pure Java. Setting the user.dir property via System.setProperty() or java -Duser.dir=... does seem to affect subsequent creations of Files, but not e.g. FileOutputStreams.

The File(String parent, String child) constructor can help if you build up your directory path separately from your file path, allowing easier swapping.

An alternative is to set up a script to run Java from a different directory, or use JNI native code as suggested below.

The relevant Sun bug was closed in 2008 as "will not fix".

回答2:

If you run your legacy program with ProcessBuilder, you will be able to specify its working directory.

回答3:

There is a way to do this using the system property "user.dir". The key part to understand is that getAbsoluteFile() must be called (as shown below) or else relative paths will be resolved against the default "user.dir" value.

import java.io.*;

public class FileUtils
{
    public static boolean setCurrentDirectory(String directory_name)
    {
        boolean result = false;  // Boolean indicating whether directory was set
        File    directory;       // Desired current working directory

        directory = new File(directory_name).getAbsoluteFile();
        if (directory.exists() || directory.mkdirs())
        {
            result = (System.setProperty("user.dir", directory.getAbsolutePath()) != null);
        }

        return result;
    }

    public static PrintWriter openOutputFile(String file_name)
    {
        PrintWriter output = null;  // File to open for writing

        try
        {
            output = new PrintWriter(new File(file_name).getAbsoluteFile());
        }
        catch (Exception exception) {}

        return output;
    }

    public static void main(String[] args) throws Exception
    {
        FileUtils.openOutputFile("DefaultDirectoryFile.txt");
        FileUtils.setCurrentDirectory("NewCurrentDirectory");
        FileUtils.openOutputFile("CurrentDirectoryFile.txt");
    }
}

回答4:

It is possible to change the PWD, using JNA/JNI to make calls to libc. The JRuby guys have a handy java library for making POSIX calls called jna-posix Here's the maven info

You can see an example of its use here (Clojure code, sorry). Look at the function chdirToRoot

回答5:

If I understand correctly, a Java program starts with a copy of the current environment variables. Any changes via System.setProperty(String, String) are modifying the copy, not the original environment variables. Not that this provides a thorough reason as to why Sun chose this behavior, but perhaps it sheds a little light...

回答6:

As mentioned you can't change the CWD of the JVM but if you were to launch another process using Runtime.exec() you can use the overloaded method that lets you specify the working directory. This is not really for running your Java program in another directory but for many cases when one needs to launch another program like a Perl script for example, you can specify the working directory of that script while leaving the working dir of the JVM unchanged.

See Runtime.exec javadocs

Specifically,

public Process exec(String[] cmdarray,String[] envp, File dir) throws IOException

where dir is the working directory to run the subprocess in

回答7:

The working directory is a operating system feature (set when the process starts). Why don't you just pass your own System property (-Dsomeprop=/my/path) and use that in your code as the parent of your File:

File f = new File ( System.getProperty("someprop"), myFilename)

回答8:

The smarter/easier thing to do here is to just change your code so that instead of opening the file assuming that it exists in the current working directory (I assume you are doing something like new File("blah.txt"), just build the path to the file yourself.

Let the user pass in the base directory, read it from a config file, fall back to user.dir if the other properties can't be found, etc. But it's a whole lot easier to improve the logic in your program than it is to change how environment variables work.

回答9:

I have tried to invoke

String oldDir = System.setProperty("user.dir", currdir.getAbsolutePath());

It seems to work. But

File myFile = new File("localpath.ext"); InputStream openit = new FileInputStream(myFile);

throws a FileNotFoundException though

myFile.getAbsolutePath()

shows the correct path. I have read this. I think the problem is:

Java knows the current directory with the new setting.
But the file handling is done by the operation system. It does not know the new set current directory, unfortunately.

The solution may be:

File myFile = new File(System.getPropety("user.dir"), "localpath.ext");

It creates a file Object as absolute one with the current directory which is known by the JVM. But that code should be existing in a used class, it needs changing of reused codes.

~~~~JcHartmut

回答10:

You can use

new File("relative/path").getAbsoluteFile()

after

System.setProperty("user.dir", "/some/directory")

System.setProperty("user.dir", "C:/OtherProject");
File file = new File("data/data.csv").getAbsoluteFile();
System.out.println(file.getPath());

Will print

C:\OtherProject\data\data.csv

回答11:

The other possible answer to this question may depend on the reason you are opening the file. Is this a property file or a file that has some configuration related to your application?

If this is the case you may consider trying to load the file through the classpath loader, this way you can load any file Java has access to.

回答12:

If you run your commands in a shell you can write something like "java -cp" and add any directories you want separated by ":" if java doesnt find something in one directory it will go try and find them in the other directories, that is what I do.

回答13:

Use FileSystemView

private FileSystemView fileSystemView;
fileSystemView = FileSystemView.getFileSystemView();
currentDirectory = new File(".");
//listing currentDirectory
File[] filesAndDirs = fileSystemView.getFiles(currentDirectory, false);
fileList = new ArrayList<File>();
dirList = new ArrayList<File>();
for (File file : filesAndDirs) {
if (file.isDirectory())
    dirList.add(file);
else
    fileList.add(file);
}
Collections.sort(dirList);
if (!fileSystemView.isFileSystemRoot(currentDirectory))
    dirList.add(0, new File(".."));
Collections.sort(fileList);
//change
currentDirectory = fileSystemView.getParentDirectory(currentDirectory);

来源：https://stackoverflow.com/questions/840190/changing-the-current-working-directory-in-java

标签

java

working-directory