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Tip of the Day
Language: Java
Expertise: Intermediate
Feb 24, 2000

Java Applications and the "Current Directory"

A recurring question in the Sun Java Forum groups is, "How do I change the current directory of my Java application?" The correct answer is that you don't! Java applications don't use the metaphor of 'current directory' in the same way native Unix and Windows applications do.

A native Unix or Windows application typically by default points at the current directory of the command shell where the program is started, and if you do an fopen("x.txt","w") your file will end up in that directory. You can then call chdir(..) to change the current directory of the application. The metaphor is that the application is like a user at the command line, cd'ing around and accessing files.

On the other hand, when you start a Java application with 'java appname', it is system dependent what the current directory is and you can't change the 'current directory' from a Java application, and the application as user metaphor does not apply.

To understand the Java solution, it is helpful to identify is the real requirement. The requirement is not to change to a new current directory just because you want to. The real requirement is to allow a program to get at files in the file system relative to the current directory. In other words, make it easy to access files in a given location without an absolute path prefix.

In Java, you use File objects to construct a relative view of the file system. Two of the constructors for the File object take a 'parent' argument that specifies a parent path that is prefixed to the path of the file itself to create the full abstract path to the file. What you do is, create a File object with the path that represents your current directory and then create all your file objects using that File object as the parent. Voila, a current directory. If you want to change the current directory, create a new File object with the desired path and use it as the parent. See the SDK documentation, File object for the specifics of the constructors.

The main catch here is that you have to specify the parent somehow, either by knowing it outright, specifying it thru some user input or figuring it out from the application environment. If you know it outright, you can hardcode it or store it as an application property or something similar. The user input could be a command line argument or some run-time input method, such as a JFileChooser object.

On some systems you can read a system property called 'user.dir' to get the path that the application was started in. This property is system dependent as to what it means or whether it exists at all, but in Unix and Windows it returns the directory the command shell was in when the java application was started. On Windows and Unix, "user.dir" is a READ ONLY property, and you can't change the 'current directory' by trying to set the "user.dir" property. You read system properties with the System.getProperty(String) method.

David Howard
 
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