public static String getRelativePath(String targetPath, String basePath, String pathSeparator) {
        // Normalize the paths
        String normalizedTargetPath = FilenameUtils.normalizeNoEndSeparator(targetPath);
        String normalizedBasePath = FilenameUtils.normalizeNoEndSeparator(basePath);

        // Undo the changes to the separators made by normalization
        if (pathSeparator.equals("/")) {
            normalizedTargetPath = FilenameUtils.separatorsToUnix(normalizedTargetPath);
            normalizedBasePath = FilenameUtils.separatorsToUnix(normalizedBasePath);

        } else if (pathSeparator.equals("\\")) {
            normalizedTargetPath = FilenameUtils.separatorsToWindows(normalizedTargetPath);
            normalizedBasePath = FilenameUtils.separatorsToWindows(normalizedBasePath);

        } else {
            throw new IllegalArgumentException("Unrecognised dir separator '" + pathSeparator + "'");
        }

        String[] base = normalizedBasePath.split(Pattern.quote(pathSeparator));
        String[] target = normalizedTargetPath.split(Pattern.quote(pathSeparator));

        // First get all the common elements. Store them as a string,
        // and also count how many of them there are.
        StringBuffer common = new StringBuffer();

        int commonIndex = 0;
        while (commonIndex < target.length && commonIndex < base.length
                && target[commonIndex].equals(base[commonIndex])) {
            common.append(target[commonIndex] + pathSeparator);
            commonIndex++;
        }

        if (commonIndex == 0) {
            // No single common path element. This most
            // likely indicates differing drive letters, like C: and D:.
            // These paths cannot be relativized.
            throw new PathResolutionException("No common path element found for '" + normalizedTargetPath + "' and '" + normalizedBasePath + "'");
        }   

        // The number of directories we have to backtrack depends on whether the base is a file or a dir
        // For example, the relative path from
        //
        // /foo/bar/baz/gg/ff to /foo/bar/baz
        // 
        // ".." if ff is a file
        // "../.." if ff is a directory
        //
        // The following is a heuristic to figure out if the base refers to a file or dir. It's not perfect, because
        // the resource referred to by this path may not actually exist, but it's the best I can do
        boolean baseIsFile = true;

        File baseResource = new File(normalizedBasePath);

        if (baseResource.exists()) {
            baseIsFile = baseResource.isFile();

        } else if (basePath.endsWith(pathSeparator)) {
            baseIsFile = false;
        }

        StringBuffer relative = new StringBuffer();

        if (base.length != commonIndex) {
            int numDirsUp = baseIsFile ? base.length - commonIndex - 1 : base.length - commonIndex;

            for (int i = 0; i < numDirsUp; i++) {
                relative.append(".." + pathSeparator);
            }
        }
        relative.append(normalizedTargetPath.substring(common.length()));
        return relative.toString();
    }
public static String getRelativePath(String targetPath, String basePath, String pathSeparator) {
        
        String normalizedTargetPath = FilenameUtils.normalizeNoEndSeparator(targetPath);
        String normalizedBasePath = FilenameUtils.normalizeNoEndSeparator(basePath);

        
        if (pathSeparator.equals("/")) {
            normalizedTargetPath = FilenameUtils.separatorsToUnix(normalizedTargetPath);
            normalizedBasePath = FilenameUtils.separatorsToUnix(normalizedBasePath);

        } else if (pathSeparator.equals("\\")) {
            normalizedTargetPath = FilenameUtils.separatorsToWindows(normalizedTargetPath);
            normalizedBasePath = FilenameUtils.separatorsToWindows(normalizedBasePath);

        } else {
            throw new IllegalArgumentException("Unrecognised dir separator '" + pathSeparator + "'");
        }

        String[] base = normalizedBasePath.split(Pattern.quote(pathSeparator));
        String[] target = normalizedTargetPath.split(Pattern.quote(pathSeparator));

        
        
        StringBuffer common = new StringBuffer();

        int commonIndex = 0;
        while (commonIndex < target.length && commonIndex < base.length
                && target[commonIndex].equals(base[commonIndex])) {
            common.append(target[commonIndex] + pathSeparator);
            commonIndex++;
        }

        if (commonIndex == 0) {
            
            
            
            throw new PathResolutionException("No common path element found for '" + normalizedTargetPath + "' and '" + normalizedBasePath + "'");
        }   

        
        
        
        
        
        
        
        
        
        
        boolean baseIsFile = true;

        File baseResource = new File(normalizedBasePath);

        if (baseResource.exists()) {
            baseIsFile = baseResource.isFile();

        } else if (basePath.endsWith(pathSeparator)) {
            baseIsFile = false;
        }

        StringBuffer relative = new StringBuffer();

        if (base.length != commonIndex) {
            int numDirsUp = baseIsFile ? base.length - commonIndex - 1 : base.length - commonIndex;

            for (int i = 0; i < numDirsUp; i++) {
                relative.append(".." + pathSeparator);
            }
        }
        relative.append(normalizedTargetPath.substring(common.length()));
        return relative.toString();
    }
public static String getRelativePath ( String targetPath , String basePath , String pathSeparator ) { // Normalize the paths String normalizedTargetPath = FilenameUtils . normalizeNoEndSeparator ( targetPath ) ; String normalizedBasePath = FilenameUtils . normalizeNoEndSeparator ( basePath ) ; // Undo the changes to the separators made by normalization if ( pathSeparator . equals ( "/" ) ) { normalizedTargetPath = FilenameUtils . separatorsToUnix ( normalizedTargetPath ) ; normalizedBasePath = FilenameUtils . separatorsToUnix ( normalizedBasePath ) ; } else if ( pathSeparator . equals ( "\\" ) ) { normalizedTargetPath = FilenameUtils . separatorsToWindows ( normalizedTargetPath ) ; normalizedBasePath = FilenameUtils . separatorsToWindows ( normalizedBasePath ) ; } else { throw new IllegalArgumentException ( "Unrecognised dir separator '" + pathSeparator + "'" ) ; } String [ ] base = normalizedBasePath . split ( Pattern . quote ( pathSeparator ) ) ; String [ ] target = normalizedTargetPath . split ( Pattern . quote ( pathSeparator ) ) ; // First get all the common elements. Store them as a string, // and also count how many of them there are. StringBuffer common = new StringBuffer ( ) ; int commonIndex = 0 ; while ( commonIndex < target . length && commonIndex < base . length && target [ commonIndex ] . equals ( base [ commonIndex ] ) ) { common . append ( target [ commonIndex ] + pathSeparator ) ; commonIndex ++ ; } if ( commonIndex == 0 ) { // No single common path element. This most // likely indicates differing drive letters, like C: and D:. // These paths cannot be relativized. throw new PathResolutionException ( "No common path element found for '" + normalizedTargetPath + "' and '" + normalizedBasePath + "'" ) ; } // The number of directories we have to backtrack depends on whether the base is a file or a dir // For example, the relative path from // // /foo/bar/baz/gg/ff to /foo/bar/baz //  // ".." if ff is a file // "../.." if ff is a directory // // The following is a heuristic to figure out if the base refers to a file or dir. It's not perfect, because // the resource referred to by this path may not actually exist, but it's the best I can do boolean baseIsFile = true ; File baseResource = new File ( normalizedBasePath ) ; if ( baseResource . exists ( ) ) { baseIsFile = baseResource . isFile ( ) ; } else if ( basePath . endsWith ( pathSeparator ) ) { baseIsFile = false ; } StringBuffer relative = new StringBuffer ( ) ; if ( base . length != commonIndex ) { int numDirsUp = baseIsFile ? base . length - commonIndex - 1 : base . length - commonIndex ; for ( int i = 0 ; i < numDirsUp ; i ++ ) { relative . append ( ".." + pathSeparator ) ; } } relative . append ( normalizedTargetPath . substring ( common . length ( ) ) ) ; return relative . toString ( ) ; }
<p>
Get the relative path from one file to another, specifying the directory
separator.
If one of the provided resources does not exist, it is assumed to be a
file unless it ends with '/' or '\'.
</p>

@param targetPath targetPath is calculated to this file
@param basePath basePath is calculated from this file
@param pathSeparator directory separator. The platform default is not assumed so that we can test Unix behaviour when running on Windows (for example)
@return
<p > Get the relative path from one file to another specifying the directory separator . If one of the provided resources does not exist it is assumed to be a file unless it ends with / or \ . < / p >