student:java:resources [JMU CS Wiki]

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===== Using Resources in Java Programs =====

==== Overview ====

One commonly wants to use icons, configuration files, and other kinds of resources in Java programs.
While there are many ways to do accomplish, doing it in a way that is portable requires some thought
because of the way different IDEs and deployment mechanisms (e.g., [[ student:java:jar | .jar files ]])
organize code.

==== Icons ====

To easiest way to refer to icons in a portable fashion is to use a ''%%URL%%'' object (from the ''%%java.net%%'' package), and the easiest way to get the appropriate ''%%URL%%'' object is to use a  ''%%Class%%'' object. For example, if you want to get the  ''%%URL%%'' for a resource named ''%%logo.png%%'' that is  in the ''%%icons%%'' directory **under the top-level directory containing the class of the object referred to by ''%%this%%''** then  you would do the following:

<code java>
URL url = this.getClass().getResource("/images/logo.png").
</code>

You could then load it as follows:

<code java>
ImageIcon icon = new ImageIcon(url);
</code>


==== Resource Bundles ====

The static ''%%getBundle()%%'' method in the ''%%ResourceBundle%%'' class can be passed a ''%%ClassLoader%%'' that it will use to locate the resources.  For example, if you want to get the  ''%%ResourceBundle%%'' with the prefix ''%%Strings%%'' that is  in the **top-level directory containing the class of the object referred to by ''%%this%%''** then  you would do the following:

<code java>
ResourceBundle strings;
strings = ResourceBundle.getBundle("Strings", Locale.getDefault(), this.getClass().getClassLoader());
</code>

==== Other Kinds of Resources ====

For other kinds of resources (e.g., configuration files), the easiest way to ensure that your code is portable is to use an ''%%InputStream%%'' object (from the ''%%java.io%%'' package) to refer to the resource and use a  ''%%Class%%'' object to retrieve the appropriate ''%%InputStream%%'' object.


For example, if you want to get the  ''%%InputStream%%'' for a text resource named ''%%default.cfg%%'' that is in the ''%%configurations%%'' directory **under the top-level directory containing the class of the object referred to by ''%%this%%''** then  you would do the following:


<code java>
InputStream is = this.getClass().getResourceAsStream("/configurations/config.txt").
</code>

You could then read the first line of this text resource as follows:

<code java>
BufferedReader in = new BufferedReader(new InputStreamReader(is));
String   line = in.readLine();
</code>


==== Copying Resources to a Temporary Directory ====

Sometimes it is necessary to copy resources from a %%.jar%% file to a temporary directory on the files system (e.g., if you want to load %%.html%% files that are in a %%.jar%% file into a browser). The following utility class can be used for this purpose.

<code java>
import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.nio.file.Files;
import java.nio.file.FileSystem;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.HashMap;
import java.util.List;
import java.util.stream.Stream;

public class ResourceCopier {

    /**
     * Copy all of the files from a resources package to a temporary directory
     * on the file system (whether or not the code and resources are in a .jar
     * file or on the file system).
     * 
     * This implementation assumes the resource package is a subpackage of the
     * package that contains this class (though that can be changed_.
     * 
     * @param id The ID to use for the temporary directory
     * @param subdir The name of the subpackage that contains the resources
     * @return The Path of the temporary directory
     * @throws IOException If something goes wrong
     * @throws URISyntaxException If something goes wrong
     */
    public static Path copyResourcesToTemp(String id, String subdir)
            throws IOException, URISyntaxException {

        // Get the location of the ResourceCopier.class file
        String canonicalName = ResourceCopier.class.getName();
        String packageName = ResourceCopier.class.getPackageName();
        String className = canonicalName.substring(packageName.length()+1) + ".class";
        String thisLocation = ResourceCopier.class.getResource(className).toString();

        // Remove the file name from the location and create a URI
        int fileStart = thisLocation.indexOf(className);
        String rootURL = thisLocation.substring(0, fileStart);
        URI sourceURI = new URI(rootURL + subdir);

        // Get the Path for source files (whether in a .jar file or the file system)
        Path sourcePath;
        FileSystem fileSystem = null;
        if (sourceURI.getScheme().equals("jar")) {
            fileSystem = FileSystems.newFileSystem(sourceURI, new HashMap<String, Object>());
            sourcePath = fileSystem.getPath("/" + packageName + "/" + subdir);
        } else {
            sourcePath = Paths.get(sourceURI);
        }

        // Get a List of all of the files in the source Path
        Stream<Path> files = Files.list(sourcePath);
        List<Path> filesList = files.toList();

        // Create a temporary directory on the local file system
        Path temp = Files.createTempDirectory(id);
        Path destinationPath = Path.of(temp.toString());

        // Copy all of the files from the source Path to the temporary directory
        for (Path file : filesList) {
            Path targetFile = Path.of(destinationPath.toString(), file.getFileName().toString());
            Files.copy(file, targetFile);
        }

        files.close();
        if (fileSystem != null) fileSystem.close();

        return destinationPath;
    }
}
</code>



==== Under the Hood ====

These technique use the ''%%ClassLoader%%'' infrastructure to  retrieve resources from the ''%%.jar%%'' file. So, it is very  important that you understand the relative locations of the  resource and the object that you call ''%%getClass()%%'' on.  A leading ''%%/%%'' will start at the root of the directory  tree,  but you still must understand where in the ''%%.jar%%''  file is the root. Omitting the leading ''%%/%%'', on the other hand, will start at the location of the ''%%.class%%'' file of the object referred to by ''%%this%%''.
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