diff --git a/phoenicis-tools/src/main/java/org/phoenicis/tools/http/Downloader.java b/phoenicis-tools/src/main/java/org/phoenicis/tools/http/Downloader.java
index f01ed802eb8..9c32430a3b5 100644
--- a/phoenicis-tools/src/main/java/org/phoenicis/tools/http/Downloader.java
+++ b/phoenicis-tools/src/main/java/org/phoenicis/tools/http/Downloader.java
@@ -29,6 +29,9 @@
 import java.net.URLConnection;
 import java.util.function.Consumer;
 
+/**
+ * file download utilities (mainly to be used from JS)
+ */
 @Safe
 public class Downloader {
     private static final String EXCEPTION_ITEM_DOWNLOAD_FAILED = "Download of %s has failed";
@@ -36,10 +39,20 @@ public class Downloader {
 
     private final FileSizeUtilities fileSizeUtilities;
 
+    /**
+     * constructor
+     * @param fileSizeUtilities
+     */
     public Downloader(FileSizeUtilities fileSizeUtilities) {
         this.fileSizeUtilities = fileSizeUtilities;
     }
 
+    /**
+     * downloads url to localFile, shows progress via onChange
+     * @param url download URL
+     * @param localFile destination of the download
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     */
     public void get(String url, String localFile, Consumer<ProgressEntity> onChange) {
         try {
             get(new URL(url), new File(localFile), onChange);
@@ -48,6 +61,12 @@ public void get(String url, String localFile, Consumer<ProgressEntity> onChange)
         }
     }
 
+    /**
+     * downloads url to localFile, shows progress via onChange
+     * @param url download URL
+     * @param localFile destination of the download
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     */
     public void get(URL url, File localFile, Consumer<ProgressEntity> onChange) {
         try {
             get(url, new FileOutputStream(localFile), onChange);
@@ -56,6 +75,12 @@ public void get(URL url, File localFile, Consumer<ProgressEntity> onChange) {
         }
     }
 
+    /**
+     * downloads url and returns downloaded content, shows progress via onChange
+     * @param url download URL
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     * @return downloaded content
+     */
     public String get(String url, Consumer<ProgressEntity> onChange) {
         ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
         try {
@@ -66,18 +91,36 @@ public String get(String url, Consumer<ProgressEntity> onChange) {
         return outputStream.toString();
     }
 
+    /**
+     * downloads url and returns downloaded content, shows progress via onChange
+     * @param url download URL
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     * @return downloaded content
+     */
     public String get(URL url, Consumer<ProgressEntity> onChange) {
         ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
         get(url, outputStream, onChange);
         return outputStream.toString();
     }
 
+    /**
+     * downloads url and returns downloaded content, shows progress via onChange
+     * @param url download URL
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     * @return downloaded content
+     */
     public byte[] getBytes(URL url, Consumer<ProgressEntity> onChange) {
         ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
         get(url, outputStream, onChange);
         return outputStream.toByteArray();
     }
 
+    /**
+     * downloads url to outputStream, shows progress via onChange
+     * @param url download URL
+     * @param outputStream file is downloaded to this stream
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     */
     private void get(URL url, OutputStream outputStream, Consumer<ProgressEntity> onChange) {
         try {
             URLConnection connection = url.openConnection();
@@ -87,6 +130,13 @@ private void get(URL url, OutputStream outputStream, Consumer<ProgressEntity> on
         }
     }
 
+    /**
+     * downloads url to outputStream, shows progress via onChange
+     * @param url download URL
+     * @param connection URLConnection which is used for the download
+     * @param outputStream file is downloaded to this stream
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     */
     private void saveConnectionToStream(URL url, URLConnection connection, OutputStream outputStream,
             Consumer<ProgressEntity> onChange) {
         float percentage = 0F;
@@ -125,6 +175,13 @@ private void saveConnectionToStream(URL url, URLConnection connection, OutputStr
         changeState(ProgressState.SUCCESS, percentage, "", onChange);
     }
 
+    /**
+     * updates the progress indicator
+     * @param state current state (e.g. if download finished)
+     * @param percentage progress percentage
+     * @param progressText e.g. downloaded x of y bytes
+     * @param onChange consumer to show the download progress (e.g. a progress bar)
+     */
     private void changeState(ProgressState state, float percentage, String progressText,
             Consumer<ProgressEntity> onChange) {
         if (onChange != null) {
@@ -134,4 +191,46 @@ private void changeState(ProgressState state, float percentage, String progressT
         }
     }
 
+    /**
+     * checks if the downloadable file has been updated more recently than localFile
+     * This can be used to avoid re-downloading existing resources if no new version is available.
+     * Note: The last modified date must be set correctly by the server.
+     * @param localFile local file (should have been downloaded from url)
+     * @param url download URL
+     * @return update for localFile is available
+     */
+    public boolean isUpdateAvailable(String localFile, String url) {
+        try {
+            return isUpdateAvailable(new File(localFile), new URL(url));
+        } catch (MalformedURLException e) {
+            throw new DownloadException(String.format(EXCEPTION_ITEM_DOWNLOAD_FAILED, url), e);
+        }
+    }
+
+    /**
+     * checks if the downloadable file has been updated more recently than localFile
+     * This can be used to avoid re-downloading existing resources if no new version is available.
+     * Note: The last modified date must be set correctly by the server.
+     * @param localFile local file (should have been downloaded from url)
+     * @param url download URL
+     * @return update for localFile is available
+     */
+    public boolean isUpdateAvailable(File localFile, URL url) {
+        if (!localFile.exists()) {
+            return true;
+        }
+        try {
+            URLConnection connection = url.openConnection();
+            connection.connect();
+            long fileLastModified = localFile.lastModified();
+            long urlLastModified = connection.getLastModified();
+            if (fileLastModified == 0 || urlLastModified == 0) {
+                // we know nothing
+                return true;
+            }
+            return localFile.lastModified() <= connection.getLastModified();
+        } catch (IOException e) {
+            throw new DownloadException(String.format(EXCEPTION_ITEM_DOWNLOAD_FAILED, url), e);
+        }
+    }
 }