Improves the readability of the backup code a bit

src/main/java/net/knarcraft/minecraftserverlauncher/userinterface/BackupGUI.java

@@ -75,19 +75,19 @@ public class BackupGUI implements ActionListener {
      * Destroys the backup GUI
      */
     public static void destroy() {
-        BackupUtil.abortBackup();
-        progressTextArea = null;
-        progressBar = null;
-        cancelButton = null;
         if (frame != null) {
             frame.dispose();
-        }
             frame = null;
+            progressBar = null;
+            progressTextArea = null;
+            cancelButton = null;
+        }
     }
 
     @Override
     public void actionPerformed(ActionEvent actionEvent) {
         if (actionEvent.getSource() == cancelButton) {
+            BackupUtil.abortBackup();
             destroy();
         }
     }

src/main/java/net/knarcraft/minecraftserverlauncher/utility/BackupUtil.java

@@ -38,9 +38,12 @@ public class BackupUtil {
      *
      * @param source         <p>The folder to copy</p>
      * @param destination    <p>Target destination</p>
+     * @param backupFileSize <p>The total file size of the backup in progress</p>
+     * @param alreadyCopied  <p>The amount of bytes already copied</p>
      * @throws IOException <p>If we can't start a file stream</p>
      */
-    private static synchronized long backupFolder(File source, File destination, long backupFileSize, long alreadyCopied) throws IOException {
+    private static synchronized long backupFolder(File source, File destination, long backupFileSize,
+                                                  long alreadyCopied) throws IOException {
         if (backupAborted) {
             return 0L;
         }
@@ -91,6 +94,8 @@ public class BackupUtil {
 
     /**
      * Copies all server directories to a folder specified by the user
+     *
+     * @param gui <p>The GUI to use for informing the user</p>
      */
     public synchronized static void backup(GUI gui) {
         backupAborted = false;
@@ -116,15 +121,46 @@ public class BackupUtil {
 
         gui.setStatus("Backing up " + (backupFileSize / 1000000) + "MB");
 
+        performBackup(gui, serverFolders, backupFileSize);
+    }
+
+    /**
+     * Performs the actual backup after checks have passed and necessary info is available
+     * @param gui <p>The GUI to use for informing the user</p>
+     * @param serverFolders <p>The folders of the servers to backup</p>
+     * @param backupFileSize <p>The total size of the folders to backup</p>
+     */
+    private static void performBackup(GUI gui, List<List<File>> serverFolders, long backupFileSize) {
         new BackupGUI();
         BackupGUI.updateProgress("Backup starting...", 0);
         long alreadyCopied = 0;
         for (List<File> serverFolder : serverFolders) {
-            if (backupAborted) {
+            if (backupAborted || !backupRunning) {
                 gui.setStatus("Backup aborted");
                 backupRunning = false;
                 return;
             }
+            alreadyCopied = backupServerFiles(gui, serverFolder, backupFileSize, alreadyCopied);
+        }
+        backupRunning = false;
+        if (backupAborted) {
+            gui.setStatus("Backup aborted");
+        } else {
+            BackupGUI.destroy();
+            gui.setStatus("Backup finished");
+        }
+    }
+
+    /**
+     * Backs up the files for a single server
+     *
+     * @param gui <p>The GUI to use for informing the user</p>
+     * @param serverFolder <p>The server's input and output folders</p>
+     * @param backupFileSize <p>The total size of the files to backup</p>
+     * @param alreadyCopied <p>The amount of bytes already copied</p>
+     * @return <p>The new amount of bytes copied</p>
+     */
+    private static long backupServerFiles(GUI gui, List<File> serverFolder, long backupFileSize, long alreadyCopied) {
         File srcFolder = serverFolder.get(0);
         File destinationFolder = serverFolder.get(1);
 
@@ -142,14 +178,7 @@ public class BackupUtil {
             gui.setStatus("Backup caused an error");
             backupRunning = false;
         }
-        }
-        backupRunning = false;
-        BackupGUI.destroy();
-        if (backupAborted) {
-            gui.setStatus("Backup aborted");
-        } else {
-            gui.setStatus("Backup finished");
-        }
+        return alreadyCopied;
     }
 
     /**
@@ -208,6 +237,7 @@ public class BackupUtil {
 
     /**
      * Gets the size of a file given its path
+     *
      * @param path <p>The path to a file</p>
      * @return <p>The size of the file in bytes, or 0 if an exception is thrown</p>
      */