Add listeners for different operating systems with different implementations

src/main/java/me/goudham/listener/ClipboardListener.java

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+package me.goudham.listener;
+
+public interface ClipboardListener {
+    void execute();
+}

src/main/java/me/goudham/listener/MacClipboardListener.java

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+package me.goudham.listener;
+
+import java.awt.Dimension;
+import java.awt.Image;
+import java.awt.Toolkit;
+import java.awt.datatransfer.Clipboard;
+import java.awt.datatransfer.Transferable;
+import java.awt.datatransfer.UnsupportedFlavorException;
+import java.awt.image.BufferedImage;
+import java.io.IOException;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+import me.goudham.model.MyClipboardContent;
+import me.goudham.util.ClipboardUtils;
+import org.jetbrains.annotations.NotNull;
+
+import static me.goudham.model.Contents.IMAGE;
+import static me.goudham.model.Contents.STRING;
+
+public class MacClipboardListener implements ClipboardListener {
+    private final Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();
+    private final @NotNull ClipboardEvent clipboardEvent;
+
+    public MacClipboardListener(@NotNull ClipboardEvent clipboardEvent) {
+        this.clipboardEvent = clipboardEvent;
+    }
+
+    @Override
+    public void execute() {
+        Transferable oldClipboardContents = clipboard.getContents(null);
+        final MyClipboardContent<?>[] myOldClipboardContentsArray = new MyClipboardContent[]{ ClipboardUtils.getClipboardContents(oldClipboardContents, clipboard) };
+
+        ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
+        executor.scheduleAtFixedRate(() -> {
+            Transferable newClipboardContents = clipboard.getContents(null);
+            MyClipboardContent<?> myNewClipboardContents = new MyClipboardContent<>("");
+
+            try {
+                if (STRING.isAvailable(clipboard)) {
+                    myNewClipboardContents.setContent(newClipboardContents.getTransferData(STRING.getDataFlavor()));
+                    if (!myNewClipboardContents.getContent().equals(myOldClipboardContentsArray[0].getContent())) {
+                        clipboardEvent.onCopyString((String) myNewClipboardContents.getContent());
+                        myOldClipboardContentsArray[0].setContent(myNewClipboardContents.getContent());
+                    }
+                } else if (IMAGE.isAvailable(clipboard)) {
+                    BufferedImage bufferedImage = ClipboardUtils.convertToBufferedImage((Image) newClipboardContents.getTransferData(IMAGE.getDataFlavor()));
+                    myNewClipboardContents.setContent(new Dimension(bufferedImage.getWidth(), bufferedImage.getHeight()));
+                    if (!myNewClipboardContents.getContent().equals(myOldClipboardContentsArray[0].getContent())) {
+                        clipboardEvent.onCopyImage(bufferedImage);
+                        myOldClipboardContentsArray[0].setContent(myNewClipboardContents.getContent());
+                    }
+                }
+            } catch (UnsupportedFlavorException | IOException exp) {
+                exp.printStackTrace();
+            }
+        }, 0, 350, TimeUnit.MILLISECONDS);
+    }
+}

src/main/java/me/goudham/listener/WindowsOrUnixClipboardListener.java

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+package me.goudham.listener;
+
+import java.awt.Image;
+import java.awt.Toolkit;
+import java.awt.datatransfer.Clipboard;
+import java.awt.datatransfer.ClipboardOwner;
+import java.awt.datatransfer.DataFlavor;
+import java.awt.datatransfer.Transferable;
+import java.awt.datatransfer.UnsupportedFlavorException;
+import java.awt.image.BufferedImage;
+import java.io.IOException;
+import me.goudham.util.ClipboardUtils;
+import org.jetbrains.annotations.NotNull;
+
+public class WindowsOrUnixClipboardListener extends Thread implements ClipboardListener, ClipboardOwner {
+    private final Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();
+    private final @NotNull ClipboardEvent clipboardEvent;
+
+    public WindowsOrUnixClipboardListener(@NotNull ClipboardEvent clipboardEvent) {
+        this.clipboardEvent = clipboardEvent;
+    }
+
+    @Override
+    public void lostOwnership(Clipboard oldClipboard, Transferable oldClipboardContents) {
+        try {
+            sleep(200);
+        } catch (InterruptedException ignored) {
+        }
+
+        Transferable newClipboardContents = oldClipboard.getContents(currentThread());
+        processContents(oldClipboard, newClipboardContents);
+        regainOwnership(oldClipboard, newClipboardContents);
+    }
+
+    public void processContents(Clipboard oldClipboard, Transferable newClipboardContents) {
+        try {
+            if (oldClipboard.isDataFlavorAvailable(DataFlavor.stringFlavor)) {
+                String stringContent = (String) newClipboardContents.getTransferData(DataFlavor.stringFlavor);
+                clipboardEvent.onCopyString(stringContent);
+            } else if (oldClipboard.isDataFlavorAvailable(DataFlavor.imageFlavor)) {
+                BufferedImage imageContent = ClipboardUtils.convertToBufferedImage((Image) newClipboardContents.getTransferData(DataFlavor.imageFlavor));
+                clipboardEvent.onCopyImage(imageContent);
+            }
+        } catch (UnsupportedFlavorException | IOException ignored) {
+        }
+    }
+
+    public void regainOwnership(Clipboard clipboard, Transferable newClipboardContents) {
+        try {
+            clipboard.setContents(newClipboardContents, this);
+        } catch (IllegalStateException ise) {
+            try {
+                sleep(200);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            regainOwnership(clipboard, newClipboardContents);
+        }
+    }
+
+    @Override
+    public void execute() {
+        Transferable currentClipboardContents = clipboard.getContents(null);
+        processContents(clipboard, currentClipboardContents);
+        regainOwnership(clipboard, currentClipboardContents);
+    }
+}