diff --git a/src/org/kde/kdeconnect/Backends/BaseLink.java b/src/org/kde/kdeconnect/Backends/BaseLink.java
index a238a389..bf9507d5 100644
--- a/src/org/kde/kdeconnect/Backends/BaseLink.java
+++ b/src/org/kde/kdeconnect/Backends/BaseLink.java
@@ -28,6 +28,8 @@ import org.kde.kdeconnect.NetworkPacket;
 import java.security.PrivateKey;
 import java.util.ArrayList;
 
+import androidx.annotation.WorkerThread;
+
 
 public abstract class BaseLink {
 
@@ -88,5 +90,6 @@ public abstract class BaseLink {
     }
 
     //TO OVERRIDE, should be sync
+    @WorkerThread
     public abstract boolean sendPacket(NetworkPacket np, Device.SendPacketStatusCallback callback);
 }
diff --git a/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java b/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java
index 2623485d..27e9f3f5 100644
--- a/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java
+++ b/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java
@@ -43,6 +43,8 @@ import java.io.Reader;
 import java.nio.charset.Charset;
 import java.util.UUID;
 
+import androidx.annotation.WorkerThread;
+
 public class BluetoothLink extends BaseLink {
     private final ConnectionMultiplexer connection;
     private final InputStream input;
@@ -150,6 +152,7 @@ public class BluetoothLink extends BaseLink {
         Log.i("BluetoothLink", "Finished sending message");
     }
 
+    @WorkerThread
     @Override
     public boolean sendPacket(NetworkPacket np, final Device.SendPacketStatusCallback callback) {
 
diff --git a/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java b/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java
index f4847fa0..c8853de3 100644
--- a/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java
+++ b/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java
@@ -44,6 +44,8 @@ import java.nio.channels.NotYetConnectedException;
 
 import javax.net.ssl.SSLSocket;
 
+import androidx.annotation.WorkerThread;
+
 public class LanLink extends BaseLink {
 
     public interface LinkDisconnectedCallback {
@@ -137,6 +139,7 @@ public class LanLink extends BaseLink {
     }
 
     //Blocking, do not call from main thread
+    @WorkerThread
     @Override
     public boolean sendPacket(NetworkPacket np, final Device.SendPacketStatusCallback callback) {
         if (socket == null) {
diff --git a/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java b/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java
index bb8bef66..9ee0a753 100644
--- a/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java
+++ b/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java
@@ -28,6 +28,8 @@ import org.kde.kdeconnect.Backends.BasePairingHandler;
 import org.kde.kdeconnect.Device;
 import org.kde.kdeconnect.NetworkPacket;
 
+import androidx.annotation.WorkerThread;
+
 public class LoopbackLink extends BaseLink {
 
     public LoopbackLink(Context context, BaseLinkProvider linkProvider) {
@@ -44,6 +46,7 @@ public class LoopbackLink extends BaseLink {
         return new LoopbackPairingHandler(device, callback);
     }
 
+    @WorkerThread
     @Override
     public boolean sendPacket(NetworkPacket in, Device.SendPacketStatusCallback callback) {
         packageReceived(in);
diff --git a/src/org/kde/kdeconnect/Device.java b/src/org/kde/kdeconnect/Device.java
index e9d6fc5b..35cdef56 100644
--- a/src/org/kde/kdeconnect/Device.java
+++ b/src/org/kde/kdeconnect/Device.java
@@ -57,6 +57,8 @@ import java.util.Vector;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 
+import androidx.annotation.AnyThread;
+import androidx.annotation.WorkerThread;
 import androidx.core.app.NotificationCompat;
 import androidx.core.content.ContextCompat;
 
@@ -621,18 +623,22 @@ public class Device implements BaseLink.PacketReceiver {
         }
     };
 
+    @AnyThread
     public void sendPacket(NetworkPacket np) {
         sendPacket(np, -1, defaultCallback);
     }
 
+    @AnyThread
     public void sendPacket(NetworkPacket np, int replaceID) {
         sendPacket(np, replaceID, defaultCallback);
     }
 
+    @WorkerThread
     public boolean sendPacketBlocking(NetworkPacket np) {
         return sendPacketBlocking(np, defaultCallback);
     }
 
+    @AnyThread
     public void sendPacket(final NetworkPacket np, final SendPacketStatusCallback callback) {
         sendPacket(np, -1, callback);
     }
@@ -643,6 +649,7 @@ public class Device implements BaseLink.PacketReceiver {
      * @param replaceID If positive, replaces all unsent packages with the same replaceID
      * @param callback A callback for success/failure
      */
+    @AnyThread
     public void sendPacket(final NetworkPacket np, int replaceID, final SendPacketStatusCallback callback) {
         if (packetQueue == null) {
             callback.onFailure(new Exception("Device disconnected!"));
@@ -665,6 +672,15 @@ public class Device implements BaseLink.PacketReceiver {
         }
     }
 
+    /**
+     * Send {@code np} over one of this device's connected {@link #links}.
+     *
+     * @param np       the packet to send
+     * @param callback a callback that can receive realtime updates
+     * @return true if the packet was sent ok, false otherwise
+     * @see BaseLink#sendPacket(NetworkPacket, SendPacketStatusCallback)
+     */
+    @WorkerThread
     public boolean sendPacketBlocking(final NetworkPacket np, final SendPacketStatusCallback callback) {
 
         /*
diff --git a/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeReceiveFileJob.java b/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeReceiveFileJob.java
index 63a67318..1a63081e 100644
--- a/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeReceiveFileJob.java
+++ b/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeReceiveFileJob.java
@@ -43,9 +43,27 @@ import java.io.OutputStream;
 import java.util.ArrayList;
 import java.util.List;
 
+import androidx.annotation.GuardedBy;
 import androidx.core.content.FileProvider;
 import androidx.documentfile.provider.DocumentFile;
 
+/**
+ * A type of {@link BackgroundJob} that reads Files from another device.
+ *
+ * <p>
+ *     We receive the requests as {@link NetworkPacket}s.
+ * </p>
+ * <p>
+ *     Each packet should have a 'filename' property and a payload. If the payload is missing,
+ *     we'll just create an empty file. You can add new packets anytime via
+ *     {@link #addNetworkPacket(NetworkPacket)}.
+ * </p>
+ * <p>
+ *     The I/O-part of this file reading is handled by {@link #receiveFile(InputStream, OutputStream)}.
+ * </p>
+ *
+ * @see CompositeUploadFileJob
+ */
 public class CompositeReceiveFileJob extends BackgroundJob<Device, Void> {
     private final ReceiveNotification receiveNotification;
     private NetworkPacket currentNetworkPacket;
@@ -56,8 +74,11 @@ public class CompositeReceiveFileJob extends BackgroundJob<Device, Void> {
     private long prevProgressPercentage;
 
     private final Object lock;                              //Use to protect concurrent access to the variables below
+    @GuardedBy("lock")
     private final List<NetworkPacket> networkPacketList;
+    @GuardedBy("lock")
     private int totalNumFiles;
+    @GuardedBy("lock")
     private long totalPayloadSize;
     private boolean isRunning;
 
diff --git a/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeUploadFileJob.java b/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeUploadFileJob.java
index a9e22827..69f6a828 100644
--- a/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeUploadFileJob.java
+++ b/src/org/kde/kdeconnect/Plugins/SharePlugin/CompositeUploadFileJob.java
@@ -31,8 +31,28 @@ import org.kde.kdeconnect_tp.R;
 import java.util.ArrayList;
 import java.util.List;
 
+import androidx.annotation.GuardedBy;
 import androidx.annotation.NonNull;
 
+/**
+ * A type of {@link BackgroundJob} that sends Files to another device.
+ *
+ * <p>
+ *     We represent the individual upload requests as {@link NetworkPacket}s.
+ * </p>
+ * <p>
+ *     Each packet should have a 'filename' property and a payload. If the payload is
+ *     missing, we'll just send an empty file. You can add new packets anytime via
+ *     {@link #addNetworkPacket(NetworkPacket)}.
+ * </p>
+ * <p>
+ *     The I/O-part of this file sending is handled by
+ *     {@link Device#sendPacketBlocking(NetworkPacket, Device.SendPacketStatusCallback)}.
+ * </p>
+ *
+ * @see CompositeReceiveFileJob
+ * @see SendPacketStatusCallback
+ */
 public class CompositeUploadFileJob extends BackgroundJob<Device, Void> {
     private boolean isRunning;
     private Handler handler;
@@ -44,10 +64,13 @@ public class CompositeUploadFileJob extends BackgroundJob<Device, Void> {
     private UploadNotification uploadNotification;
 
     private final Object lock;                              //Use to protect concurrent access to the variables below
+    @GuardedBy("lock")
     private final List<NetworkPacket> networkPacketList;
     private NetworkPacket currentNetworkPacket;
     private final Device.SendPacketStatusCallback sendPacketStatusCallback;
+    @GuardedBy("lock")
     private int totalNumFiles;
+    @GuardedBy("lock")
     private long totalPayloadSize;
 
     CompositeUploadFileJob(@NonNull Device device, @NonNull Callback<Void> callback) {
@@ -169,6 +192,9 @@ public class CompositeUploadFileJob extends BackgroundJob<Device, Void> {
         }
     }
 
+    /**
+     * Use this to send metadata ahead of all the other {@link #networkPacketList packets}.
+     */
     private void sendUpdatePacket() {
         NetworkPacket np = new NetworkPacket(SharePlugin.PACKET_TYPE_SHARE_REQUEST_UPDATE);
 
diff --git a/src/org/kde/kdeconnect/Plugins/SharePlugin/SharePlugin.java b/src/org/kde/kdeconnect/Plugins/SharePlugin/SharePlugin.java
index fac25b1c..2d3f99a3 100644
--- a/src/org/kde/kdeconnect/Plugins/SharePlugin/SharePlugin.java
+++ b/src/org/kde/kdeconnect/Plugins/SharePlugin/SharePlugin.java
@@ -49,6 +49,13 @@ import androidx.annotation.NonNull;
 import androidx.annotation.WorkerThread;
 import androidx.core.content.ContextCompat;
 
+/**
+ * A Plugin for sharing and receiving files and uris.
+ * <p>
+ *     All of the associated I/O work is scheduled on background
+ *     threads by {@link BackgroundJobHandler}.
+ * </p>
+ */
 @PluginFactory.LoadablePlugin
 public class SharePlugin extends Plugin {
     final static String ACTION_CANCEL_SHARE = "org.kde.kdeconnect.Plugins.SharePlugin.CancelShare";
diff --git a/src/org/kde/kdeconnect/async/BackgroundJobHandler.java b/src/org/kde/kdeconnect/async/BackgroundJobHandler.java
index b2891833..ad4172e7 100644
--- a/src/org/kde/kdeconnect/async/BackgroundJobHandler.java
+++ b/src/org/kde/kdeconnect/async/BackgroundJobHandler.java
@@ -37,6 +37,13 @@ import java.util.concurrent.TimeUnit;
 
 import androidx.annotation.Nullable;
 
+/**
+ * Scheduler for {@link BackgroundJob} objects.
+ * <p>
+ *     We use an internal {@link ThreadPoolExecutor} to catch Exceptions and
+ *     pass them along to {@link #handleUncaughtException(Future, Throwable)}.
+ * </p>
+ */
 public class BackgroundJobHandler {
     private static final String TAG = BackgroundJobHandler.class.getSimpleName();