Simplify code relating to UI thread synchronization

* java/org/gnu/emacs/EmacsContextMenu.java (display):

* java/org/gnu/emacs/EmacsDialog.java (display):

* java/org/gnu/emacs/EmacsService.java (getEmacsView)
(getLocationOnScreen, getClipboardManager)
(requestDirectoryAccess): Replace manual synchronization within
Runnable objects by usage of FutureTask.
(syncRunnable): Accept FutureTask<V> in place of Runnables, and
obtain and return results from calls to its get method.

3 files changed, 115 insertions, 146 deletions

diff --git a/java/org/gnu/emacs/EmacsContextMenu.java b/java/org/gnu/emacs/EmacsContextMenu.java
index 2652f35b545..b6c63c3cbe1 100644
--- a/java/org/gnu/emacs/EmacsContextMenu.java
+++ b/java/org/gnu/emacs/EmacsContextMenu.java
@@ -22,6 +22,9 @@ package org.gnu.emacs;
 import java.util.List;
 import java.util.ArrayList;
 
+import java.util.concurrent.Callable;
+import java.util.concurrent.FutureTask;
+
 import android.content.Context;
 import android.content.Intent;
 
@@ -344,8 +347,7 @@ public final class EmacsContextMenu
   display (final EmacsWindow window, final int xPosition,
            final int yPosition, final int serial)
   {
-    Runnable runnable;
-    final EmacsHolder<Boolean> rc;
+    FutureTask<Boolean> task;
 
     /* Android will permanently cease to display any popup menus at
        all if the list of menu items is empty.  Prevent this by
@@ -354,25 +356,17 @@ public final class EmacsContextMenu
     if (menuItems.isEmpty ())
       return false;
 
-    rc = new EmacsHolder<Boolean> ();
-    rc.thing = false;
-
-    runnable = new Runnable () {
+    task = new FutureTask<Boolean> (new Callable<Boolean> () {
         @Override
-        public void
-        run ()
+        public Boolean
+        call ()
         {
-          synchronized (this)
-            {
-              lastMenuEventSerial = serial;
-              rc.thing = display1 (window, xPosition, yPosition);
-              notify ();
-            }
+          lastMenuEventSerial = serial;
+          return display1 (window, xPosition, yPosition);
         }
-      };
+      });
 
-    EmacsService.syncRunnable (runnable);
-    return rc.thing;
+    return EmacsService.<Boolean>syncRunnable (task);
   }
 
   /* Dismiss this context menu.  WINDOW is the window where the
diff --git a/java/org/gnu/emacs/EmacsDialog.java b/java/org/gnu/emacs/EmacsDialog.java
index bad1ddde227..7552b16b370 100644
--- a/java/org/gnu/emacs/EmacsDialog.java
+++ b/java/org/gnu/emacs/EmacsDialog.java
@@ -22,6 +22,9 @@ package org.gnu.emacs;
 import java.util.List;
 import java.util.ArrayList;
 
+import java.util.concurrent.Callable;
+import java.util.concurrent.FutureTask;
+
 import android.app.AlertDialog;
 
 import android.content.Context;
@@ -388,26 +391,18 @@ public final class EmacsDialog implements DialogInterface.OnDismissListener
   public boolean
   display ()
   {
-    Runnable runnable;
-    final EmacsHolder<Boolean> rc;
+    FutureTask<Boolean> task;
 
-    rc = new EmacsHolder<Boolean> ();
-    rc.thing = false;
-    runnable = new Runnable () {
+    task = new FutureTask<Boolean> (new Callable<Boolean> () {
         @Override
-        public void
-        run ()
+        public Boolean
+        call ()
         {
-          synchronized (this)
-            {
-              rc.thing = display1 ();
-              notify ();
-            }
+          return display1 ();
         }
-      };
+      });
 
-    EmacsService.syncRunnable (runnable);
-    return rc.thing;
+    return EmacsService.<Boolean>syncRunnable (task);
   }
 
 
diff --git a/java/org/gnu/emacs/EmacsService.java b/java/org/gnu/emacs/EmacsService.java
index c71670b3e47..7934d6f9cd3 100644
--- a/java/org/gnu/emacs/EmacsService.java
+++ b/java/org/gnu/emacs/EmacsService.java
@@ -27,6 +27,10 @@ import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
 
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.FutureTask;
+
 import java.util.concurrent.atomic.AtomicInteger;
 
 import android.database.Cursor;
@@ -331,52 +335,45 @@ public final class EmacsService extends Service
                 final boolean isFocusedByDefault)
   {
     Runnable runnable;
-    final EmacsHolder<EmacsView> view;
-
-    view = new EmacsHolder<EmacsView> ();
+    FutureTask<EmacsView> task;
 
-    runnable = new Runnable () {
+    task = new FutureTask<EmacsView> (new Callable<EmacsView> () {
         @Override
-        public void
-        run ()
+        public EmacsView
+        call ()
         {
-          synchronized (this)
-            {
-              view.thing = new EmacsView (window);
-              view.thing.setVisibility (visibility);
+          EmacsView view;
 
-              /* The following function is only present on Android 26
-                 or later.  */
-              if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
-                view.thing.setFocusedByDefault (isFocusedByDefault);
+          view = new EmacsView (window);
+          view.setVisibility (visibility);
 
-              notify ();
-            }
+          /* The following function is only present on Android 26
+             or later.  */
+          if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
+            view.setFocusedByDefault (isFocusedByDefault);
+
+          return view;
         }
-      };
+      });
 
-    syncRunnable (runnable);
-    return view.thing;
+    return EmacsService.<EmacsView>syncRunnable (task);
   }
 
   public void
   getLocationOnScreen (final EmacsView view, final int[] coordinates)
   {
-    Runnable runnable;
+    FutureTask<Void> task;
 
-    runnable = new Runnable () {
-        public void
-        run ()
+    task = new FutureTask<Void> (new Callable<Void> () {
+        public Void
+        call ()
         {
-          synchronized (this)
-            {
-              view.getLocationOnScreen (coordinates);
-              notify ();
-            }
+          view.getLocationOnScreen (coordinates);
+          return null;
         }
-      };
+      });
 
-    syncRunnable (runnable);
+    EmacsService.<Void>syncRunnable (task);
   }
 
 
@@ -702,28 +699,17 @@ public final class EmacsService extends Service
   public ClipboardManager
   getClipboardManager ()
   {
-    final EmacsHolder<ClipboardManager> manager;
-    Runnable runnable;
+    FutureTask<Object> task;
 
-    manager = new EmacsHolder<ClipboardManager> ();
-
-    runnable = new Runnable () {
-        public void
-        run ()
+    task = new FutureTask<Object> (new Callable<Object> () {
+        public Object
+        call ()
         {
-          Object tem;
-
-          synchronized (this)
-            {
-              tem = getSystemService (Context.CLIPBOARD_SERVICE);
-              manager.thing = (ClipboardManager) tem;
-              notify ();
-            }
+          return getSystemService (Context.CLIPBOARD_SERVICE);
         }
-      };
+      });
 
-    syncRunnable (runnable);
-    return manager.thing;
+    return (ClipboardManager) EmacsService.<Object>syncRunnable (task);
   }
 
   public void
@@ -738,33 +724,37 @@ public final class EmacsService extends Service
     System.exit (0);
   }
 
-  /* Wait synchronously for the specified RUNNABLE to complete in the
-     UI thread.  Must be called from the Emacs thread.  */
+  /* Wait synchronously for the specified TASK to complete in the UI
+     thread, then return its result.  Must be called from the Emacs
+     thread.  */
 
-  public static void
-  syncRunnable (Runnable runnable)
+  public static <V> V
+  syncRunnable (FutureTask<V> task)
   {
+    V object;
+
     EmacsNative.beginSynchronous ();
+    SERVICE.runOnUiThread (task);
 
-    synchronized (runnable)
+    try
       {
-        SERVICE.runOnUiThread (runnable);
-
-        while (true)
-          {
-            try
-              {
-                runnable.wait ();
-                break;
-              }
-            catch (InterruptedException e)
-              {
-                continue;
-              }
-          }
+        object = task.get ();
+      }
+    catch (ExecutionException exception)
+      {
+        /* Wrap this exception in a RuntimeException and signal it to
+           the caller.  */
+        throw new RuntimeException (exception.getCause ());
+      }
+    catch (InterruptedException exception)
+      {
+        EmacsNative.emacsAbort ();
+        object = null;
       }
 
     EmacsNative.endSynchronous ();
+
+    return object;
   }
 
 
@@ -1283,71 +1273,61 @@ public final class EmacsService extends Service
   public int
   requestDirectoryAccess ()
   {
-    Runnable runnable;
-    final EmacsHolder<Integer> rc;
+    FutureTask<Integer> task;
 
     /* Return 1 if Android is too old to support this feature.  */
 
     if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP)
       return 1;
 
-    rc = new EmacsHolder<Integer> ();
-    rc.thing = Integer.valueOf (1);
-
-    runnable = new Runnable () {
+    task = new FutureTask<Integer> (new Callable<Integer> () {
         @Override
-        public void
-        run ()
+        public Integer
+        call ()
         {
           EmacsActivity activity;
           Intent intent;
-          int id;
+          int id, rc;
+
+          /* Try to obtain an activity that will receive the response
+             from the file chooser dialog.  */
 
-          synchronized (this)
+          if (EmacsActivity.focusedActivities.isEmpty ())
             {
-              /* Try to obtain an activity that will receive the
-                 response from the file chooser dialog.  */
+              /* If focusedActivities is empty then this dialog may
+                 have been displayed immediately after another popup
+                 dialog was dismissed.  Try the EmacsActivity to be
+                 focused.  */
 
-              if (EmacsActivity.focusedActivities.isEmpty ())
-                {
-                  /* If focusedActivities is empty then this dialog
-                     may have been displayed immediately after another
-                     popup dialog was dismissed.  Try the
-                     EmacsActivity to be focused.  */
-
-                  activity = EmacsActivity.lastFocusedActivity;
-
-                  if (activity == null)
-                    {
-                      /* Still no luck.  Return failure.  */
-                      notify ();
-                      return;
-                    }
-                }
-              else
-                activity = EmacsActivity.focusedActivities.get (0);
+              activity = EmacsActivity.lastFocusedActivity;
 
-              /* Now create the intent.  */
-              intent = new Intent (Intent.ACTION_OPEN_DOCUMENT_TREE);
+              if (activity == null)
+                /* Still no luck.  Return failure.  */
+                return 1;
+            }
+          else
+            activity = EmacsActivity.focusedActivities.get (0);
 
-              try
-                {
-                  id = EmacsActivity.ACCEPT_DOCUMENT_TREE;
-                  activity.startActivityForResult (intent, id, null);
-                  rc.thing = Integer.valueOf (0);
-                }
-              catch (Exception e)
-                {
-                  e.printStackTrace ();
-                }
+          /* Now create the intent.  */
+          intent = new Intent (Intent.ACTION_OPEN_DOCUMENT_TREE);
+          rc = 1;
 
-              notify ();
+          try
+            {
+              id = EmacsActivity.ACCEPT_DOCUMENT_TREE;
+              activity.startActivityForResult (intent, id, null);
+              rc = 0;
             }
+          catch (Exception e)
+            {
+              e.printStackTrace ();
+            }
+
+          return rc;
         }
-      };
+      });
 
-    syncRunnable (runnable);
-    return rc.thing;
+    return EmacsService.<Integer>syncRunnable (task);
   }
 
   /* Return an array of each tree provided by the document PROVIDER
@@ -1969,7 +1949,7 @@ public final class EmacsService extends Service
           /* Now request these permissions.  */
           activity.requestPermissions (new String[] { permission,
                                                       permission1, },
-            0);
+                                       0);
         }
       };