diff --git a/android/guava-testlib/src/com/google/common/testing/TearDownStack.java b/android/guava-testlib/src/com/google/common/testing/TearDownStack.java
index c22b54c40486..b16869865907 100644
--- a/android/guava-testlib/src/com/google/common/testing/TearDownStack.java
+++ b/android/guava-testlib/src/com/google/common/testing/TearDownStack.java
@@ -19,6 +19,7 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.Lists;
 import com.google.errorprone.annotations.concurrent.GuardedBy;
 import java.util.ArrayList;
@@ -40,7 +41,9 @@
 public class TearDownStack implements TearDownAccepter {
   private static final Logger logger = Logger.getLogger(TearDownStack.class.getName());
 
-  @GuardedBy("stack")
+  @VisibleForTesting final Object lock = new Object();
+
+  @GuardedBy("lock")
   final LinkedList<TearDown> stack = new LinkedList<>();
 
   private final boolean suppressThrows;
@@ -55,7 +58,7 @@ public TearDownStack(boolean suppressThrows) {
 
   @Override
   public final void addTearDown(TearDown tearDown) {
-    synchronized (stack) {
+    synchronized (lock) {
       stack.addFirst(checkNotNull(tearDown));
     }
   }
@@ -64,7 +67,7 @@ public final void addTearDown(TearDown tearDown) {
   public final void runTearDown() {
     List<Throwable> exceptions = new ArrayList<>();
     List<TearDown> stackCopy;
-    synchronized (stack) {
+    synchronized (lock) {
       stackCopy = Lists.newArrayList(stack);
       stack.clear();
     }
diff --git a/android/guava-testlib/src/com/google/common/testing/TestLogHandler.java b/android/guava-testlib/src/com/google/common/testing/TestLogHandler.java
index e481a2f19e9b..a08ed550c231 100644
--- a/android/guava-testlib/src/com/google/common/testing/TestLogHandler.java
+++ b/android/guava-testlib/src/com/google/common/testing/TestLogHandler.java
@@ -16,7 +16,6 @@
 
 package com.google.common.testing;
 
-
 import com.google.common.annotations.GwtCompatible;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -55,14 +54,18 @@
 @GwtCompatible
 @ElementTypesAreNonnullByDefault
 public class TestLogHandler extends Handler {
+  private final Object lock = new Object();
+
   /** We will keep a private list of all logged records */
   private final List<LogRecord> list = new ArrayList<>();
 
   /** Adds the most recently logged record to our list. */
   @Override
-  public synchronized void publish(@Nullable LogRecord record) {
-    if (record != null) {
-      list.add(record);
+  public void publish(@Nullable LogRecord record) {
+    synchronized (lock) {
+      if (record != null) {
+        list.add(record);
+      }
     }
   }
 
@@ -72,8 +75,10 @@ public void flush() {}
   @Override
   public void close() {}
 
-  public synchronized void clear() {
-    list.clear();
+  public void clear() {
+    synchronized (lock) {
+      list.clear();
+    }
   }
 
   /** Returns a snapshot of the logged records. */
@@ -84,8 +89,10 @@ public synchronized void clear() {
    * TODO(cpovirk): consider renaming this method to reflect that it takes a snapshot (and/or return
    * an ImmutableList)
    */
-  public synchronized List<LogRecord> getStoredLogRecords() {
-    List<LogRecord> result = new ArrayList<>(list);
-    return Collections.unmodifiableList(result);
+  public List<LogRecord> getStoredLogRecords() {
+    synchronized (lock) {
+      List<LogRecord> result = new ArrayList<>(list);
+      return Collections.unmodifiableList(result);
+    }
   }
 }
diff --git a/android/guava-testlib/test/com/google/common/testing/TearDownStackTest.java b/android/guava-testlib/test/com/google/common/testing/TearDownStackTest.java
index 63e162f75ee9..369563e04755 100644
--- a/android/guava-testlib/test/com/google/common/testing/TearDownStackTest.java
+++ b/android/guava-testlib/test/com/google/common/testing/TearDownStackTest.java
@@ -117,7 +117,7 @@ private TearDownStack buildTearDownStack() {
 
           @Override
           public void tearDown() throws Exception {
-            synchronized (result.stack) {
+            synchronized (result.lock) {
               assertEquals(
                   "The test should have cleared the stack (say, by virtue of running runTearDown)",
                   0,
diff --git a/android/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java b/android/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java
index e234b0f163d4..5192bb8297e5 100644
--- a/android/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java
+++ b/android/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java
@@ -134,6 +134,7 @@ public void testOrderingUnmodifiable() {
     assertOrderingReadOnly(Multimaps.unmodifiableMultimap(multimap));
   }
 
+  @J2ktIncompatible // Synchronized
   public void testOrderingSynchronized() {
     Multimap<String, Integer> multimap = initializeMultimap5();
     assertOrderingReadOnly(Multimaps.synchronizedMultimap(multimap));
diff --git a/android/guava-tests/test/com/google/common/collect/MapsTest.java b/android/guava-tests/test/com/google/common/collect/MapsTest.java
index 07657d350d6e..92d02944fd5c 100644
--- a/android/guava-tests/test/com/google/common/collect/MapsTest.java
+++ b/android/guava-tests/test/com/google/common/collect/MapsTest.java
@@ -1324,6 +1324,7 @@ public void testImmutableEntryNull() {
   }
 
   /** See {@link SynchronizedBiMapTest} for more tests. */
+  @J2ktIncompatible // Synchronized
   public void testSynchronizedBiMap() {
     BiMap<String, Integer> bimap = HashBiMap.create();
     bimap.put("one", 1);
diff --git a/android/guava-tests/test/com/google/common/collect/MultimapsTest.java b/android/guava-tests/test/com/google/common/collect/MultimapsTest.java
index 2a66620ba75c..171f7a48687e 100644
--- a/android/guava-tests/test/com/google/common/collect/MultimapsTest.java
+++ b/android/guava-tests/test/com/google/common/collect/MultimapsTest.java
@@ -167,6 +167,7 @@ public void testSerializingUnmodifiableTreeMultimap() {
   }
 
   @GwtIncompatible // slow (~10s)
+  @J2ktIncompatible // Synchronized
   public void testUnmodifiableSynchronizedArrayListMultimap() {
     checkUnmodifiableMultimap(
         Multimaps.synchronizedListMultimap(
@@ -187,6 +188,7 @@ public void testSerializingUnmodifiableSynchronizedArrayListMultimap() {
   }
 
   @GwtIncompatible // slow (~10s)
+  @J2ktIncompatible // Synchronized
   public void testUnmodifiableSynchronizedHashMultimap() {
     checkUnmodifiableMultimap(
         Multimaps.synchronizedSetMultimap(
@@ -207,6 +209,7 @@ public void testSerializingUnmodifiableSynchronizedHashMultimap() {
   }
 
   @GwtIncompatible // slow (~10s)
+  @J2ktIncompatible // Synchronized
   public void testUnmodifiableSynchronizedTreeMultimap() {
     TreeMultimap<String, Integer> delegate =
         TreeMultimap.create(Ordering.<String>natural(), INT_COMPARATOR);
@@ -932,11 +935,13 @@ public String transformEntry(String key, Integer value) {
     assertEquals("{a=[a1, a4, a4], b=[b6]}", transformed.toString());
   }
 
+  @J2ktIncompatible // Synchronized
   public void testSynchronizedMultimapSampleCodeCompilation() {
     // Extra indirection for J2KT, to avoid error: not enough information to infer type variable K
     this.<@Nullable Object, @Nullable Object>genericTestSynchronizedMultimapSampleCodeCompilation();
   }
 
+  @J2ktIncompatible // Synchronized
   private <K extends @Nullable Object, V extends @Nullable Object>
       void genericTestSynchronizedMultimapSampleCodeCompilation() {
     K key = null;
diff --git a/android/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java b/android/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java
index 0e85a24bb295..54ed07152f5e 100644
--- a/android/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java
+++ b/android/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java
@@ -132,8 +132,6 @@ public boolean isEmpty() {
       return super.isEmpty();
     }
 
-    /* Don't test iterator(); it may or may not hold the mutex. */
-
     @Override
     public boolean remove(@Nullable Object o) {
       assertTrue(Thread.holdsLock(mutex));
diff --git a/android/guava/src/com/google/common/base/Suppliers.java b/android/guava/src/com/google/common/base/Suppliers.java
index 9ef29af08933..c7cc3d0e23bd 100644
--- a/android/guava/src/com/google/common/base/Suppliers.java
+++ b/android/guava/src/com/google/common/base/Suppliers.java
@@ -121,6 +121,9 @@ public String toString() {
 
   @VisibleForTesting
   static class MemoizingSupplier<T extends @Nullable Object> implements Supplier<T>, Serializable {
+    private final Object lock =
+        new Integer(1); // something serializable
+
     final Supplier<T> delegate;
     transient volatile boolean initialized;
     // "value" does not need to be volatile; visibility piggy-backs
@@ -136,7 +139,7 @@ static class MemoizingSupplier<T extends @Nullable Object> implements Supplier<T
     public T get() {
       // A 2-field variant of Double Checked Locking.
       if (!initialized) {
-        synchronized (this) {
+        synchronized (lock) {
           if (!initialized) {
             T t = delegate.get();
             value = t;
@@ -161,6 +164,8 @@ public String toString() {
 
   @VisibleForTesting
   static class NonSerializableMemoizingSupplier<T extends @Nullable Object> implements Supplier<T> {
+    private final Object lock = new Object();
+
     @SuppressWarnings("UnnecessaryLambda") // Must be a fixed singleton object
     private static final Supplier<Void> SUCCESSFULLY_COMPUTED =
         () -> {
@@ -181,7 +186,7 @@ static class NonSerializableMemoizingSupplier<T extends @Nullable Object> implem
     public T get() {
       // Because Supplier is read-heavy, we use the "double-checked locking" pattern.
       if (delegate != SUCCESSFULLY_COMPUTED) {
-        synchronized (this) {
+        synchronized (lock) {
           if (delegate != SUCCESSFULLY_COMPUTED) {
             T t = delegate.get();
             value = t;
@@ -272,6 +277,9 @@ public String toString() {
   @SuppressWarnings("GoodTime") // lots of violations
   static class ExpiringMemoizingSupplier<T extends @Nullable Object>
       implements Supplier<T>, Serializable {
+    private final Object lock =
+        new Integer(1); // something serializable
+
     final Supplier<T> delegate;
     final long durationNanos;
     @CheckForNull transient volatile T value;
@@ -295,7 +303,7 @@ public T get() {
       long nanos = expirationNanos;
       long now = System.nanoTime();
       if (nanos == 0 || now - nanos >= 0) {
-        synchronized (this) {
+        synchronized (lock) {
           if (nanos == expirationNanos) { // recheck for lost race
             T t = delegate.get();
             value = t;
@@ -367,11 +375,13 @@ public String toString() {
    * Returns a supplier whose {@code get()} method synchronizes on {@code delegate} before calling
    * it, making it thread-safe.
    */
+  @J2ktIncompatible
   public static <T extends @Nullable Object> Supplier<T> synchronizedSupplier(
       Supplier<T> delegate) {
     return new ThreadSafeSupplier<>(delegate);
   }
 
+  @J2ktIncompatible
   private static class ThreadSafeSupplier<T extends @Nullable Object>
       implements Supplier<T>, Serializable {
     final Supplier<T> delegate;
diff --git a/android/guava/src/com/google/common/collect/Maps.java b/android/guava/src/com/google/common/collect/Maps.java
index 15c8c1ab8935..5ba7ff2b2283 100644
--- a/android/guava/src/com/google/common/collect/Maps.java
+++ b/android/guava/src/com/google/common/collect/Maps.java
@@ -1624,6 +1624,7 @@ public String toString() {
    * @param bimap the bimap to be wrapped in a synchronized view
    * @return a synchronized view of the specified bimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       BiMap<K, V> synchronizedBiMap(BiMap<K, V> bimap) {
     return Synchronized.biMap(bimap, null);
@@ -3609,6 +3610,7 @@ public NavigableMap<K, V> tailMap(@ParametricNullness K fromKey, boolean inclusi
    * @since 13.0
    */
   @GwtIncompatible // NavigableMap
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       NavigableMap<K, V> synchronizedNavigableMap(NavigableMap<K, V> navigableMap) {
     return Synchronized.navigableMap(navigableMap);
diff --git a/android/guava/src/com/google/common/collect/Multimaps.java b/android/guava/src/com/google/common/collect/Multimaps.java
index d5c027fbfa1f..441c6c2d0089 100644
--- a/android/guava/src/com/google/common/collect/Multimaps.java
+++ b/android/guava/src/com/google/common/collect/Multimaps.java
@@ -639,6 +639,7 @@ M invertFrom(Multimap<? extends V, ? extends K> source, M dest) {
    * @param multimap the multimap to be wrapped in a synchronized view
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       Multimap<K, V> synchronizedMultimap(Multimap<K, V> multimap) {
     return Synchronized.multimap(multimap, null);
@@ -899,6 +900,7 @@ public Comparator<? super V> valueComparator() {
    * @param multimap the multimap to be wrapped
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       SetMultimap<K, V> synchronizedSetMultimap(SetMultimap<K, V> multimap) {
     return Synchronized.setMultimap(multimap, null);
@@ -946,6 +948,7 @@ public static <K, V> SetMultimap<K, V> unmodifiableSetMultimap(
    * @param multimap the multimap to be wrapped
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       SortedSetMultimap<K, V> synchronizedSortedSetMultimap(SortedSetMultimap<K, V> multimap) {
     return Synchronized.sortedSetMultimap(multimap, null);
@@ -978,6 +981,7 @@ SortedSetMultimap<K, V> unmodifiableSortedSetMultimap(SortedSetMultimap<K, V> de
    * @param multimap the multimap to be wrapped
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       ListMultimap<K, V> synchronizedListMultimap(ListMultimap<K, V> multimap) {
     return Synchronized.listMultimap(multimap, null);
diff --git a/android/guava/src/com/google/common/collect/Queues.java b/android/guava/src/com/google/common/collect/Queues.java
index 4525f2623897..3f5d9b6ff520 100644
--- a/android/guava/src/com/google/common/collect/Queues.java
+++ b/android/guava/src/com/google/common/collect/Queues.java
@@ -407,6 +407,7 @@ public static <E> int drainUninterruptibly(
    * @return a synchronized view of the specified queue
    * @since 14.0
    */
+  @J2ktIncompatible // Synchronized
   public static <E extends @Nullable Object> Queue<E> synchronizedQueue(Queue<E> queue) {
     return Synchronized.queue(queue, null);
   }
@@ -440,6 +441,7 @@ public static <E> int drainUninterruptibly(
    * @return a synchronized view of the specified deque
    * @since 15.0
    */
+  @J2ktIncompatible // Synchronized
   public static <E extends @Nullable Object> Deque<E> synchronizedDeque(Deque<E> deque) {
     return Synchronized.deque(deque, null);
   }
diff --git a/android/guava/src/com/google/common/collect/Sets.java b/android/guava/src/com/google/common/collect/Sets.java
index 943e71957d64..0cecc57cf84a 100644
--- a/android/guava/src/com/google/common/collect/Sets.java
+++ b/android/guava/src/com/google/common/collect/Sets.java
@@ -1923,6 +1923,7 @@ public NavigableSet<E> tailSet(@ParametricNullness E fromElement, boolean inclus
    * @since 13.0
    */
   @GwtIncompatible // NavigableSet
+  @J2ktIncompatible // Synchronized
   public static <E extends @Nullable Object> NavigableSet<E> synchronizedNavigableSet(
       NavigableSet<E> navigableSet) {
     return Synchronized.navigableSet(navigableSet);
diff --git a/android/guava/src/com/google/common/collect/Synchronized.java b/android/guava/src/com/google/common/collect/Synchronized.java
index adb94fd03461..dcf409d42b03 100644
--- a/android/guava/src/com/google/common/collect/Synchronized.java
+++ b/android/guava/src/com/google/common/collect/Synchronized.java
@@ -55,6 +55,7 @@
  * @author Mike Bostock
  * @author Jared Levy
  */
+@J2ktIncompatible
 @GwtCompatible(emulated = true)
 @ElementTypesAreNonnullByDefault
 /*
diff --git a/android/guava/src/com/google/common/collect/Tables.java b/android/guava/src/com/google/common/collect/Tables.java
index 789a57e1e2c5..69fe5897e4c3 100644
--- a/android/guava/src/com/google/common/collect/Tables.java
+++ b/android/guava/src/com/google/common/collect/Tables.java
@@ -21,6 +21,7 @@
 import static com.google.common.collect.NullnessCasts.uncheckedCastNullableTToT;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.annotations.J2ktIncompatible;
 import com.google.common.base.Function;
 import com.google.common.base.Objects;
 import com.google.common.base.Supplier;
@@ -730,6 +731,7 @@ public Map<Object, Object> apply(Map<Object, Object> input) {
    * @return a synchronized view of the specified table
    * @since 22.0
    */
+  @J2ktIncompatible // Synchronized
   public static <R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>
       Table<R, C, V> synchronizedTable(Table<R, C, V> table) {
     return Synchronized.table(table, null);
diff --git a/android/guava/src/com/google/common/util/concurrent/LazyLogger.java b/android/guava/src/com/google/common/util/concurrent/LazyLogger.java
index 9ae0f784e57e..ef1b56957d52 100644
--- a/android/guava/src/com/google/common/util/concurrent/LazyLogger.java
+++ b/android/guava/src/com/google/common/util/concurrent/LazyLogger.java
@@ -22,6 +22,8 @@
 @GwtCompatible
 @ElementTypesAreNonnullByDefault
 final class LazyLogger {
+  private final Object lock = new Object();
+
   private final String loggerName;
   private volatile @Nullable Logger logger;
 
@@ -45,7 +47,7 @@ Logger get() {
     if (local != null) {
       return local;
     }
-    synchronized (this) {
+    synchronized (lock) {
       local = logger;
       if (local != null) {
         return local;
diff --git a/guava-testlib/src/com/google/common/collect/testing/testers/CollectionSpliteratorTester.java b/guava-testlib/src/com/google/common/collect/testing/testers/CollectionSpliteratorTester.java
index a4ee0b56a4fa..45fa49a82f53 100644
--- a/guava-testlib/src/com/google/common/collect/testing/testers/CollectionSpliteratorTester.java
+++ b/guava-testlib/src/com/google/common/collect/testing/testers/CollectionSpliteratorTester.java
@@ -46,41 +46,31 @@ public class CollectionSpliteratorTester<E> extends AbstractCollectionTester<E>
 
   @CollectionFeature.Require(absent = KNOWN_ORDER)
   public void testSpliteratorUnknownOrder() {
-    synchronized (collection) {
-      SpliteratorTester.of(collection::spliterator).expect(getSampleElements());
-    }
+    SpliteratorTester.of(collection::spliterator).expect(getSampleElements());
   }
 
   @CollectionFeature.Require(KNOWN_ORDER)
   public void testSpliteratorKnownOrder() {
-    synchronized (collection) {
-      SpliteratorTester.of(collection::spliterator).expect(getOrderedElements()).inOrder();
-    }
+    SpliteratorTester.of(collection::spliterator).expect(getOrderedElements()).inOrder();
   }
 
   @CollectionFeature.Require(ALLOWS_NULL_VALUES)
   @CollectionSize.Require(absent = ZERO)
   public void testSpliteratorNullable() {
     initCollectionWithNullElement();
-    synchronized (collection) { // for Collections.synchronized
-      assertFalse(collection.spliterator().hasCharacteristics(Spliterator.NONNULL));
-    }
+    assertFalse(collection.spliterator().hasCharacteristics(Spliterator.NONNULL));
   }
 
   @CollectionFeature.Require(SUPPORTS_ADD)
   public void testSpliteratorNotImmutable_CollectionAllowsAdd() {
     // If add is supported, verify that IMMUTABLE is not reported.
-    synchronized (collection) { // for Collections.synchronized
-      assertFalse(collection.spliterator().hasCharacteristics(Spliterator.IMMUTABLE));
-    }
+    assertFalse(collection.spliterator().hasCharacteristics(Spliterator.IMMUTABLE));
   }
 
   @CollectionFeature.Require(SUPPORTS_REMOVE)
   public void testSpliteratorNotImmutable_CollectionAllowsRemove() {
     // If remove is supported, verify that IMMUTABLE is not reported.
-    synchronized (collection) { // for Collections.synchronized
-      assertFalse(collection.spliterator().hasCharacteristics(Spliterator.IMMUTABLE));
-    }
+    assertFalse(collection.spliterator().hasCharacteristics(Spliterator.IMMUTABLE));
   }
 
   @J2ktIncompatible
diff --git a/guava-testlib/src/com/google/common/collect/testing/testers/CollectionStreamTester.java b/guava-testlib/src/com/google/common/collect/testing/testers/CollectionStreamTester.java
index 38bbb90a0900..2cf0e7236506 100644
--- a/guava-testlib/src/com/google/common/collect/testing/testers/CollectionStreamTester.java
+++ b/guava-testlib/src/com/google/common/collect/testing/testers/CollectionStreamTester.java
@@ -41,22 +41,16 @@ public class CollectionStreamTester<E> extends AbstractCollectionTester<E> {
 
   @CollectionFeature.Require(absent = KNOWN_ORDER)
   public void testStreamToArrayUnknownOrder() {
-    synchronized (collection) { // to allow Collections.synchronized* tests to pass
-      Helpers.assertEqualIgnoringOrder(
-          getSampleElements(), Arrays.asList(collection.stream().toArray()));
-    }
+    Helpers.assertEqualIgnoringOrder(
+        getSampleElements(), Arrays.asList(collection.stream().toArray()));
   }
 
   @CollectionFeature.Require(KNOWN_ORDER)
   public void testStreamToArrayKnownOrder() {
-    synchronized (collection) { // to allow Collections.synchronized* tests to pass
-      assertEquals(getOrderedElements(), Arrays.asList(collection.stream().toArray()));
-    }
+    assertEquals(getOrderedElements(), Arrays.asList(collection.stream().toArray()));
   }
 
   public void testStreamCount() {
-    synchronized (collection) { // to allow Collections.synchronized* tests to pass
-      assertEquals(getNumElements(), collection.stream().count());
-    }
+    assertEquals(getNumElements(), collection.stream().count());
   }
 }
diff --git a/guava-testlib/src/com/google/common/testing/TearDownStack.java b/guava-testlib/src/com/google/common/testing/TearDownStack.java
index c22b54c40486..b16869865907 100644
--- a/guava-testlib/src/com/google/common/testing/TearDownStack.java
+++ b/guava-testlib/src/com/google/common/testing/TearDownStack.java
@@ -19,6 +19,7 @@
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.Lists;
 import com.google.errorprone.annotations.concurrent.GuardedBy;
 import java.util.ArrayList;
@@ -40,7 +41,9 @@
 public class TearDownStack implements TearDownAccepter {
   private static final Logger logger = Logger.getLogger(TearDownStack.class.getName());
 
-  @GuardedBy("stack")
+  @VisibleForTesting final Object lock = new Object();
+
+  @GuardedBy("lock")
   final LinkedList<TearDown> stack = new LinkedList<>();
 
   private final boolean suppressThrows;
@@ -55,7 +58,7 @@ public TearDownStack(boolean suppressThrows) {
 
   @Override
   public final void addTearDown(TearDown tearDown) {
-    synchronized (stack) {
+    synchronized (lock) {
       stack.addFirst(checkNotNull(tearDown));
     }
   }
@@ -64,7 +67,7 @@ public final void addTearDown(TearDown tearDown) {
   public final void runTearDown() {
     List<Throwable> exceptions = new ArrayList<>();
     List<TearDown> stackCopy;
-    synchronized (stack) {
+    synchronized (lock) {
       stackCopy = Lists.newArrayList(stack);
       stack.clear();
     }
diff --git a/guava-testlib/src/com/google/common/testing/TestLogHandler.java b/guava-testlib/src/com/google/common/testing/TestLogHandler.java
index e481a2f19e9b..a08ed550c231 100644
--- a/guava-testlib/src/com/google/common/testing/TestLogHandler.java
+++ b/guava-testlib/src/com/google/common/testing/TestLogHandler.java
@@ -16,7 +16,6 @@
 
 package com.google.common.testing;
 
-
 import com.google.common.annotations.GwtCompatible;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -55,14 +54,18 @@
 @GwtCompatible
 @ElementTypesAreNonnullByDefault
 public class TestLogHandler extends Handler {
+  private final Object lock = new Object();
+
   /** We will keep a private list of all logged records */
   private final List<LogRecord> list = new ArrayList<>();
 
   /** Adds the most recently logged record to our list. */
   @Override
-  public synchronized void publish(@Nullable LogRecord record) {
-    if (record != null) {
-      list.add(record);
+  public void publish(@Nullable LogRecord record) {
+    synchronized (lock) {
+      if (record != null) {
+        list.add(record);
+      }
     }
   }
 
@@ -72,8 +75,10 @@ public void flush() {}
   @Override
   public void close() {}
 
-  public synchronized void clear() {
-    list.clear();
+  public void clear() {
+    synchronized (lock) {
+      list.clear();
+    }
   }
 
   /** Returns a snapshot of the logged records. */
@@ -84,8 +89,10 @@ public synchronized void clear() {
    * TODO(cpovirk): consider renaming this method to reflect that it takes a snapshot (and/or return
    * an ImmutableList)
    */
-  public synchronized List<LogRecord> getStoredLogRecords() {
-    List<LogRecord> result = new ArrayList<>(list);
-    return Collections.unmodifiableList(result);
+  public List<LogRecord> getStoredLogRecords() {
+    synchronized (lock) {
+      List<LogRecord> result = new ArrayList<>(list);
+      return Collections.unmodifiableList(result);
+    }
   }
 }
diff --git a/guava-testlib/test/com/google/common/testing/TearDownStackTest.java b/guava-testlib/test/com/google/common/testing/TearDownStackTest.java
index 63e162f75ee9..369563e04755 100644
--- a/guava-testlib/test/com/google/common/testing/TearDownStackTest.java
+++ b/guava-testlib/test/com/google/common/testing/TearDownStackTest.java
@@ -117,7 +117,7 @@ private TearDownStack buildTearDownStack() {
 
           @Override
           public void tearDown() throws Exception {
-            synchronized (result.stack) {
+            synchronized (result.lock) {
               assertEquals(
                   "The test should have cleared the stack (say, by virtue of running runTearDown)",
                   0,
diff --git a/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java b/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java
index 625e1892c992..1a1382bd801b 100644
--- a/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java
+++ b/guava-tests/test/com/google/common/collect/LinkedHashMultimapTest.java
@@ -135,6 +135,7 @@ public void testOrderingUnmodifiable() {
     assertOrderingReadOnly(Multimaps.unmodifiableMultimap(multimap));
   }
 
+  @J2ktIncompatible // Synchronized
   public void testOrderingSynchronized() {
     Multimap<String, Integer> multimap = initializeMultimap5();
     assertOrderingReadOnly(Multimaps.synchronizedMultimap(multimap));
diff --git a/guava-tests/test/com/google/common/collect/MapsTest.java b/guava-tests/test/com/google/common/collect/MapsTest.java
index ac9bcc2b078b..31cdee432fde 100644
--- a/guava-tests/test/com/google/common/collect/MapsTest.java
+++ b/guava-tests/test/com/google/common/collect/MapsTest.java
@@ -1369,6 +1369,7 @@ public void testImmutableEntryNull() {
   }
 
   /** See {@link SynchronizedBiMapTest} for more tests. */
+  @J2ktIncompatible // Synchronized
   public void testSynchronizedBiMap() {
     BiMap<String, Integer> bimap = HashBiMap.create();
     bimap.put("one", 1);
diff --git a/guava-tests/test/com/google/common/collect/MultimapsTest.java b/guava-tests/test/com/google/common/collect/MultimapsTest.java
index a821d77d5052..3e79c97d99da 100644
--- a/guava-tests/test/com/google/common/collect/MultimapsTest.java
+++ b/guava-tests/test/com/google/common/collect/MultimapsTest.java
@@ -236,6 +236,7 @@ public void testSerializingUnmodifiableTreeMultimap() {
   }
 
   @GwtIncompatible // slow (~10s)
+  @J2ktIncompatible // Synchronized
   public void testUnmodifiableSynchronizedArrayListMultimap() {
     checkUnmodifiableMultimap(
         Multimaps.synchronizedListMultimap(
@@ -256,6 +257,7 @@ public void testSerializingUnmodifiableSynchronizedArrayListMultimap() {
   }
 
   @GwtIncompatible // slow (~10s)
+  @J2ktIncompatible // Synchronized
   public void testUnmodifiableSynchronizedHashMultimap() {
     checkUnmodifiableMultimap(
         Multimaps.synchronizedSetMultimap(
@@ -276,6 +278,7 @@ public void testSerializingUnmodifiableSynchronizedHashMultimap() {
   }
 
   @GwtIncompatible // slow (~10s)
+  @J2ktIncompatible // Synchronized
   public void testUnmodifiableSynchronizedTreeMultimap() {
     TreeMultimap<String, Integer> delegate =
         TreeMultimap.create(Ordering.<String>natural(), INT_COMPARATOR);
@@ -1001,11 +1004,13 @@ public String transformEntry(String key, Integer value) {
     assertEquals("{a=[a1, a4, a4], b=[b6]}", transformed.toString());
   }
 
+  @J2ktIncompatible // Synchronized
   public void testSynchronizedMultimapSampleCodeCompilation() {
     // Extra indirection for J2KT, to avoid error: not enough information to infer type variable K
     this.<@Nullable Object, @Nullable Object>genericTestSynchronizedMultimapSampleCodeCompilation();
   }
 
+  @J2ktIncompatible // Synchronized
   private <K extends @Nullable Object, V extends @Nullable Object>
       void genericTestSynchronizedMultimapSampleCodeCompilation() {
     K key = null;
diff --git a/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java b/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java
index 9db478cbb0be..1c42b92a7c08 100644
--- a/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java
+++ b/guava-tests/test/com/google/common/collect/SynchronizedSetTest.java
@@ -25,6 +25,8 @@
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.Spliterator;
+import java.util.stream.Stream;
 import junit.framework.Test;
 import junit.framework.TestCase;
 import org.checkerframework.checker.nullness.qual.Nullable;
@@ -130,7 +132,32 @@ public boolean isEmpty() {
       return super.isEmpty();
     }
 
-    /* Don't test iterator(); it may or may not hold the mutex. */
+    /*
+     * We don't assert that the lock is held during calls to iterator(), stream(), and spliterator:
+     * `Synchronized` doesn't guarantee that it will hold the mutex for those calls because callers
+     * are responsible for taking the mutex themselves:
+     * https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/util/Collections.html#synchronizedCollection(java.util.Collection)
+     *
+     * Similarly, we avoid having those methods *implemented* in terms of *other* TestSet methods
+     * that will perform holdsLock assertions:
+     *
+     * - For iterator(), we can accomplish that by not overriding iterator() at all. That way, we
+     *   inherit an implementation that forwards to the delegate collection, which performs no
+     *   holdsLock assertions.
+     *
+     * - For stream() and spliterator(), we have to forward to the delegate ourselves because
+     *   ForwardingSet does not forward `default` methods, as discussed in its Javadoc.
+     */
+
+    @Override
+    public Stream<E> stream() {
+      return delegate.stream();
+    }
+
+    @Override
+    public Spliterator<E> spliterator() {
+      return delegate.spliterator();
+    }
 
     @Override
     public boolean remove(@Nullable Object o) {
diff --git a/guava/src/com/google/common/base/Suppliers.java b/guava/src/com/google/common/base/Suppliers.java
index 9ef29af08933..c7cc3d0e23bd 100644
--- a/guava/src/com/google/common/base/Suppliers.java
+++ b/guava/src/com/google/common/base/Suppliers.java
@@ -121,6 +121,9 @@ public String toString() {
 
   @VisibleForTesting
   static class MemoizingSupplier<T extends @Nullable Object> implements Supplier<T>, Serializable {
+    private final Object lock =
+        new Integer(1); // something serializable
+
     final Supplier<T> delegate;
     transient volatile boolean initialized;
     // "value" does not need to be volatile; visibility piggy-backs
@@ -136,7 +139,7 @@ static class MemoizingSupplier<T extends @Nullable Object> implements Supplier<T
     public T get() {
       // A 2-field variant of Double Checked Locking.
       if (!initialized) {
-        synchronized (this) {
+        synchronized (lock) {
           if (!initialized) {
             T t = delegate.get();
             value = t;
@@ -161,6 +164,8 @@ public String toString() {
 
   @VisibleForTesting
   static class NonSerializableMemoizingSupplier<T extends @Nullable Object> implements Supplier<T> {
+    private final Object lock = new Object();
+
     @SuppressWarnings("UnnecessaryLambda") // Must be a fixed singleton object
     private static final Supplier<Void> SUCCESSFULLY_COMPUTED =
         () -> {
@@ -181,7 +186,7 @@ static class NonSerializableMemoizingSupplier<T extends @Nullable Object> implem
     public T get() {
       // Because Supplier is read-heavy, we use the "double-checked locking" pattern.
       if (delegate != SUCCESSFULLY_COMPUTED) {
-        synchronized (this) {
+        synchronized (lock) {
           if (delegate != SUCCESSFULLY_COMPUTED) {
             T t = delegate.get();
             value = t;
@@ -272,6 +277,9 @@ public String toString() {
   @SuppressWarnings("GoodTime") // lots of violations
   static class ExpiringMemoizingSupplier<T extends @Nullable Object>
       implements Supplier<T>, Serializable {
+    private final Object lock =
+        new Integer(1); // something serializable
+
     final Supplier<T> delegate;
     final long durationNanos;
     @CheckForNull transient volatile T value;
@@ -295,7 +303,7 @@ public T get() {
       long nanos = expirationNanos;
       long now = System.nanoTime();
       if (nanos == 0 || now - nanos >= 0) {
-        synchronized (this) {
+        synchronized (lock) {
           if (nanos == expirationNanos) { // recheck for lost race
             T t = delegate.get();
             value = t;
@@ -367,11 +375,13 @@ public String toString() {
    * Returns a supplier whose {@code get()} method synchronizes on {@code delegate} before calling
    * it, making it thread-safe.
    */
+  @J2ktIncompatible
   public static <T extends @Nullable Object> Supplier<T> synchronizedSupplier(
       Supplier<T> delegate) {
     return new ThreadSafeSupplier<>(delegate);
   }
 
+  @J2ktIncompatible
   private static class ThreadSafeSupplier<T extends @Nullable Object>
       implements Supplier<T>, Serializable {
     final Supplier<T> delegate;
diff --git a/guava/src/com/google/common/collect/Maps.java b/guava/src/com/google/common/collect/Maps.java
index 0876c9a7b0b7..2622db163a80 100644
--- a/guava/src/com/google/common/collect/Maps.java
+++ b/guava/src/com/google/common/collect/Maps.java
@@ -1654,6 +1654,7 @@ public String toString() {
    * @param bimap the bimap to be wrapped in a synchronized view
    * @return a synchronized view of the specified bimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       BiMap<K, V> synchronizedBiMap(BiMap<K, V> bimap) {
     return Synchronized.biMap(bimap, null);
@@ -3799,6 +3800,7 @@ public NavigableMap<K, V> tailMap(@ParametricNullness K fromKey, boolean inclusi
    * @since 13.0
    */
   @GwtIncompatible // NavigableMap
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       NavigableMap<K, V> synchronizedNavigableMap(NavigableMap<K, V> navigableMap) {
     return Synchronized.navigableMap(navigableMap);
diff --git a/guava/src/com/google/common/collect/Multimaps.java b/guava/src/com/google/common/collect/Multimaps.java
index 58d727010d75..c0aaa2cba375 100644
--- a/guava/src/com/google/common/collect/Multimaps.java
+++ b/guava/src/com/google/common/collect/Multimaps.java
@@ -638,6 +638,7 @@ M invertFrom(Multimap<? extends V, ? extends K> source, M dest) {
    * @param multimap the multimap to be wrapped in a synchronized view
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       Multimap<K, V> synchronizedMultimap(Multimap<K, V> multimap) {
     return Synchronized.multimap(multimap, null);
@@ -903,6 +904,7 @@ public Comparator<? super V> valueComparator() {
    * @param multimap the multimap to be wrapped
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       SetMultimap<K, V> synchronizedSetMultimap(SetMultimap<K, V> multimap) {
     return Synchronized.setMultimap(multimap, null);
@@ -950,6 +952,7 @@ public static <K, V> SetMultimap<K, V> unmodifiableSetMultimap(
    * @param multimap the multimap to be wrapped
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       SortedSetMultimap<K, V> synchronizedSortedSetMultimap(SortedSetMultimap<K, V> multimap) {
     return Synchronized.sortedSetMultimap(multimap, null);
@@ -982,6 +985,7 @@ SortedSetMultimap<K, V> unmodifiableSortedSetMultimap(SortedSetMultimap<K, V> de
    * @param multimap the multimap to be wrapped
    * @return a synchronized view of the specified multimap
    */
+  @J2ktIncompatible // Synchronized
   public static <K extends @Nullable Object, V extends @Nullable Object>
       ListMultimap<K, V> synchronizedListMultimap(ListMultimap<K, V> multimap) {
     return Synchronized.listMultimap(multimap, null);
diff --git a/guava/src/com/google/common/collect/Queues.java b/guava/src/com/google/common/collect/Queues.java
index e6cf8d6e30e5..253c9052b5fd 100644
--- a/guava/src/com/google/common/collect/Queues.java
+++ b/guava/src/com/google/common/collect/Queues.java
@@ -452,6 +452,7 @@ public static <E> int drainUninterruptibly(
    * @return a synchronized view of the specified queue
    * @since 14.0
    */
+  @J2ktIncompatible // Synchronized
   public static <E extends @Nullable Object> Queue<E> synchronizedQueue(Queue<E> queue) {
     return Synchronized.queue(queue, null);
   }
@@ -485,6 +486,7 @@ public static <E> int drainUninterruptibly(
    * @return a synchronized view of the specified deque
    * @since 15.0
    */
+  @J2ktIncompatible // Synchronized
   public static <E extends @Nullable Object> Deque<E> synchronizedDeque(Deque<E> deque) {
     return Synchronized.deque(deque, null);
   }
diff --git a/guava/src/com/google/common/collect/Sets.java b/guava/src/com/google/common/collect/Sets.java
index 386633f977a8..d46bc06715b3 100644
--- a/guava/src/com/google/common/collect/Sets.java
+++ b/guava/src/com/google/common/collect/Sets.java
@@ -1989,6 +1989,7 @@ public NavigableSet<E> tailSet(@ParametricNullness E fromElement, boolean inclus
    * @since 13.0
    */
   @GwtIncompatible // NavigableSet
+  @J2ktIncompatible // Synchronized
   public static <E extends @Nullable Object> NavigableSet<E> synchronizedNavigableSet(
       NavigableSet<E> navigableSet) {
     return Synchronized.navigableSet(navigableSet);
diff --git a/guava/src/com/google/common/collect/Synchronized.java b/guava/src/com/google/common/collect/Synchronized.java
index 26fbd034dad0..b3972cfd096f 100644
--- a/guava/src/com/google/common/collect/Synchronized.java
+++ b/guava/src/com/google/common/collect/Synchronized.java
@@ -64,6 +64,7 @@
  * @author Mike Bostock
  * @author Jared Levy
  */
+@J2ktIncompatible
 @GwtCompatible(emulated = true)
 @ElementTypesAreNonnullByDefault
 /*
diff --git a/guava/src/com/google/common/collect/Tables.java b/guava/src/com/google/common/collect/Tables.java
index d8aabde00ab3..a55d691014f3 100644
--- a/guava/src/com/google/common/collect/Tables.java
+++ b/guava/src/com/google/common/collect/Tables.java
@@ -21,6 +21,7 @@
 import static com.google.common.collect.NullnessCasts.uncheckedCastNullableTToT;
 
 import com.google.common.annotations.GwtCompatible;
+import com.google.common.annotations.J2ktIncompatible;
 import com.google.common.base.Function;
 import com.google.common.base.Objects;
 import com.google.common.base.Supplier;
@@ -737,6 +738,7 @@ public Map<Object, Object> apply(Map<Object, Object> input) {
    * @return a synchronized view of the specified table
    * @since 22.0
    */
+  @J2ktIncompatible // Synchronized
   public static <R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>
       Table<R, C, V> synchronizedTable(Table<R, C, V> table) {
     return Synchronized.table(table, null);
diff --git a/guava/src/com/google/common/util/concurrent/LazyLogger.java b/guava/src/com/google/common/util/concurrent/LazyLogger.java
index 9ae0f784e57e..ef1b56957d52 100644
--- a/guava/src/com/google/common/util/concurrent/LazyLogger.java
+++ b/guava/src/com/google/common/util/concurrent/LazyLogger.java
@@ -22,6 +22,8 @@
 @GwtCompatible
 @ElementTypesAreNonnullByDefault
 final class LazyLogger {
+  private final Object lock = new Object();
+
   private final String loggerName;
   private volatile @Nullable Logger logger;
 
@@ -45,7 +47,7 @@ Logger get() {
     if (local != null) {
       return local;
     }
-    synchronized (this) {
+    synchronized (lock) {
       local = logger;
       if (local != null) {
         return local;