Excessive locking in TypeCachingBytecodeGenerator#BOOTSTRAP_LOCK

Changed locking scheme in TypeCachingBytecodeGenerator from a
single global lock to cacheLocks with size 16.
The used TypeCachingLock from cacheLocks depends on
the hashcode of MockitoMockKey, which aggregates the types and
settings to mock.

Fixes #3035

src/main/java/org/mockito/internal/creation/bytebuddy/TypeCachingBytecodeGenerator.java

@@ -6,6 +6,7 @@
 
 import java.lang.ref.ReferenceQueue;
 import java.util.Set;
+import java.util.concurrent.Callable;
 import java.util.concurrent.locks.ReadWriteLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 
@@ -14,38 +15,68 @@
 
 class TypeCachingBytecodeGenerator extends ReferenceQueue<ClassLoader>
         implements BytecodeGenerator {
-
-    private static final Object BOOTSTRAP_LOCK = new Object();
+    /**
+     * The size of the {@link #cacheLocks}.
+     * <p>Caution: This must match the {@link #CACHE_LOCK_MASK}.
+     */
+    private static final int CACHE_LOCK_SIZE = 16;
+
+    /**
+     * The mask to use to mask out the {@link MockitoMockKey#hashCode()} to find the {@link #cacheLocks}.
+     * <p>Caution: this must match the bits of the {@link #CACHE_LOCK_SIZE}.
+     */
+    private static final int CACHE_LOCK_MASK = 0x0F;
 
     private final BytecodeGenerator bytecodeGenerator;
 
     private final TypeCache<MockitoMockKey> typeCache;
 
     private final ReadWriteLock lock = new ReentrantReadWriteLock();
 
+    /**
+     * This array contains {@link TypeCachingLock} instances, which are used as java monitor locks for
+     * {@link TypeCache#findOrInsert(ClassLoader, Object, Callable, Object)}.
+     * The locks spread the lock to acquire over multiple locks instead of using a single lock
+     * {@code BOOTSTRAP_LOCK} for all {@link MockitoMockKey}.
+     *
+     * <p>Note: We can't simply use the mockedType class lock as a lock,
+     * because the {@link MockitoMockKey}, will be the same for different mockTypes + interfaces.
+     *
+     * <p>#3035: Excessive locking in TypeCachingBytecodeGenerator#BOOTSTRAP_LOCK
+     */
+    private final TypeCachingLock[] cacheLocks;
+
     public TypeCachingBytecodeGenerator(BytecodeGenerator bytecodeGenerator, boolean weak) {
         this.bytecodeGenerator = bytecodeGenerator;
         typeCache =
                 new TypeCache.WithInlineExpunction<>(
                         weak ? TypeCache.Sort.WEAK : TypeCache.Sort.SOFT);
+
+        this.cacheLocks = new TypeCachingLock[CACHE_LOCK_SIZE];
+        for (int i = 0; i < CACHE_LOCK_SIZE; i++) {
+            cacheLocks[i] = new TypeCachingLock();
+        }
     }
 
     @SuppressWarnings("unchecked")
     @Override
     public <T> Class<T> mockClass(final MockFeatures<T> params) {
         lock.readLock().lock();
         try {
-            ClassLoader classLoader = params.mockedType.getClassLoader();
+            Class<T> mockedType = params.mockedType;
+            ClassLoader classLoader = mockedType.getClassLoader();
+            MockitoMockKey key =
+                    new MockitoMockKey(
+                            mockedType,
+                            params.interfaces,
+                            params.serializableMode,
+                            params.stripAnnotations);
             return (Class<T>)
                     typeCache.findOrInsert(
                             classLoader,
-                            new MockitoMockKey(
-                                    params.mockedType,
-                                    params.interfaces,
-                                    params.serializableMode,
-                                    params.stripAnnotations),
+                            key,
                             () -> bytecodeGenerator.mockClass(params),
-                            BOOTSTRAP_LOCK);
+                            getCacheLockForKey(key));
         } catch (IllegalArgumentException exception) {
             Throwable cause = exception.getCause();
             if (cause instanceof RuntimeException) {
@@ -58,6 +89,20 @@ public <T> Class<T> mockClass(final MockFeatures<T> params) {
         }
     }
 
+    /**
+     * Returns a {@link TypeCachingLock}, which locks the {@link TypeCache#findOrInsert(ClassLoader, Object, Callable, Object)}.
+     *
+     * @param key the key to lock
+     * @return the {@link TypeCachingLock} to use to lock the {@link TypeCache}
+     */
+    private TypeCachingLock getCacheLockForKey(MockitoMockKey key) {
+        int hashCode = key.hashCode();
+        // Try to spread some higher bits with XOR to lower bits, because we only use lower bits.
+        hashCode = hashCode ^ (hashCode >>> 16);
+        int index = hashCode & CACHE_LOCK_MASK;
+        return cacheLocks[index];
+    }
+
     @Override
     public void mockClassStatic(Class<?> type) {
         bytecodeGenerator.mockClassStatic(type);
@@ -79,6 +124,8 @@ public void clearAllCaches() {
         }
     }
 
+    private static final class TypeCachingLock {}
+
     private static class MockitoMockKey extends TypeCache.SimpleKey {
 
         private final SerializableMode serializableMode;