Share internal StandardExecutionContext delegates

This commit makes sure that the per-operation execution context for
caching and event listening does not recreate the default internal
delegates, but rather get initialized with a shared state.

This reduces the number of instances created per operation execution,
reducing the GC pressure as a result. This also makes sure that any
cache, such as the one in StandardTypeLocator, is reused.

Closes gh-31617

spring-expression/src/main/java/org/springframework/expression/spel/support/StandardEvaluationContext.java

@@ -55,6 +55,7 @@ import org.springframework.util.Assert;
  * @author Andy Clement
  * @author Juergen Hoeller
  * @author Sam Brannen
+ * @author Stephane Nicoll
  * @since 3.0
  * @see SimpleEvaluationContext
  * @see ReflectivePropertyAccessor
@@ -90,9 +91,9 @@ public class StandardEvaluationContext implements EvaluationContext {
 	@Nullable
 	private TypeConverter typeConverter;
 
-	private TypeComparator typeComparator = new StandardTypeComparator();
+	private TypeComparator typeComparator = StandardTypeComparator.INSTANCE;
 
-	private OperatorOverloader operatorOverloader = new StandardOperatorOverloader();
+	private OperatorOverloader operatorOverloader = StandardOperatorOverloader.INSTANCE;
 
 	private final Map<String, Object> variables = new ConcurrentHashMap<>();
 
@@ -329,6 +330,29 @@ public class StandardEvaluationContext implements EvaluationContext {
 		resolver.registerMethodFilter(type, filter);
 	}
 
+	/**
+	 * Apply the internal delegates of this instance to the specified
+	 * {@code evaluationContext}. Typically invoked right after the new context
+	 * instance has been created to reuse the delegates. Do not modify the
+	 * {@linkplain #setRootObject(Object) root object} or any registered
+	 * {@linkplain #setVariables(Map) variables}.
+	 * @param evaluationContext the evaluation context to update
+	 * @since 6.1.1
+	 */
+	public void applyDelegatesTo(StandardEvaluationContext evaluationContext) {
+		// Triggers initialization for default delegates
+		evaluationContext.setConstructorResolvers(new ArrayList<>(this.getConstructorResolvers()));
+		evaluationContext.setMethodResolvers(new ArrayList<>(this.getMethodResolvers()));
+		evaluationContext.setPropertyAccessors(new ArrayList<>(this.getPropertyAccessors()));
+		evaluationContext.setTypeLocator(this.getTypeLocator());
+		evaluationContext.setTypeConverter(this.getTypeConverter());
+
+		evaluationContext.beanResolver = this.beanResolver;
+		evaluationContext.operatorOverloader = this.operatorOverloader;
+		evaluationContext.reflectiveMethodResolver = this.reflectiveMethodResolver;
+		evaluationContext.typeComparator = this.typeComparator;
+	}
+
 
 	private List<PropertyAccessor> initPropertyAccessors() {
 		List<PropertyAccessor> accessors = this.propertyAccessors;

spring-expression/src/main/java/org/springframework/expression/spel/support/StandardOperatorOverloader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,6 +29,8 @@ import org.springframework.lang.Nullable;
  */
 public class StandardOperatorOverloader implements OperatorOverloader {
 
+	static final StandardOperatorOverloader INSTANCE = new StandardOperatorOverloader();
+
 	@Override
 	public boolean overridesOperation(Operation operation, @Nullable Object leftOperand, @Nullable Object rightOperand)
 			throws EvaluationException {

spring-expression/src/main/java/org/springframework/expression/spel/support/StandardTypeComparator.java

@@ -37,6 +37,8 @@ import org.springframework.util.NumberUtils;
  */
 public class StandardTypeComparator implements TypeComparator {
 
+	static final StandardTypeComparator INSTANCE = new StandardTypeComparator();
+
 	@Override
 	public boolean canCompare(@Nullable Object left, @Nullable Object right) {
 		if (left == null || right == null) {

spring-expression/src/test/java/org/springframework/expression/spel/support/StandardEvaluationContextTests.java

@@ -0,0 +1,122 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.expression.spel.support;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Consumer;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.expression.BeanResolver;
+import org.springframework.expression.ConstructorResolver;
+import org.springframework.expression.MethodResolver;
+import org.springframework.expression.OperatorOverloader;
+import org.springframework.expression.PropertyAccessor;
+import org.springframework.expression.TypeConverter;
+import org.springframework.expression.TypeLocator;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link StandardEvaluationContext}.
+ *
+ * @author Stephane Nicoll
+ */
+class StandardEvaluationContextTests {
+
+	private final StandardEvaluationContext evaluationContext = new StandardEvaluationContext();
+
+	@Test
+	void applyDelegatesToSetDelegatesToTarget() {
+		StandardEvaluationContext target = new StandardEvaluationContext();
+		this.evaluationContext.applyDelegatesTo(target);
+		assertThat(target).hasFieldOrProperty("reflectiveMethodResolver").isNotNull();
+		assertThat(target.getBeanResolver()).isSameAs(this.evaluationContext.getBeanResolver());
+		assertThat(target.getTypeLocator()).isSameAs(this.evaluationContext.getTypeLocator());
+		assertThat(target.getTypeConverter()).isSameAs(this.evaluationContext.getTypeConverter());
+		assertThat(target.getOperatorOverloader()).isSameAs(this.evaluationContext.getOperatorOverloader());
+		assertThat(target.getPropertyAccessors()).satisfies(hasSameElements(
+				this.evaluationContext.getPropertyAccessors()));
+		assertThat(target.getConstructorResolvers()).satisfies(hasSameElements(
+				this.evaluationContext.getConstructorResolvers()));
+		assertThat(target.getMethodResolvers()).satisfies(hasSameElements(
+				this.evaluationContext.getMethodResolvers()));
+	}
+
+	@Test
+	void applyDelegatesToSetOverrideDelegatesInTarget() {
+		StandardEvaluationContext target = new StandardEvaluationContext();
+		target.setBeanResolver(mock(BeanResolver.class));
+		target.setTypeLocator(mock(TypeLocator.class));
+		target.setTypeConverter(mock(TypeConverter.class));
+		target.setOperatorOverloader(mock(OperatorOverloader.class));
+		target.setPropertyAccessors(new ArrayList<>());
+		target.setConstructorResolvers(new ArrayList<>());
+		target.setMethodResolvers(new ArrayList<>());
+		this.evaluationContext.applyDelegatesTo(target);
+		assertThat(target).hasFieldOrProperty("reflectiveMethodResolver").isNotNull();
+		assertThat(target.getBeanResolver()).isSameAs(this.evaluationContext.getBeanResolver());
+		assertThat(target.getTypeLocator()).isSameAs(this.evaluationContext.getTypeLocator());
+		assertThat(target.getTypeConverter()).isSameAs(this.evaluationContext.getTypeConverter());
+		assertThat(target.getOperatorOverloader()).isSameAs(this.evaluationContext.getOperatorOverloader());
+		assertThat(target.getPropertyAccessors()).satisfies(hasSameElements(
+				this.evaluationContext.getPropertyAccessors()));
+		assertThat(target.getConstructorResolvers()).satisfies(hasSameElements(
+				this.evaluationContext.getConstructorResolvers()));
+		assertThat(target.getMethodResolvers()).satisfies(hasSameElements(
+				this.evaluationContext.getMethodResolvers()));
+	}
+
+	@Test
+	void applyDelegatesToMakesACopyOfPropertyAccessors() {
+		StandardEvaluationContext target = new StandardEvaluationContext();
+		this.evaluationContext.applyDelegatesTo(target);
+		PropertyAccessor propertyAccessor = mock(PropertyAccessor.class);
+		this.evaluationContext.getPropertyAccessors().add(propertyAccessor);
+		assertThat(target.getPropertyAccessors()).doesNotContain(propertyAccessor);
+	}
+
+	@Test
+	void applyDelegatesToMakesACopyOfConstructorResolvers() {
+		StandardEvaluationContext target = new StandardEvaluationContext();
+		this.evaluationContext.applyDelegatesTo(target);
+		ConstructorResolver methodResolver = mock(ConstructorResolver.class);
+		this.evaluationContext.getConstructorResolvers().add(methodResolver);
+		assertThat(target.getConstructorResolvers()).doesNotContain(methodResolver);
+	}
+
+	@Test
+	void applyDelegatesToMakesACopyOfMethodResolvers() {
+		StandardEvaluationContext target = new StandardEvaluationContext();
+		this.evaluationContext.applyDelegatesTo(target);
+		MethodResolver methodResolver = mock(MethodResolver.class);
+		this.evaluationContext.getMethodResolvers().add(methodResolver);
+		assertThat(target.getMethodResolvers()).doesNotContain(methodResolver);
+	}
+
+	private Consumer<List<?>> hasSameElements(List<?> candidates) {
+		return actual -> {
+			assertThat(actual.size()).isEqualTo(candidates.size());
+			for (int i = 0; i < candidates.size(); i++) {
+				assertThat(candidates.get(i)).isSameAs(actual.get(i));
+			}
+		};
+	}
+
+}