Provide a flag to disable SpEL support

This commit introduces a spring.spel.ignore system property
which when set to true avoid initializing SpEL infrastructure.

A typical use case is optimizing GraalVM native image footprint
for applications not using SpEL. In order to be effective, those
classes should be initialized at build time:

- org.springframework.context.support.AbstractApplicationContext
- org.springframework.core.SpringProperties
- org.springframework.context.event.EventListenerMethodProcessor

Closes gh-25153

spring-context/src/main/java/org/springframework/context/event/ApplicationListenerMethodAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -142,7 +142,7 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 	/**
 	 * Initialize this instance.
 	 */
-	void init(ApplicationContext applicationContext, EventExpressionEvaluator evaluator) {
+	void init(ApplicationContext applicationContext, @Nullable EventExpressionEvaluator evaluator) {
 		this.applicationContext = applicationContext;
 		this.evaluator = evaluator;
 	}

spring-context/src/main/java/org/springframework/context/event/EventListenerMethodProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -40,6 +40,7 @@ import org.springframework.context.ApplicationContextAware;
 import org.springframework.context.ApplicationListener;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.MethodIntrospector;
+import org.springframework.core.SpringProperties;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.core.annotation.AnnotationUtils;
@@ -56,6 +57,7 @@ import org.springframework.util.CollectionUtils;
  *
  * @author Stephane Nicoll
  * @author Juergen Hoeller
+ * @author Sebastien Deleuze
  * @since 4.2
  * @see EventListenerFactory
  * @see DefaultEventListenerFactory
@@ -63,6 +65,14 @@ import org.springframework.util.CollectionUtils;
 public class EventListenerMethodProcessor
 		implements SmartInitializingSingleton, ApplicationContextAware, BeanFactoryPostProcessor {
 
+	/**
+	 * Boolean flag controlled by a {@code spring.spel.ignore} system property that instructs Spring to
+	 * ignore SpEL, i.e. to not initialize the SpEL infrastructure.
+	 * <p>The default is "false".
+	 */
+	private static final boolean shouldIgnoreSpel = SpringProperties.getFlag("spring.spel.ignore");
+
+
 	protected final Log logger = LogFactory.getLog(getClass());
 
 	@Nullable
@@ -74,11 +84,21 @@ public class EventListenerMethodProcessor
 	@Nullable
 	private List<EventListenerFactory> eventListenerFactories;
 
-	private final EventExpressionEvaluator evaluator = new EventExpressionEvaluator();
+	@Nullable
+	private final EventExpressionEvaluator evaluator;
 
 	private final Set<Class<?>> nonAnnotatedClasses = Collections.newSetFromMap(new ConcurrentHashMap<>(64));
 
 
+	public EventListenerMethodProcessor() {
+		if (shouldIgnoreSpel) {
+			this.evaluator = null;
+		}
+		else {
+			this.evaluator = new EventExpressionEvaluator();
+		}
+	}
+
 	@Override
 	public void setApplicationContext(ApplicationContext applicationContext) {
 		Assert.isTrue(applicationContext instanceof ConfigurableApplicationContext,

spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java

@@ -67,6 +67,7 @@ import org.springframework.context.expression.StandardBeanExpressionResolver;
 import org.springframework.context.weaving.LoadTimeWeaverAware;
 import org.springframework.context.weaving.LoadTimeWeaverAwareProcessor;
 import org.springframework.core.ResolvableType;
+import org.springframework.core.SpringProperties;
 import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -117,6 +118,7 @@ import org.springframework.util.ReflectionUtils;
  * @author Mark Fisher
  * @author Stephane Nicoll
  * @author Sam Brannen
+ * @author Sebastien Deleuze
  * @since January 21, 2001
  * @see #refreshBeanFactory
  * @see #getBeanFactory
@@ -152,6 +154,13 @@ public abstract class AbstractApplicationContext extends DefaultResourceLoader
 	 */
 	public static final String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";
 
+	/**
+	 * Boolean flag controlled by a {@code spring.spel.ignore} system property that instructs Spring to
+	 * ignore SpEL, i.e. to not initialize the SpEL infrastructure.
+	 * <p>The default is "false".
+	 */
+	private static final boolean shouldIgnoreSpel = SpringProperties.getFlag("spring.spel.ignore");
+
 
 	static {
 		// Eagerly load the ContextClosedEvent class to avoid weird classloader issues
@@ -647,7 +656,9 @@ public abstract class AbstractApplicationContext extends DefaultResourceLoader
 	protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
 		// Tell the internal bean factory to use the context's class loader etc.
 		beanFactory.setBeanClassLoader(getClassLoader());
+		if (!shouldIgnoreSpel) {
 			beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
+		}
 		beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
 
 		// Configure the bean factory with context callbacks.