StandardEvaluationContext supports concurrent variable modification

Issue: SPR-17448

(cherry picked from commit 59fa647e2d)

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

@@ -18,9 +18,9 @@ package org.springframework.expression.spel.support;
 
 import java.lang.reflect.Method;
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.expression.BeanResolver;
@@ -88,7 +88,7 @@ public class StandardEvaluationContext implements EvaluationContext {
 
 	private OperatorOverloader operatorOverloader = new StandardOperatorOverloader();
 
-	private final Map<String, Object> variables = new HashMap<>();
+	private final Map<String, Object> variables = new ConcurrentHashMap<>();
 
 
 	/**
@@ -230,11 +230,16 @@ public class StandardEvaluationContext implements EvaluationContext {
 
 	@Override
 	public void setVariable(String name, @Nullable Object value) {
+		if (value != null) {
 			this.variables.put(name, value);
 		}
+		else {
+			this.variables.remove(name);
+		}
+	}
 
-	public void setVariables(Map<String,Object> variables) {
-		this.variables.putAll(variables);
+	public void setVariables(Map<String, Object> variables) {
+		variables.forEach(this::setVariable);
 	}
 
 	public void registerFunction(String name, Method method) {