Introduce withAssignmentDisabled() option for SimpleEvaluationContext

To support additional use cases, this commit introduces a
withAssignmentDisabled() method in the Builder for
SimpleEvaluationContext.

Closes gh-33319

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

@@ -64,8 +64,9 @@ import org.springframework.lang.Nullable;
  * read-only access to properties via {@link DataBindingPropertyAccessor}. Similarly,
  * {@link SimpleEvaluationContext#forReadWriteDataBinding()} enables read and write access
  * to properties. Alternatively, configure custom accessors via
- * {@link SimpleEvaluationContext#forPropertyAccessors} and potentially activate method
- * resolution and/or a type converter through the builder.
+ * {@link SimpleEvaluationContext#forPropertyAccessors}, potentially
+ * {@linkplain Builder#withAssignmentDisabled() disable assignment}, and optionally
+ * activate method resolution and/or a type converter through the builder.
  *
  * <p>Note that {@code SimpleEvaluationContext} is typically not configured
  * with a default root object. Instead it is meant to be created once and
@@ -264,9 +265,8 @@ public final class SimpleEvaluationContext implements EvaluationContext {
 	 * ({@code ++}), and decrement ({@code --}) operators are disabled.
 	 * @return {@code true} if assignment is enabled; {@code false} otherwise
 	 * @since 5.3.38
-	 * @see #forPropertyAccessors(PropertyAccessor...)
 	 * @see #forReadOnlyDataBinding()
-	 * @see #forReadWriteDataBinding()
+	 * @see Builder#withAssignmentDisabled()
 	 */
 	@Override
 	public boolean isAssignmentEnabled() {
@@ -275,15 +275,18 @@ public final class SimpleEvaluationContext implements EvaluationContext {
 
 	/**
 	 * Create a {@code SimpleEvaluationContext} for the specified {@link PropertyAccessor}
-	 * delegates: typically a custom {@code PropertyAccessor} specific to a use case
-	 * (e.g. attribute resolution in a custom data structure), potentially combined with
-	 * a {@link DataBindingPropertyAccessor} if property dereferences are needed as well.
-	 * <p>Assignment is enabled within expressions evaluated by the context created via
-	 * this factory method.
+	 * delegates: typically a custom {@code PropertyAccessor} specific to a use case &mdash;
+	 * for example, for attribute resolution in a custom data structure &mdash; potentially
+	 * combined with a {@link DataBindingPropertyAccessor} if property dereferences are
+	 * needed as well.
+	 * <p>By default, assignment is enabled within expressions evaluated by the context
+	 * created via this factory method; however, assignment can be disabled via
+	 * {@link Builder#withAssignmentDisabled()}.
 	 * @param accessors the accessor delegates to use
 	 * @see DataBindingPropertyAccessor#forReadOnlyAccess()
 	 * @see DataBindingPropertyAccessor#forReadWriteAccess()
 	 * @see #isAssignmentEnabled()
+	 * @see Builder#withAssignmentDisabled()
 	 */
 	public static Builder forPropertyAccessors(PropertyAccessor... accessors) {
 		for (PropertyAccessor accessor : accessors) {
@@ -292,7 +295,7 @@ public final class SimpleEvaluationContext implements EvaluationContext {
 						"ReflectivePropertyAccessor. Consider using DataBindingPropertyAccessor or a custom subclass.");
 			}
 		}
-		return new Builder(true, accessors);
+		return new Builder(accessors);
 	}
 
 	/**
@@ -303,22 +306,26 @@ public final class SimpleEvaluationContext implements EvaluationContext {
 	 * @see DataBindingPropertyAccessor#forReadOnlyAccess()
 	 * @see #forPropertyAccessors
 	 * @see #isAssignmentEnabled()
+	 * @see Builder#withAssignmentDisabled()
 	 */
 	public static Builder forReadOnlyDataBinding() {
-		return new Builder(false, DataBindingPropertyAccessor.forReadOnlyAccess());
+		return new Builder(DataBindingPropertyAccessor.forReadOnlyAccess()).withAssignmentDisabled();
 	}
 
 	/**
 	 * Create a {@code SimpleEvaluationContext} for read-write access to
 	 * public properties via {@link DataBindingPropertyAccessor}.
-	 * <p>Assignment is enabled within expressions evaluated by the context created via
-	 * this factory method.
+	 * <p>By default, assignment is enabled within expressions evaluated by the context
+	 * created via this factory method. Assignment can be disabled via
+	 * {@link Builder#withAssignmentDisabled()}; however, it is preferable to use
+	 * {@link #forReadOnlyDataBinding()} if you desire read-only access.
 	 * @see DataBindingPropertyAccessor#forReadWriteAccess()
 	 * @see #forPropertyAccessors
 	 * @see #isAssignmentEnabled()
+	 * @see Builder#withAssignmentDisabled()
 	 */
 	public static Builder forReadWriteDataBinding() {
-		return new Builder(true, DataBindingPropertyAccessor.forReadWriteAccess());
+		return new Builder(DataBindingPropertyAccessor.forReadWriteAccess());
 	}
 
 
@@ -337,13 +344,22 @@ public final class SimpleEvaluationContext implements EvaluationContext {
 		@Nullable
 		private TypedValue rootObject;
 
-		private final boolean assignmentEnabled;
+		private boolean assignmentEnabled = true;
 
-		private Builder(boolean assignmentEnabled, PropertyAccessor... accessors) {
-			this.assignmentEnabled = assignmentEnabled;
+		private Builder(PropertyAccessor... accessors) {
 			this.accessors = Arrays.asList(accessors);
 		}
 
+		/**
+		 * Disable assignment within expressions evaluated by this evaluation context.
+		 * @since 5.3.38
+		 * @see SimpleEvaluationContext#isAssignmentEnabled()
+		 */
+		public Builder withAssignmentDisabled() {
+			this.assignmentEnabled = false;
+			return this;
+		}
+
 		/**
 		 * Register the specified {@link MethodResolver} delegates for
 		 * a combination of property access and method resolution.

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

@@ -187,6 +187,59 @@ class SimpleEvaluationContextTests {
 		assertIncrementAndDecrementWritesForIndexedStructures(context);
 	}
 
+	@Test
+	void forPropertyAccessorsWithAssignmentDisabled() {
+		SimpleEvaluationContext context = SimpleEvaluationContext
+				.forPropertyAccessors(new CompilableMapAccessor(), DataBindingPropertyAccessor.forReadOnlyAccess())
+				.withAssignmentDisabled()
+				.build();
+
+		assertCommonReadOnlyModeBehavior(context);
+
+		// Map -- with key as property name supported by CompilableMapAccessor
+
+		Expression expression;
+		expression = parser.parseExpression("map.yellow");
+		// setValue() is supported even though assignment is not.
+		expression.setValue(context, model, "pineapple");
+		assertThat(expression.getValue(context, model, String.class)).isEqualTo("pineapple");
+
+		// WRITE -- via assignment operator
+
+		// Variable
+		assertAssignmentDisabled(context, "#myVar = 'rejected'");
+
+		// Property
+		assertAssignmentDisabled(context, "name = 'rejected'");
+		assertAssignmentDisabled(context, "map.yellow = 'rejected'");
+		assertIncrementDisabled(context, "count++");
+		assertIncrementDisabled(context, "++count");
+		assertDecrementDisabled(context, "count--");
+		assertDecrementDisabled(context, "--count");
+
+		// Array Index
+		assertAssignmentDisabled(context, "array[0] = 'rejected'");
+		assertIncrementDisabled(context, "numbers[0]++");
+		assertIncrementDisabled(context, "++numbers[0]");
+		assertDecrementDisabled(context, "numbers[0]--");
+		assertDecrementDisabled(context, "--numbers[0]");
+
+		// List Index
+		assertAssignmentDisabled(context, "list[0] = 'rejected'");
+
+		// Map Index -- key as String
+		assertAssignmentDisabled(context, "map['red'] = 'rejected'");
+
+		// Map Index -- key as pseudo property name
+		assertAssignmentDisabled(context, "map[yellow] = 'rejected'");
+
+		// String Index
+		assertAssignmentDisabled(context, "name[0] = 'rejected'");
+
+		// Object Index
+		assertAssignmentDisabled(context, "['name'] = 'rejected'");
+	}
+
 
 	private void assertReadWriteMode(SimpleEvaluationContext context) {
 		// Variables can always be set programmatically within an EvaluationContext.