Improve diagnostics in SpEL for repeated text

Attempting to create repeated text in a SpEL expression using the
repeat operator can result in errors that are not very helpful to the
user.

This commit improves the diagnostics in SpEL for the repeat operator by
throwing a SpelEvaluationException with a meaningful error message in
order to better assist the user.

Closes gh-30143

spring-expression/src/main/java/org/springframework/expression/spel/SpelMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 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.
@@ -264,7 +264,11 @@ public enum SpelMessage {
 
 	/** @since 5.3.17 */
 	MAX_ARRAY_ELEMENTS_THRESHOLD_EXCEEDED(Kind.ERROR, 1075,
-			"Array declares too many elements, exceeding the threshold of ''{0}''");
+			"Array declares too many elements, exceeding the threshold of ''{0}''"),
+
+	/** @since 5.3.26 */
+	MAX_REPEATED_TEXT_SIZE_EXCEEDED(Kind.ERROR, 1076,
+			"Repeated text results in too many characters, exceeding the threshold of ''{0}''");
 
 
 	private final Kind kind;

spring-expression/src/main/java/org/springframework/expression/spel/ast/OpMultiply.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 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.
@@ -25,6 +25,8 @@
 import org.springframework.expression.TypedValue;
 import org.springframework.expression.spel.CodeFlow;
 import org.springframework.expression.spel.ExpressionState;
+import org.springframework.expression.spel.SpelEvaluationException;
+import org.springframework.expression.spel.SpelMessage;
 import org.springframework.util.Assert;
 import org.springframework.util.NumberUtils;
 
@@ -52,6 +54,13 @@
  */
 public class OpMultiply extends Operator {
 
+	/**
+	 * Maximum number of characters permitted in repeated text.
+	 * @since 5.3.26
+	 */
+	private static final int MAX_REPEATED_TEXT_SIZE = 256;
+
+
 	public OpMultiply(int startPos, int endPos, SpelNodeImpl... operands) {
 		super("*", startPos, endPos, operands);
 	}
@@ -109,17 +118,27 @@ else if (CodeFlow.isIntegerForNumericOp(leftNumber) || CodeFlow.isIntegerForNume
 		}
 
 		if (leftOperand instanceof String && rightOperand instanceof Integer) {
-			int repeats = (Integer) rightOperand;
-			StringBuilder result = new StringBuilder();
-			for (int i = 0; i < repeats; i++) {
-				result.append(leftOperand);
+			String text = (String) leftOperand;
+			int count = (Integer) rightOperand;
+			int requestedSize = text.length() * count;
+			checkRepeatedTextSize(requestedSize);
+			StringBuilder result = new StringBuilder(requestedSize);
+			for (int i = 0; i < count; i++) {
+				result.append(text);
 			}
 			return new TypedValue(result.toString());
 		}
 
 		return state.operate(Operation.MULTIPLY, leftOperand, rightOperand);
 	}
 
+	private void checkRepeatedTextSize(int requestedSize) {
+		if (requestedSize > MAX_REPEATED_TEXT_SIZE) {
+			throw new SpelEvaluationException(getStartPosition(),
+					SpelMessage.MAX_REPEATED_TEXT_SIZE_EXCEEDED, MAX_REPEATED_TEXT_SIZE);
+		}
+	}
+
 	@Override
 	public boolean isCompilable() {
 		if (!getLeftOperand().isCompilable()) {

spring-expression/src/test/java/org/springframework/expression/spel/OperatorTests.java

@@ -21,17 +21,20 @@
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.expression.Expression;
 import org.springframework.expression.spel.ast.Operator;
 import org.springframework.expression.spel.standard.SpelExpression;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.expression.spel.SpelMessage.MAX_REPEATED_TEXT_SIZE_EXCEEDED;
 
 /**
  * Tests the evaluation of expressions using various operators.
  *
  * @author Andy Clement
  * @author Juergen Hoeller
  * @author Giovanni Dall'Oglio Risso
+ * @author Sam Brannen
  */
 class OperatorTests extends AbstractExpressionTests {
 
@@ -324,11 +327,6 @@ void realLiteral() {
 		evaluate("3.5", 3.5d, Double.class);
 	}
 
-	@Test
-	void multiplyStringInt() {
-		evaluate("'a' * 5", "aaaaa", String.class);
-	}
-
 	@Test
 	void multiplyDoubleDoubleGivesDouble() {
 		evaluate("3.0d * 5.0d", 15.0d, Double.class);
@@ -576,6 +574,19 @@ void strings() {
 		evaluate("'abc' != 'def'", true, Boolean.class);
 	}
 
+	@Test
+	void stringRepeat() {
+		evaluate("'abc' * 0", "", String.class);
+		evaluate("'abc' * 1", "abc", String.class);
+		evaluate("'abc' * 2", "abcabc", String.class);
+
+		Expression expr = parser.parseExpression("'a' * 256");
+		assertThat(expr.getValue(context, String.class)).hasSize(256);
+
+		// 4 is the position of the '*' (repeat operator)
+		evaluateAndCheckError("'a' * 257", String.class, MAX_REPEATED_TEXT_SIZE_EXCEEDED, 4);
+	}
+
 	@Test
 	void longs() {
 		evaluate("3L == 4L", false, Boolean.class);