Ensure SpEL's InlineList is immutable in compiled mode

Prior to this commit, SpEL's InlineList was cached as a mutable list in
compiled mode (i.e., in a static field in the generated byte code).

To address that, this commit modifies InlineList's generateClinitCode()
method so that it wraps both top-level and nested inline lists using
Collections.unmodifiableList(), analogous to what we already do in
createList().

Closes gh-37001

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

@@ -176,9 +176,10 @@ public class InlineList extends SpelNodeImpl {
 			else {
 				mv.visitInsn(DUP);
 			}
-			// The children might be further lists if they are not constants. In this
-			// situation do not call back into generateCode() because it will register another clinit adder.
-			// Instead, directly build the list here:
+			// Nested InlineList children are always constant (guaranteed by isCompilable/isConstant).
+			// Thus, we call generateClinitCode() directly rather than generateCode() to avoid registering
+			// a separate static field and clinit entry for each nested list. In other words, we build each
+			// nested list inline within the current clinit sequence.
 			if (this.children[c] instanceof InlineList inlineList) {
 				inlineList.generateClinitCode(clazzname, constantFieldName, mv, codeflow, true);
 			}
@@ -192,6 +193,18 @@ public class InlineList extends SpelNodeImpl {
 			mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
 			mv.visitInsn(POP);
 		}
+		// Wrap the mutable ArrayList in an unmodifiable list, matching the behavior
+		// of the interpreted mode (see createList()). For the non-nested case, retrieve
+		// the list from the static field first, then store the wrapped list back. For
+		// the nested case, the list is already on the stack for the caller to use.
+		if (!nested) {
+			mv.visitFieldInsn(GETSTATIC, clazzname, constantFieldName, "Ljava/util/List;");
+		}
+		mv.visitMethodInsn(INVOKESTATIC, "java/util/Collections", "unmodifiableList",
+				"(Ljava/util/List;)Ljava/util/List;", false);
+		if (!nested) {
+			mv.visitFieldInsn(PUTSTATIC, clazzname, constantFieldName, "Ljava/util/List;");
+		}
 	}
 
 }

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

@@ -1619,6 +1619,55 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 			o = expression.getValue();
 			assertThat(o).isEqualTo("op");
 		}
+
+		@Test  // gh-37001
+		@SuppressWarnings("unchecked")
+		void compiledInlineListIsUnmodifiable() {
+			expression = parser.parseExpression("{1, 2, 3}");
+			List<Object> interpreted = (List<Object>) expression.getValue();
+			assertThat(interpreted).containsExactly(1, 2, 3);
+
+			// Interpreted --> unmodifiable
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> interpreted.add("boom"));
+
+			assertCanCompile(expression);
+
+			List<Object> compiled = (List<Object>) expression.getValue();
+			assertThat(compiled).containsExactly(1, 2, 3);
+
+			// Compiled --> unmodifiable
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> compiled.add("boom"));
+
+			// Same list reference must be returned on every evaluation
+			assertThat(expression.getValue()).isSameAs(compiled);
+		}
+
+		@Test  // gh-37001
+		@SuppressWarnings("unchecked")
+		void compiledNestedInlineListsAreUnmodifiable() {
+			expression = parser.parseExpression("{{1, 2}, {3, 4}}");
+			List<Object> interpreted = (List<Object>) expression.getValue();
+			assertThat(interpreted.toString()).isEqualTo("[[1, 2], [3, 4]]");
+
+			// Interpreted --> unmodifiable
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> interpreted.add("boom"));
+			List<Object> nested1Interpreted = (List<Object>) interpreted.get(0);
+			List<Object> nested2Interpreted = (List<Object>) interpreted.get(1);
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> nested1Interpreted.add("boom"));
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> nested2Interpreted.add("boom"));
+
+			assertCanCompile(expression);
+
+			List<Object> compiled = (List<Object>) expression.getValue();
+			assertThat(compiled.toString()).isEqualTo("[[1, 2], [3, 4]]");
+
+			// Compiled --> unmodifiable
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> compiled.add("boom"));
+			List<Object> nested1Compiled = (List<Object>) compiled.get(0);
+			List<Object> nested2Compiled = (List<Object>) compiled.get(1);
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> nested1Compiled.add("boom"));
+			assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> nested2Compiled.add("boom"));
+		}
 	}
 
 	@Nested