feat: allowlist enum method invocation and return types

src/main/java/com/hubspot/jinjava/el/ext/AllowlistMethodValidator.java

@@ -51,6 +51,12 @@ public Method validateMethod(Method m) {
       m1 -> {
         Class<?> clazz = m1.getDeclaringClass();
         String canonicalClassName = clazz.getCanonicalName();
+        if (canonicalClassName == null) {
+          Class<?> superclass = clazz.getSuperclass();
+          if (superclass != null && superclass.isEnum()) {
+            canonicalClassName = superclass.getCanonicalName();
+          }
+        }
         if (canonicalClassName == null) {
           return false;
         }

src/main/java/com/hubspot/jinjava/el/ext/AllowlistReturnTypeValidator.java

@@ -50,7 +50,7 @@ public Object validateReturnType(Object o) {
     if (o instanceof String || o instanceof Number || o instanceof Boolean) {
       return o;
     }
-    Class<?> clazz = o.getClass();
+    Class<?> clazz = o instanceof Enum ? ((Enum<?>) o).getDeclaringClass() : o.getClass();
     if (clazz.isArray() && allowArrays) {
       return o;
     }

src/test/java/com/hubspot/jinjava/el/ext/AllowlistEnumTest.java

@@ -0,0 +1,95 @@
+package com.hubspot.jinjava.el.ext;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.hubspot.jinjava.BaseJinjavaTest;
+import com.hubspot.jinjava.testobjects.OperationEnum;
+import com.hubspot.jinjava.testobjects.SimpleColorEnum;
+import java.lang.reflect.Method;
+import java.time.Month;
+import java.util.HashMap;
+import java.util.Map;
+import org.junit.Test;
+
+public class AllowlistEnumTest extends BaseJinjavaTest {
+
+  private static final AllowlistReturnTypeValidator ENUM_RETURN_TYPE_VALIDATOR =
+    AllowlistReturnTypeValidator.create(
+      ReturnTypeValidatorConfig
+        .builder()
+        .addAllowedCanonicalClassNames(
+          SimpleColorEnum.class.getCanonicalName(),
+          OperationEnum.class.getCanonicalName()
+        )
+        .build()
+    );
+
+  private static final AllowlistMethodValidator ENUM_METHOD_VALIDATOR =
+    AllowlistMethodValidator.create(
+      MethodValidatorConfig
+        .builder()
+        .addAllowedDeclaredMethodsFromCanonicalClassNames(
+          SimpleColorEnum.class.getCanonicalName(),
+          OperationEnum.class.getCanonicalName()
+        )
+        .build()
+    );
+
+  @Test
+  public void itAllowsReturningSimpleEnumValue() {
+    assertThat(ENUM_RETURN_TYPE_VALIDATOR.validateReturnType(SimpleColorEnum.RED))
+      .isEqualTo(SimpleColorEnum.RED);
+  }
+
+  @Test
+  public void itAllowsReturningConstantSpecificBodyEnumValue() {
+    assertThat(ENUM_RETURN_TYPE_VALIDATOR.validateReturnType(OperationEnum.PLUS))
+      .isEqualTo(OperationEnum.PLUS);
+  }
+
+  @Test
+  public void itRejectsReturningNonAllowlistedEnumValue() {
+    assertThat(ENUM_RETURN_TYPE_VALIDATOR.validateReturnType(Month.JANUARY)).isNull();
+  }
+
+  @Test
+  public void itAllowsInvokingSimpleEnumGetter() throws NoSuchMethodException {
+    Method getLabel = SimpleColorEnum.class.getMethod("getLabel");
+    assertThat(ENUM_METHOD_VALIDATOR.validateMethod(getLabel)).isEqualTo(getLabel);
+  }
+
+  @Test
+  public void itAllowsInvokingConstantSpecificBodyEnumMethod()
+    throws NoSuchMethodException {
+    Method apply = OperationEnum.PLUS.getClass().getMethod("apply", int.class, int.class);
+    assertThat(apply.getDeclaringClass().getCanonicalName()).isNull();
+    assertThat(ENUM_METHOD_VALIDATOR.validateMethod(apply)).isEqualTo(apply);
+  }
+
+  @Test
+  public void itRejectsInvokingNonAllowlistedEnumMethod() throws NoSuchMethodException {
+    Method getValue = Month.class.getMethod("getValue");
+    assertThat(ENUM_METHOD_VALIDATOR.validateMethod(getValue)).isNull();
+  }
+
+  @Test
+  public void itRendersSimpleEnumGetter() {
+    Map<String, Object> context = new HashMap<>();
+    context.put("color", SimpleColorEnum.RED);
+    assertThat(jinjava.render("{{ color.label }}", context)).isEqualTo("red-label");
+  }
+
+  @Test
+  public void itRendersConstantSpecificBodyEnumMethodInvocation() {
+    Map<String, Object> context = new HashMap<>();
+    context.put("op", OperationEnum.PLUS);
+    assertThat(jinjava.render("{{ op.apply(2, 3) }}", context)).isEqualTo("5");
+  }
+
+  @Test
+  public void itRendersEnumValueAsName() {
+    Map<String, Object> context = new HashMap<>();
+    context.put("op", OperationEnum.TIMES);
+    assertThat(jinjava.render("{{ op }}", context)).isEqualTo("TIMES");
+  }
+}

src/test/java/com/hubspot/jinjava/testobjects/OperationEnum.java

@@ -0,0 +1,18 @@
+package com.hubspot.jinjava.testobjects;
+
+public enum OperationEnum {
+  PLUS {
+    @Override
+    public int apply(int a, int b) {
+      return a + b;
+    }
+  },
+  TIMES {
+    @Override
+    public int apply(int a, int b) {
+      return a * b;
+    }
+  };
+
+  public abstract int apply(int a, int b);
+}

src/test/java/com/hubspot/jinjava/testobjects/SimpleColorEnum.java

@@ -0,0 +1,16 @@
+package com.hubspot.jinjava.testobjects;
+
+public enum SimpleColorEnum {
+  RED("red-label"),
+  GREEN("green-label");
+
+  private final String label;
+
+  SimpleColorEnum(String label) {
+    this.label = label;
+  }
+
+  public String getLabel() {
+    return label;
+  }
+}