Added new static utility method

api/src/main/java/org/fuin/esc/api/HasSerializedDataTypeConstantValidator.java

@@ -4,6 +4,7 @@
 import jakarta.validation.ConstraintValidatorContext;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 
 /**
@@ -20,39 +21,61 @@ public void initialize(HasSerializedDataTypeConstant annotation) {
 
     @Override
     public boolean isValid(Object obj, ConstraintValidatorContext context) {
+        final Result result = analyze(obj.getClass(), name);
+        if (result.message() == null) {
+            return true;
+        }
+        error(context, result.message());
+        return false;
+    }
+
+    private void error(ConstraintValidatorContext context, String message) {
+        context.disableDefaultConstraintViolation();
+        context.buildConstraintViolationWithTemplate(message).addConstraintViolation();
+    }
+
+    private static Result analyze(final Class<?> clasz, final String name) {
         try {
-            final Field field = obj.getClass().getField(name);
+            final Field field = clasz.getField(name);
             final int modifiers = field.getModifiers();
             if (!Modifier.isStatic(modifiers)) {
-                error(context, "Field '" + name + "' is not static (#1)");
-                return false;
+                return new Result("Field '" + name + "' is not static (#1)", null);
             }
             if (field.getType() != SerializedDataType.class) {
-                error(context, "Expected constant '" + name + "' to be of type '" + SerializedDataType.class.getName() + "', but was: " + field.getType().getName() + " (#3)");
-                return false;
+                return new Result("Expected constant '" + name + "' to be of type '" + SerializedDataType.class.getName() + "', but was: " + field.getType().getName() + " (#3)", null);
             }
-            final Object value = field.get(obj);
+            final Object value = field.get(clasz);
             if (value == null) {
-                error(context, "Constant '" + name + "' is expected to be a non-null value (#4)");
-                return false;
+                return new Result("Constant '" + name + "' is expected to be a non-null value (#4)", null);
             }
             if (!Modifier.isFinal(modifiers)) {
-                error(context, "Constant '" + name + "' is not not final (#5)");
-                return false;
+                return new Result("Constant '" + name + "' is not not final (#5)", null);
             }
-            return true;
+            return new Result(null, value);
         } catch (final NoSuchFieldException ex) {
-            error(context, "The field '" + name + "' is undefined or it is not public (#2)");
-            return false;
+            return new Result("The field '" + name + "' is undefined or it is not public (#2)", null);
         } catch (final IllegalAccessException ex) {
             throw new IllegalStateException("Failed to execute method", ex);
         }
+    }
 
+    private record Result(String message, Object value) {
     }
 
-    private void error(ConstraintValidatorContext context, String message) {
-        context.disableDefaultConstraintViolation();
-        context.buildConstraintViolationWithTemplate(message).addConstraintViolation();
+    /**
+     * Returns a constant of type {@link SerializedDataType} in a class. Throws an {@link IllegalArgumentException}
+     * in case there is a problem with the field.
+     *
+     * @param clasz      Class to inspect.
+     * @param fieldName Name of the public static field of type {@link SerializedDataType}.
+     * @return Value of the constant.
+     */
+    public static SerializedDataType extractValue(final Class<?> clasz, final String fieldName) {
+        final Result result = analyze(clasz, fieldName);
+        if (result.message() == null) {
+            return (SerializedDataType) result.value();
+        }
+        throw new IllegalArgumentException(result.message());
     }
 
 }

api/src/test/java/org/fuin/esc/api/HasSerializedDataTypeConstantValidatorTest.java

@@ -27,9 +27,11 @@
 import java.util.Set;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 public final class HasSerializedDataTypeConstantValidatorTest {
 
+    public static final String FIELD_NAME = "SER_TYPE";
     private static Validator validator;
 
     @BeforeAll
@@ -42,36 +44,80 @@ static void beforeAll() {
     @Test
     public final void testValid() {
         assertThat(validator.validate(new MyClassValid())).isEmpty();
+        assertThat(HasSerializedDataTypeConstantValidator
+                .extractValue(MyClassValid.class, FIELD_NAME)).isEqualTo(new SerializedDataType("XYZ"));
     }
 
     @Test
     public final void testNotStatic() {
+
         assertThat(first(validator.validate(new MyClassNotStatic()))).contains("#1");
+
+        assertThatThrownBy(
+                () -> HasSerializedDataTypeConstantValidator.extractValue(MyClassNotStatic.class, FIELD_NAME))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("#1");
+
     }
 
     @Test
     public final void testNotPublic() {
+
         assertThat(first(validator.validate(new MyClassNotPublic()))).contains("#2");
+
+        assertThatThrownBy(
+                () -> HasSerializedDataTypeConstantValidator.extractValue(MyClassNotPublic.class, FIELD_NAME))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("#2");
+
     }
 
     @Test
     public final void testWrongReturnType() {
+
         assertThat(first(validator.validate(new MyClassWrongType()))).contains("#3");
+
+        assertThatThrownBy(
+                () -> HasSerializedDataTypeConstantValidator.extractValue(MyClassWrongType.class, FIELD_NAME))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("#3");
+
     }
 
     @Test
     public final void testWrongReturn() {
+
         assertThat(first(validator.validate(new MyClassNullValue()))).contains("#4");
+
+        assertThatThrownBy(
+                () -> HasSerializedDataTypeConstantValidator.extractValue(MyClassNullValue.class, FIELD_NAME))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("#4");
+
     }
 
     @Test
-    public final void testNoMethod() {
+    public final void testNoField() {
+
         assertThat(first(validator.validate(new MyClassNoField()))).contains("#2");
+
+        assertThatThrownBy(
+                () -> HasSerializedDataTypeConstantValidator.extractValue(MyClassNoField.class, FIELD_NAME))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("#2");
+
     }
 
     @Test
     public final void testNotFinal() {
+
         assertThat(first(validator.validate(new MyClassNotFinal()))).contains("#5");
+
+        assertThatThrownBy(
+                () -> HasSerializedDataTypeConstantValidator.extractValue(MyClassNotFinal.class, FIELD_NAME))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("#5");
+
     }
 
     private static String first(Set<?> violations) {