Add closed check at Cursor.iterator

Fixes #1297

Author: kazuki43zoo

src/main/java/org/apache/ibatis/cursor/defaults/DefaultCursor.java

@@ -94,6 +94,9 @@ public int getCurrentIndex() {
 
     @Override
     public Iterator<T> iterator() {
+        if (isClosed()) {
+            throw new IllegalStateException("A Cursor is already closed.");
+        }
         if (iteratorRetrieved) {
             throw new IllegalStateException("Cannot open more than one iterator on a Cursor");
         }

src/test/java/org/apache/ibatis/submitted/cursor_simple/CursorSimpleTest.java

@@ -361,4 +361,18 @@ public void shouldGetAllUserUsingAnnotationBasedMapper() {
         }
     }
 
+    @Test
+    public void shouldThrowIllegalStateExceptionUsingIteratorOnSessionClosed() {
+        Cursor<User> usersCursor;
+        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
+            usersCursor = sqlSession.getMapper(Mapper.class).getAllUsers();
+        }
+        try {
+            usersCursor.iterator();
+            Assert.fail("Should throws the IllegalStateException when call the iterator method after session is closed.");
+        } catch (IllegalStateException e) {
+            Assert.assertEquals("A Cursor is already closed.", e.getMessage());
+        }
+    }
+
 }