Fix multi-threaded empty read for JdbcPagingItemReader

On empty input, the JdbcPagingItemReader cannot derive
a start value for the sort key to be used in further
queries. For multi-threaded steps, it is thus necessary
to prevent the reader from trying to read further pages
if the first page is empty.

Issue #3898

Author: hpoettker

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java

@@ -19,7 +19,7 @@
 import java.sql.ResultSet;
 import java.sql.SQLException;
 import java.util.ArrayList;
-import java.util.Collection;
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -179,7 +179,6 @@ public void afterPropertiesSet() throws Exception {
 	}
 
 	@Override
-	@SuppressWarnings("unchecked")
 	protected void doReadPage() {
 		if (results == null) {
 			results = new CopyOnWriteArrayList<>();
@@ -190,7 +189,7 @@ protected void doReadPage() {
 
 		PagingRowMapper rowCallback = new PagingRowMapper();
 
-		List<?> query;
+		List<T> query;
 
 		if (getPage() == 0) {
 			if (logger.isDebugEnabled()) {
@@ -211,7 +210,7 @@ protected void doReadPage() {
 			}
 
 		}
-		else {
+		else if (startAfterValues != null) {
 			previousStartAfterValues = startAfterValues;
 			if (logger.isDebugEnabled()) {
 				logger.debug("SQL used for reading remaining pages: [" + remainingPagesSql + "]");
@@ -225,9 +224,11 @@ protected void doReadPage() {
 						getParameterList(parameterValues, startAfterValues).toArray(), rowCallback);
 			}
 		}
+		else {
+			query = Collections.emptyList();
+		}
 
-		Collection<T> result = (Collection<T>) query;
-		results.addAll(result);
+		results.addAll(query);
 	}
 
 	@Override

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/database/JdbcPagingItemReaderAsyncTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2009-2014 the original author or authors.
+ * Copyright 2009-2021 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.
@@ -80,7 +80,8 @@ public class JdbcPagingItemReaderAsyncTests {
 	@Before
 	public void init() {
 		JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource);
-		maxId = jdbcTemplate.queryForObject("SELECT MAX(ID) from T_FOOS", Integer.class);
+		Integer maxIdResult = jdbcTemplate.queryForObject("SELECT MAX(ID) from T_FOOS", Integer.class);
+		maxId = maxIdResult == null ? 0 : maxIdResult;
 		for (int i = maxId + 1; i <= ITEM_COUNT; i++) {
 			jdbcTemplate.update("INSERT into T_FOOS (ID,NAME,VALUE) values (?, ?, ?)", i, "foo" + i, i);
 		}

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/database/JdbcPagingItemReaderEmptyResultSetTests.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.batch.item.database;
+
+import static org.junit.Assert.assertNull;
+
+import java.util.Collections;
+
+import javax.sql.DataSource;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.batch.item.ItemReader;
+import org.springframework.batch.item.database.support.HsqlPagingQueryProvider;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.jdbc.core.SingleColumnRowMapper;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+@RunWith(SpringJUnit4ClassRunner.class)
+@ContextConfiguration(locations = "JdbcPagingItemReaderCommonTests-context.xml")
+public class JdbcPagingItemReaderEmptyResultSetTests {
+
+	private static final int PAGE_SIZE = 2;
+	private static final int EMPTY_READS = PAGE_SIZE + 1;
+
+	@Autowired
+	private DataSource dataSource;
+
+	@Test
+	public void testMultiplePageReadsOnEmptyResultSet() throws Exception {
+		final ItemReader<Long> reader = getItemReader();
+		for (int i = 0; i < EMPTY_READS; i++) {
+			assertNull(reader.read());
+		}
+	}
+
+	private ItemReader<Long> getItemReader() throws Exception {
+		JdbcPagingItemReader<Long> reader = new JdbcPagingItemReader<>();
+		reader.setDataSource(dataSource);
+		HsqlPagingQueryProvider queryProvider = new HsqlPagingQueryProvider();
+		queryProvider.setSelectClause("select ID");
+		queryProvider.setFromClause("from T_FOOS");
+		queryProvider.setWhereClause("1 = 0");
+		queryProvider.setSortKeys(Collections.singletonMap("ID", Order.ASCENDING));
+		reader.setQueryProvider(queryProvider);
+		reader.setRowMapper(new SingleColumnRowMapper<>());
+		reader.setPageSize(PAGE_SIZE);
+		reader.afterPropertiesSet();
+		reader.setSaveState(false);
+
+		return reader;
+	}
+}