INT-4147: Fix NPE in the FileReadingMessageSource

JIRA: https://jira.spring.io/browse/INT-4147

By default `FileReadingMessageSource` is created without `filter`, at the same time internal `WatchServiceDirectoryScanner` is supplied with default `filter` by its `DefaultDirectoryScanner` super class.
A `DELETE` watch event condition around `FileReadingMessageSource.this.filter` is wrong, because it is `null` by default. Therefore `DELETE` events never succeed

* Modify `FileInboundTransactionTests` to accept `DELETE` watch event as well and verify that `ResettableFileListFilter.remove(File)` is performed
* Call newly created getter for `DefaultDirectoryScanner.filter` in the `WatchServiceDirectoryScanner` to assert against supplied `filter`

Polishing

**Cherry-pick to 4.3.x**

spring-integration-file/src/main/java/org/springframework/integration/file/DefaultDirectoryScanner.java

@@ -62,11 +62,19 @@ public class DefaultDirectoryScanner implements DirectoryScanner {
 		this.filter = filter;
 	}
 
+	protected FileListFilter<File> getFilter() {
+		return this.filter;
+	}
+
 	@Override
 	public final void setLocker(FileLocker locker) {
 		this.locker = locker;
 	}
 
+	protected FileLocker getLocker() {
+		return this.locker;
+	}
+
 	/**
 	 * This class takes the minimal implementation and merely delegates to the locker if set.
 	 * @param file the file to try to claim.

spring-integration-file/src/main/java/org/springframework/integration/file/FileReadingMessageSource.java

@@ -490,8 +490,8 @@ public class FileReadingMessageSource extends IntegrationObjectSupport implement
 						}
 
 						if (event.kind() == StandardWatchEventKinds.ENTRY_DELETE) {
-							if (FileReadingMessageSource.this.filter instanceof ResettableFileListFilter) {
-								((ResettableFileListFilter<File>) FileReadingMessageSource.this.filter).remove(file);
+							if (getFilter() != null && getFilter() instanceof ResettableFileListFilter) {
+								((ResettableFileListFilter<File>) getFilter()).remove(file);
 							}
 							boolean fileRemoved = files.remove(file);
 							if (fileRemoved && logger.isDebugEnabled()) {

spring-integration-file/src/test/java/org/springframework/integration/file/FileInboundTransactionTests-context.xml

@@ -14,7 +14,8 @@
 	<int-file:inbound-channel-adapter id="pseudoTx"
 		channel="input" auto-startup="false"
 						directory="#{T (org.springframework.integration.file.FileInboundTransactionTests).tmpDir.root}/si-test1"
-						use-watch-service="true">
+						use-watch-service="true"
+						watch-events="CREATE,DELETE">
 		<int:poller fixed-rate="500">
 			<int:transactional synchronization-factory="syncFactoryA"/>
 		</int:poller>

spring-integration-file/src/test/java/org/springframework/integration/file/FileInboundTransactionTests.java

@@ -22,6 +22,8 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.verify;
 
 import java.io.File;
 import java.util.concurrent.CountDownLatch;
@@ -32,8 +34,10 @@ import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
 import org.junit.runner.RunWith;
 
+import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.integration.endpoint.SourcePollingChannelAdapter;
+import org.springframework.integration.file.filters.ResettableFileListFilter;
 import org.springframework.integration.test.util.TestUtils;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessagingException;
@@ -84,6 +88,16 @@ public class FileInboundTransactionTests {
 
 	@Test
 	public void testNoTx() throws Exception {
+
+		Object scanner = TestUtils.getPropertyValue(pseudoTx.getMessageSource(), "scanner");
+		assertThat(scanner.getClass().getName(), containsString("FileReadingMessageSource$WatchServiceDirectoryScanner"));
+
+		@SuppressWarnings("unchecked")
+		ResettableFileListFilter<File> fileListFilter =
+				spy(TestUtils.getPropertyValue(scanner, "filter", ResettableFileListFilter.class));
+
+		new DirectFieldAccessor(scanner).setPropertyValue("filter", fileListFilter);
+
 		final CountDownLatch latch = new CountDownLatch(1);
 		final AtomicBoolean crash = new AtomicBoolean();
 		input.subscribe(message -> {
@@ -110,8 +124,7 @@ public class FileInboundTransactionTests {
 		assertFalse(transactionManager.getCommitted());
 		assertFalse(transactionManager.getRolledBack());
 
-		Object scanner = TestUtils.getPropertyValue(pseudoTx.getMessageSource(), "scanner");
-		assertThat(scanner.getClass().getName(), containsString("FileReadingMessageSource$WatchServiceDirectoryScanner"));
+		verify(fileListFilter).remove(new File(tmpDir.getRoot(), "si-test1/foo"));
 	}
 
 	@Test