GH-3076: NioFileLocker: add file existence check

Fixes spring-projects/spring-integration#3076

* Return `null` instead of throwing an exception

* Fix license header

* Add missing `@since`

* Avoid over-engineered test logic

* Fix code style

* Test non-existent file does not get created

* Fix: remove unused import

spring-integration-file/src/test/java/org/springframework/integration/file/locking/FileChannelCacheTests.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2019 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.integration.file.locking;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.File;
+import java.io.IOException;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+/**
+ * @author Emmanuel Roux
+ * @since 4.3.22
+ */
+public class FileChannelCacheTests {
+
+	@Rule
+	public TemporaryFolder temp = new TemporaryFolder();
+
+	@Test
+	public void throwsExceptionWhenFileNotExists() throws IOException {
+		File testFile = new File(temp.getRoot(), "test0");
+		assertThat(testFile.exists()).isFalse();
+		assertThat(FileChannelCache.tryLockFor(testFile)).isNull();
+		assertThat(testFile.exists()).isFalse();
+	}
+
+	@Test
+	public void fileLocked() throws IOException {
+		File testFile = temp.newFile("test1");
+		testFile.createNewFile();
+		assertThat(testFile.exists()).isTrue();
+		assertThat(FileChannelCache.tryLockFor(testFile)).isNotNull();
+		FileChannelCache.closeChannelFor(testFile);
+	}
+
+}

spring-integration-file/src/test/java/org/springframework/integration/file/locking/NioFileLockerTests.java

@@ -34,6 +34,7 @@ import org.springframework.integration.test.util.TestUtils;
 /**
  * @author Iwein Fuld
  * @author Gary Russell
+ * @author Emmanuel Roux
  */
 public class NioFileLockerTests {
 
@@ -76,4 +77,21 @@ public class NioFileLockerTests {
 		filter1.unlock(testFile);
 	}
 
+	@Test
+	public void fileLockedWhenNotAlreadyLockedAndExists() throws IOException {
+		NioFileLocker locker = new NioFileLocker();
+		File testFile = new File(workdir, "test2");
+		testFile.createNewFile();
+		assertThat(locker.lock(testFile)).isTrue();
+		locker.unlock(testFile);
+	}
+
+	@Test
+	public void fileNotLockedWhenNotExists() throws IOException {
+		NioFileLocker locker = new NioFileLocker();
+		File testFile = new File(workdir, "test3");
+		assertThat(locker.lock(testFile)).isFalse();
+		assertThat(testFile.exists()).isFalse();
+	}
+
 }