Use Java Files utilities in FileCopyUtils

Use helpers from java.nio.file.Files for some methods in FileCopyUtils.

Additionally, add unit tests for the various file-related methods.

Signed-off-by: Patrick Strawderman <pstrawderman@netflix.com>

spring-core/src/test/java/org/springframework/util/FileCopyUtilsTests.java

@@ -21,9 +21,12 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.StringReader;
 import java.io.StringWriter;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.Arrays;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -87,4 +90,29 @@ class FileCopyUtilsTests {
 		assertThat(result).isEqualTo(content);
 	}
 
+	@Test
+	void copyFile(@TempDir Path tempDir) throws IOException {
+		Path source = tempDir.resolve("src");
+		Path target = tempDir.resolve("target");
+		Files.write(source, "content".getBytes());
+		int bytesWritten = FileCopyUtils.copy(source.toFile(), target.toFile());
+		assertThat(bytesWritten).isEqualTo(7);
+		assertThat(target).exists();
+		assertThat(target).content().isEqualTo("content");
+	}
+
+	@Test
+	void copyFileToByteArray(@TempDir Path tempDir) throws IOException {
+		Path source = tempDir.resolve("src");
+		Files.write(source, "content".getBytes());
+		assertThat(FileCopyUtils.copyToByteArray(source.toFile())).asString().isEqualTo("content");
+	}
+
+	@Test
+	void copyByteArrayToFile(@TempDir Path tempDir) throws IOException {
+		Path target = tempDir.resolve("target");
+		FileCopyUtils.copy("content".getBytes(), target.toFile());
+		assertThat(target).exists();
+		assertThat(target).content().isEqualTo("content");
+	}
 }