Add writableChannel to WritableResource

This commit introduces a `writableChannel()` method to
`WritableResource`, defaulting to `Channels.newChannel`, but with
overrides for file-based resources.

Author: poutsma

spring-core/src/main/java/org/springframework/core/io/FileSystemResource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -25,6 +25,7 @@
 import java.net.URI;
 import java.net.URL;
 import java.nio.channels.ReadableByteChannel;
+import java.nio.channels.WritableByteChannel;
 
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -178,6 +179,15 @@ public ReadableByteChannel readableChannel() throws IOException {
 		return new FileInputStream(this.file).getChannel();
 	}
 
+	/**
+	 * This implementation opens a FileChannel for the underlying file.
+	 * @see java.nio.channels.FileChannel
+	 */
+	@Override
+	public WritableByteChannel writableChannel() throws IOException {
+		return new FileOutputStream(this.file).getChannel();
+	}
+
 	/**
 	 * This implementation returns the underlying File's length.
 	 */

spring-core/src/main/java/org/springframework/core/io/PathResource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -24,6 +24,7 @@
 import java.net.URI;
 import java.net.URL;
 import java.nio.channels.ReadableByteChannel;
+import java.nio.channels.WritableByteChannel;
 import java.nio.file.Files;
 import java.nio.file.OpenOption;
 import java.nio.file.Path;
@@ -197,14 +198,23 @@ public File getFile() throws IOException {
 	}
 
 	/**
-	 * This implementation opens a InputStream for the underlying file.
-	 * @see java.nio.file.spi.FileSystemProvider#newInputStream(Path, OpenOption...)
+	 * This implementation opens a Channel for the underlying file.
+	 * @see Files#newByteChannel(Path, OpenOption...)
 	 */
 	@Override
 	public ReadableByteChannel readableChannel() throws IOException {
 		return Files.newByteChannel(this.path, StandardOpenOption.READ);
 	}
 
+	/**
+	 * This implementation opens a Channel for the underlying file.
+	 * @see Files#newByteChannel(Path, OpenOption...)
+	 */
+	@Override
+	public WritableByteChannel writableChannel() throws IOException {
+		return Files.newByteChannel(this.path, StandardOpenOption.WRITE);
+	}
+
 	/**
 	 * This implementation returns the underlying File's length.
 	 */

spring-core/src/main/java/org/springframework/core/io/WritableResource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -18,6 +18,8 @@
 
 import java.io.IOException;
 import java.io.OutputStream;
+import java.nio.channels.Channels;
+import java.nio.channels.WritableByteChannel;
 
 /**
  * Extended interface for a resource that supports writing to it.
@@ -51,4 +53,19 @@ default boolean isWritable() {
 	 */
 	OutputStream getOutputStream() throws IOException;
 
+	/**
+	 * Return a {@link WritableByteChannel}.
+	 * <p>It is expected that each call creates a <i>fresh</i> channel.
+	 * <p>The default implementation returns {@link Channels#newChannel(OutputStream)}
+	 * with the result of {@link #getOutputStream()}.
+	 * @return the byte channel for the underlying resource (must not be {@code null})
+	 * @throws java.io.FileNotFoundException if the underlying resource doesn't exist
+	 * @throws IOException if the content channel could not be opened
+	 * @since 5.0
+	 * @see #getOutputStream()
+	 */
+	default WritableByteChannel writableChannel() throws IOException {
+		return Channels.newChannel(getOutputStream());
+	}
+
 }

spring-core/src/test/java/org/springframework/core/io/PathResourceTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -21,6 +21,8 @@
 import java.net.URI;
 import java.nio.ByteBuffer;
 import java.nio.channels.ReadableByteChannel;
+import java.nio.channels.WritableByteChannel;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.AccessDeniedException;
 import java.nio.file.NoSuchFileException;
 import java.nio.file.Path;
@@ -272,7 +274,7 @@ public void directoryIsNotWritable() throws Exception {
 	@Test
 	public void outputStream() throws Exception {
 		PathResource resource = new PathResource(temporaryFolder.newFile("test").toPath());
-		FileCopyUtils.copy("test".getBytes(), resource.getOutputStream());
+		FileCopyUtils.copy("test".getBytes(StandardCharsets.UTF_8), resource.getOutputStream());
 		assertThat(resource.contentLength(), equalTo(4L));
 	}
 
@@ -328,4 +330,21 @@ public void getReadableByteChannelDoesNotExist() throws Exception {
 		resource.readableChannel();
 	}
 
+	@Test
+	public void getWritableChannel() throws Exception {
+		PathResource resource = new PathResource(temporaryFolder.newFile("test").toPath());
+		ByteBuffer buffer = ByteBuffer.wrap("test".getBytes(StandardCharsets.UTF_8));
+		WritableByteChannel channel = null;
+		try {
+			channel = resource.writableChannel();
+			channel.write(buffer);
+		}
+		finally {
+			if (channel != null) {
+				channel.close();
+			}
+		}
+		assertThat(resource.contentLength(), equalTo(4L));
+	}
+
 }