Add writableChannel to WritableResource

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

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.io.OutputStream;
 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 class FileSystemResource extends AbstractResource implements WritableReso
 		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.io.OutputStream;
 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 class PathResource extends AbstractResource implements WritableResource {
 	}
 
 	/**
-	 * 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 @@ package org.springframework.core.io;
 
 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 @@ public interface WritableResource extends Resource {
 	 */
 	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());
+	}
+
 }