Add Resource.readableChannel()

Added readableChannel() to Resource, which returns a
java.nio.ReadableByteChannel. The default implementation uses
Channels.newChannel() to create a channel based on what is returned from
getInputStream(). Subclasses have more effecient, file-based
implementations.

Issue: SPR-14698

spring-core/src/main/java/org/springframework/core/codec/ResourceEncoder.java

@@ -17,7 +17,7 @@
 package org.springframework.core.codec;
 
 import java.io.IOException;
-import java.io.InputStream;
+import java.nio.channels.ReadableByteChannel;
 import java.util.Map;
 
 import reactor.core.publisher.Flux;
@@ -66,8 +66,8 @@ public class ResourceEncoder extends AbstractSingleValueEncoder<Resource> {
 	protected Flux<DataBuffer> encode(Resource resource, DataBufferFactory dataBufferFactory,
 			ResolvableType type, MimeType mimeType, Map<String, Object> hints) throws IOException {
 
-		InputStream is = resource.getInputStream();
-		return DataBufferUtils.read(is, dataBufferFactory, bufferSize);
+		ReadableByteChannel channel = resource.readableChannel();
+		return DataBufferUtils.read(channel, dataBufferFactory, bufferSize);
 	}
 
 }

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

@@ -17,12 +17,14 @@
 package org.springframework.core.io;
 
 import java.io.File;
+import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.HttpURLConnection;
 import java.net.URI;
 import java.net.URL;
 import java.net.URLConnection;
+import java.nio.channels.ReadableByteChannel;
 
 import org.springframework.util.ResourceUtils;
 
@@ -115,6 +117,22 @@ public abstract class AbstractFileResolvingResource extends AbstractResource {
 		return ResourceUtils.getFile(uri, getDescription());
 	}
 
+	/**
+	 * This implementation returns a FileChannel for the given URI-identified
+	 * resource, provided that it refers to a file in the file system.
+	 * @since 5.0
+	 * @see #getFile(URI)
+	 */
+	@Override
+	public ReadableByteChannel readableChannel() throws IOException {
+		if (isFile()) {
+			return new FileInputStream(getFile()).getChannel();
+		}
+		else {
+			return super.readableChannel();
+		}
+	}
+
 
 	@Override
 	public boolean exists() {

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

@@ -23,6 +23,8 @@ import java.io.InputStream;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.net.URL;
+import java.nio.channels.Channels;
+import java.nio.channels.ReadableByteChannel;
 
 import org.springframework.core.NestedIOException;
 import org.springframework.util.Assert;
@@ -122,6 +124,15 @@ public abstract class AbstractResource implements Resource {
 		throw new FileNotFoundException(getDescription() + " cannot be resolved to absolute file path");
 	}
 
+	/**
+	 * This implementation returns {@link Channels#newChannel(InputStream)} with the result of
+	 * {@link #getInputStream()}.
+	 */
+	@Override
+	public ReadableByteChannel readableChannel() throws IOException {
+		return Channels.newChannel(getInputStream());
+	}
+
 	/**
 	 * This implementation reads the entire InputStream to calculate the
 	 * content length. Subclasses will almost always be able to provide

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

@@ -24,6 +24,7 @@ import java.io.InputStream;
 import java.io.OutputStream;
 import java.net.URI;
 import java.net.URL;
+import java.nio.channels.ReadableByteChannel;
 
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -168,6 +169,15 @@ public class FileSystemResource extends AbstractResource implements WritableReso
 		return this.file;
 	}
 
+	/**
+	 * This implementation opens a FileChannel for the underlying file.
+	 * @see java.nio.channels.FileChannel
+	 */
+	@Override
+	public ReadableByteChannel readableChannel() throws IOException {
+		return new FileInputStream(this.file).getChannel();
+	}
+
 	/**
 	 * This implementation returns the underlying File's length.
 	 */

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

@@ -23,10 +23,12 @@ import java.io.InputStream;
 import java.io.OutputStream;
 import java.net.URI;
 import java.net.URL;
+import java.nio.channels.ReadableByteChannel;
 import java.nio.file.Files;
 import java.nio.file.OpenOption;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.nio.file.StandardOpenOption;
 
 import org.springframework.util.Assert;
 
@@ -194,6 +196,15 @@ 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...)
+	 */
+	@Override
+	public ReadableByteChannel readableChannel() throws IOException {
+		return Files.newByteChannel(this.path, StandardOpenOption.READ);
+	}
+
 	/**
 	 * This implementation returns the underlying File's length.
 	 */

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

@@ -18,8 +18,11 @@ package org.springframework.core.io;
 
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
 import java.net.URI;
 import java.net.URL;
+import java.nio.channels.Channels;
+import java.nio.channels.ReadableByteChannel;
 
 /**
  * Interface for a resource descriptor that abstracts from the actual
@@ -111,6 +114,19 @@ public interface Resource extends InputStreamSource {
 	 */
 	File getFile() throws IOException;
 
+	/**
+	 * Return a {@link ReadableByteChannel}.
+	 * <p>It is expected that each call creates a <i>fresh</i> channel.
+	 * <p>The default implementation returns {@link Channels#newChannel(InputStream)} with the
+	 * result of {@link #getInputStream()}.
+	 * @return the byte channel for the underlying resource (must not be {@code null})
+	 * @throws IOException if the channel could not be opened
+	 * @since 5.0
+	 */
+	default ReadableByteChannel readableChannel() throws IOException {
+		return Channels.newChannel(getInputStream());
+	}
+
 	/**
 	 * Determine the content length for this resource.
 	 * @throws IOException if the resource cannot be resolved