8329829: HttpClient: Add a BodyPublishers.ofFileChannel method

src/java.net.http/share/classes/java/net/http/HttpRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -29,10 +29,9 @@
 import java.io.InputStream;
 import java.net.URI;
 import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
-import java.nio.file.Files;
-import java.nio.file.OpenOption;
 import java.nio.file.Path;
 import java.time.Duration;
 import java.util.Iterator;
@@ -720,6 +719,34 @@ public static BodyPublisher ofFile(Path path) throws FileNotFoundException {
             return RequestPublishers.FilePublisher.create(path);
         }
 
+        /**
+         * {@return a request body publisher whose body is the {@code length}
+         * content bytes read from the provided file {@code channel} starting
+         * from the specified {@code offset}}
+         * <p>
+         * The {@linkplain FileChannel file channel} will be read using
+         * {@link FileChannel#read(ByteBuffer, long) FileChannel.read(ByteBuffer buffer, long position)},
+         * which does not modify the channel's position. Thus, the same file
+         * channel may be shared between several publishers passed to
+         * concurrent requests.
+         * <p>
+         * The file channel will not be closed upon completion. The caller is
+         * expected to manage the life cycle of the channel, and close it
+         * appropriately when not needed anymore.
+         *
+         * @param channel a file channel
+         * @param offset the offset of the first byte
+         * @param length the number of bytes to read from the file channel
+         *
+         * @throws IndexOutOfBoundsException if the specified byte range is
+         * found to be out of bounds compared with the size of the file
+         * referred by the channel
+         */
+        public static BodyPublisher ofFileChannel(FileChannel channel, long offset, long length) {
+            Objects.requireNonNull(channel, "channel");
+            return new RequestPublishers.FileChannelPublisher(channel, offset, length);
+        }
+
         /**
          * A request body publisher that takes data from an {@code Iterable}
          * of byte arrays. An {@link Iterable} is provided which supplies

src/java.net.http/share/classes/jdk/internal/net/http/RequestPublishers.java

@@ -32,6 +32,7 @@
 import java.lang.reflect.UndeclaredThrowableException;
 import java.net.http.HttpRequest.BodyPublisher;
 import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
 import java.nio.charset.Charset;
 import java.nio.file.Files;
 import java.nio.file.NoSuchFileException;
@@ -418,6 +419,90 @@ public long contentLength() {
         }
     }
 
+    public static final class FileChannelPublisher implements BodyPublisher {
+
+        private final FileChannel channel;
+
+        private final long position;
+
+        private final long limit;
+
+        public FileChannelPublisher(FileChannel channel, long offset, long length) {
+            this.channel = Objects.requireNonNull(channel, "channel");
+            long fileSize = fileSize(channel);
+            Objects.checkFromIndexSize(offset, length, fileSize);
+            this.position = offset;
+            this.limit = offset + length;
+        }
+
+        private static long fileSize(FileChannel channel) {
+            try {
+                return channel.size();
+            } catch (IOException ioe) {
+                throw new UncheckedIOException(ioe);
+            }
+        }
+
+        @Override
+        public long contentLength() {
+            return limit - position;
+        }
+
+        @Override
+        public void subscribe(Flow.Subscriber<? super ByteBuffer> subscriber) {
+            Iterable<ByteBuffer> iterable = () -> new FileChannelIterator(channel, position, limit);
+            new PullPublisher<>(iterable).subscribe(subscriber);
+        }
+
+    }
+
+    private static final class FileChannelIterator implements Iterator<ByteBuffer> {
+
+        private final FileChannel channel;
+
+        private final long limit;
+
+        private long position;
+
+        private boolean terminated;
+
+        private FileChannelIterator(FileChannel channel, long position, long limit) {
+            this.channel = channel;
+            this.position = position;
+            this.limit = limit;
+        }
+
+        @Override
+        public synchronized boolean hasNext() {
+            return position < limit && !terminated;
+        }
+
+        @Override
+        public synchronized ByteBuffer next() {
+            if (!hasNext()) {
+                throw new NoSuchElementException();
+            }
+            long remaining = limit - position;
+            ByteBuffer buffer = Utils.getBufferWithAtMost(remaining);
+            try {
+                int readLength = channel.read(buffer, position);
+                // Short-circuit if `read()` has failed, e.g., due to file content being changed in the meantime
+                if (readLength < 0) {
+                    // We *must* throw to signal that the request needs to be cancelled.
+                    // Otherwise, the server will continue waiting data.
+                    throw new IOException("Unexpected EOF (position=%s)".formatted(position));
+                } else {
+                    position += readLength;
+                }
+            } catch (IOException ioe) {
+                terminated = true;
+                throw new UncheckedIOException(ioe);
+            }
+            return buffer.flip();
+        }
+
+    }
+
     public static final class PublisherAdapter implements BodyPublisher {
 
         private final Publisher<? extends ByteBuffer> publisher;
@@ -430,12 +515,12 @@ public PublisherAdapter(Publisher<? extends ByteBuffer> publisher,
         }
 
         @Override
-        public final long contentLength() {
+        public long contentLength() {
             return contentLength;
         }
 
         @Override
-        public final void subscribe(Flow.Subscriber<? super ByteBuffer> subscriber) {
+        public void subscribe(Flow.Subscriber<? super ByteBuffer> subscriber) {
             publisher.subscribe(subscriber);
         }
     }

src/java.net.http/share/classes/jdk/internal/net/http/common/Utils.java

@@ -367,10 +367,32 @@ public static String describeOps(int interestOps) {
     public static IllegalArgumentException newIAE(String message, Object... args) {
         return new IllegalArgumentException(format(message, args));
     }
+
+    /**
+     * {@return a new {@link ByteBuffer} instance of configured capacity for the HTTP Client}
+     */
     public static ByteBuffer getBuffer() {
         return ByteBuffer.allocate(BUFSIZE);
     }
 
+    /**
+     * {@return a new {@link ByteBuffer} instance whose capacity is set to the
+     * smaller of the specified {@code maxCapacity} and the default
+     * ({@value BUFSIZE})}
+     *
+     * @param maxCapacity a buffer capacity, in bytes
+     * @throws IllegalArgumentException if {@code capacity < 0}
+     */
+    public static ByteBuffer getBufferWithAtMost(long maxCapacity) {
+        if (maxCapacity < 0) {
+            throw new IllegalArgumentException(
+                    // Match the message produced by `ByteBuffer::createCapacityException`
+                    "capacity < 0: (%s < 0)".formatted(maxCapacity));
+        }
+        int effectiveCapacity = (int) Math.min(maxCapacity, BUFSIZE);
+        return ByteBuffer.allocate(effectiveCapacity);
+    }
+
     public static Throwable getCompletionCause(Throwable x) {
         Throwable cause = x;
         while ((cause instanceof CompletionException)