Avoid extra byte copy inside FileSystemImpl.readFileInternal (#5730)

* Avoid extra byte copy inside FileSystemImpl.readFileInternal

Closes #5677

The improvement consists in creating the buffer upfront and using Netty API to read the file channel.

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

* Added ReadFileBenchmark

On my machine, the advantage grows with file size. At 1MB, using the Netty FileChannel method is nearly 2x faster and allocates half as much as the Files.readAllBytes approach.

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

---------

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

Author: tsegismont

vertx-core/src/main/java/io/vertx/core/file/impl/FileSystemImpl.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2019 Contributors to the Eclipse Foundation
+ * Copyright (c) 2011-2026 Contributors to the Eclipse Foundation
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -14,45 +14,21 @@
 import io.vertx.codegen.annotations.Nullable;
 import io.vertx.core.Future;
 import io.vertx.core.buffer.Buffer;
-import io.vertx.core.file.AsyncFile;
-import io.vertx.core.file.CopyOptions;
-import io.vertx.core.file.FileProps;
+import io.vertx.core.file.*;
 import io.vertx.core.file.FileSystem;
 import io.vertx.core.file.FileSystemException;
-import io.vertx.core.file.FileSystemProps;
-import io.vertx.core.file.OpenOptions;
 import io.vertx.core.internal.ContextInternal;
 import io.vertx.core.internal.VertxInternal;
+import io.vertx.core.internal.buffer.BufferInternal;
 
 import java.io.File;
 import java.io.FilenameFilter;
 import java.io.IOException;
 import java.io.RandomAccessFile;
-import java.nio.file.CopyOption;
-import java.nio.file.FileAlreadyExistsException;
-import java.nio.file.FileStore;
-import java.nio.file.FileVisitOption;
-import java.nio.file.FileVisitResult;
-import java.nio.file.Files;
-import java.nio.file.LinkOption;
-import java.nio.file.Path;
-import java.nio.file.Paths;
-import java.nio.file.SimpleFileVisitor;
-import java.nio.file.StandardCopyOption;
-import java.nio.file.attribute.BasicFileAttributes;
-import java.nio.file.attribute.FileAttribute;
-import java.nio.file.attribute.GroupPrincipal;
-import java.nio.file.attribute.PosixFileAttributeView;
-import java.nio.file.attribute.PosixFilePermission;
-import java.nio.file.attribute.PosixFilePermissions;
-import java.nio.file.attribute.UserPrincipal;
-import java.nio.file.attribute.UserPrincipalLookupService;
-import java.util.ArrayList;
-import java.util.EnumSet;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Objects;
-import java.util.Set;
+import java.nio.channels.FileChannel;
+import java.nio.file.*;
+import java.nio.file.attribute.*;
+import java.util.*;
 import java.util.concurrent.Callable;
 import java.util.regex.Pattern;
 
@@ -876,8 +852,17 @@ private BlockingAction<Buffer> readFileInternal(String path) {
       public Buffer perform() {
         try {
           Path target = resolveFile(path).toPath();
-          byte[] bytes = Files.readAllBytes(target);
-          return Buffer.buffer(bytes);
+          try (FileChannel fc = FileChannel.open(target, StandardOpenOption.READ)) {
+            long size = fc.size();
+            if (size > (long) Integer.MAX_VALUE) {
+              // Throwing OOM as Files#readAllLines would in this case
+              throw new OutOfMemoryError("File is too big");
+            }
+            int len = (int) size;
+            BufferInternal res = BufferInternal.buffer(len);
+            res.unwrap().writeBytes(fc, 0, len);
+            return res;
+          }
         } catch (IOException e) {
           throw new FileSystemException(getFileAccessErrorMessage("read", path), e);
         }

vertx-core/src/test/java/io/vertx/benchmarks/ReadFileBenchmark.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2011-2026 Contributors to the Eclipse Foundation
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
+ * which is available at https://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
+ */
+
+package io.vertx.benchmarks;
+
+import io.netty.util.ResourceLeakDetector;
+import io.vertx.core.buffer.Buffer;
+import io.vertx.core.internal.buffer.BufferInternal;
+import org.openjdk.jmh.annotations.*;
+
+import java.io.IOException;
+import java.nio.channels.FileChannel;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.concurrent.TimeUnit;
+
+@State(Scope.Thread)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@Warmup(iterations = 20, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 10, time = 2, timeUnit = TimeUnit.SECONDS)
+@Fork(5)
+public class ReadFileBenchmark {
+
+  static {
+    ResourceLeakDetector.setLevel(ResourceLeakDetector.Level.DISABLED);
+  }
+
+  @Param({"128", "4096", "65536", "1048576"})
+  private int fileSize;
+
+  private Path tempFile;
+
+  @Setup
+  public void setup() throws IOException {
+    tempFile = Files.createTempFile("vertx-bench-", ".dat");
+    byte[] data = new byte[fileSize];
+    ThreadLocalRandom.current().nextBytes(data);
+    Files.write(tempFile, data);
+  }
+
+  @TearDown
+  public void tearDown() throws IOException {
+    Files.deleteIfExists(tempFile);
+  }
+
+  @Benchmark
+  public Buffer readAllBytes() throws IOException {
+    byte[] bytes = Files.readAllBytes(tempFile);
+    return Buffer.buffer(bytes);
+  }
+
+  @Benchmark
+  public Buffer fileChannel() throws IOException {
+    try (FileChannel fc = FileChannel.open(tempFile, StandardOpenOption.READ)) {
+      int len = (int) fc.size();
+      BufferInternal res = BufferInternal.buffer(len);
+      res.unwrap().writeBytes(fc, 0, len);
+      return res;
+    }
+  }
+}