Fix readNBytes single-byte path treating 0xFF as EOF

src/java.base/share/classes/java/io/FileInputStream.java

@@ -360,10 +360,10 @@ public byte[] readNBytes(int len) throws IOException {
                 remaining -= n;
             } else if (n == 0) {
                 // Block until a byte is read or EOF is detected
-                byte b = (byte)read();
-                if (b == -1 )
+                int b = read();
+                if (b == -1)
                     break;
-                buf[nread++] = b;
+                buf[nread++] = (byte) b;
                 remaining--;
             }
         } while (n >= 0 && remaining > 0);

src/java.base/share/classes/sun/nio/ch/ChannelInputStream.java

@@ -198,10 +198,10 @@ public byte[] readNBytes(int len) throws IOException {
                 remaining -= n;
             } else if (n == 0) {
                 // Block until a byte is read or EOF is detected
-                byte b = (byte)read();
-                if (b == -1 )
+                int b = read();
+                if (b == -1)
                     break;
-                buf[nread++] = b;
+                buf[nread++] = (byte) b;
                 remaining--;
             }
         } while (n >= 0 && remaining > 0);

test/jdk/java/io/FileInputStream/ReadNBytesOxFF.java

@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug     8XXXXXXX
+ * @summary readNBytes should not treat byte value 0xFF as EOF
+ * @run main ReadNBytesOxFF
+ */
+
+import java.io.*;
+import java.nio.channels.*;
+import java.nio.file.*;
+import java.util.Arrays;
+
+/**
+ * Tests that FileInputStream.readNBytes() and ChannelInputStream.readNBytes()
+ * correctly handle the byte value 0xFF (255). Previously, the single-byte
+ * retry path cast read() to byte before comparing with -1:
+ *
+ *     byte b = (byte)read();  // (byte)255 == -1
+ *     if (b == -1) break;     // incorrectly treats 0xFF as EOF
+ *
+ * This caused readNBytes() to return truncated data when encountering 0xFF
+ * bytes in the single-byte fallback path (entered when read(byte[],int,int)
+ * returns 0).
+ */
+public class ReadNBytesOxFF {
+
+    public static void main(String[] args) throws Exception {
+        // Create a file filled with 0xFF bytes
+        byte[] data = new byte[256];
+        Arrays.fill(data, (byte) 0xFF);
+        // Also include some 0xFF bytes interspersed with other values
+        for (int i = 0; i < data.length; i += 2) {
+            data[i] = (byte) 0xFF;
+            if (i + 1 < data.length) {
+                data[i + 1] = (byte) i;
+            }
+        }
+
+        Path tmpFile = Files.createTempFile("readnbytes-0xff-", ".bin");
+        try {
+            Files.write(tmpFile, data);
+
+            // Test FileInputStream.readNBytes()
+            testFileInputStream(tmpFile, data);
+
+            // Test ChannelInputStream.readNBytes() via Channels.newInputStream()
+            testChannelInputStream(tmpFile, data);
+
+            System.out.println("Test passed.");
+        } finally {
+            Files.deleteIfExists(tmpFile);
+        }
+    }
+
+    static void testFileInputStream(Path path, byte[] expected) throws Exception {
+        try (FileInputStream fis = new FileInputStream(path.toFile())) {
+            byte[] result = fis.readNBytes(expected.length);
+            if (result.length != expected.length) {
+                throw new RuntimeException(
+                    "FileInputStream.readNBytes: expected " + expected.length +
+                    " bytes but got " + result.length +
+                    " (0xFF byte likely misinterpreted as EOF)");
+            }
+            if (!Arrays.equals(result, expected)) {
+                throw new RuntimeException(
+                    "FileInputStream.readNBytes: content mismatch");
+            }
+        }
+    }
+
+    static void testChannelInputStream(Path path, byte[] expected) throws Exception {
+        try (FileChannel fc = FileChannel.open(path, StandardOpenOption.READ);
+             InputStream cis = Channels.newInputStream(fc)) {
+            byte[] result = cis.readNBytes(expected.length);
+            if (result.length != expected.length) {
+                throw new RuntimeException(
+                    "ChannelInputStream.readNBytes: expected " + expected.length +
+                    " bytes but got " + result.length +
+                    " (0xFF byte likely misinterpreted as EOF)");
+            }
+            if (!Arrays.equals(result, expected)) {
+                throw new RuntimeException(
+                    "ChannelInputStream.readNBytes: content mismatch");
+            }
+        }
+    }
+}