--- a/jdk/src/java.base/share/native/libjava/RandomAccessFile.c	Thu Jan 07 10:54:41 2016 +0100
+++ b/jdk/src/java.base/share/native/libjava/RandomAccessFile.c	Fri Dec 18 16:06:26 2015 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2015, 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
@@ -103,23 +103,19 @@
 
 JNIEXPORT jlong JNICALL
 Java_java_io_RandomAccessFile_length(JNIEnv *env, jobject this) {
+
     FD fd;
-    jlong cur = jlong_zero;
-    jlong end = jlong_zero;
+    jlong length = jlong_zero;
 
     fd = GET_FD(this, raf_fd);
     if (fd == -1) {
         JNU_ThrowIOException(env, "Stream Closed");
         return -1;
     }
-    if ((cur = IO_Lseek(fd, 0L, SEEK_CUR)) == -1) {
-        JNU_ThrowIOExceptionWithLastError(env, "Seek failed");
-    } else if ((end = IO_Lseek(fd, 0L, SEEK_END)) == -1) {
-        JNU_ThrowIOExceptionWithLastError(env, "Seek failed");
-    } else if (IO_Lseek(fd, cur, SEEK_SET) == -1) {
-        JNU_ThrowIOExceptionWithLastError(env, "Seek failed");
+    if ((length = IO_GetLength(fd)) == -1) {
+        JNU_ThrowIOExceptionWithLastError(env, "GetLength failed");
     }
-    return end;
+    return length;
 }
 
 JNIEXPORT void JNICALL

--- a/jdk/src/java.base/unix/native/libjava/io_util_md.c	Thu Jan 07 10:54:41 2016 +0100
+++ b/jdk/src/java.base/unix/native/libjava/io_util_md.c	Fri Dec 18 16:06:26 2015 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2015, 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
@@ -22,7 +22,6 @@
  * or visit www.oracle.com if you need additional information or have any
  * questions.
  */
-
 #include "jni.h"
 #include "jni_util.h"
 #include "jvm.h"
@@ -219,3 +218,14 @@
     RESTARTABLE(ftruncate64(fd, length), result);
     return result;
 }
+
+jlong
+handleGetLength(FD fd)
+{
+    struct stat64 sb;
+    if (fstat64(fd, &sb) == 0) {
+        return sb.st_size;
+    } else {
+        return -1;
+    }
+}

--- a/jdk/src/java.base/unix/native/libjava/io_util_md.h	Thu Jan 07 10:54:41 2016 +0100
+++ b/jdk/src/java.base/unix/native/libjava/io_util_md.h	Fri Dec 18 16:06:26 2015 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2015, 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
@@ -39,7 +39,7 @@
 ssize_t handleRead(FD fd, void *buf, jint len);
 jint handleAvailable(FD fd, jlong *pbytes);
 jint handleSetLength(FD fd, jlong length);
-
+jlong handleGetLength(FD fd);
 FD handleOpen(const char *path, int oflag, int mode);
 
 /*
@@ -72,6 +72,7 @@
 #define IO_Append handleWrite
 #define IO_Available handleAvailable
 #define IO_SetLength handleSetLength
+#define IO_GetLength handleGetLength
 
 #ifdef _ALLBSD_SOURCE
 #define open64 open

--- a/jdk/src/java.base/windows/native/libjava/io_util_md.c	Thu Jan 07 10:54:41 2016 +0100
+++ b/jdk/src/java.base/windows/native/libjava/io_util_md.c	Fri Dec 18 16:06:26 2015 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2015, 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
@@ -584,3 +584,14 @@
     }
     return long_to_jlong(pos.QuadPart);
 }
+
+jlong
+handleGetLength(FD fd) {
+    HANDLE h = (HANDLE) fd;
+    LARGE_INTEGER length;
+    if (GetFileSizeEx(h, &length) != 0) {
+        return long_to_jlong(length.QuadPart);
+    } else {
+        return -1;
+    }
+}

--- a/jdk/src/java.base/windows/native/libjava/io_util_md.h	Thu Jan 07 10:54:41 2016 +0100
+++ b/jdk/src/java.base/windows/native/libjava/io_util_md.h	Fri Dec 18 16:06:26 2015 +0530
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2015, 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
@@ -44,6 +44,7 @@
 int handleAvailable(FD fd, jlong *pbytes);
 int handleSync(FD fd);
 int handleSetLength(FD fd, jlong length);
+jlong handleGetLength(FD fd);
 JNIEXPORT jint handleRead(FD fd, void *buf, jint len);
 jint handleWrite(FD fd, const void *buf, jint len);
 jint handleAppend(FD fd, const void *buf, jint len);
@@ -84,6 +85,7 @@
 #define IO_Lseek handleLseek
 #define IO_Available handleAvailable
 #define IO_SetLength handleSetLength
+#define IO_GetLength handleGetLength
 
 /*
  * Setting the handle field in Java_java_io_FileDescriptor_set for

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/test/java/io/RandomAccessFile/FileLengthTest.java	Fri Dec 18 16:06:26 2015 +0530
@@ -0,0 +1,137 @@
+/*
+ * Copyright (c) 2015, 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 randomAccessFile 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 4823133
+ * @summary optimize RandomAccessFile.length() and length() is thread safe now.
+ */
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+
+/**
+ *
+ * @author vyom.tewari@oracle.com
+ */
+public class FileLengthTest {
+
+    private static final int BUF_SIZE = 4096;
+    private static RandomAccessFile randomAccessFile;
+    private static Thread fileLengthCaller;
+    private static Thread fileContentReader;
+    private static StringBuilder fileContents;
+    private static volatile boolean isFailed = false;
+
+    /**
+     * this thread will call length() in loop
+     */
+    private static void startLengthThread() {
+        if (randomAccessFile == null) {
+            return;
+        }
+        fileLengthCaller = new Thread(() -> {
+            while (true) {
+                try {
+                    long length = randomAccessFile.length();
+                    if (length < 0) {
+                        return;
+                    }
+                } catch (IOException ex) {
+                    return;
+                }
+            }
+        });
+        fileLengthCaller.setName("RandomAccessFile-length-caller");
+        fileLengthCaller.setDaemon(true);
+        fileLengthCaller.start();
+    }
+
+    /**
+     * this thread will call read() and store the content in internal buffer.
+     */
+    private static void startReaderThread() {
+        if (randomAccessFile == null) {
+            return;
+        }
+        fileContentReader = new Thread(() -> {
+            StringBuilder sb = new StringBuilder(BUF_SIZE);
+            int i;
+            byte arr[] = new byte[8];
+            try {
+                while ((i = randomAccessFile.read(arr)) != -1) {
+                    sb.append(new String(arr, 0, i));
+                }
+                if (!sb.toString().equals(fileContents.toString())) {
+                    isFailed = true;
+                }
+            } catch (IOException ex) {
+            }
+        });
+        fileContentReader.setName("RandomAccessFile-content-reader");
+        fileContentReader.setDaemon(true);
+        fileContentReader.start();
+    }
+
+    public static void main(String args[]) {
+        byte arr[] = new byte[BUF_SIZE];
+        String testFile = "testfile.txt";
+        try {
+            createDummyFile(testFile);
+            File file = new File(testFile);
+            file.deleteOnExit();
+            randomAccessFile = new RandomAccessFile(file, "r");
+            int count = randomAccessFile.read(arr);
+            randomAccessFile.seek(0);
+            fileContents = new StringBuilder(BUF_SIZE);
+            fileContents.append(new String(arr, 0, count));
+            startLengthThread();
+            startReaderThread();
+            fileContentReader.join();
+        } catch (FileNotFoundException | InterruptedException ex) {
+        } catch (IOException ex) {
+        } finally {
+            try {
+                randomAccessFile.close();
+            } catch (IOException ex) {
+            }
+        }
+        if (isFailed) {
+            throw new RuntimeException("RandomAccessFile.length() changed the underlying file pointer.");
+        }
+    }
+
+    private static void createDummyFile(String fileName) throws FileNotFoundException, IOException {
+        try (FileOutputStream outputStream = new FileOutputStream(new File(fileName))) {
+            String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
+            int count = 0;
+            while ((count + str.length()) < BUF_SIZE) {
+                outputStream.write(str.getBytes());
+                count += str.length();
+            }
+            outputStream.flush();
+        }
+    }
+}