Java ByteBuffer Set readBlock(ByteBuffer buffer, String fileName, long startOffset, Logger log)

Description

Read one block from data file.

License

Open Source License

Parameter

Parameter	Description
buffer	Buffer ready for reading from position to limit. Need to set the limit before call. On successful return it has the position and limit ready for reading.
fileName	a parameter
startOffset	0-based start position in file, inclusive.
log	Optional.

Return

Number of read bytes or 0 on error: missing file, end of file, buffer is not large enough, etc.

Declaration

public static int readBlock(ByteBuffer buffer, String fileName, long startOffset, Logger log) 

Method Source Code

//package com.java2s;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.ReadableByteChannel;

import java.util.logging.Level;
import java.util.logging.Logger;

public class Main {
    /**/* w  w w.  j a va  2s . c  o  m*/
     * Read one block from data file.
     * 
     * @param fileName
     * @param startOffset
     *            0-based start position in file, inclusive.
     * @param length
     * @param log
     * @return Null if failed to read form some reason.
     */
    public static ByteBuffer readBlock(String fileName, long startOffset, int length, ByteOrder byteOrder,
            Logger log) {
        File file = new File(fileName);

        //
        // File is supposed to be on disk
        //
        if (!file.exists()) {
            if (log != null)
                log.log(Level.WARNING, "Cannot read file {0}, that does not exist", fileName);
            return null;
        }

        //
        // Make sure that position and length match the file size
        //
        if (startOffset + length > file.length()) {
            if (log != null)
                log.log(Level.SEVERE,
                        "Wrong position and/or length when trying to read file \"" + fileName + "\", file length "
                                + file.length() + ", offset " + startOffset + ", data length " + length);
            return null;
        }

        RandomAccessFile randomAccessFile;
        //
        // Create the file descriptor
        //
        try {
            // It can fail if file does not exist or there is a security problem
            randomAccessFile = new RandomAccessFile(file, "rw");
        } catch (Exception e) {
            if (log != null)
                log.log(Level.WARNING, "Failed to open file \"" + fileName + "\" for read:\n" + e);
            return null;
        }

        //
        // Move writing head to the right position
        //
        try {
            randomAccessFile.seek(startOffset);
        } catch (Exception e) {
            if (log != null)
                log.log(Level.SEVERE, "Failed to set position when trying to read file \"" + fileName
                        + "\", file length " + file.length() + ", offset " + startOffset + ":\n" + e);
            try {
                randomAccessFile.close();
            } catch (Exception e2) {
            }
            return null;
        }

        // Allocate new buffer
        ByteBuffer buffer = ByteBuffer.allocate(length).order(byteOrder);

        ReadableByteChannel channel = randomAccessFile.getChannel();

        //
        // Read the data
        //
        int readBytes = 0;
        try {
            readBytes = channel.read(buffer);
        } catch (IOException e) {
            if (log != null)
                log.log(Level.SEVERE, "Failed to read data from \"" + fileName + "\", file length " + file.length()
                        + ", offset " + startOffset + ", data length " + length + ":\n" + e);
            try {
                randomAccessFile.close();
            } catch (Exception e2) {
            }
            return null;
        }

        //
        // Close the file
        //
        try {
            randomAccessFile.close();
        } catch (Exception e2) {
        }

        //
        // Check if all bytes were written
        //
        if (readBytes != length) {
            if (log != null)
                log.log(Level.SEVERE, "Failed to read data from \"" + fileName + "\", read only " + readBytes
                        + " instead of " + length);
            return null;
        }

        buffer.flip();

        return buffer;
    }

    /**
     * Read one block from data file.
     * 
     * @param buffer
     *            Buffer ready for reading from position to limit. Need to set
     *            the limit before call. On successful return it has the
     *            position and limit ready for reading.
     * @param fileName
     * @param startOffset
     *            0-based start position in file, inclusive.
     * @param log
     *            Optional.
     * @return Number of read bytes or 0 on error: missing file, end of file,
     *         buffer is not large enough, etc.
     */
    public static int readBlock(ByteBuffer buffer, String fileName, long startOffset, Logger log) {
        File file = new File(fileName);

        //
        // File is supposed to be on disk
        //
        if (!file.exists()) {
            if (log != null)
                log.log(Level.WARNING, "Cannot read file {0}, that does not exist", fileName);
            return 0;
        }

        //
        // Make sure that position and length match the file size
        //
        if (startOffset + buffer.remaining() > file.length()) {
            int length = (int) (file.length() - startOffset);
            if (length <= 0) {
                if (log != null)
                    log.log(Level.SEVERE,
                            "Wrong position and/or length when trying to read file \"" + fileName
                                    + "\", file length " + file.length() + ", offset " + startOffset
                                    + ", data length " + length);
                return 0;
            }

            buffer.limit(buffer.position() + length);
        }

        RandomAccessFile randomAccessFile;
        //
        // Create the file descriptor
        //
        try {
            // It can fail if file does not exist or there is a security problem
            randomAccessFile = new RandomAccessFile(file, "rw");
        } catch (Exception e) {
            if (log != null)
                log.log(Level.WARNING, "Failed to open file \"" + fileName + "\" for read:\n" + e);
            return 0;
        }

        //
        // Move writing head to the right position
        //
        try {
            randomAccessFile.seek(startOffset);
        } catch (Exception e) {
            if (log != null)
                log.log(Level.SEVERE, "Failed to set position when trying to read file \"" + fileName
                        + "\", file length " + file.length() + ", offset " + startOffset + ":\n" + e);
            try {
                randomAccessFile.close();
            } catch (Exception e2) {
            }
            return 0;
        }

        ReadableByteChannel channel = randomAccessFile.getChannel();

        //
        // Read the data
        //
        int readBytes = 0;
        try {
            readBytes = channel.read(buffer);
        } catch (IOException e) {
            if (log != null)
                log.log(Level.SEVERE, "Failed to read data from \"" + fileName + "\", file length " + file.length()
                        + ", offset " + startOffset + ", data length " + buffer.remaining() + ":\n" + e);
            try {
                randomAccessFile.close();
            } catch (Exception e2) {
            }
            return 0;
        }

        //
        // Close the file
        //
        try {
            randomAccessFile.close();
        } catch (Exception e2) {
        }

        buffer.limit(buffer.position());
        buffer.position(buffer.position() - readBytes);

        return readBytes;
    }
}

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2. parseToByteBuffer(Object value)
3. pourBufferToArray(ByteBuffer source, byte[] destination, int offset, int sizeRequested)
4. readBitset(ByteBuffer bb, int numbytes)
5. readBitSet(ByteBuffer buf, int len)
6. readFromChannel(FileChannel fc, ByteBuffer buffer, int bufferOffset, int num)
7. readKey(ByteBuffer index, int indexByteOffset, byte[] foundKey)
8. readPackageName(ByteBuffer buffer, int offset)
9. redByteArray(ByteBuffer buffer, int offset, int length)