org.apache.lucene.codecs

## Class CodecUtil

- java.lang.Object
- - org.apache.lucene.codecs.CodecUtil

-     public final class CodecUtil
      extends Object

  Utility class for reading and writing versioned headers.

  Writing codec headers is useful to ensure that a file is in the format you think it is.

  WARNING: This API is experimental and might change in incompatible ways in the next release.

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `CODEC_MAGIC` Constant to identify the start of a codec header.
    `static int` | `FOOTER_MAGIC` Constant to identify the start of a codec footer.

  - ### Method Summary

    All Methods Static Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `static long` | `checkFooter(ChecksumIndexInput in)` Validates the codec footer previously written by `writeFooter(org.apache.lucene.store.IndexOutput)`.
    `static void` | `checkFooter(ChecksumIndexInput in, Throwable priorException)` Validates the codec footer previously written by `writeFooter(org.apache.lucene.store.IndexOutput)`, optionally passing an unexpected exception that has already occurred.
    `static int` | `checkHeader(DataInput in, String codec, int minVersion, int maxVersion)` Reads and validates a header previously written with `writeHeader(DataOutput, String, int)`.
    `static int` | `checkHeaderNoMagic(DataInput in, String codec, int minVersion, int maxVersion)` Like `checkHeader(DataInput,String,int,int)` except this version assumes the first int has already been read and validated from the input.
    `static int` | `checkIndexHeader(DataInput in, String codec, int minVersion, int maxVersion, byte[] expectedID, String expectedSuffix)` Reads and validates a header previously written with `writeIndexHeader(DataOutput, String, int, byte[], String)`.
    `static byte[]` | `checkIndexHeaderID(DataInput in, byte[] expectedID)` Expert: just reads and verifies the object ID of an index header
    `static String` | `checkIndexHeaderSuffix(DataInput in, String expectedSuffix)` Expert: just reads and verifies the suffix of an index header
    `static long` | `checksumEntireFile(IndexInput input)` Clones the provided input, reads all bytes from the file, and calls `checkFooter(org.apache.lucene.store.ChecksumIndexInput)`
    `static int` | `footerLength()` Computes the length of a codec footer.
    `static int` | `headerLength(String codec)` Computes the length of a codec header.
    `static int` | `indexHeaderLength(String codec, String suffix)` Computes the length of an index header.
    `static byte[]` | `readFooter(IndexInput in)` Retrieves the full footer from the provided `IndexInput`.
    `static byte[]` | `readIndexHeader(IndexInput in)` Retrieves the full index header from the provided `IndexInput`.
    `static long` | `retrieveChecksum(IndexInput in)` Returns (but does not validate) the checksum previously written by `checkFooter(org.apache.lucene.store.ChecksumIndexInput)`.
    `static void` | `verifyAndCopyIndexHeader(IndexInput in, DataOutput out, byte[] expectedID)` Expert: verifies the incoming `IndexInput` has an index header and that its segment ID matches the expected one, and then copies that index header into the provided `DataOutput`.
    `static void` | `writeFooter(IndexOutput out)` Writes a codec footer, which records both a checksum algorithm ID and a checksum.
    `static void` | `writeHeader(DataOutput out, String codec, int version)` Writes a codec header, which records both a string to identify the file and a version number.
    `static void` | `writeIndexHeader(DataOutput out, String codec, int version, byte[] id, String suffix)` Writes a codec header for an index file, which records both a string to identify the format of the file, a version number, and data to identify the file instance (ID and auxiliary suffix such as generation).

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### CODEC_MAGIC

          public static final int CODEC_MAGIC

      Constant to identify the start of a codec header.
      See Also:
        Constant Field Values

    - #### FOOTER_MAGIC

          public static final int FOOTER_MAGIC

      Constant to identify the start of a codec footer.
      See Also:
        Constant Field Values

  - ### Method Detail

    - #### writeHeader

          public static void writeHeader(DataOutput out,
                                         String codec,
                                         int version)
                                  throws IOException

      Writes a codec header, which records both a string to identify the file and a version number. This header can be parsed and validated with `checkHeader()`.

      CodecHeader --> Magic,CodecName,Version

      - Magic --> `Uint32`. This identifies the start of the header. It is always 1071082519.
      - CodecName --> `String`. This is a string to identify this file.
      - Version --> `Uint32`. Records the version of the file.

      Note that the length of a codec header depends only upon the name of the codec, so this length can be computed at any time with `headerLength(String)`.

      Parameters:
        `out` - Output stream
        `codec` - String to identify this file. It should be simple ASCII, less than 128 characters in length.
        `version` - Version number
      Throws:
        `IOException` - If there is an I/O error writing to the underlying medium.
        `IllegalArgumentException` - If the codec name is not simple ASCII, or is more than 127 characters in length

    - #### writeIndexHeader

          public static void writeIndexHeader(DataOutput out,
                                              String codec,
                                              int version,
                                              byte[] id,
                                              String suffix)
                                       throws IOException

      Writes a codec header for an index file, which records both a string to identify the format of the file, a version number, and data to identify the file instance (ID and auxiliary suffix such as generation).

      This header can be parsed and validated with `checkIndexHeader()`.

      IndexHeader --> CodecHeader,ObjectID,ObjectSuffix

      - CodecHeader --> `writeHeader(org.apache.lucene.store.DataOutput, java.lang.String, int)`
      - ObjectID --> `byte`16
      - ObjectSuffix --> SuffixLength,SuffixBytes
      - SuffixLength --> `byte`
      - SuffixBytes --> `byte`SuffixLength

      Note that the length of an index header depends only upon the name of the codec and suffix, so this length can be computed at any time with `indexHeaderLength(String,String)`.

      Parameters:
        `out` - Output stream
        `codec` - String to identify the format of this file. It should be simple ASCII, less than 128 characters in length.
        `id` - Unique identifier for this particular file instance.
        `suffix` - auxiliary suffix information for the file. It should be simple ASCII, less than 256 characters in length.
        `version` - Version number
      Throws:
        `IOException` - If there is an I/O error writing to the underlying medium.
        `IllegalArgumentException` - If the codec name is not simple ASCII, or is more than 127 characters in length, or if id is invalid, or if the suffix is not simple ASCII, or more than 255 characters in length.

    - #### headerLength

          public static int headerLength(String codec)

      Computes the length of a codec header.
      Parameters:
        `codec` - Codec name.
      Returns:
        length of the entire codec header.
      See Also:
        `writeHeader(DataOutput, String, int)`

    - #### indexHeaderLength

          public static int indexHeaderLength(String codec,
                                              String suffix)

      Computes the length of an index header.
      Parameters:
        `codec` - Codec name.
      Returns:
        length of the entire index header.
      See Also:
        `writeIndexHeader(DataOutput, String, int, byte[], String)`

    - #### checkHeader

          public static int checkHeader(DataInput in,
                                        String codec,
                                        int minVersion,
                                        int maxVersion)
                                 throws IOException

      Reads and validates a header previously written with `writeHeader(DataOutput, String, int)`.

      When reading a file, supply the expected `codec` and an expected version range (`minVersion to maxVersion`).

      Parameters:
        `in` - Input stream, positioned at the point where the header was previously written. Typically this is located at the beginning of the file.
        `codec` - The expected codec name.
        `minVersion` - The minimum supported expected version number.
        `maxVersion` - The maximum supported expected version number.
      Returns:
        The actual version found, when a valid header is found that matches `codec`, with an actual version where `minVersion <= actual <= maxVersion`. Otherwise an exception is thrown.
      Throws:
        `CorruptIndexException` - If the first four bytes are not `CODEC_MAGIC`, or if the actual codec found is not `codec`.
        `IndexFormatTooOldException` - If the actual version is less than `minVersion`.
        `IndexFormatTooNewException` - If the actual version is greater than `maxVersion`.
        `IOException` - If there is an I/O error reading from the underlying medium.
      See Also:
        `writeHeader(DataOutput, String, int)`

    - #### checkHeaderNoMagic

          public static int checkHeaderNoMagic(DataInput in,
                                               String codec,
                                               int minVersion,
                                               int maxVersion)
                                        throws IOException

      Like `checkHeader(DataInput,String,int,int)` except this version assumes the first int has already been read and validated from the input.
      Throws:
        `IOException`

    - #### checkIndexHeader

          public static int checkIndexHeader(DataInput in,
                                             String codec,
                                             int minVersion,
                                             int maxVersion,
                                             byte[] expectedID,
                                             String expectedSuffix)
                                      throws IOException

      Reads and validates a header previously written with `writeIndexHeader(DataOutput, String, int, byte[], String)`.

      When reading a file, supply the expected `codec`, expected version range (`minVersion to maxVersion`), and object ID and suffix.

      Parameters:
        `in` - Input stream, positioned at the point where the header was previously written. Typically this is located at the beginning of the file.
        `codec` - The expected codec name.
        `minVersion` - The minimum supported expected version number.
        `maxVersion` - The maximum supported expected version number.
        `expectedID` - The expected object identifier for this file.
        `expectedSuffix` - The expected auxiliary suffix for this file.
      Returns:
        The actual version found, when a valid header is found that matches `codec`, with an actual version where `minVersion <= actual <= maxVersion`, and matching `expectedID` and `expectedSuffix` Otherwise an exception is thrown.
      Throws:
        `CorruptIndexException` - If the first four bytes are not `CODEC_MAGIC`, or if the actual codec found is not `codec`, or if the `expectedID` or `expectedSuffix` do not match.
        `IndexFormatTooOldException` - If the actual version is less than `minVersion`.
        `IndexFormatTooNewException` - If the actual version is greater than `maxVersion`.
        `IOException` - If there is an I/O error reading from the underlying medium.
      See Also:
        `writeIndexHeader(DataOutput, String, int, byte[],String)`

    - #### verifyAndCopyIndexHeader

          public static void verifyAndCopyIndexHeader(IndexInput in,
                                                      DataOutput out,
                                                      byte[] expectedID)
                                               throws IOException

      Expert: verifies the incoming `IndexInput` has an index header and that its segment ID matches the expected one, and then copies that index header into the provided `DataOutput`. This is useful when building compound files.
      Parameters:
        `in` - Input stream, positioned at the point where the index header was previously written. Typically this is located at the beginning of the file.
        `out` - Output stream, where the header will be copied to.
        `expectedID` - Expected segment ID
      Throws:
        `CorruptIndexException` - If the first four bytes are not `CODEC_MAGIC`, or if the `expectedID` does not match.
        `IOException` - If there is an I/O error reading from the underlying medium.
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### readIndexHeader

          public static byte[] readIndexHeader(IndexInput in)
                                        throws IOException

      Retrieves the full index header from the provided `IndexInput`. This throws `CorruptIndexException` if this file does not appear to be an index file.
      Throws:
        `IOException`

    - #### readFooter

          public static byte[] readFooter(IndexInput in)
                                   throws IOException

      Retrieves the full footer from the provided `IndexInput`. This throws `CorruptIndexException` if this file does not have a valid footer.
      Throws:
        `IOException`

    - #### checkIndexHeaderID

          public static byte[] checkIndexHeaderID(DataInput in,
                                                  byte[] expectedID)
                                           throws IOException

      Expert: just reads and verifies the object ID of an index header
      Throws:
        `IOException`

    - #### checkIndexHeaderSuffix

          public static String checkIndexHeaderSuffix(DataInput in,
                                                      String expectedSuffix)
                                               throws IOException

      Expert: just reads and verifies the suffix of an index header
      Throws:
        `IOException`

    - #### writeFooter

          public static void writeFooter(IndexOutput out)
                                  throws IOException

      Writes a codec footer, which records both a checksum algorithm ID and a checksum. This footer can be parsed and validated with `checkFooter()`.

      CodecFooter --> Magic,AlgorithmID,Checksum

      - Magic --> `Uint32`. This identifies the start of the footer. It is always -1071082520.
      - AlgorithmID --> `Uint32`. This indicates the checksum algorithm used. Currently this is always 0, for zlib-crc32.
      - Checksum --> `Uint64`. The actual checksum value for all previous bytes in the stream, including the bytes from Magic and AlgorithmID.
      Parameters:
        `out` - Output stream
      Throws:
        `IOException` - If there is an I/O error writing to the underlying medium.

    - #### footerLength

          public static int footerLength()

      Computes the length of a codec footer.
      Returns:
        length of the entire codec footer.
      See Also:
        `writeFooter(IndexOutput)`

    - #### checkFooter

          public static long checkFooter(ChecksumIndexInput in)
                                  throws IOException

      Validates the codec footer previously written by `writeFooter(org.apache.lucene.store.IndexOutput)`.
      Returns:
        actual checksum value
      Throws:
        `IOException` - if the footer is invalid, if the checksum does not match, or if `in` is not properly positioned before the footer at the end of the stream.

    - #### checkFooter

          public static void checkFooter(ChecksumIndexInput in,
                                         Throwable priorException)
                                  throws IOException

      Validates the codec footer previously written by `writeFooter(org.apache.lucene.store.IndexOutput)`, optionally passing an unexpected exception that has already occurred.

      When a `priorException` is provided, this method will add a suppressed exception indicating whether the checksum for the stream passes, fails, or cannot be computed, and rethrow it. Otherwise it behaves the same as `checkFooter(ChecksumIndexInput)`.

      Example usage:

           try (ChecksumIndexInput input = ...) {
             Throwable priorE = null;
             try {
               // ... read a bunch of stuff ...
             } catch (Throwable exception) {
               priorE = exception;
             } finally {
               CodecUtil.checkFooter(input, priorE);
             }
           }

      Throws:
        `IOException`

    - #### retrieveChecksum

          public static long retrieveChecksum(IndexInput in)
                                       throws IOException

      Returns (but does not validate) the checksum previously written by `checkFooter(org.apache.lucene.store.ChecksumIndexInput)`.
      Returns:
        actual checksum value
      Throws:
        `IOException` - if the footer is invalid

    - #### checksumEntireFile

          public static long checksumEntireFile(IndexInput input)
                                         throws IOException

      Clones the provided input, reads all bytes from the file, and calls `checkFooter(org.apache.lucene.store.ChecksumIndexInput)`

      Note that this method may be slow, as it must process the entire file. If you just need to extract the checksum value, call `retrieveChecksum(org.apache.lucene.store.IndexInput)`.

      Throws:
        `IOException`

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