Package java.nio.charset

Class CharsetDecoder

java.lang.Object
java.nio.charset.CharsetDecoder
public abstract class CharsetDecoder
extends Object
A converter that can convert a byte sequence from a charset into a 16-bit Unicode character sequence.

The input byte sequence is wrapped by a ByteBuffer and the output character sequence is a CharBuffer. A decoder instance should be used in the following sequence, which is referred to as a decoding operation:

  1. invoking the reset method to reset the decoder if the decoder has been used;
  2. invoking the decode method until the additional input is not needed, the endOfInput parameter must be set to false, the input buffer must be filled and the output buffer must be flushed between invocations;
  3. invoking the decode method for the last time, and then the endOfInput parameter must be set to true;
  4. invoking the flush method to flush the output.

The decode method will convert as many bytes as possible, and the process won't stop until the input bytes have run out, the output buffer has been filled or some error has happened. A CoderResult instance will be returned to indicate the stop reason, and the invoker can identify the result and choose further action, which includes filling the input buffer, flushing the output buffer or recovering from an error and trying again.

There are two common decoding errors. One is named malformed and it is returned when the input byte sequence is illegal for the current specific charset, the other is named unmappable character and it is returned when a problem occurs mapping a legal input byte sequence to its Unicode character equivalent.

Both errors can be handled in three ways, the default one is to report the error to the invoker by a CoderResult instance, and the alternatives are to ignore it or to replace the erroneous input with the replacement string. The replacement string is "�" by default and can be changed by invoking replaceWith method. The invoker of this decoder can choose one way by specifying a CodingErrorAction instance for each error type via onMalformedInput method and onUnmappableCharacter method.

This is an abstract class and encapsulates many common operations of the decoding process for all charsets. Decoders for a specific charset should extend this class and need only to implement the decodeLoop method for the basic decoding. If a subclass maintains an internal state, it should override the implFlush method and the implReset method in addition.

This class is not thread-safe.

See Also:
Charset, CharsetEncoder

  • Constructor Details

    • CharsetDecoder

      protected CharsetDecoder​(Charset charset, float averageCharsPerByte, float maxCharsPerByte)
      Constructs a new CharsetDecoder using the given Charset, average number and maximum number of characters created by this decoder for one input byte, and the default replacement string "�".
      Parameters:
      charset - the Charset to be used by this decoder.
      averageCharsPerByte - the average number of characters created by this decoder for one input byte, must be positive.
      maxCharsPerByte - the maximum number of characters created by this decoder for one input byte, must be positive.
      Throws:
      IllegalArgumentException - if averageCharsPerByte <= 0 || maxCharsPerByte <= 0 || averageCharsPerByte > maxCharsPerByte.

  • Method Details

    • averageCharsPerByte

      public final float averageCharsPerByte()
      Returns the average number of characters created by this decoder for a single input byte.

    • charset

      public final Charset charset()
      Returns the Charset which this decoder uses.

    • decode

      public final CharBuffer decode​(ByteBuffer in) throws CharacterCodingException
      This is a facade method for the decoding operation.

      This method decodes the remaining byte sequence of the given byte buffer into a new character buffer. This method performs a complete decoding operation, resets at first, then decodes, and flushes at last.

      This method should not be invoked while another decode operation is ongoing.

      Parameters:
      in - the input buffer.
      Returns:
      a new CharBuffer containing the the characters produced by this decoding operation. The buffer's limit will be the position of the last character in the buffer, and the position will be zero.
      Throws:
      IllegalStateException - if another decoding operation is ongoing.
      MalformedInputException - if an illegal input byte sequence for this charset was encountered, and the action for malformed error is CodingErrorAction.REPORT
      UnmappableCharacterException - if a legal but unmappable input byte sequence for this charset was encountered, and the action for unmappable character error is CodingErrorAction.REPORT. Unmappable means the byte sequence at the input buffer's current position cannot be mapped to a Unicode character sequence.
      CharacterCodingException - if another exception happened during the decode operation.

    • decode

      public final CoderResult decode​(ByteBuffer in, CharBuffer out, boolean endOfInput)
      Decodes bytes starting at the current position of the given input buffer, and writes the equivalent character sequence into the given output buffer from its current position.

      The buffers' position will be changed with the reading and writing operation, but their limits and marks will be kept intact.

      A CoderResult instance will be returned according to following rules:

      • CoderResult.OVERFLOW indicates that even though not all of the input has been processed, the buffer the output is being written to has reached its capacity. In the event of this code being returned this method should be called once more with an out argument that has not already been filled.
      • CoderResult.UNDERFLOW indicates that as many bytes as possible in the input buffer have been decoded. If there is no further input and no remaining bytes in the input buffer then this operation may be regarded as complete. Otherwise, this method should be called once more with additional input.
      • A malformed input result indicates that some malformed input error has been encountered, and the erroneous bytes start at the input buffer's position and their number can be got by result's length. This kind of result can be returned only if the malformed action is CodingErrorAction.REPORT.
      • A unmappable character result indicates that some unmappable character error has been encountered, and the erroneous bytes start at the input buffer's position and their number can be got by result's length. This kind of result can be returned only if the unmappable character action is CodingErrorAction.REPORT.

      The endOfInput parameter indicates that the invoker cannot provide further input. This parameter is true if and only if the bytes in current input buffer are all inputs for this decoding operation. Note that it is common and won't cause an error if the invoker sets false and then can't provide more input, while it may cause an error if the invoker always sets true in several consecutive invocations. This would make the remaining input to be treated as malformed input.

      This method invokes the decodeLoop method to implement the basic decode logic for a specific charset.

      Parameters:
      in - the input buffer.
      out - the output buffer.
      endOfInput - true if all the input characters have been provided.
      Returns:
      a CoderResult instance which indicates the reason of termination.
      Throws:
      IllegalStateException - if decoding has started or no more input is needed in this decoding progress.
      CoderMalfunctionError - if the decodeLoop method threw an BufferUnderflowException or BufferOverflowException.

    • decodeLoop

      protected abstract CoderResult decodeLoop​(ByteBuffer in, CharBuffer out)
      Decodes bytes into characters. This method is called by the decode method.

      This method will implement the essential decoding operation, and it won't stop decoding until either all the input bytes are read, the output buffer is filled, or some exception is encountered. Then it will return a CoderResult object indicating the result of current decoding operation. The rules to construct the CoderResult are the same as for decode. When an exception is encountered in the decoding operation, most implementations of this method will return a relevant result object to the decode method, and some performance optimized implementation may handle the exception and implement the error action itself.

      The buffers are scanned from their current positions, and their positions will be modified accordingly, while their marks and limits will be intact. At most in.remaining() characters will be read, and out.remaining() bytes will be written.

      Note that some implementations may pre-scan the input buffer and return a CoderResult.UNDERFLOW until it receives sufficient input.

      Parameters:
      in - the input buffer.
      out - the output buffer.
      Returns:
      a CoderResult instance indicating the result.

    • detectedCharset

      public Charset detectedCharset()
      Gets the charset detected by this decoder; this method is optional.

      If implementing an auto-detecting charset, then this decoder returns the detected charset from this method when it is available. The returned charset will be the same for the rest of the decode operation.

      If insufficient bytes have been read to determine the charset, an IllegalStateException will be thrown.

      The default implementation always throws UnsupportedOperationException, so it should be overridden by a subclass if needed.

      Returns:
      the charset detected by this decoder, or null if it is not yet determined.
      Throws:
      UnsupportedOperationException - if this decoder does not implement an auto-detecting charset.
      IllegalStateException - if insufficient bytes have been read to determine the charset.

    • flush

      public final CoderResult flush​(CharBuffer out)
      Flushes this decoder. This method will call implFlush. Some decoders may need to write some characters to the output buffer when they have read all input bytes; subclasses can override implFlush to perform the writing operation.

      The maximum number of written bytes won't be larger than out.remaining(). If some decoder wants to write more bytes than an output buffer's remaining space allows, then a CoderResult.OVERFLOW will be returned, and this method must be called again with a character buffer that has more remaining space. Otherwise this method will return CoderResult.UNDERFLOW, which means one decoding process has been completed successfully.

      During the flush, the output buffer's position will be changed accordingly, while its mark and limit will be intact.

      Parameters:
      out - the given output buffer.
      Returns:
      CoderResult.UNDERFLOW or CoderResult.OVERFLOW.
      Throws:
      IllegalStateException - if this decoder isn't already flushed or at end of input.

    • implFlush

      protected CoderResult implFlush​(CharBuffer out)
      Flushes this decoder. The default implementation does nothing and always returns CoderResult.UNDERFLOW; this method can be overridden if needed.
      Parameters:
      out - the output buffer.
      Returns:
      CoderResult.UNDERFLOW or CoderResult.OVERFLOW.

    • implOnMalformedInput

      protected void implOnMalformedInput​(CodingErrorAction newAction)
      Notifies that this decoder's CodingErrorAction specified for malformed input error has been changed. The default implementation does nothing; this method can be overridden if needed.
      Parameters:
      newAction - the new action.

    • implOnUnmappableCharacter

      protected void implOnUnmappableCharacter​(CodingErrorAction newAction)
      Notifies that this decoder's CodingErrorAction specified for unmappable character error has been changed. The default implementation does nothing; this method can be overridden if needed.
      Parameters:
      newAction - the new action.

    • implReplaceWith

      protected void implReplaceWith​(String newReplacement)
      Notifies that this decoder's replacement has been changed. The default implementation does nothing; this method can be overridden if needed.
      Parameters:
      newReplacement - the new replacement string.

    • implReset

      protected void implReset()
      Reset this decoder's charset related state. The default implementation does nothing; this method can be overridden if needed.

    • isAutoDetecting

      public boolean isAutoDetecting()
      Indicates whether this decoder implements an auto-detecting charset.
      Returns:
      true if this decoder implements an auto-detecting charset.

    • isCharsetDetected

      public boolean isCharsetDetected()
      Indicates whether this decoder has detected a charset; this method is optional.

      If this decoder implements an auto-detecting charset, then this method may start to return true during decoding operation to indicate that a charset has been detected in the input bytes and that the charset can be retrieved by invoking the detectedCharset method.

      Note that a decoder that implements an auto-detecting charset may still succeed in decoding a portion of the given input even when it is unable to detect the charset. For this reason users should be aware that a false return value does not indicate that no decoding took place.

      The default implementation always throws an UnsupportedOperationException; it should be overridden by a subclass if needed.

      Returns:
      true if this decoder has detected a charset.
      Throws:
      UnsupportedOperationException - if this decoder doesn't implement an auto-detecting charset.

    • malformedInputAction

      public CodingErrorAction malformedInputAction()
      Returns this decoder's CodingErrorAction when malformed input occurred during the decoding process.

    • maxCharsPerByte

      public final float maxCharsPerByte()
      Returns the maximum number of characters which can be created by this decoder for one input byte, must be positive.

    • onMalformedInput

      public final CharsetDecoder onMalformedInput​(CodingErrorAction newAction)
      Sets this decoder's action on malformed input errors. This method will call the implOnMalformedInput method with the given new action as argument.
      Parameters:
      newAction - the new action on malformed input error.
      Returns:
      this decoder.
      Throws:
      IllegalArgumentException - if newAction == null.

    • onUnmappableCharacter

      public final CharsetDecoder onUnmappableCharacter​(CodingErrorAction newAction)
      Sets this decoder's action on unmappable character errors. This method will call the implOnUnmappableCharacter method with the given new action as argument.
      Parameters:
      newAction - the new action on unmappable character error.
      Returns:
      this decoder.
      Throws:
      IllegalArgumentException - if newAction == null.

    • replacement

      public final String replacement()
      Returns the replacement string, which is never null or empty.

    • replaceWith

      public final CharsetDecoder replaceWith​(String replacement)
      Sets the new replacement string. This method first checks the given replacement's validity, then changes the replacement value, and at last calls the implReplaceWith method with the given new replacement as argument.
      Parameters:
      replacement - the replacement string cannot be null, empty, or longer than maxCharsPerByte().
      Returns:
      this decoder.
      Throws:
      IllegalArgumentException - if the given replacement cannot satisfy the requirement mentioned above.

    • reset

      public final CharsetDecoder reset()
      Resets this decoder. This method will reset the internal state, and then calls implReset() to reset any state related to the specific charset.

    • unmappableCharacterAction

      public CodingErrorAction unmappableCharacterAction()
      Returns this decoder's CodingErrorAction when an unmappable character error occurred during the decoding process.