Deflater

public class Deflater extends Object

This class provides support for general purpose compression using the popular ZLIB compression library. The ZLIB compression library was initially developed as part of the PNG graphics standard and is not protected by patents. It is fully described in the specifications at the java.util.zip package description.

The following code fragment demonstrates a trivial compression and decompression of a string using Deflater and Inflater.

 try {
     // Encode a String into bytes
     String inputString = "blahblahblah";
     byte[] input = inputString.getBytes("UTF-8");

     // Compress the bytes
     byte[] output = new byte[100];
     Deflater compresser = new Deflater();
     compresser.setInput(input);
     compresser.finish();
     int compressedDataLength = compresser.deflate(output);
     compresser.end();

     // Decompress the bytes
     Inflater decompresser = new Inflater();
     decompresser.setInput(output, 0, compressedDataLength);
     byte[] result = new byte[100];
     int resultLength = decompresser.inflate(result);
     decompresser.end();

     // Decode the bytes into a String
     String outputString = new String(result, 0, resultLength, "UTF-8");
 } catch(java.io.UnsupportedEncodingException ex) {
     // handle
 } catch (java.util.zip.DataFormatException ex) {
     // handle
 }
 

See Also

Constant Summary

int BEST_COMPRESSION Compression level for best compression.
int BEST_SPEED Compression level for fastest compression.
int DEFAULT_COMPRESSION Default compression level.
int DEFAULT_STRATEGY Default compression strategy.
int DEFLATED Compression method for the deflate algorithm (the only one currently supported).
int FILTERED Compression strategy best used for data consisting mostly of small values with a somewhat random distribution.
int FULL_FLUSH Compression flush mode used to flush out all pending output and reset the deflater.
int HUFFMAN_ONLY Compression strategy for Huffman coding only.
int NO_COMPRESSION Compression level for no compression.
int NO_FLUSH Compression flush mode used to achieve best compression result.
int SYNC_FLUSH Compression flush mode used to flush out all pending output; may degrade compression for some compression algorithms.

Public Constructor Summary

Deflater(int level, boolean nowrap)
Creates a new compressor using the specified compression level.
Deflater(int level)
Creates a new compressor using the specified compression level.
Deflater()
Creates a new compressor with the default compression level.

Public Method Summary

int
deflate(byte[] b, int off, int len)
Compresses the input data and fills specified buffer with compressed data.
int
deflate(byte[] b)
Compresses the input data and fills specified buffer with compressed data.
int
deflate(byte[] b, int off, int len, int flush)
Compresses the input data and fills the specified buffer with compressed data.
void
end()
Closes the compressor and discards any unprocessed input.
void
finish()
When called, indicates that compression should end with the current contents of the input buffer.
boolean
finished()
Returns true if the end of the compressed data output stream has been reached.
int
getAdler()
Returns the ADLER-32 value of the uncompressed data.
long
getBytesRead()
Returns the total number of uncompressed bytes input so far.
long
getBytesWritten()
Returns the total number of compressed bytes output so far.
int
getTotalIn()
Returns the total number of uncompressed bytes input so far.
int
getTotalOut()
Returns the total number of compressed bytes output so far.
boolean
needsInput()
Returns true if the input data buffer is empty and setInput() should be called in order to provide more input.
void
reset()
Resets deflater so that a new set of input data can be processed.
void
setDictionary(byte[] b, int off, int len)
Sets preset dictionary for compression.
void
setDictionary(byte[] b)
Sets preset dictionary for compression.
void
setInput(byte[] b, int off, int len)
Sets input data for compression.
void
setInput(byte[] b)
Sets input data for compression.
void
setLevel(int level)
Sets the current compression level to the specified value.
void
setStrategy(int strategy)
Sets the compression strategy to the specified value.

Protected Method Summary

void
finalize()
Closes the compressor when garbage is collected.

Inherited Method Summary

Constants

public static final int BEST_COMPRESSION

Compression level for best compression.

Constant Value: 9

public static final int BEST_SPEED

Compression level for fastest compression.

Constant Value: 1

public static final int DEFAULT_COMPRESSION

Default compression level.

Constant Value: -1

public static final int DEFAULT_STRATEGY

Default compression strategy.

Constant Value: 0

public static final int DEFLATED

Compression method for the deflate algorithm (the only one currently supported).

Constant Value: 8

public static final int FILTERED

Compression strategy best used for data consisting mostly of small values with a somewhat random distribution. Forces more Huffman coding and less string matching.

Constant Value: 1

public static final int FULL_FLUSH

Compression flush mode used to flush out all pending output and reset the deflater. Using this mode too often can seriously degrade compression.

Constant Value: 3

public static final int HUFFMAN_ONLY

Compression strategy for Huffman coding only.

Constant Value: 2

public static final int NO_COMPRESSION

Compression level for no compression.

Constant Value: 0

public static final int NO_FLUSH

Compression flush mode used to achieve best compression result.

Constant Value: 0

public static final int SYNC_FLUSH

Compression flush mode used to flush out all pending output; may degrade compression for some compression algorithms.

Constant Value: 2

Public Constructors

public Deflater (int level, boolean nowrap)

Creates a new compressor using the specified compression level. If 'nowrap' is true then the ZLIB header and checksum fields will not be used in order to support the compression format used in both GZIP and PKZIP.

Parameters
level the compression level (0-9)
nowrap if true then use GZIP compatible compression

public Deflater (int level)

Creates a new compressor using the specified compression level. Compressed data will be generated in ZLIB format.

Parameters
level the compression level (0-9)

public Deflater ()

Creates a new compressor with the default compression level. Compressed data will be generated in ZLIB format.

Public Methods

public int deflate (byte[] b, int off, int len)

Compresses the input data and fills specified buffer with compressed data. Returns actual number of bytes of compressed data. A return value of 0 indicates that needsInput should be called in order to determine if more input data is required.

This method uses NO_FLUSH as its compression flush mode. An invocation of this method of the form deflater.deflate(b, off, len) yields the same result as the invocation of deflater.deflate(b, off, len, Deflater.NO_FLUSH).

Parameters
b the buffer for the compressed data
off the start offset of the data
len the maximum number of bytes of compressed data
Returns
  • the actual number of bytes of compressed data written to the output buffer

public int deflate (byte[] b)

Compresses the input data and fills specified buffer with compressed data. Returns actual number of bytes of compressed data. A return value of 0 indicates that needsInput should be called in order to determine if more input data is required.

This method uses NO_FLUSH as its compression flush mode. An invocation of this method of the form deflater.deflate(b) yields the same result as the invocation of deflater.deflate(b, 0, b.length, Deflater.NO_FLUSH).

Parameters
b the buffer for the compressed data
Returns
  • the actual number of bytes of compressed data written to the output buffer

public int deflate (byte[] b, int off, int len, int flush)

Compresses the input data and fills the specified buffer with compressed data. Returns actual number of bytes of data compressed.

Compression flush mode is one of the following three modes:

  • NO_FLUSH: allows the deflater to decide how much data to accumulate, before producing output, in order to achieve the best compression (should be used in normal use scenario). A return value of 0 in this flush mode indicates that needsInput() should be called in order to determine if more input data is required.
  • SYNC_FLUSH: all pending output in the deflater is flushed, to the specified output buffer, so that an inflater that works on compressed data can get all input data available so far (In particular the needsInput() returns true after this invocation if enough output space is provided). Flushing with SYNC_FLUSH may degrade compression for some compression algorithms and so it should be used only when necessary.
  • FULL_FLUSH: all pending output is flushed out as with SYNC_FLUSH. The compression state is reset so that the inflater that works on the compressed output data can restart from this point if previous compressed data has been damaged or if random access is desired. Using FULL_FLUSH too often can seriously degrade compression.

In the case of FULL_FLUSH or SYNC_FLUSH, if the return value is len, the space available in output buffer b, this method should be invoked again with the same flush parameter and more output space.

Parameters
b the buffer for the compressed data
off the start offset of the data
len the maximum number of bytes of compressed data
flush the compression flush mode
Returns
  • the actual number of bytes of compressed data written to the output buffer
Throws
IllegalArgumentException if the flush mode is invalid

public void end ()

Closes the compressor and discards any unprocessed input. This method should be called when the compressor is no longer being used, but will also be called automatically by the finalize() method. Once this method is called, the behavior of the Deflater object is undefined.

public void finish ()

When called, indicates that compression should end with the current contents of the input buffer.

public boolean finished ()

Returns true if the end of the compressed data output stream has been reached.

Returns
  • true if the end of the compressed data output stream has been reached

public int getAdler ()

Returns the ADLER-32 value of the uncompressed data.

Returns
  • the ADLER-32 value of the uncompressed data

public long getBytesRead ()

Returns the total number of uncompressed bytes input so far.

Returns
  • the total (non-negative) number of uncompressed bytes input so far

public long getBytesWritten ()

Returns the total number of compressed bytes output so far.

Returns
  • the total (non-negative) number of compressed bytes output so far

public int getTotalIn ()

Returns the total number of uncompressed bytes input so far.

Since the number of bytes may be greater than Integer.MAX_VALUE, the getBytesRead() method is now the preferred means of obtaining this information.

Returns
  • the total number of uncompressed bytes input so far

public int getTotalOut ()

Returns the total number of compressed bytes output so far.

Since the number of bytes may be greater than Integer.MAX_VALUE, the getBytesWritten() method is now the preferred means of obtaining this information.

Returns
  • the total number of compressed bytes output so far

public boolean needsInput ()

Returns true if the input data buffer is empty and setInput() should be called in order to provide more input.

Returns
  • true if the input data buffer is empty and setInput() should be called in order to provide more input

public void reset ()

Resets deflater so that a new set of input data can be processed. Keeps current compression level and strategy settings.

public void setDictionary (byte[] b, int off, int len)

Sets preset dictionary for compression. A preset dictionary is used when the history buffer can be predetermined. When the data is later uncompressed with Inflater.inflate(), Inflater.getAdler() can be called in order to get the Adler-32 value of the dictionary required for decompression.

Parameters
b the dictionary data bytes
off the start offset of the data
len the length of the data

public void setDictionary (byte[] b)

Sets preset dictionary for compression. A preset dictionary is used when the history buffer can be predetermined. When the data is later uncompressed with Inflater.inflate(), Inflater.getAdler() can be called in order to get the Adler-32 value of the dictionary required for decompression.

Parameters
b the dictionary data bytes

public void setInput (byte[] b, int off, int len)

Sets input data for compression. This should be called whenever needsInput() returns true indicating that more input data is required.

Parameters
b the input data bytes
off the start offset of the data
len the length of the data
See Also

public void setInput (byte[] b)

Sets input data for compression. This should be called whenever needsInput() returns true indicating that more input data is required.

Parameters
b the input data bytes
See Also

public void setLevel (int level)

Sets the current compression level to the specified value.

Parameters
level the new compression level (0-9)
Throws
IllegalArgumentException if the compression level is invalid

public void setStrategy (int strategy)

Sets the compression strategy to the specified value.

Parameters
strategy the new compression strategy
Throws
IllegalArgumentException if the compression strategy is invalid

Protected Methods

protected void finalize ()

Closes the compressor when garbage is collected.