org.apache.beam.sdk.coders

Class LengthPrefixCoder<T>

java.lang.Object

org.apache.beam.sdk.coders.Coder<T>

org.apache.beam.sdk.coders.StructuredCoder<T>

org.apache.beam.sdk.coders.LengthPrefixCoder<T>

Type Parameters:

T - the type of the values being transcoded

All Implemented Interfaces:

java.io.Serializable

public class LengthPrefixCoder<T>
extends StructuredCoder<T>

A Coder which is able to take any existing coder and wrap it such that it is only invoked in the outer context. The data representing the element is prefixed with a length using a variable integer encoding.

See Also:

Serialized Form

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.beam.sdk.coders.Coder

Coder.Context, Coder.NonDeterministicException

Method Summary

Modifier and Type	Method and Description
boolean	consistentWithEquals() LengthPrefixCoder is consistent with equals if the nested Coder is.
T	decode(java.io.InputStream inStream) Decodes a value of type T from the given input stream in the given context.
void	encode(T value, java.io.OutputStream outStream) Encodes the given value of type T onto the given output stream.
java.util.List<? extends Coder<?>>	getCoderArguments() If this is a Coder for a parameterized type, returns the list of Coders being used for each of the parameters in the same order they appear within the parameterized type's type signature.
protected long	getEncodedElementByteSize(T value) Overridden to short-circuit the default StructuredCoder behavior of encoding and counting the bytes.
Coder<T>	getValueCoder() Gets the value coder that will be prefixed by the length.
boolean	isRegisterByteSizeObserverCheap(T value) LengthPrefixCoder is cheap if valueCoder is cheap.
static <T> LengthPrefixCoder<T>	of(Coder<T> valueCoder)
void	verifyDeterministic() LengthPrefixCoder is deterministic if the nested Coder is.

Methods inherited from class org.apache.beam.sdk.coders.StructuredCoder

equals, getComponents, hashCode, toString

Methods inherited from class org.apache.beam.sdk.coders.Coder

decode, encode, getEncodedTypeDescriptor, registerByteSizeObserver, structuralValue, verifyDeterministic, verifyDeterministic

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Method Detail

of

public static <T> LengthPrefixCoder<T> of(Coder<T> valueCoder)

encode

public void encode(T value,
                   java.io.OutputStream outStream)
            throws CoderException,
                   java.io.IOException

Description copied from class: Coder

Encodes the given value of type T onto the given output stream. Multiple elements can be encoded next to each other on the output stream, each coder should encode information to know how many bytes to read when decoding. A common approach is to prefix the encoding with the element's encoded length.

Specified by:

encode in class Coder<T>

Throws:

CoderException - if the value could not be encoded for some reason

java.io.IOException - if writing to the OutputStream fails for some reason

decode

public T decode(java.io.InputStream inStream)
         throws CoderException,
                java.io.IOException

Description copied from class: Coder

Decodes a value of type T from the given input stream in the given context. Returns the decoded value. Multiple elements can be encoded next to each other on the input stream, each coder should encode information to know how many bytes to read when decoding. A common approach is to prefix the encoding with the element's encoded length.

Specified by:

decode in class Coder<T>

Throws:

CoderException - if the value could not be decoded for some reason

java.io.IOException - if reading from the InputStream fails for some reason

getCoderArguments

public java.util.List<? extends Coder<?>> getCoderArguments()

Description copied from class: Coder

If this is a Coder for a parameterized type, returns the list of Coders being used for each of the parameters in the same order they appear within the parameterized type's type signature. If this cannot be done, or this Coder does not encode/decode a parameterized type, returns the empty list.

Specified by:

getCoderArguments in class Coder<T>

getValueCoder

public Coder<T> getValueCoder()

Gets the value coder that will be prefixed by the length.

verifyDeterministic

public void verifyDeterministic()
                         throws Coder.NonDeterministicException

LengthPrefixCoder is deterministic if the nested Coder is.

Throw Coder.NonDeterministicException if the coding is not deterministic.

In order for a Coder to be considered deterministic, the following must be true:

• two values that compare as equal (via Object.equals() or Comparable.compareTo(), if supported) have the same encoding.
• the Coder always produces a canonical encoding, which is the same for an instance of an object even if produced on different computers at different times.

Specified by:

verifyDeterministic in class Coder<T>

Throws:

Coder.NonDeterministicException - if this coder is not deterministic.

consistentWithEquals

public boolean consistentWithEquals()

LengthPrefixCoder is consistent with equals if the nested Coder is.

Returns true if this Coder is injective with respect to Object.equals(java.lang.Object).

Whenever the encoded bytes of two values are equal, then the original values are equal according to Objects.equals(). Note that this is well-defined for null.

This condition is most notably false for arrays. More generally, this condition is false whenever equals() compares object identity, rather than performing a semantic/structural comparison.

By default, returns false.

Overrides:

consistentWithEquals in class Coder<T>

getEncodedElementByteSize

protected long getEncodedElementByteSize(T value)
                                  throws java.lang.Exception

Overridden to short-circuit the default StructuredCoder behavior of encoding and counting the bytes. The size is known to be the size of the value plus the number of bytes required to prefix the length.

Returns the size in bytes of the encoded value using this coder.

Overrides:

getEncodedElementByteSize in class Coder<T>

Throws:

java.lang.Exception

isRegisterByteSizeObserverCheap

public boolean isRegisterByteSizeObserverCheap(T value)

LengthPrefixCoder is cheap if valueCoder is cheap.

Returns whether Coder.registerByteSizeObserver(T, org.apache.beam.sdk.util.common.ElementByteSizeObserver) cheap enough to call for every element, that is, if this Coder can calculate the byte size of the element to be coded in roughly constant time (or lazily).

Not intended to be called by user code, but instead by PipelineRunner implementations.

By default, returns false. The default Coder.registerByteSizeObserver(T, org.apache.beam.sdk.util.common.ElementByteSizeObserver) implementation invokes Coder.getEncodedElementByteSize(T) which requires re-encoding an element unless it is overridden. This is considered expensive.

Overrides:

isRegisterByteSizeObserverCheap in class Coder<T>