Package org.acemq.amqp.core

Class CompositeCodec

java.lang.Object

org.acemq.amqp.core.CompositeCodec

All Implemented Interfaces:

Codec

public final class CompositeCodec
extends Object
implements Codec

Writes in one format and reads several.

This is what makes changing format survivable. A queue during a cutover holds messages written by the old producers and the new ones at the same time, and a consumer that understands only one of them either loses the others or has to be deployed in lockstep with every producer. Reading both for a while, and writing only the new one, turns a flag day into two ordinary releases: deploy the consumers that read both, then the producers that write the new format, then drop the old codec once the queue has drained.

Encoding always uses the first codec, and there is no way to configure otherwise. Choosing an encoding per message would need a rule for choosing, and every such rule turns into a question the caller has to answer at each publish; one format out, several in, is the shape that migrations actually need.

A message whose content type matches nothing is offered to every codec in turn, because a sender that set no content type has told us nothing rather than told us it is unreadable, and something on the wire is usually still readable by one of them.

Method Summary

Modifier and Type	Method	Description
boolean	canDecode(@Nullable String contentType)
List<Codec>	codecs()
String	contentType()
<T> T	decode(byte[] body, Class<T> target)
<T> T	decode(byte[] body, Class<T> target, @Nullable String contentType)	Decodes a received message, told what the sender said it was.
byte[]	encode(Object payload)
static CompositeCodec	of(Codec... codecs)
String	toString()

Methods inherited from class java.lang.Object

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

Method Details

of

public static CompositeCodec of(Codec... codecs)

Parameters:

codecs - codecs to read with; the first is also the one written with

Returns:

a codec that writes the first format and reads any of them

contentType

public String contentType()

Specified by:

contentType in interface Codec

Returns:

the content type this codec produces, for example application/json

encode

public byte[] encode(Object payload)

Specified by:

encode in interface Codec

Parameters:

payload - object to encode; never null

Returns:

encoded bytes

decode

public <T> T decode(byte[] body,
 Class<T> target)

Specified by:

decode in interface Codec

Type Parameters:

T - target type

Parameters:

body - encoded bytes

target - type to produce

Returns:

the decoded payload

decode

public <T> T decode(byte[] body,
 Class<T> target,
 @Nullable String contentType)

Description copied from interface: Codec

Decodes a received message, told what the sender said it was.

The engine calls this one, because a codec that dispatches between several formats cannot choose without knowing the content type. A codec that handles a single format has no use for it and need not override this.

Specified by:

decode in interface Codec

Type Parameters:

T - target type

Parameters:

body - encoded bytes

target - type to produce

contentType - content type the message arrived with, or null when the sender did not set one

Returns:

the decoded payload

canDecode

public boolean canDecode(@Nullable String contentType)

Specified by:

canDecode in interface Codec

Parameters:

contentType - content type of a received message, or null when the sender did not set one

Returns:

whether this codec can decode it

codecs

public List<Codec> codecs()

Returns:

the codecs this one reads with, in the order they are tried

toString

public String toString()

Overrides:

toString in class Object