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From "Vincenz Priesnitz (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1341) Allow controlling avro via java annotations when using reflection.
Date Wed, 03 Jul 2013 17:14:20 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1341?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13699189#comment-13699189

Vincenz Priesnitz commented on AVRO-1341:

Thanks for reviewing the Patch. 

I think it is clearer if there was an extra Ignore Annotation for Avro. Also, the general
java Annotations might collide with other Services that use Annotations. 

The AvroEncoder Annotation should indded only used by Experts; I will add such warnings. 

I'd like to mention Issue AVRO-1347 here, which adds another Annotation for adding Aliases.

> Allow controlling avro via java annotations when using reflection. 
> -------------------------------------------------------------------
>                 Key: AVRO-1341
>                 URL: https://issues.apache.org/jira/browse/AVRO-1341
>             Project: Avro
>          Issue Type: New Feature
>          Components: java
>            Reporter: Vincenz Priesnitz
>            Assignee: Vincenz Priesnitz
>         Attachments: AVRO-1341.patch, AVRO-1341.patch
> It would be great if one could control avro with java annotations. As of now, it is already
possible to mark fields as Nullable or classes being encoded as a String. I propose a bigger
set of annotations to control the behavior of avro on fields and classes. Such annotations
have proven useful with jacksons json serialization and morphias mongoDB serialization.
> I propose the following additional annotations: 
> @AvroName("alternativeName")
> @AvroIgnore
> @AvroMeta(key="K", value="V")
> @AvroEncode(using=CustomEncoding.class)
> Java fields with the @AvroName("alternativeName") annotation will be renamed in the induced
schema. When reading an avro file via reflection, the reflection reader will look for fields
in the schema with "alternativeName". 
> For example:
> {code}
>    @AvroName("foo")
>    int bar;  
> {code}
> is serialized as
> {code}
>   { "name" : "foo", "type" : "int" } 
> {code}
> Fields with the @AvroIgnore annotation will be treated as if they had a transient modifier,
i.e. they will not be written to or read from avro files. 
> The @AvroMeta(key="K", value="V") annotation allows you to store an arbitrary key : value
pair at every node in the schema.
> {code}
>    @AvroMeta(key="fieldKey", value="fieldValue")
>    int foo;  
> {code}
> will create the following schema
> {code}
> {"name" : "foo", "type" : "int", "fieldKey" : "fieldValue" } 
> {code}
> Fields can be custom encoded with the AvroEncode(using=CustomEncoding.class) annotation.
This annotation is a generalization of the @Stringable annotation. The @Stringable annotation
is limited to classes with string argument constructors. Some classes can be similarly reduced
to a smaller class or even a single primitive, but dont fit the requirements for @Stringable.
A prominent example is java.util.Date, which instances can essentially be described with a
single long. Such classes can now be encoded with a CustomEncoding, which reads and writes
directly from the encoder/decoder. 
> One simply extends the abstract CustomEncodings class by implementing a schema, a read
method and a write method. A java field can then be annotated like this:
> {code}
> @AvroEncode(using=DateAslongEncoding.class)
> Date date;
> {code}
> The custom encoding implementation would look like 
> {code}
> public class DateAsLongEncoding extends CustomEncoding<Date> {
>   {
>     schema = Schema.create(Schema.Type.LONG);
>     schema.addProp("CustomEncoding", "DateAsLongEncoding");
>   }
>   @Override
>   public void write(Object datum, Encoder out) throws IOException {
>     out.writeLong(((Date)datum).getTime());
>   }
>   @Override
>   public Date read(Object reuse, Decoder in) throws IOException {
>     if (reuse != null) {
>       ((Date)reuse).setTime(in.readLong());
>       return (Date)reuse;
>     }
>     else return new Date(in.readLong());
>   }
> }
> {code}
> I implemented said annotations and a custom encoding for java.util.Date as a proof of
concept and also extended the @Stringable annotations to fields.
> This issue is a followup of AVRO-1328 and AVRO-1330.

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