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Date: Thu, 2 Apr 2015 18:41:54 +0000 (UTC)
From: "Ryan Blue (JIRA)" <jira@apache.org>
To: dev@avro.apache.org
Message-ID: <JIRA.12649704.1369749170000.109365.1428000114700@Atlassian.JIRA>
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Subject: [jira] [Commented] (AVRO-1341) Allow controlling avro via java
 annotations when using reflection.
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    [ https://issues.apache.org/jira/browse/AVRO-1341?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14393138#comment-14393138 ] 

Ryan Blue commented on AVRO-1341:
---------------------------------

[~sunzhaonan], there isn't an @AvroDoc annotation, but its a great idea. Could you open an issue to add it? It shouldn't be too much work, either.

> 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
>             Fix For: 1.7.5
>
>         Attachments: AVRO-1341.patch, AVRO-1341.patch, AVRO-1341.patch, AVRO-1341.patch, AVRO-1341.patch, 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")
> @AvroAlias(alias="alias", space="space")
> @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}
> The @AvroAlias annotation will add a new alias to the induced schema of a record, enum or field. The space parameter is optional and defaults to the namespace of the named schema the alias is added to.
> 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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