blocks|key|1682851|text|我决定使用Schema在运行时从类生成ReflectData，然后使用ReflectDatumWriter进行序列化。反射的使用将会更慢。但是，看起来该模式是在内部缓存的。如果我看到性能问题，我会报告的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1682852|Schema+schema+=+ReflectData.AllowNull.get().getSchema(sourceObject.getClass());
ReflectDatumWriter<T>+reflectDatumWriter+=+new+ReflectDatumWriter<>(schema);

DataFileWriter<T>+writer+=+new+DataFileWriter<>(reflectDatumWriter);
try+{
++++writer.setCodec(CodecFactory.snappyCodec());
++++writer.create(schema,+new+File("data.avro"));
++++writer.append(sourceObject);
++++writer.close();
}
catch+(IOException+e)+{
++++//+log+exception
}|code-block|syntax|javascript|1682853|entityMap^0|J|B|Z|I|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

I decided to use the <code>ReflectData</code> API to generate the Schema from the class at runtime and then use the <code>ReflectDatumWriter</code> for serialization. 
Use of reflection will be slower. But, it looks like the schema is cached internally. I will report back if I see performance issues.

<pre><code>Schema schema = ReflectData.AllowNull.get().getSchema(sourceObject.getClass());
ReflectDatumWriter&lt;T&gt; reflectDatumWriter = new ReflectDatumWriter&lt;&gt;(schema);

DataFileWriter&lt;T&gt; writer = new DataFileWriter&lt;&gt;(reflectDatumWriter);
try {
 writer.setCodec(CodecFactory.snappyCodec());
 writer.create(schema, new File("data.avro"));
 writer.append(sourceObject);
 writer.close();
}
catch (IOException e) {
 // log exception
}
</code></pre>

blocks|key|1896513|text|我找到了一个更好的方法来解决这个问题。查看Avro中的Schema生成源，我发现在内部类生成逻辑使用Velocity模式来生成类。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1896514|我修改了record.vm模板以实现我的特定接口。有一种方法可以使用maven构建插件中的templateDirectory配置来指定velocity目录的位置。|offset|length|style|CODE|1896515|我也改用SpecificDatumWriter而不是reflectDatumWriter。|1896516|<plugin>
++<groupId>org.apache.avro</groupId>
++<artifactId>avro-maven-plugin</artifactId>
+++<version>${avro.version}</version>
+++<executions>
++++<execution>
++++++<phase>generate-sources</phase>
++++++<goals>
++++++++<goal>schema</goal>
++++++</goals>
++++++<configuration>
+++++++++<sourceDirectory>${basedir}/src/main/resources/avro/schema</sourceDirectory>
+++++++++<outputDirectory>${basedir}/target/java-gen</outputDirectory>
+++++++++<fieldVisibility>private</fieldVisibility>
+++++++++<stringType>String</stringType>
+++++++++<templateDirectory>${basedir}/src/main/resources/avro/velocity-templates/</templateDirectory>
+++++++</configuration>
++++</execution>
++</executions>
</plugin>|code-block|syntax|javascript|1896517|entityMap^0|0|4|9|19|H|0|4|J|Q|I|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]|$D|U|E|V|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@$D|X|E|Y|F|G]|$D|Z|E|10|F|G]]|9|@]|A|$]]|$1|J|3|K|5|L|7|11|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|P|$]]

I found a better way to solve this problem. Looking at the Schema generation source in Avro, I figured out that internally the class generation logic uses Velocity schemas to generate the classes. 

I modified the <code>record.vm</code> template to also implement my specific interface. There is a way to specify the location of velocity directory using the <code>templateDirectory</code> configuration in the maven build plugin. 

I also switched to using <code>SpecificDatumWriter</code> instead of <code>reflectDatumWriter</code>.

<pre><code>&lt;plugin&gt;
 &lt;groupId&gt;org.apache.avro&lt;/groupId&gt;
 &lt;artifactId&gt;avro-maven-plugin&lt;/artifactId&gt;
 &lt;version&gt;${avro.version}&lt;/version&gt;
 &lt;executions&gt;
 &lt;execution&gt;
 &lt;phase&gt;generate-sources&lt;/phase&gt;
 &lt;goals&gt;
 &lt;goal&gt;schema&lt;/goal&gt;
 &lt;/goals&gt;
 &lt;configuration&gt;
 &lt;sourceDirectory&gt;${basedir}/src/main/resources/avro/schema&lt;/sourceDirectory&gt;
 &lt;outputDirectory&gt;${basedir}/target/java-gen&lt;/outputDirectory&gt;
 &lt;fieldVisibility&gt;private&lt;/fieldVisibility&gt;
 &lt;stringType&gt;String&lt;/stringType&gt;
 &lt;templateDirectory&gt;${basedir}/src/main/resources/avro/velocity-templates/&lt;/templateDirectory&gt;
 &lt;/configuration&gt;
 &lt;/execution&gt;
 &lt;/executions&gt;
&lt;/plugin&gt;
</code></pre>

blocks|key|1683072|text|我发现这个问题也有类似的问题。在我的例子中，我只需要对某些类型施加标记接口(稍后区分特定的类)。感谢你的回答，我深入研究了record.vm模板的结构。我发现可以在"javaAnnotation":+"my.full.AnnotationName"定义JSON中定义.avsc键。然后将@my.full.AnnotationName添加到生成的类中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1683073|诚然，这个解决方案最终不是建立在标记接口上的，尽管对于我的目的来说已经足够好了，保持模板不变是一个很大的优势。|1683074|entityMap^0|1P|9|2A|16|3P|5|3Z|N|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

I found this question having similar problem. In my case I needed just to impose marker interface and only to some types (to distinguish particular classes later). Thanks to your answer, I dug deeper into structure of <code>record.vm</code> template. I found out it's possible to define <code>"javaAnnotation": "my.full.AnnotationName"</code> key in <code>.avsc</code> definition JSON. <code>@my.full.AnnotationName</code> is then added to generated class. 

Admittedly, this solution is not built on marker interface finally, though for my purpose is good enough and keeping template untouched is big advantage.

blocks|key|1683295|text|我遵循https://www.infoq.com/articles/ApacheAvro/来实现继承。这也给多态性带来了希望(这是我所需要的)。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1683296|一分。在声明{"name":+"user",+"type":+com.navteq.avro.FacebookUser+},时，请确保使用双引号，如{"name":+"user",+"type":+"com.navteq.avro.FacebookUser"+},|style|CODE|1683297|如果我不这样做，我会得到如下的错误|1683298|>+org.apache.avro.SchemaParseException:+org.codehaus.jackson.JsonParseException:+Unexpected+character+('c'+(code+99)):+expected+a+valid+value+(number,+String,+array,+object,+'true',+'false'+or+'null')|1683299|entityMap|0|LINK|mutability|MUTABLE|url|https://www.infoq.com/articles/ApacheAvro/^0|3|16|0|0|6|1K|22|1M|0|0|0|5K|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|D|3|E|5|6|7|X|8|@$A|Y|B|Z|F|G]|$A|10|B|11|F|G]]|9|@]|C|$]]|$1|H|3|I|5|6|7|12|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|13|8|@$A|14|B|15|F|G]]|9|@]|C|$]]|$1|L|3|-4|5|6|7|16|8|@]|9|@]|C|$]]]|M|$N|$5|O|P|Q|C|$R|S]]]]

I followed <a href="https://www.infoq.com/articles/ApacheAvro/" rel="nofollow noreferrer">https://www.infoq.com/articles/ApacheAvro/</a> for implementing inheritance. This put lights on polymorphism as well(which i needed). 

One point. 
While declaring 
<code>{"name": "user", "type": com.navteq.avro.FacebookUser },</code>
make sure you double quote , like <code>{"name": "user", "type": "com.navteq.avro.FacebookUser" },</code>

If i do not do that, then i was getting error like below

<code>&gt; org.apache.avro.SchemaParseException: org.codehaus.jackson.JsonParseException: Unexpected character ('c' (code 99)): expected a valid value (number, String, array, object, 'true', 'false' or 'null')</code>

blocks|key|1896940|text|如果我在这里写下我为这个案例创建的maven插件--+https://github.com/tunguski/interfacer，我希望它能对其他人有所帮助。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1896941|它检查自动生成的类，并检查它们是否符合特定包中类路径上的接口。如果是，则将接口添加到类中。它适用于泛型接口，至少在我不得不处理的基本示例中是这样。|1896942|该插件不是avro特定的，作为生成的代码后处理器，所以它也可以在其他情况下使用。|1896943|
<plugin>
++++<groupId>pl.matsuo.interfacer</groupId>
++++<artifactId>interfacer-maven-plugin</artifactId>
++++<version>0.0.6</version>
++++<executions>
++++++++<execution>
++++++++++++<configuration>
++++++++++++++++<interfacesDirectory>${project.basedir}/src/main/java</interfacesDirectory>
++++++++++++++++<interfacePackage>pl.matsuo.interfacer.showcase</interfacePackage>
++++++++++++</configuration>
++++++++++++<goals>
++++++++++++++++<goal>add-interfaces</goal>
++++++++++++</goals>
++++++++</execution>
++++</executions>
</plugin>|code-block|syntax|javascript|1896944|//+src/main/java+manually+defined+interface
public+interface+HasName+{
++String+getName();
}

//+target/generated-sources/avro
public+class+Person+{

++String+name;

++public+String+getName()+{
++++return+name;
++}
++//+[...]
}

public+class+Company+{

++String+name;

++public+String+getName()+{
++++return+name;
++}
++//+[...]
}

//+after+this+plugin+run

//+target/generated-sources/avro
public+class+Person+implements+HasName+{

++String+name;

++public+String+getName()+{
++++return+name;
++}
++//+[...]
}

public+class+Company+implements+HasName+{

++String+name;

++public+String+getName()+{
++++return+name;
++}
++//+[...]
}|1896945|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/tunguski/interfacer^0|R|12|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]]|C|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|11|8|@]|9|@]|C|$]]|$1|H|3|I|5|J|7|12|8|@]|9|@]|C|$K|L]]|$1|M|3|N|5|J|7|13|8|@]|9|@]|C|$K|L]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|C|$]]]|P|$Q|$5|R|S|T|C|$U|V]]]]

I hope it will be helpful for others if I'll write it here that I've created maven plugin for exactly this case - <a href="https://github.com/tunguski/interfacer" rel="nofollow noreferrer">https://github.com/tunguski/interfacer</a>.
It goes through auto generated classes and check do they conform to interfaces found on classpath in specific package. If yes, interface is added to the class. It works with generic interfaces, at least in basic examples I had to deal with.
The plugin is not avro specific, works as a generated code post processor, so it may be used in other cases too.
<pre class="lang-xml prettyprint-override"><code>&lt;!-- 
 post process avro generated sources and add interfaces from package
 pl.matsuo.interfacer.showcase to every generated class that has 
 all methods from specific interface
 --&gt;
&lt;plugin&gt;
 &lt;groupId&gt;pl.matsuo.interfacer&lt;/groupId&gt;
 &lt;artifactId&gt;interfacer-maven-plugin&lt;/artifactId&gt;
 &lt;version&gt;0.0.6&lt;/version&gt;
 &lt;executions&gt;
 &lt;execution&gt;
 &lt;configuration&gt;
 &lt;interfacesDirectory&gt;${project.basedir}/src/main/java&lt;/interfacesDirectory&gt;
 &lt;interfacePackage&gt;pl.matsuo.interfacer.showcase&lt;/interfacePackage&gt;
 &lt;/configuration&gt;
 &lt;goals&gt;
 &lt;goal&gt;add-interfaces&lt;/goal&gt;
 &lt;/goals&gt;
 &lt;/execution&gt;
 &lt;/executions&gt;
&lt;/plugin&gt;
</code></pre>
<pre class="lang-java prettyprint-override"><code>// src/main/java manually defined interface
public interface HasName {
 String getName();
}

// target/generated-sources/avro
public class Person {

 String name;

 public String getName() {
 return name;
 }
 // [...]
}

public class Company {

 String name;

 public String getName() {
 return name;
 }
 // [...]
}

// after this plugin run

// target/generated-sources/avro
public class Person implements HasName {

 String name;

 public String getName() {
 return name;
 }
 // [...]
}

public class Company implements HasName {

 String name;

 public String getName() {
 return name;
 }
 // [...]
}
</code></pre>

Is it possible to write an Avro schema/IDL that will generate a Java class that either extends a base class or implements an interface? 
It seems like the generated Java class extends the <code>org.apache.avro.specific.SpecificRecordBase</code>. So, the implements might be the way to go. But, I don't know if this is possible.

I have seen examples with suggestions to define an explicit "type" field in each specific schema, with more of an association than inheritance semantics.

I use my base class heavily in my factory classes and other parts of the code with generics like <code>&lt;T extends BaseObject&gt;</code>. Currently, I had it code generated from the JSON Schema, which supports inheritance.

Another side question: can you use IDL to define just records without the protocol definition? I think the answer is no because the compiler complains about the missing protocol keyword.

Help appreciated! Thanks.

Polymorphism and inheritance in Avro schemas

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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有没有可能编写一个Avro schema/IDL来生成一个扩展基类或实现接口的Java类？看起来生成的Java类扩展了org.apache.avro.specific.SpecificRecordBase。因此，工具可能是要走的路。但是，我不知道这是否可能。我见过一些例子，建议在每个特定的模式中定义一个显式的"type...

问Avro模式中的多态性和继承
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