[GitHub] [pulsar] codelipenghui commented on issue #5503: [Schema]GenericData$Record cannot be cast

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codelipenghui commented on issue #5503: [Schema]GenericData$Record cannot be 
cast 
URL: https://github.com/apache/pulsar/issues/5503#issuecomment-590084502
 
 
   The problem is we should specify the correct namespace and name in the JSON 
definition. I have written a demo in java:
   
   ```java
   package org.apache.pulsar.client.impl;
   
   import org.apache.pulsar.client.api.schema.SchemaDefinition;
   import org.apache.pulsar.client.impl.schema.AvroSchema;
   import org.testng.Assert;
   import org.testng.annotations.Test;
   
   import java.util.Objects;
   
   public class SchemaTest {
   
       @Test
       public void test() {
           SchemaDefinition<User> sd = 
SchemaDefinition.<User>builder().withJsonDef("{\n" +
                   "    \"type\": \"record\",\n" +
                   "    \"name\": \"User\",\n" +
                   "    \"namespace\": 
\"org.apache.pulsar.client.impl.SchemaTest\",\n" +
                   "    \"fields\": [\n" +
                   "      {\n" +
                   "        \"name\": \"id\",\n" +
                   "        \"type\": [\n" +
                   "          \"null\",\n" +
                   "          \"int\"\n" +
                   "        ]\n" +
                   "      },\n" +
                   "      {\n" +
                   "        \"name\": \"name\",\n" +
                   "        \"type\": [\n" +
                   "          \"null\",\n" +
                   "          \"string\"\n" +
                   "        ]\n" +
                   "      }\n" +
                   "    ]\n" +
                   "  }").build();
   
           AvroSchema<User> schema = AvroSchema.of(sd);
           User user = new User();
           user.setId(1);
           user.setName("penghui");
           byte[] encoded = schema.encode(user);
           User decoded = schema.decode(encoded);
           Assert.assertEquals(user, decoded);
       }
   
       private static class User {
           private Integer id;
           private String name;
   
           public Integer getId() {
               return id;
           }
   
           public void setId(Integer id) {
               this.id = id;
           }
   
           public String getName() {
               return name;
           }
   
           public void setName(String name) {
               this.name = name;
           }
   
           @Override
           public boolean equals(Object o) {
               if (this == o) return true;
               if (o == null || getClass() != o.getClass()) return false;
               User user = (User) o;
               return Objects.equals(id, user.id) &&
                       Objects.equals(name, user.name);
           }
   
           @Override
           public int hashCode() {
               return Objects.hash(id, name);
           }
       }
   }
   
   ```
   Avro use full name(`namespace` + "." + `name`) to get Class for decoding 
data. GenericData uses If class not found.  Here is source code of 
`SpecificData` in avro, and `SpecificData` is `GenericData`'s subclass, if the 
class not found, will call `supper.newRecord`, this will introduce 
`GenericRecord`.
   ```
   public Object newRecord(Object old, Schema schema) {
       Class c = getClass(schema);
       if (c == null)
         return super.newRecord(old, schema); // punt to generic
       return (c.isInstance(old) ? old : newInstance(c, schema));
     }
   ```

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