[英]Avro backward schema evolution throws ClassCastException
我在嘗試使用簡單的 Java 程序測試 Avro 模式演變時遇到ClassCastException
。
Avro 版本: 1.10.0
customer-v1.avsc
{
"type": "record",
"namespace": "com.practice.kafka",
"name": "Customer",
"doc": "Avro schema for Customer",
"fields": [
{"name": "first_name", "type": "string", "doc": "Customer first name"},
{"name": "last_name", "type": "string", "doc": "Customer last name"},
{"name": "automated_email", "type": "boolean", "default": true, "doc": "Receive marketing emails or not"}
]
}
customer-v2.avsc
{
"type": "record",
"namespace": "com.practice.kafka",
"name": "CustomerV2",
"doc": "Avro schema for Customer",
"fields": [
{"name": "first_name", "type": "string", "doc": "Customer first name"},
{"name": "last_name", "type": "string", "doc": "Customer last name"},
{"name": "phone_number", "type": ["null","boolean"], "default": null, "doc": "Optional phone number"},
{"name": "email", "type": "string", "default": "missing@example.com", "doc": "Optional email address"}
]
}
Program to serialize v1 and deserialize v2
package com.practice.kafka;
import org.apache.avro.file.DataFileReader;
import org.apache.avro.file.DataFileWriter;
import org.apache.avro.io.DatumReader;
import org.apache.avro.io.DatumWriter;
import org.apache.avro.specific.SpecificDatumReader;
import org.apache.avro.specific.SpecificDatumWriter;
import java.io.File;
import java.io.IOException;
public class BackwardSchemaEvolutionSample {
public static void main(String[] args) {
// Step 1 - Create specific record
Customer customer = Customer.newBuilder().setFirstName("John").setLastName("Doe").setAutomatedEmail(false).build();
// Step 2 - Write specific record to a file
final DatumWriter<Customer> datumWriter = new SpecificDatumWriter<>();
try (DataFileWriter<Customer> dataFileWriter = new DataFileWriter<>(datumWriter)) {
dataFileWriter.create(customer.getSchema(), new File("customer-v1.avro"));
dataFileWriter.append(customer);
} catch (IOException e) {
e.printStackTrace();
}
// Step 3 - Read specific record from a file
final File file = new File("customer-v1.avro");
final DatumReader<CustomerV2> datumReader = new SpecificDatumReader<>();
CustomerV2 customerRecord;
try (DataFileReader<CustomerV2> dataFileReader = new DataFileReader<>(file, datumReader)) {
customerRecord = dataFileReader.next();
System.out.println(customerRecord.toString());
} catch (IOException e) {
e.printStackTrace();
}
}
}
Result
Exception in thread "main" java.lang.ClassCastException: class com.practice.kafka.Customer cannot be cast to class com.practice.kafka.CustomerV2 (com.practice.kafka.Customer and com.practice.kafka.CustomerV2 are in unnamed module of loader 'app')
at com.practice.kafka.SchemaEvolutionSample.main(SchemaEvolutionSample.java:34)
你能告訴我如何解決這個錯誤嗎?
您定義了 2 個數據類型Customer
和Customer2
並且您不能進行任何轉換,因為它們沒有繼承關系。 所以 Java 不能進行轉換,而你得到的是ClassCastException
。 在您的代碼中,唯一的解決方案是捕獲ClassCastException
並在 catch 塊中將 Customer 轉換為 Customer2。
我假設您正在模擬 Kafka 環境中架構的更改。 在這種情況下,您將通過添加新字段或刪除舊字段來擴展現有的 avro 模式。
只要類的名稱保持不變,avro 架構更改將起作用。
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