Apache Avro：使用聯合數據類型時出現 UnresolvedUnionException

Question

我正在使用Apache Avro進行對象序列化。

我有一個用於School對象的 Avro 架構：

{"namespace": "com.my.model",
 "type": "record",
 "name": "School",
 "fields": [
     {"name": "sid",  "type": "int"},
     {"name": "size",  "type": "long"},
     {"name": "other", "type": ["null", "Teacher", "Student"]}
   ]
}

正如你在上面看到的， "other"字段包含一個聯合數據類型，它可以是null或者一個Teacher實例或一個Student實例。

教師對象架構：

{"namespace": "com.my.model",
"type": "record",
"name": "Teacher",
    "fields": [
        {"name": "isMale", "type": "boolean"}
    ]
}

Student對象架構：

{"namespace": "com.my.model",
"type": "record",
"name": "Student",
    "fields": [
        {"name": "age", "type": "int"}
    ]
}

我用 Avro 工具編譯了上面的模式，Avro 自動為我生成了所有的 Java 類。

然后，在我的 java 程序中，我通過以下方式創建School實例：

School school = new School();
school.setSid(3);
school.setSize(2000);

//create a student object
Student student = new Student();
student.setAge(18);

//set student into school instance
school.setOther(student);

正如您在上面看到的，學校實例的other字段包含一個學生對象。 然而，當我編譯我的代碼時，我得到了UnresolvedUnionException 。 它抱怨School模式的other字段的聯合數據類型。 看來它無法解決student我設置為school在我的Java代碼。 為什么會出現這種異常？ 堆棧跟蹤是：

org.apache.avro.UnresolvedUnionException: Not in union ["null",{"type":"record","name":"Student","namespace":"com.my.model","fields":[{"name":"age","type":"long"}]},{"type":"record","name":"Teacher","namespace":"com.model","fields":[{"name":"isMale","type":"boolean"}]}]: false
    at org.apache.avro.generic.GenericData.resolveUnion(GenericData.java:561)
    at org.apache.avro.generic.GenericDatumWriter.resolveUnion(GenericDatumWriter.java:144)
    at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:71)
    at org.apache.avro.generic.GenericDatumWriter.writeRecord(GenericDatumWriter.java:106)
    at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:66)
    at org.apache.avro.generic.GenericDatumWriter.writeArray(GenericDatumWriter.java:131)
    at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:68)
    at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:73)
    at org.apache.avro.generic.GenericDatumWriter.writeRecord(GenericDatumWriter.java:106)
    at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:66)
    at org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:58)

Answer 1

我可能弄錯了，但我認為聯合類型應該只包含一個非默認值：

{
    "name": "other",
    "type": [
        "null",
        "Teacher",
        "Student"
    ]
}

應該是：

{
    "name": "other",
    "type": [
        "null",
        {
            "name": "Visitors",
            "symbols": [
                "Student",
                "Teacher"
            ],
            "type": "enum"
        }
    ]
}