Apache Avro：使用联合数据类型时出现 UnresolvedUnionException答案

【问题标题】：Apache Avro : UnresolvedUnionException when using union data typeApache Avro：使用联合数据类型时出现 UnresolvedUnionException
【发布时间】：2013-12-02 16:44:46
【问题描述】：

我正在使用Apache Avro 进行对象序列化。

我有一个 School 对象的 Avro 架构：

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

正如您在上面看到的，"other" 字段包含一个联合数据类型，它可以是null 或Teacher 实例或Student 实例。

Teacher 对象架构：

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

Student 对象架构：

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

我使用 Avro 工具编译了上述架构，Avro 自动为我生成了所有 Java 类。

然后，在我的 java 程序中，我通过以下方式创建 School 实例：

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，学校实例的@987654332@ 字段包含一个学生对象。然而，当我编译我的代码时，我得到了 UnresolvedUnionException 。它抱怨School 模式的other 字段的联合数据类型。似乎它无法解析我在 Java 代码中设置为 school 的 student。为什么会出现这个异常？堆栈跟踪是：

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)

【问题讨论】：

标签： java apache avro

【解决方案1】：

我可能弄错了，但我认为 Union 类型应该只包含一个非默认值：

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

应该是：

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

【讨论】：