简单 XML 框架：回退到未知枚举值的默认值

Question

假设一个 XML 提要由多个消息组成，例如：

<feed>
    <message type="FOO_TYPE">
        <!-- possibly some message content here -->
    </message>
    <!-- more messages -->
</feed>

feed和message分别被翻译成两个不同的Java类； type 被翻译成 Java enum.

反序列化代码：

// custom strategy to avoid name collisions (we use `class` as an attribute name)
Strategy XML_STRATEGY = new TreeStrategy("simpleXmlClass", "simpleXmlLength");
Serializer XML_SERIALIZER = new Persister(XML_STRATEGY);
Feed feed = XML_SERIALIZER.read(Feed.class, inputStream);

现在，如果 Feed 包含一条带有未知 type 的消息（即未为相应的 enum 定义的消息），将抛出异常。如果我将对 Serializer#read() 的调用包装到一个异常处理程序中，那将导致整个提要被丢弃。

我想要实现的是每当遇到未知值时替换默认值（或可能为空）（并可能在更高的地方做一些进一步的过滤，但现在超出范围）。但是，我不知道在哪里将该逻辑插入到 Strategy/Serializer 构造中，文档也没有太大帮助。

典型的异常如下所示：

java.lang.IllegalArgumentException: No enum constant org.traffxml.traff.TraffEvent.Type.CONSTRUCTION_BRIDGE_DEMOLITION
    at java.lang.Enum.valueOf(Enum.java:238)
    at org.simpleframework.xml.transform.EnumTransform.read(EnumTransform.java:58)
    at org.simpleframework.xml.transform.EnumTransform.read(EnumTransform.java:29)
    at org.simpleframework.xml.transform.Transformer.read(Transformer.java:106)
    at org.simpleframework.xml.core.Support.read(Support.java:372)
    at org.simpleframework.xml.core.PrimitiveFactory.getInstance(PrimitiveFactory.java:105)
    at org.simpleframework.xml.core.Primitive.readTemplate(Primitive.java:231)
    at org.simpleframework.xml.core.Primitive.read(Primitive.java:171)
    at org.simpleframework.xml.core.Primitive.read(Primitive.java:126)
    at org.simpleframework.xml.core.Composite.readVariable(Composite.java:623)
    at org.simpleframework.xml.core.Composite.readInstance(Composite.java:573)
    at org.simpleframework.xml.core.Composite.readAttribute(Composite.java:497)
    at org.simpleframework.xml.core.Composite.readAttributes(Composite.java:413)
    at org.simpleframework.xml.core.Composite.access0(Composite.java:59)
    at org.simpleframework.xml.core.Composite$Injector.read(Composite.java:1432)
    at org.simpleframework.xml.core.Composite.read(Composite.java:201)
    at org.simpleframework.xml.core.Composite.read(Composite.java:148)
    at org.simpleframework.xml.core.Traverser.read(Traverser.java:92)
    at org.simpleframework.xml.core.CompositeInlineList.read(CompositeInlineList.java:190)
    at org.simpleframework.xml.core.CompositeInlineList.read(CompositeInlineList.java:167)
    at org.simpleframework.xml.core.CompositeInlineList.read(CompositeInlineList.java:124)
    at org.simpleframework.xml.core.Composite.readVariable(Composite.java:623)
    at org.simpleframework.xml.core.Composite.readInstance(Composite.java:573)
    at org.simpleframework.xml.core.Composite.readUnion(Composite.java:549)
    at org.simpleframework.xml.core.Composite.readElement(Composite.java:532)
    at org.simpleframework.xml.core.Composite.readElements(Composite.java:445)
    at org.simpleframework.xml.core.Composite.readSection(Composite.java:327)
    at org.simpleframework.xml.core.Composite.readElements(Composite.java:443)
    at org.simpleframework.xml.core.Composite.access0(Composite.java:59)
    at org.simpleframework.xml.core.Composite$Injector.read(Composite.java:1433)
    at org.simpleframework.xml.core.Composite.read(Composite.java:201)
    at org.simpleframework.xml.core.Composite.read(Composite.java:148)
    at org.simpleframework.xml.core.Traverser.read(Traverser.java:92)
    at org.simpleframework.xml.core.CompositeInlineList.read(CompositeInlineList.java:190)
    at org.simpleframework.xml.core.CompositeInlineList.read(CompositeInlineList.java:167)
    at org.simpleframework.xml.core.CompositeInlineList.read(CompositeInlineList.java:124)
    at org.simpleframework.xml.core.Composite.readVariable(Composite.java:623)
    at org.simpleframework.xml.core.Composite.readInstance(Composite.java:573)
    at org.simpleframework.xml.core.Composite.readUnion(Composite.java:549)
    at org.simpleframework.xml.core.Composite.readElement(Composite.java:532)
    at org.simpleframework.xml.core.Composite.readElements(Composite.java:445)
    at org.simpleframework.xml.core.Composite.access0(Composite.java:59)
    at org.simpleframework.xml.core.Composite$Injector.read(Composite.java:1433)
    at org.simpleframework.xml.core.Composite.read(Composite.java:201)
    at org.simpleframework.xml.core.Composite.read(Composite.java:148)
    at org.simpleframework.xml.core.Traverser.read(Traverser.java:92)
    at org.simpleframework.xml.core.Persister.read(Persister.java:625)
    at org.simpleframework.xml.core.Persister.read(Persister.java:606)
    at org.simpleframework.xml.core.Persister.read(Persister.java:584)
    at org.simpleframework.xml.core.Persister.read(Persister.java:543)
    at com.example.myowncode.MyClass.doStuff(MyClass.java:42)

由于堆栈中的最后一个异常包括来自 EnumTransformer 的一些异常，它是 Transform<Enum> 的一个实现，我想知道我是否可以为那个特定的类型插入我的自定义 Transform 类型枚举到代码中；我只是不知道如何实现。

Answer 1

假设枚举名为MyEnum，首先为它编写一个转换class。 class 必须实现 Transform<MyEnum> ；我们在这里称它为 MyEnumTransform。像这样实现自定义 read 方法：

public MyEnum read(String value) throws Exception {
    try {
        return MyEnum.valueOf(value);
    } catch (IllegalArgumentException e) {
        return MyEnum.FALLBACK_VALUE; // or null, at your option
    }
}

你还需要实现write，你可以直接从EnumTransform复制：

public String write(MyEnum value) throws Exception {
    return value.name();
}

现在将您的反序列化代码更改如下：

// custom strategy to avoid name collisions (we use `class` as an attribute name)
RegistryMatcher XML_MATCHER = new RegistryMatcher();
XML_MATCHER.bind(MyEnum.class, MyEnumTransform.class);
Strategy XML_STRATEGY = new TreeStrategy("simpleXmlClass", "simpleXmlLength");
Serializer XML_SERIALIZER = new Persister(XML_STRATEGY, XML_MATCHER);
Feed feed = XML_SERIALIZER.read(Feed.class, inputStream);

本质上，您是在为序列化程序提供自定义匹配器。否则，序列化程序将回退到 EmptyMatcher 实例（动态创建），其中 returns null 作为任何转换，导致简单 XML 回退到默认转换类型（并在遇到非法的枚举值时抛出异常）。

然后，您可能需要在反序列化具有 null/default 类型的消息时进行一些进一步的一致性检查（例如完全丢弃消息），但这超出了此处的范围。

简单 XML 框架：回退到未知枚举值的默认值

Simple XML Framework: fall back to default for unknown Enum values

java

simple-framework