简单XML框架：对于未知的Enum值，请恢复默认设置

问题描述

假设一个XML Feed 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分别转换为两个不同的java类； type转换为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中包含一条带有未知type的消息（即未为相应enum定义的消息），则会引发异常。如果将对Serializer#read()的调用包装到异常处理程序中，则会导致整个Feed都被丢弃。

我想要实现的是在遇到未知值时替换默认值（或可能为null）（并可能进一步进行进一步过滤，但这暂时不在范围之内）。但是，我不知道将该逻辑插入Strategy / Serializer构造中的位置，并且文档也无济于事。

典型的异常如下：

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.access$300(Composite.java:59)
    at org.simpleframework.xml.core.Composite$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.access$400(Composite.java:59)
    at org.simpleframework.xml.core.Composite$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.access$400(Composite.java:59)
    at org.simpleframework.xml.core.Composite$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EnumTransformer中的一些异常，它是Transform<Enum>的一种实现，因此我想知道是否可以将针对该特定枚举的自定义Transform类型插入到其中编码;我不知道该怎么实现。

解决方法

假设枚举称为MyEnum，首先为其编写一个转换类。该类必须实现Transform<MyEnum>；我们在这里将其称为MyEnumTransform。实现这样的自定义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
    }
}

您还需要实现write，可以直接从EnumTransform复制：

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

现在按如下所示更改反序列化代码：

// 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);

基本上，您正在为序列化程序提供自定义匹配器。否则，序列化程序将退回到EmptyMatcher实例（动态创建），该实例返回null作为任何内容的转换，从而导致Simple XML退回该类型的默认转换（并在发生异常时遇到非法的枚举值。

然后，您可能需要在反序列化具有null / default类型的消息（例如，将消息全部丢弃）时进行一些进一步的一致性检查，但这在本文范围之外。

java java simple-framework