如何扩展ToAttributedValueConverter以在content为null时跳过序列化

解决方案 #

你可以通过继承ToAttributedValueConverter重写序列化逻辑实现需求，具体步骤如下：

1. 自定义Converter

import com.thoughtworks.xstream.converters.MarshallingContext;
import com.thoughtworks.xstream.converters.extended.ToAttributedValueConverter;
import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
import com.thoughtworks.xstream.mapper.Mapper;
import java.lang.reflect.Field;

public class SkipNullValueConverter extends ToAttributedValueConverter {
    private final String valueFieldName;

    // 构造方法接收value字段名，和原生Converter参数规则一致
    public SkipNullValueConverter(Class type, Mapper mapper, String valueFieldName) {
        super(type, mapper, valueFieldName);
        this.valueFieldName = valueFieldName;
    }

    @Override
    public void marshal(Object source, HierarchicalStreamWriter writer, MarshallingContext context) {
        try {
            // 读取指定的value字段值
            Field field = source.getClass().getDeclaredField(valueFieldName);
            field.setAccessible(true);
            Object value = field.get(source);
            // 值为null直接返回，不输出任何标签内容
            if (value == null) {
                return;
            }
        } catch (NoSuchFieldException | IllegalAccessException e) {
            // 异常场景可根据业务需要调整处理逻辑
            e.printStackTrace();
        }
        // 值非空时走原生序列化逻辑
        super.marshal(source, writer, context);
    }
}

2. 修改POJO注解

将原来POJO上的@XStreamConverter指定的Converter替换为你自定义的实现即可：

@XStreamAlias("MyPojo")
@XStreamConverter(value = SkipNullValueConverter.class, strings = {"content"})
public class MyPojo{

  private final String content;
  private final String attribute1;
  private final String attribute2;

  // 注意原代码构造方法名有误，应和类名保持一致为MyPojo
  public MyPojo(String content, String attribute1, String attribute2) {
    this.content = content;
    this.attribute1 = attribute1;
    this.attribute2 = attribute2;
  }
}

注意事项

• 如果你不想用反射读取字段，可以给POJO添加对应value字段的getter方法，在marshal方法中直接强转对象调用getter判断，性能更好
• 该自定义Converter是通用实现，其他同类需求的POJO只需在@XStreamConverter的strings参数中指定对应value字段名即可复用

内容的提问来源于stack exchange，提问作者Walter