使用springfox-swagger2:3.0.0时，某次项目启动出现报错提示：

03-25 10:37:50.773 [main] ERROR s.d.spring.web.plugins.DocumentationPluginsBootstrapper - Unable to scan documentation context default
java.lang.NullPointerException: null
    at springfox.documentation.schema.Example.equals(Example.java:131)
    at java.base/java.util.Objects.equals(Objects.java:77)
    at springfox.documentation.service.RequestParameter.equals(RequestParameter.java:132)
    at java.base/java.util.HashMap.putVal(HashMap.java:630)
    at java.base/java.util.HashMap.put(HashMap.java:607)
    at java.base/java.util.HashSet.add(HashSet.java:220)
    at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
    at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:177)
    at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
    at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654)
    at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
    at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
    at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
    at springfox.documentation.spring.web.readers.operation.OperationParameterReader.apply(OperationParameterReader.java:93)
    at springfox.documentation.spring.web.plugins.DocumentationPluginsManager.operation(DocumentationPluginsManager.java:144)
    at springfox.documentation.spring.web.readers.operation.ApiOperationReader.read(ApiOperationReader.java:72)

逐个接口排查，发现某个接口定义的参数对象里面的属性，使用了public修饰，并且使用@Data生成了对应的getter、setter方法:

@Data
@ApiModel("demoRequestModel")
public DemoRequest{
 @ApiModelProperty("testProperty")
  public String a;
}

当把该属性设置为private后（或者去除@Data注解），问题解决。
为什么呢？
调试代码发现，在springfox.documentation.spring.web.readers.parameter.ModelAttributeParameterExpander类中的168行，存在如下代码：

  private List<ModelAttributeField> allModelAttributes(
      Map<Method, PropertyDescriptor> propertyLookupByGetter,
      Iterable<ResolvedMethod> getters,
      Map<String, ResolvedField> fieldsByName,
      AlternateTypeProvider alternateTypeProvider,
      Collection<Class> ignorables) {

    Stream<ModelAttributeField> modelAttributesFromGetters =
        StreamSupport.stream(getters.spliterator(), false)
            .filter(method -> !ignored(alternateTypeProvider, method, ignorables))
            .map(toModelAttributeField(fieldsByName, propertyLookupByGetter, alternateTypeProvider));

    Stream<ModelAttributeField> modelAttributesFromFields =
        fieldsByName.values().stream()
            .filter(ResolvedMember::isPublic)
            .filter(ResolvedMember::isPublic)
            .map(toModelAttributeField(alternateTypeProvider));

    return Stream.concat(
        modelAttributesFromFields,
        modelAttributesFromGetters)
        .collect(toList());
  }

跟踪代码可知，swagger在生成接口参数记录对象时，会保留参数对象中存在getter方法的属性，和使用public修饰的属性，如果同时满足这两个条件，则会造成冲突。