Question

有没有办法在Java中获取字段的泛型类型？

我有以下对象变量：

protected ScheduleView<WantedClass> scheduleLine1;
protected ScheduleView<SomeOtherClass> scheduleLine2;

现在我尝试使用反射来获取ScheduleView类型为WantedClass作为泛型类型的所有对象变量：

Arrays.asList(this.getClass().getDeclaredFields()).stream().map(field -> {
    ScheduleView<WantedClass> retValue = null;

    System.out.println(field.getGenericType()); // prints control.ScheduleView<dto.WantedClass>

    try {
        if (field.getType() == ScheduleView.class) { // here I also want to check if the generic type is WantedClass
            retValue = (ScheduleView<WantedClass>) field.get(this);
        } else {
            retValue = null;
        }
    } catch (IllegalAccessException e) {
        retValue = null;
    } finally {
        return retValue;
    }
}).filter(scheduleView -> scheduleView != null).forEach(scheduleView -> {
        /* some more code */
});

问题是，如果泛型类型为WantedClass，我还想检查if-Statement。我也尝试使用getGenericType()方法，但似乎这样的事情是不可能的：

field.getGenericType() == ScheduleView<WantedClass>.class

那么有没有办法获得字段的泛型类型？

Answer 1

您必须检查该字段是否为ParameterizedType，然后检查getActualTypeArguments的内容，例如

    for (Field field : Ideone.class.getDeclaredFields()) {
        Type type = field.getGenericType();
        if (type instanceof ParameterizedType) {
            ParameterizedType ptype = (ParameterizedType) type;
            if (ptype.getRawType() == ScheduledView.class) {
                if (ptype.getActualTypeArguments().length == 1
                    && ptype.getActualTypeArguments()[0] == WantedClass.class) {
                  // Do whatever with the field.
                  System.out.println(field.getName());
                }
            }
        }
    }

Ideone demo

Answer 2

使用流来实现@Andy Turner的答案：

// For each declared field in the class...
Arrays.stream(Ideone.class.getDeclaredFields())
  // select fields...
  .filter(
    // that have a generic type...
    field -> Stream.of(field.getGenericType())
      // that is an instance of ParametrizedType...
      .filter(ParameterizedType.class::isInstance)
      .map(ParameterizedType.class::cast)
      // whose raw type is ScheduledView...
      .filter(ptype -> ptype.getRawType().equals(ScheduledView.class))
      // and whose actual type arguments...
      .map(ParameterizedType::getActualTypeArguments)
      .flatMap(Arrays::stream)
      // include WantedClass...
      .anyMatch(WantedClass.class::equals)
  )
  // then get their names...
  .map(Field::getName)
  // and print them.
  .forEach(System.out::println)
;

使用反射获取字段的泛型类型

2 个答案: