如何确定 Java 中通用字段的类型? [英] How can I determine the type of a generic field in Java?

问题描述

我一直在尝试确定类中字段的类型.我已经看到了所有的内省方法，但还没有完全弄清楚如何去做.这将用于从 Java 类生成 xml/json.我在这里查看了许多问题，但还没有找到我真正需要的.

I have been trying to determine the type of a field in a class. I've seen all the introspection methods but haven't quite figured out how to do it. This is going to be used to generate xml/json from a java class. I've looked at a number of the questions here but haven't found exactly what I need.

示例:

class Person {
    public final String name;
    public final List<Person> children;
}

当我编组这个对象时，我需要知道 chidren 字段是一个类型为 Person 的对象的列表，这样我才能正确地编组它.

When I marshall this object, I need to know that the chidren field is a list of objects of type Person, so I can marshall it properly.

我试过了

for (Field field : Person.class.getDeclaredFields()) {
    System.out.format("Type: %s%n", field.getType());
}

但这只会告诉我它是一个List，而不是Persons

But this will only tell me that it's a List, not a List of Persons

谢谢

推荐答案

看看 获取字段类型来自 Java 教程 Trail: The反射 API.

Have a look at Obtaining Field Types from the Java Tutorial Trail: The Reflection API.

基本上，您需要做的是获取所有 java.lang.reflect.Field 你的班级 并调用 Field#getType() 在他们每个人上(检查下面编辑).要获取所有对象字段，包括公共、受保护、包和私有访问字段，只需使用 Class.getDeclaredFields().像这样:

Basically, what you need to do is to get all java.lang.reflect.Field of your class and call Field#getType() on each of them (check edit below). To get all object fields including public, protected, package and private access fields, simply use Class.getDeclaredFields(). Something like this:

for (Field field : Person.class.getDeclaredFields()) {
    System.out.format("Type: %s%n", field.getType());
    System.out.format("GenericType: %s%n", field.getGenericType());
}

正如 wowest 在评论中指出的那样，您实际上需要调用Field#getGenericType()，检查返回的Type 是一个 ParameterizedType 然后相应地获取参数.使用 ParameterizedType#getRawType() 和 ParameterizedType#getActualTypeArgument() 分别获取ParameterizedType 的原始类型和类型参数数组.以下代码演示了这一点:

As pointed out by wowest in a comment, you actually need to call Field#getGenericType(), check if the returned Type is a ParameterizedType and then grab the parameters accordingly. Use ParameterizedType#getRawType() and ParameterizedType#getActualTypeArgument() to get the raw type and an array of the types argument of a ParameterizedType respectively. The following code demonstrates this:

for (Field field : Person.class.getDeclaredFields()) {
    System.out.print("Field: " + field.getName() + " - ");
    Type type = field.getGenericType();
    if (type instanceof ParameterizedType) {
        ParameterizedType pType = (ParameterizedType)type;
        System.out.print("Raw type: " + pType.getRawType() + " - ");
        System.out.println("Type args: " + pType.getActualTypeArguments()[0]);
    } else {
        System.out.println("Type: " + field.getType());
    }
}

并且会输出:

Field: name - Type: class java.lang.String
Field: children - Raw type: interface java.util.List - Type args: class foo.Person

这篇关于如何确定 Java 中通用字段的类型?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！