Java泛型：获取泛型方法的返回类型的类
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Java generics: get class of generic method's return type
提问by Peter Jaric
Background
背景
I once wrote this method:
我曾经写过这个方法：
private <T> SortedSet<T> createSortedSet() {
return new TreeSet<T>();
}
It's supposed to be called like this:
应该这样称呼：
Set<String> set = createSortedSet();
This works fine (although I've seen in answers here when researching the current question that this is error-prone).
这很好用（尽管我在研究当前问题时在这里的答案中看到了这很容易出错）。
The current situation
目前的情况
Anyway, now I am writing the following code (in a class that extends javax.servlet.jsp.tagext.TagSupport):
无论如何，现在我正在编写以下代码（在扩展 javax.servlet.jsp.tagext.TagSupport 的类中）：
private <T> T evaluate(String expression) {
ExpressionEvaluator evaluator = pageContext.getExpressionEvaluator();
return evaluator.evaluate(expression, T, null, pageContext.getVariableResolver());
}
The purpose is to be able to call this like:
目的是能够像这样调用它：
Integer width = evaluate(widthStr);
The code in my evaluate method obviously doesn't work. The second argument to evaluator.evaluate()is supposed to be a Class object. This leads me to:
我的评估方法中的代码显然不起作用。的第二个参数evaluator.evaluate()应该是一个 Class 对象。这导致我：
My question
我的问题
How can I get the Class of a generic (return) type? What should I write in place of T as the second argument to evaluate?
如何获取泛型（返回）类型的类？我应该写什么来代替 T 作为要评估的第二个参数？
EDIT: Conclusion
编辑：结论
Nicolas seems to be right, it can not be done and I need to pass the class as an argument to my method. The upside is that since his solution makes the argument parametrized on the generic type I get a compilation check on that argument.
Nicolas 似乎是对的，它无法完成，我需要将类作为参数传递给我的方法。好处是，由于他的解决方案使参数在泛型类型上参数化，因此我对该参数进行了编译检查。
采纳答案by Nicolas
Unfortunately, you will certainly have to change your method to:
不幸的是，您肯定必须将方法更改为：
private <T> T evaluate(Class<T> clazz, String expression)
and then pass clazzas your second parameter. Not as short as you expected.
然后clazz作为您的第二个参数传递。没有你想象的那么短。
回答by Fabrice TIERCELIN
You can first create a generic object and then retrieve its parameterized type:
您可以先创建一个通用对象，然后检索其参数化类型：
private <T> T evaluate(String expression) {
List<T> dummy = new ArrayList<>(0);
Type[] actualTypeArguments = ((ParameterizedType) dummy.getClass().getGenericSuperclass()).getActualTypeArguments();
Type clazz = actualTypeArguments[0];
Class<T> theClass = (Class<T>) clazz.getClass();
ExpressionEvaluator evaluator = pageContext.getExpressionEvaluator();
return evaluator.evaluate(expression, theClass, null, pageContext.getVariableResolver());
}
Beware! You don't get a Class<>object, you get a TypeVariableImplsubclass object, which may behave differently.
谨防！你得到的不是一个Class<>对象，而是一个TypeVariableImpl子类对象，它的行为可能不同。
