由于属性必须是常量表达式错误，Java代码将无法编译 [英] Java code won't compile due to attribute must be a constant expression error

问题描述

我无法弄清楚为什么以下不会编译。 IDE给我的错误是注释属性RequestParam.defaultValue的值必须是一个常量表达式。

I can't figure out why the following won't compile. The error the IDE gives me is "The value for annotation attribute RequestParam.defaultValue must be a constant expression".

我的项目涉及Spring和Maven，它包括以下内容：

My project involves Spring and Maven, and it goes the following:

private static final String MAX_LONG_AS_STRING = Long.toString(Long.MAX_VALUE);

@RequestMapping(method=RequestMethod.GET)
public List<Spittle> spittles(
        @RequestParam(value="max",
                    defaultValue=MAX_LONG_AS_STRING) long max,
        @RequestParam(value="count", defaultValue="20") int count) {
    return spittleRepository.findSpittles(max, count);
}

我认为错误来自于Long转换为String，但是我不知道如何解决它。我将不胜感激任何帮助，我是注释和Spring的新手。

I'm thinking the error comes from the conversion of Long to String, but I do not know how to fix it. I will appreciate any help, I am new to annotations and Spring.

推荐答案

Java规则说当你有一个注释时，它有一个参数，需要一个原始类型（如 int ）或 String ，该值必须是常量表达式。 [这与Spring无关。]粗略地说，常量表达式是编译器在编译时可以计算出来的值。但是，有什么构成常量表达式的规则。这些规则在 JLS 15.28 。在常量表达式中只能使用某些类型的操作。方法调用（例如 Long.toString（））不是其中之一。所以使用它会使你的表达式不是一个常量表达式，即使它看起来应该是这样。 （它看起来像你，因为你知道 Long.toString 是什么。但是，编译器没有保留所有方法的目录来知道哪些是常量 可以在编译时计算其值的方法。

The Java rules say that when you have an annotation, and it has a parameter that expects a primitive type (such as an int) or a String, the value must be a constant expression. [This has nothing to do with Spring.] Roughly speaking, a constant expression is one whose value the compiler can figure out at compile time. However, there are rules for what constitutes a constant expression. These rules are in JLS 15.28. Only certain types of operations can be used in a constant expression. A method call, such as Long.toString(), isn't one of those. So using that makes your expression not a constant expression, even though it looks like it should be. (It looks like it to you, because you know what Long.toString does. However, the compiler doesn't keep a catalog of all methods to know which ones are "constant" methods whose values can be figured out at compile time.)

但是，链接上的示例显示 + 运算符可以使用，即使其中一个参数不是字符串，因此隐式调用 toString（）方法。这表明您可能能够使这样的工作：

However, the example at the link shows that the + operator can be used, even when one of the arguments is not a string and therefore a toString() method is implicitly called. This suggests that you might be able to make things work like this:

private static final String MAX_LONG_AS_STRING = "" + Long.MAX_VALUE;

我还没试过。

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