kotlin错误的可空性推论没有任何泛型

Question

在春天mvc(5.1.3)我试图做:

val url : String? = null
val matcher: ResultMatcher = MockMvcResultMatchers.forwardedUrl(url)

我得到第二行的编译错误.

来自intellij(kotlinc-jvm 1.3.11):

Error:(230, 56) Kotlin: Null can not be a value of a non-null type String

或者从gradle(kotlin 1.2.71):

 Type mismatch: inferred type is String? but String was expected

spring方法的java源代码是:

/**
 * Asserts the request was forwarded to the given URL.
 * <p>This method accepts only exact matches.
 * @param expectedUrl the exact URL expected
 */
public static ResultMatcher forwardedUrl(String expectedUrl) {
    return result -> assertEquals("Forwarded URL", expectedUrl, result.getResponse().getForwardedUrl());
}

intellij显示javadoc:

org.springframework.test.web.servlet.result.MockMvcResultMatchers @NotNull 
@Contract(pure = true) 
public static org.springframework.test.web.servlet.ResultMatcher forwardedUrl(@Nullable String expectedUrl)

那么为什么编译器仍然需要非可空类型以及如何绕过该要求呢？

Answer 1

参数上没有明确的可空性注释MockMvcResultMatchers.forwardedUrl,因此它将默认为@NonNullApi,如下所示package-info.java:

/**
 * Contains built-in {@code ResultMatcher} and {@code ResultHandler} implementations.
 * <p>Use {@link org.springframework.test.web.servlet.result.MockMvcResultMatchers}
 * and {@link org.springframework.test.web.servlet.result.MockMvcResultHandlers}
 * to access instances of those implementations.
 */
@NonNullApi
@NonNullFields
package org.springframework.test.web.servlet.result;

import org.springframework.lang.NonNullApi;
import org.springframework.lang.NonNullFields;

有没有办法来形容空性在Java中,因此默认情况下同时传递String和String?成接受一个Java方法String是可行的.但是,Kotlin编译器尊重不同版本的可空性注释(例如javax.annotation.*,Android的注释,Spring的注释),以允许Java代码向Kotlin调用者提供可空性信息.

如何绕过这个？你不能.-你传递了错误类型的参数String,并String?在科特林是不同的类型.