Kotlin如何能够使用JDK 1.8调用Java 11代码？

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英文:

How is Kotlin able to call Java 11 code using JDK 1.8?

问题

我正在玩一个 Kotlin 项目，其中有一些 Java 11 的调用，但注意到它似乎可以使用 Java 8 来构建和运行。

特别的调用是 java.lang.String#isBlank()，这是在 Java 11 中引入的。

我正在使用 Gradle 来构建和运行测试。构建的 JDK 设置为 JDK 1.8，而 Kotlin 的编译目标也是 1.8：

  1. tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
  2.     kotlinOptions {
  3.         freeCompilerArgs += listOf("-Xjsr305=strict")
  4.         jvmTarget = "1.8"
  5.     }
  6. }
  1. gradle -v
  3. ------------------------------------------------------------
  4. Gradle 6.6
  5. ------------------------------------------------------------
  7. Build time:   2020-08-10 22:06:19 UTC
  8. Revision:     d119144684a0c301aea027b79857815659e431b9
  10. Kotlin:       1.3.72
  11. Groovy:       2.5.12
  12. Ant:          Apache Ant(TM) version 1.10.8 compiled on May 10 2020
  13. JVM:          1.8.0_265 (AdoptOpenJDK 25.265-b01)
  14. OS:           Mac OS X 10.15.6 x86_64

这是一个单元测试，调用了 Java 11，但仍然能够编译和运行：

  1. import org.junit.jupiter.api.Test
  3. class BlankTest {
  5.     @Test
  6.     fun testBlank() {
  7.         val s = java.lang.String("test string")
  8.         val blank = s.isBlank()
  9.         println("Input string blank? : $blank")
  10.     }
  11. }

这是如何可能的呢？

英文:

I was playing around with a Kotlin project which has some Java 11 calls and noticed that it seems to build and run fine using Java 8.

The call in particular is to java.lang.String#isBlank(), which was introduced in Java 11.

I'm using Gradle to build and run tests. Build JDK is set to JDK 1.8 and Kotlin compile target is also 1.8:

  1. tasks.withType&lt;org.jetbrains.kotlin.gradle.tasks.KotlinCompile&gt; {
  2.     kotlinOptions {
  3.         freeCompilerArgs += listOf(&quot;-Xjsr305=strict&quot;)
  4.         jvmTarget = &quot;1.8&quot;
  5.     }
  6. }
  1. gradle -v
  3. ------------------------------------------------------------
  4. Gradle 6.6
  5. ------------------------------------------------------------
  7. Build time:   2020-08-10 22:06:19 UTC
  8. Revision:     d119144684a0c301aea027b79857815659e431b9
  10. Kotlin:       1.3.72
  11. Groovy:       2.5.12
  12. Ant:          Apache Ant(TM) version 1.10.8 compiled on May 10 2020
  13. JVM:          1.8.0_265 (AdoptOpenJDK 25.265-b01)
  14. OS:           Mac OS X 10.15.6 x86_64

This is a unit test which calls Java 11 but still compiles and runs:

  1. import org.junit.jupiter.api.Test
  3. class BlankTest {
  5.     @Test
  6.     fun testBlank() {
  7.         val s = java.lang.String(&quot;test string&quot;)
  8.         val blank = s.isBlank()
  9.         println(&quot;Input string blank? : $blank&quot;)
  10.     }
  11. }

How is this possible?

答案1

得分: 6

答案是，实际上你并没有使用 Java 11 的 isBlank() 方法，你在调用的是 Kotlin 的 isBlank 扩展函数，该函数可以应用于 java.lang.String。

英文:

The answer is that you're not actually using Java 11's isBlank() method, you're calling Kotlin's isBlank extension function that can be applied to java.lang.String.

huangapple
  • 本文由 发表于 2020年8月17日 22:52:01
  • 转载请务必保留本文链接：https://go.coder-hub.com/63453344.html
  • gradle
  • java
  • kotlin
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