Why does Kotlin byte code reference java.util.function.BiConsumer?

问题

From what I understand Kotlin should be able to run using JRE 6. But this code with a foreach on a map fails because of a reference to a Java 8 class (java/util/function/BiConsumer)

CompilerTest.kt:

fun main(args: Array<String>) {
  val aMap = mapOf("bar" to "bat")
  aMap.forEach { k, v -> println("$k -> $v")}
}

Compile the Kotlin code:

» kotlinc CompilerTest.kt -jvm-target 1.6 -include-runtime -d compilerTest.jar

Testing the compiled code on JRE 6:

» docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp openjdk:6-jdk-slim java -jar compilerTest.jar
Exception in thread "main" java.lang.NoClassDefFoundError: java/util/function/BiConsumer
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:643)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:268)
    at CompilerTestKt.main(CompilerTest.kt:5)
Caused by: java.lang.ClassNotFoundException: java.util.function.BiConsumer
    at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:268)
    ... 12 more

Version Information:

» kotlinc -version         
info: kotlinc-jvm 1.1.4-3 (JRE 1.8.0_131-b11)

回答1:

Turns out that kotlin Map inherits from java.util.Map. When you call aMap.forEach { k, v -> println("$k -> $v")}, you are calling the Java version of the method.

However, if you change the code to take an entry: aMap.forEach { entry -> println("$entry.key -> $entry.value")}, you are calling the Kotlin version and the code will run on JRE6.

You can flag the dependency on the JRE by compiling with the -no-jdk flag to the compiler.

» kotlinc CompilerTest.kt -no-jdk -jvm-target 1.6 -include-runtime -d compilerTest.jar
CompilerTest.kt:5:10: error: type inference failed: inline fun <K, V> Map<out K, V>.forEach(action: (Map.Entry<K, V>) -> Unit): Unit
cannot be applied to
receiver: Map<String, String>  arguments: ((Map.Entry<String, String>, ???) -> Unit)

    aMap.forEach { k, v -> println("$k -> $v")}
         ^
...

回答2:

Try to disable the Instant run, under

Preferences > Build, Execution, Deployment

It was the only thing that worked for me!

回答3:

kotlinc-jvm 1.1.4-3 (JRE 1.8.0_131-b11)

It seems like you are using JDK 8 version. If you are using gradle, you should check your build.gradle.

compile "org.jetbrains.kotlin:kotlin-stdlib" // JDK 6
compile "org.jetbrains.kotlin:kotlin-stdlib-jre7" // JDK 7
compile "org.jetbrains.kotlin:kotlin-stdlib-jre8" // JDK 8

来源：https://stackoverflow.com/questions/46396751/why-does-kotlin-byte-code-reference-java-util-function-biconsumer

标签

java

compilation

kotlin

java-6