使用 IntelliJ 将 Java lambda 转换为 Kotlin Lamba 生成语法错误

Question

I have a Java class that configure the embebed tomcat of Spring Boot 2.0.6 for use HTTP2 protocol:

package com.talleres.paco.mako.config;

import org.apache.catalina.connector.Connector;
import org.apache.coyote.http2.Http2Protocol;
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class TomcatHttp2Config {
    @Bean
    public ConfigurableServletWebServerFactory tomcatCustomizer() {
        TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
        factory.addConnectorCustomizers((Connector connector) -> {
            connector.addUpgradeProtocol(new Http2Protocol());
        });
        return factory;
    }
}

When I use IntelliJ for convert this snippet code to Kotlin I obtain a compilation error. Here is the Kotlin code that IntelliJ return:

package com.talleres.paco.mako.config

import org.apache.catalina.connector.Connector
import org.apache.coyote.http2.Http2Protocol
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory
import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration

@Configuration
class TomcatHttp2Config {
    @Bean
    fun tomcatCustomizer(): ConfigurableServletWebServerFactory {
        val factory = TomcatServletWebServerFactory()
        factory.addConnectorCustomizers({ connector: Connector -> connector.addUpgradeProtocol(Http2Protocol()) })
        return factory
    }
}

I obtain this error:

Type mismatch.
Required:
TomcatConnectorCustomizer!
Found:
(Connector) → Unit

At line that appears the lambda:

    factory.addConnectorCustomizers({ connector: Connector -> connector.addUpgradeProtocol(Http2Protocol()) })

How I can translate the Java code above to Kotlin. Thanks in advance.

Answer 1

You could look into SAM conversions for Kotlin .

If for some reason the version with the method doesn't work

factory.addConnectorCustomizers { it.addUpgradeProtocol(Http2Protocol()) }

You can try to implement the Java interface easily as

factory.addConnectorCustomizers(TomcatConnectorCustomizer { it.addUpgradeProtocol(Http2Protocol()) })

Answer 2

It's a probable Kotlin compiler bug that manifests itself when the functional argument is a vararg. Here's a minimal reproducer:

Java:

public class Runner {
    void addTasks(Runnable... tasks) { }
}

Kotlin:

fun main(args: Array<String>) {
    Runner().addTasks({ println("success") })
}

Error:(4, 26) Kotlin: Type mismatch: inferred type is () -> Unit but Runnable! was expected

If we change java:

public class Runner {
    void addTask(Runnable task) { }
}

the error goes away.