try-with-resources: “use” extension function in Ko

问题:

I had some trouble expressing the Java's try-with-resources construct in Kotlin. In my understanding, every expression that is an instance of AutoClosable should provide the use extension function.

Here is a complete example:

import java.io.BufferedReader;
import java.io.FileReader;

import org.openrdf.query.TupleQuery;
import org.openrdf.query.TupleQueryResult;

public class Test {

    static String foo(String path) throws Throwable {
        try (BufferedReader r =
           new BufferedReader(new FileReader(path))) {
          return "";
        }
    }

    static String bar(TupleQuery query) throws Throwable {
        try (TupleQueryResult r = query.evaluate()) {
          return "";
        }
    }
}

The Java-to-Kotlin converter creates this output:

import java.io.BufferedReader
import java.io.FileReader

import org.openrdf.query.TupleQuery
import org.openrdf.query.TupleQueryResult

object Test {

    @Throws(Throwable::class)
    internal fun foo(path: String): String {
        BufferedReader(FileReader(path)).use { r -> return "" }
    }

    @Throws(Throwable::class)
    internal fun bar(query: TupleQuery): String {
        query.evaluate().use { r -> return "" } // ERROR
    }
}

foo works fine, but the code in bar does not compile:

Error:(16, 26) Kotlin: Unresolved reference.
None of the following candidates is applicable
because of receiver type mismatch: 
public inline fun <T : java.io.Closeable, R>
???.use(block: (???) -> ???): ??? defined in kotlin.io

query.evaluate() is from Sesame and implements AutoClosable. Is it a Kotlin bug, or is there a reason why it does not work?

---

I am using IDEA 15.0.3 with Kotlin 1.0.0-beta-4584-IJ143-12 and the following sasame-runtime version:

<groupId>org.openrdf.sesame</groupId>
<artifactId>sesame-runtime</artifactId>
<version>4.0.2</version>

回答1:

Kotlin targets Java 6 at the moment, so its standard library does not use the AutoCloseable interface. The use function only supports the Java 6 Closeable interface. See the issue tracker for reference.

You can create a copy of the use function in your project and modify it to replace Closeable with AutoCloseable:

public inline fun <T : AutoCloseable, R> T.use(block: (T) -> R): R {
    var closed = false
    try {
        return block(this)
    } catch (e: Exception) {
        closed = true
        try {
            close()
        } catch (closeException: Exception) {
            e.addSuppressed(closeException)
        }
        throw e
    } finally {
        if (!closed) {
            close()
        }
    }
}

回答2:

Kotlin 1.1+ has a standard library that targets Java 8 to support Closeable resource pattern - kotlin-stdlib-jre8

Gradle

compile "org.jetbrains.kotlin:kotlin-stdlib:1.1.1"
compile "org.jetbrains.kotlin:kotlin-stdlib-jre8:1.1.1"

Maven

<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-stdlib</artifactId>
    <version>1.1.1</version>
</dependency>
<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-stdlib-jre8</artifactId>
    <version>1.1.1</version>
</dependency>

Sample

val resource: AutoCloseable = getCloseableResource() 
resource.use { r -> //play with r }

回答3:

For classes that do not support the "use" function, I have done the next homemade try-with-resources:

inline fun <T:AutoCloseable,R> trywr(closeable: T, block: (T) -> R): R {
    try {
        return block(closeable);
    } finally {
        closeable.close()
    }
}

Then you can use it the next way:

fun countEvents(sc: EventSearchCriteria?): Long {
    return trywr(connection.prepareStatement("SELECT COUNT(*) FROM event")) {
        var rs = it.executeQuery()
        rs.next()
        rs.getLong(1)
    }
}