try-with-resources："use" Kotlin 中的扩展函数并不总是有效

Question

我在用 Kotlin 表达 Java 的 try-with-resources 结构时遇到了一些问题。在我的理解中，每个作为 AutoClosable 实例的表达式都应该提供 use 扩展函数。

这是一个完整的例子：

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 "";
        }
    }
}

Java-to-Kotlin 转换器创建此输出：

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 工作正常，但 bar 中的代码无法编译：

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() 来自 Sesame 并实现了 AutoClosable。是 Kotlin 错误，还是它不起作用的原因？

我将 IDEA 15.0.3 与 Kotlin 1.0.0-beta-4584-IJ143-12 和以下 sasame-runtime 版本一起使用：

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

Answer 1

Kotlin 目前的目标是 Java 6，所以它的标准库不使用 AutoCloseable 接口。 use函数只支持Java6Closeable接口。请参阅 the issue tracker 以供参考。

您可以在项目中创建 use 函数的副本并修改它以将 Closeable 替换为 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()
        }
    }
}

Answer 2

对于不支持"use"功能的类，我做了下自制的try-with-resources：

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

那你就可以用下面的方法了：

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

Answer 3

Kotlin 1.1+ 有一个标准库，目标是 Java 8 以支持 Closeable resource 模式 - 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>

示例

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

Answer 4

只需确保类路径中有kotlin-stdlib-jdk7.jar，默认不添加

try-with-resources："use" Kotlin 中的扩展函数并不总是有效

try-with-resources: "use" extension function in Kotlin does not always work

java

try-with-resources

kotlin