An attempt to use resources in Kotlin

Question

When I tried to write the equivalent of Java try -with-resources code in Kotlin, it did not work for me.

I tried different options for the following:

 try (writer = OutputStreamWriter(r.getOutputStream())) { // ... } 

But not one works.

Does anyone know what should be used instead? Obviously, Kotlin’s grammar has no definition for such a construction, but maybe I missed something. It defines the grammar for the try block as follows:

 try : "try" block catchBlock* finallyBlock?; 

Answer 1

There is a use function ( src ) in kotlin stdlib.

How to use it:

 OutputStreamWriter(r.getOutputStream()).use { // by `it` value you can get your OutputStreamWriter it.write('a') } 

Answer 2

TL; DR

No special syntax, but use function

Unlike Java, Kotlin has no special syntax for this. Instead, try-with-resources is suggested as a standard use library function.

 FileInputStream("filename").use { fis -> //or implicit 'it' //use stream here } 

Implementation use

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

Closeable? this function defined as a common extension for all Closeable? types. Closeable is a Java interface that lets you try resources with Java SE7 .
The function accepts a functional literal block that is executed in try . As with try-with-resources in Java, Closeable closes in finally .

Also, crashes that occur inside the block are executed at close , where possible exceptions are literally "suppressed", simply ignoring them. This is different from try-with-resources because such exceptions may be thrown in a Java solution.

How to use it

The use extension is available for any type of Closeable , i.e. Closeable , readers, etc.

 FileInputStream("filename").use { //use your stream by referring to 'it' or explicitly give a name. } 

The part in braces that becomes block in use (lambda is passed as an argument here). After the block is done, you can be sure that the FileInputStream been closed.

Answer 3

Edit : The following answer is still valid for Kotlin 1.0.x. For Kotlin 1.1, there is support for the standard library for Java 8 to support the lockable resource template.

For other classes that do not support the "use" function, I made the following home-made attempt with resources:

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

Then you can use it as follows:

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