How to Create a Temporary File in Kotlin

1. Introduction

Creating temporary files is a common task in software development, especially when dealing with large data pipelines, caching, or managing intermediate outputs in applications. Kotlin offers a concise syntax and interoperability with Java, providing robust tools for file operations to create temporary files.

This tutorial explores various methods to create temporary files in Kotlin.

2. Using Kotlin’s kotlin.io.path Extensions

Kotlin’s kotlin.io.path package offers a set of extensions that streamline file operations in a way that is easy to use and leverages Java’s powerful NIO (Non-blocking I/O) file package. This approach enhances the readability and conciseness of file management tasks:

fun createTempFileWithKotlinExtensions(): Path { 
    val tempFile = createTempFile(prefix = "kotlinTemp", suffix = ".tmp")
    println("Temporary file created with Kotlin extensions at: ${tempFile.toAbsolutePath()}")
    tempFile.writeText("Kotlin Path Data") 
    return tempFile 
}

This code snippet utilizes createTempFile() from kotlin.io.path to create a temporary file. We echo the path to the console for verification. To prove we can use the file, we call writeText() to populate the file with some content.

To delete the temp file on exit, we need to transform the Path to a File and then specify that it should be deleted on exit:

tempFile.toFile().deleteOnExit()

2.1. Customizing Temp File Attributes

While the createTempFile() method is effective, sometimes we need to set additional properties on the temp file. For example, we may need to specify a custom directory, prefix, or suffix for the temporary file:

fun createCustomTempFileWithKotlinExtensions(): Path {
    val customDir = Paths.get(System.getProperty("java.io.tmpdir"))
    val tempFile = createTempFile(
                       directory = customDir,
                       prefix = "customKotlinTemp",
                       suffix = ".tmp"
                   )
    println("Custom temporary file created with Kotlin extensions at: ${tempFile.toAbsolutePath()}")

    tempFile.writeText("Custom Kotlin Path Data")

    return tempFile
}

This example shows how to customize the file name and location, in case we need to save the temp file in a specific place or with a particular naming pattern.

2.2. Deprecated Original Kotlin Way

The createTempFile() used to live in the kotlin.io package, but this method has been deprecated due to permissions issues. It’s important to be aware of this if we encounter older code:

@Deprecated("This method is deprecated due to permissioning issues. Use kotlin.io.path.createTempFile instead.")
fun deprecatedCreateTempFile(): File {
    val tempFile = kotlin.io.createTempFile()
    println("Deprecated temporary file created at: ${tempFile.absolutePath}")
    tempFile.writeText("Deprecated Data")
    return tempFile
}

This method was deprecated because the files it creates have permissions that could allow other users on the system to read the files. We should prefer kotlin.io.path.createTempFile() instead.

3. Using java.io.File.createTempFile()

We can also use Java interop methods to create a temporary file in Kotlin with the createTempFile() method from the java.io.File class. To ensure the temporary file is deleted when the JVM terminates, the deleteOnExit() method needs to be called:

fun createTempFile(): File {
    val tempFile = File.createTempFile("temp", ".tmp")
    println("Temporary file created at: ${tempFile.absolutePath}")
    tempFile.writeText("Sample Data")
    tempFile.deleteOnExit()
    return tempFile
}

This code snippet demonstrates the creation of a temporary file, writing some data to it, and printing its path.

3.1. Customizing Temporary File Creation

Sometimes, we need to specify alternative locations or naming patterns for temp files. This createTempFile() also allows specifying the directory, prefix, and suffix for the temp file. Let’s adapt createTempFile() with these customizations:

fun createCustomTempFile(): File {
    val tempDir = System.getProperty("java.io.tmpdir")
    val tempFile = File.createTempFile("customTemp", ".tmp", File(tempDir))
    tempFile.deleteOnExit()
    println("Custom temporary file created at: ${tempFile.absolutePath}")
    tempFile.writeText("Custom Data")
    return tempFile
}

4. Conclusion

Kotlin provides multiple avenues for creating and managing temporary files, suitable for a variety of scenarios. Whether using the straightforward approach of java.io.File.createTempFile() or the Kotlin-specific kotlin.io.path.createTempFile(), we can easily create temp files to suit our needs. We should select the strategy that best fits the application’s needs and scope.

As always, the code used in this article is available over on GitHub.