Defer in Groovy

In Groovy, we can use the try-finally block to ensure that certain code is executed at the end of a method, similar to the defer keyword in other languages. Here’s how we can implement the same functionality:

import groovy.transform.CompileStatic

@CompileStatic
class DeferExample {
    static void main(String[] args) {
        def f = createFile("/tmp/defer.txt")
        try {
            writeFile(f)
        } finally {
            closeFile(f)
        }
    }

    static FileWriter createFile(String path) {
        println "creating"
        try {
            return new FileWriter(path)
        } catch (IOException e) {
            System.err.println "Error: ${e.message}"
            System.exit(1)
        }
    }

    static void writeFile(FileWriter f) {
        println "writing"
        f.writeLine("data")
    }

    static void closeFile(FileWriter f) {
        println "closing"
        try {
            f.close()
        } catch (IOException e) {
            System.err.println "Error: ${e.message}"
            System.exit(1)
        }
    }
}

In this Groovy example, we’re using a try-finally block to ensure that the file is closed after we’re done writing to it. The finally block is always executed, regardless of whether an exception is thrown or not, which gives us similar behavior to the defer keyword.

Here’s a breakdown of the code:

1. We define a main method that creates a file, writes to it, and then closes it.

2. The createFile method creates a new FileWriter object. If an error occurs during file creation, we print the error message and exit the program.

3. The writeFile method writes some data to the file.

4. The closeFile method closes the file. It’s important to check for errors when closing a file, so we wrap the close() call in a try-catch block.

5. In the main method, we use a try-finally block to ensure that closeFile is called even if an exception occurs during writeFile.

Running this program would produce output similar to:

creating
writing
closing

This approach ensures that resources are properly cleaned up, similar to using defer in other languages. The try-finally block in Groovy (and Java) is a common idiom for resource management, especially when dealing with I/O operations.

Note: In more recent versions of Groovy (and Java), you could also use the try-with-resources statement for automatic resource management, which would further simplify this code.