java-design-patterns/execute-around/README.md

title, category, language, tags

title	category	language	tags
Execute Around	Idiom	en	Extensibility

Intent

Execute Around idiom frees the user from certain actions that should always be executed before and after the business method. A good example of this is resource allocation and deallocation leaving the user to specify only what to do with the resource.

Explanation

Real-world example

A class needs to be provided for writing text strings to files. To make it easy for the user, the service class opens and closes the file automatically. The user only has to specify what is written into which file.

In plain words

Execute Around idiom handles boilerplate code before and after business method.

Stack Overflow says

Basically it's the pattern where you write a method to do things which are always required, e.g. resource allocation and clean-up, and make the caller pass in "what we want to do with the resource".

Programmatic Example

SimpleFileWriter class implements the Execute Around idiom. It takes FileWriterAction as a constructor argument allowing the user to specify what gets written into the file.

@FunctionalInterface
public interface FileWriterAction {
  void writeFile(FileWriter writer) throws IOException;
}

@Slf4j
public class SimpleFileWriter {
    public SimpleFileWriter(String filename, FileWriterAction action) throws IOException {
        LOGGER.info("Opening the file");
        try (var writer = new FileWriter(filename)) {
            LOGGER.info("Executing the action");
            action.writeFile(writer);
            LOGGER.info("Closing the file");
        }
    }
}

The following code demonstrates how SimpleFileWriter is used. Scanner is used to print the file contents after the writing finishes.

FileWriterAction writeHello = writer -> {
    writer.write("Gandalf was here");
};
new SimpleFileWriter("testfile.txt", writeHello);

var scanner = new Scanner(new File("testfile.txt"));
while (scanner.hasNextLine()) {
LOGGER.info(scanner.nextLine());
}

Here's the console output.

21:18:07.185 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Opening the file
21:18:07.188 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Executing the action
21:18:07.189 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Closing the file
21:18:07.199 [main] INFO com.iluwatar.execute.around.App - Gandalf was here

Class diagram

Applicability

Use the Execute Around idiom when

• An API requires methods to be called in pairs such as open/close or allocate/deallocate.

Credits

• Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions