java-design-patterns/execute-around

title, category, language, tag

title	category	language	tag
Execute Around	Behavioral	en	Closure Code simplification Encapsulation Functional decomposition Resource management

Also known as

• Around Method Pattern
• Resource Block Management

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.

// create the file writer and execute the custom action
FileWriterAction writeHello = writer -> writer.write("Gandalf was here");
new SimpleFileWriter("testfile.txt", writeHello);

// print the file contents
try (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

• Useful in scenarios requiring repetitive setup and cleanup activities, particularly in resource management (e.g., files, network connections, database sessions).
• Ideal for ensuring proper resource handling and cleanup in the face of exceptions, ensuring resources do not leak.
• Suitable in any Java application where the same preparation and finalization steps are executed around varying core functionalities.

Known Uses

• Java's try-with-resources statement, which ensures that resources are closed after execution regardless of whether an exception was thrown.
• Frameworks like Spring for managing database transactions, where predefined cleanup or rollback operations are performed depending on the execution outcome.

Consequences

Benefits:

• Reduces boilerplate code by abstracting routine setup and cleanup tasks.
• Increases code clarity and maintainability by separating business logic from resource management.
• Ensures robustness by automatically handling resource cleanup, even in error situations.

Trade-offs:

• Introduces additional abstraction layers, which might increase complexity and obscure control flow for some developers.
• May require more sophisticated understanding of closures and functional interfaces in Java.

Related Patterns

• Template Method: Similar in concept but differs in that it uses inheritance and abstract classes, while Execute Around typically uses interfaces and lambdas.
• Decorator: Shares the concept of adding functionality around a core component; can be extended to wrap additional behaviors dynamically.

Credits

• Effective Java
• Java Design Patterns: A Hands-On Experience with Real-World Examples
• Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions