07ee94d671
* Refactor execute around the idiom * fix checkstyle errors Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>
97 lines
3.0 KiB
Markdown
97 lines
3.0 KiB
Markdown
---
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layout: pattern
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title: Execute Around
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folder: execute-around
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permalink: /patterns/execute-around/
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categories: Idiom
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language: en
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tags:
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- Extensibility
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---
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## Intent
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Execute Around idiom frees the user from certain actions that should always be executed before and
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after the business method. A good example of this is resource allocation and deallocation leaving
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the user to specify only what to do with the resource.
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## Explanation
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Real-world example
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> A class needs to be provided for writing text strings to files. To make it easy for
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> the user, the service class opens and closes the file automatically. The user only has to
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> specify what is written into which file.
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In plain words
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> Execute Around idiom handles boilerplate code before and after business method.
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[Stack Overflow](https://stackoverflow.com/questions/341971/what-is-the-execute-around-idiom) says
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> Basically it's the pattern where you write a method to do things which are always required, e.g.
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> resource allocation and clean-up, and make the caller pass in "what we want to do with the
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> resource".
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**Programmatic Example**
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`SimpleFileWriter` class implements the Execute Around idiom. It takes `FileWriterAction` as a
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constructor argument allowing the user to specify what gets written into the file.
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```java
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@FunctionalInterface
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public interface FileWriterAction {
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void writeFile(FileWriter writer) throws IOException;
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}
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@Slf4j
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public class SimpleFileWriter {
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public SimpleFileWriter(String filename, FileWriterAction action) throws IOException {
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LOGGER.info("Opening the file");
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try (var writer = new FileWriter(filename)) {
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LOGGER.info("Executing the action");
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action.writeFile(writer);
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LOGGER.info("Closing the file");
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}
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}
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}
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```
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The following code demonstrates how `SimpleFileWriter` is used. `Scanner` is used to print the file
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contents after the writing finishes.
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```java
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FileWriterAction writeHello = writer -> {
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writer.write("Gandalf was here");
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};
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new SimpleFileWriter("testfile.txt", writeHello);
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var scanner = new Scanner(new File("testfile.txt"));
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while (scanner.hasNextLine()) {
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LOGGER.info(scanner.nextLine());
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}
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```
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Here's the console output.
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```
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21:18:07.185 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Opening the file
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21:18:07.188 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Executing the action
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21:18:07.189 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Closing the file
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21:18:07.199 [main] INFO com.iluwatar.execute.around.App - Gandalf was here
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```
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## Class diagram
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## Applicability
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Use the Execute Around idiom when
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* An API requires methods to be called in pairs such as open/close or allocate/deallocate.
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## Credits
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* [Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions](https://www.amazon.com/gp/product/1937785467/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1937785467&linkCode=as2&tag=javadesignpat-20&linkId=7e4e2fb7a141631491534255252fd08b)
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