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feature: Add gateway pattern (iluwatar#1297) (iluwatar#2734)
* Implement Gateway pattern * Fixed docstrings * Fixed README.md * Fixed App * Fixed pom.xml * Fixed pom.xml * Fixed pom.xml * Fixed pom.xml * fixed checkstyle errors and directory names * fixed pom.xml * fixed checkstyle errors * Bug fixed * Bug fixed * Bug fixed * Bug fixed and code improvement * Bug fixed and code improvement * Bug fixed and update README.md * update APP.java * update gateway pattern * Revert "update gateway pattern" This reverts commit f31a00c. * update gateway pattern --------- Co-authored-by: Finn <[email protected]>
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‎gateway/README.md‎

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---
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title: Gateway
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category: Structural
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language: en
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tag:
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- Gang of Four
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- Decoupling
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---
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## Intent
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Provide a interface to access a set of external systems or functionalities. Gateway provides a simple uniform view of
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external resources to the internals of an application.
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## Explanation
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Real-world example
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> Gateway acts like a real front gate of a certain city. The people inside the city are called
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> internal system, and different outside cities are called external services. The gateway is here
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> to provide access for internal system to different external services.
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In plain words
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> Gateway can provide an interface which lets internal system to utilize external service.
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Wikipedia says
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> A server that acts as an API front-end, receives API requests, enforces throttling and security
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> policies, passes requests to the back-end service and then passes the response back to the requester.
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**Programmatic Example**
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The main class in our example is the `ExternalService` that contains items.
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```java
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class ExternalServiceA implements Gateway {
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@Override
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public void execute() throws Exception {
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System.out.println("Executing Service A");
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// Simulate a time-consuming task
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Thread.sleep(1000);
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}
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}
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/**
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* ExternalServiceB is one of external services.
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*/
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class ExternalServiceB implements Gateway {
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@Override
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public void execute() throws Exception {
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System.out.println("Executing Service B");
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// Simulate a time-consuming task
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Thread.sleep(1000);
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}
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}
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/**
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* ExternalServiceC is one of external services.
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*/
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class ExternalServiceC implements Gateway {
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@Override
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public void execute() throws Exception {
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System.out.println("Executing Service C");
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// Simulate a time-consuming task
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Thread.sleep(1000);
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}
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public void error() throws Exception {
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// Simulate an exception
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throw new RuntimeException("Service C encountered an error");
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}
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}
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```
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To operate these external services, Here's the `App` class:
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```java
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public class App {
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/**
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* Simulate an application calling external services.
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*/
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public static void main(String[] args) throws Exception {
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GatewayFactory gatewayFactory = new GatewayFactory();
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// Register different gateways
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gatewayFactory.registerGateway("ServiceA", new ExternalServiceA());
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gatewayFactory.registerGateway("ServiceB", new ExternalServiceB());
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gatewayFactory.registerGateway("ServiceC", new ExternalServiceC());
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// Use an executor service for asynchronous execution
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Gateway serviceA = gatewayFactory.getGateway("ServiceA");
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Gateway serviceB = gatewayFactory.getGateway("ServiceB");
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Gateway serviceC = gatewayFactory.getGateway("ServiceC");
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// Execute external services
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try {
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serviceA.execute();
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serviceB.execute();
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serviceC.execute();
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} catch (ThreadDeath e) {
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System.out.println("Interrupted!" + e);
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throw e;
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}
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}
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}
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```
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The `Gateway` interface is extremely simple.
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```java
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interface Gateway {
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void execute() throws Exception;
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}
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```
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Program output:
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```java
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Executing Service A
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Executing Service B
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Executing Service C
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```
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## Class diagram
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![alt text](./etc/gateway.urm.png "gateway")
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## Applicability
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Use the Gateway pattern
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* To access an aggregate object's contents without exposing its internal representation.
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* To integration with multiple external services or APIs.
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* To provide a uniform interface for traversing different aggregate structures.
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## Tutorials
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* [Pattern: API Gateway / Backends for Frontends](https://microservices.io/patterns/apigateway.html)
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## Known uses
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* [API Gateway](https://java-design-patterns.com/patterns/api-gateway/)
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* [10 most common use cases of an API Gateway](https://apisix.apache.org/blog/2022/10/27/ten-use-cases-api-gateway/)
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## Credits
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* [Gateway](https://martinfowler.com/articles/gateway-pattern.html)
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* [What is the difference between Facade and Gateway design patterns?](https://stackoverflow.com/questions/4422211/what-is-the-difference-between-facade-and-gateway-design-patterns)

‎gateway/etc/gateway.urm.png‎

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‎gateway/etc/gateway.urm.puml‎

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@startuml GatewayPattern
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package com.iluwatar.gateway{
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class App {
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+main(args: String[]): void
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}
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class GatewayFactory {
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-gateways: Map<String, Gateway>
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+registerGateway(key: String, gateway: Gateway): void
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+getGateway(key: String): Gateway
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}
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interface Gateway {
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{abstract} +execute(): void
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}
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class ExternalServiceA {
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+execute(): void
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}
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class ExternalServiceB {
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+execute(): void
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}
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class ExternalServiceC {
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+execute(): void
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+error(): void
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}
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App --> GatewayFactory : Uses
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GatewayFactory --> Gateway : Creates
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GatewayFactory --> ExternalServiceA : Registers
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GatewayFactory --> ExternalServiceB : Registers
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GatewayFactory --> ExternalServiceC : Registers
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ExternalServiceA --> Gateway : Implements
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ExternalServiceB --> Gateway : Implements
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ExternalServiceC --> Gateway : Implements
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@enduml

‎gateway/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
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The MIT License
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Copyright © 2014-2022 Ilkka Seppälä
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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THE SOFTWARE.
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-->
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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.26.0-SNAPSHOT</version>
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</parent>
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<packaging>jar</packaging>
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<artifactId>gateway</artifactId>
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<dependencies>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-assembly-plugin</artifactId>
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<executions>
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<execution>
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<configuration>
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<archive>
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<manifest>
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<mainClass>com.iluwatar.gateway.App</mainClass>
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</manifest>
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</archive>
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</configuration>
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</execution>
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</executions>
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</plugin>
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</plugins>
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</build>
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</project>

‎gateway/src/main/java/com/iluwatar/gateway/App.java‎

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/*
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* This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
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*
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* The MIT License
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* Copyright © 2014-2022 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.gateway;
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import lombok.extern.slf4j.Slf4j;
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/**
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* the Gateway design pattern is a structural design pattern that provides a unified interface to a set of
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* interfaces in a subsystem. It involves creating a Gateway interface that serves as a common entry point for
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* interacting with various services, and concrete implementations of this interface for different external services.
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*
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* <p>In this example, GateFactory is the factory class, and it provides a method to create different kinds of external
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* services. ExternalServiceA, B, and C are virtual implementations of the external services. Each service provides its
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* own implementation of the execute() method. The Gateway interface is the common interface for all external services.
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* The App class serves as the main entry point for the application implementing the Gateway design pattern. Through
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* the Gateway interface, the App class could call each service with much less complexity.
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*/
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@Slf4j
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public class App {
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/**
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* Simulate an application calling external services.
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*/
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public static void main(String[] args) throws Exception {
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GatewayFactory gatewayFactory = new GatewayFactory();
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// Register different gateways
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gatewayFactory.registerGateway("ServiceA", new ExternalServiceA());
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gatewayFactory.registerGateway("ServiceB", new ExternalServiceB());
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gatewayFactory.registerGateway("ServiceC", new ExternalServiceC());
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// Use an executor service for execution
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Gateway serviceA = gatewayFactory.getGateway("ServiceA");
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Gateway serviceB = gatewayFactory.getGateway("ServiceB");
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Gateway serviceC = gatewayFactory.getGateway("ServiceC");
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// Execute external services
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try {
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serviceA.execute();
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serviceB.execute();
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serviceC.execute();
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} catch (ThreadDeath e) {
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LOGGER.info("Interrupted!" + e);
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throw e;
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}
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}
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}

‎gateway/src/main/java/com/iluwatar/gateway/ExternalServiceA.java‎

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/*
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* This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
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*
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* The MIT License
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* Copyright © 2014-2022 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.gateway;
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import lombok.extern.slf4j.Slf4j;
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import org.slf4j.Logger;
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/**
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* ExternalServiceA is one of external services.
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*/
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@Slf4j
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class ExternalServiceA implements Gateway {
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@Override
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public void execute() throws Exception {
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LOGGER.info("Executing Service A");
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// Simulate a time-consuming task
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Thread.sleep(1000);
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}
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}
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