feature: Add gateway pattern (#1297)

gateway/README.md

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

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

gateway/pom.xml

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

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

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

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

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