[BAEL-7257] Monkey-Patching in Java (#15700)

patterns-modules/monkey-patching/README.md

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+### Relevant Articles:

patterns-modules/monkey-patching/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<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>
+    <groupId>com.baeldung</groupId>
+    <artifactId>monkey-patching</artifactId>
+    <version>1.0.0-SNAPSHOT</version>
+    <name>monkey-patching</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>patterns-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-web</artifactId>
+            <version>2.7.0</version>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <version>2.7.0</version>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-aop</artifactId>
+            <version>2.7.0</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
+            </plugin>
+        </plugins>
+    </build>
+</project>

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/Application.java

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+package com.baeldung.monkey.patching;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class Application {
+
+    public static void main(String[] args) {
+        SpringApplication.run(Application.class, args);
+    }
+}

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/aop/BeanConfiguration.java

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+package com.baeldung.monkey.patching.aop;
+
+import com.baeldung.monkey.patching.converter.MoneyConverter;
+import com.baeldung.monkey.patching.converter.MoneyConverterImpl;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+public class BeanConfiguration {
+
+    @Bean
+    public MoneyConverter moneyConverter() {
+        return new MoneyConverterImpl();
+    }
+}

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/aop/LoggingAspect.java

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+package com.baeldung.monkey.patching.aop;
+
+import org.aspectj.lang.JoinPoint;
+import org.aspectj.lang.annotation.After;
+import org.aspectj.lang.annotation.Aspect;
+import org.aspectj.lang.annotation.Before;
+import org.springframework.stereotype.Component;
+
+@Aspect
+@Component
+public class LoggingAspect {
+
+    @Before("execution(* com.baeldung.monkey.patching.converter.MoneyConverter.convertEURtoUSD(..))")
+    public void beforeConvertEURtoUSD(JoinPoint joinPoint) {
+        System.out.println("Before method: " + joinPoint.getSignature().getName());
+    }
+
+    @After("execution(* com.baeldung.monkey.patching.converter.MoneyConverter.convertEURtoUSD(..))")
+    public void afterConvertEURtoUSD(JoinPoint joinPoint) {
+        System.out.println("After method: " + joinPoint.getSignature().getName());
+    }
+}

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/converter/MoneyConverter.java

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+package com.baeldung.monkey.patching.converter;
+
+public interface MoneyConverter {
+    double convertEURtoUSD(double amount);
+}

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/converter/MoneyConverterImpl.java

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+package com.baeldung.monkey.patching.converter;
+
+public class MoneyConverterImpl implements MoneyConverter {
+
+    private final double conversionRate;
+
+    public MoneyConverterImpl() {
+        this.conversionRate = 1.10;
+    }
+
+    @Override
+    public double convertEURtoUSD(double amount) {
+        return amount * conversionRate;
+    }
+}

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/decorator/MoneyConverterDecorator.java

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+package com.baeldung.monkey.patching.decorator;
+
+import com.baeldung.monkey.patching.converter.MoneyConverter;
+
+public class MoneyConverterDecorator implements MoneyConverter {
+
+    private final MoneyConverter moneyConverter;
+
+    public MoneyConverterDecorator(MoneyConverter moneyConverter) {
+        this.moneyConverter = moneyConverter;
+    }
+
+    @Override
+    public double convertEURtoUSD(double amount) {
+
+        System.out.println("Before method: convertEURtoUSD");
+        double result = moneyConverter.convertEURtoUSD(amount);
+        System.out.println("After method: convertEURtoUSD");
+        return result;
+    }
+}

patterns-modules/monkey-patching/src/main/java/com/baeldung/monkey/patching/proxy/LoggingInvocationHandler.java

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+package com.baeldung.monkey.patching.proxy;
+
+import java.lang.reflect.InvocationHandler;
+import java.lang.reflect.Method;
+
+public class LoggingInvocationHandler implements InvocationHandler {
+
+    private final Object target;
+
+    public LoggingInvocationHandler(Object target) {
+        this.target = target;
+    }
+
+    @Override
+    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
+        System.out.println("Before method: " + method.getName());
+        Object result = method.invoke(target, args);
+        System.out.println("After method: " + method.getName());
+        return result;
+    }
+}

patterns-modules/monkey-patching/src/test/java/com/baeldung/monkey/patching/aop/LoggingAspectIntegrationTest.java

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+package com.baeldung.monkey.patching.aop;
+
+import com.baeldung.monkey.patching.converter.MoneyConverter;
+import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
+import static org.junit.Assert.assertTrue;
+
+@SpringBootTest
+@RunWith(SpringRunner.class)
+public class LoggingAspectIntegrationTest {
+
+    @Autowired
+    private MoneyConverter moneyConverter;
+
+    @Test
+    public void whenMethodCalled_thenSurroundedByLogs() {
+        ByteArrayOutputStream logOutputStream = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(logOutputStream));
+
+        double result = moneyConverter.convertEURtoUSD(10);
+
+        Assertions.assertEquals(11, result);
+        String logOutput = logOutputStream.toString();
+        assertTrue(logOutput.contains("Before method: convertEURtoUSD"));
+        assertTrue(logOutput.contains("After method: convertEURtoUSD"));
+    }
+}

patterns-modules/monkey-patching/src/test/java/com/baeldung/monkey/patching/converter/MoneyConverterUnitTest.java

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+package com.baeldung.monkey.patching.converter;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class MoneyConverterUnitTest {
+
+    @Test
+    void whenMoneyConverter_thenResultIsCorrect() {
+        MoneyConverterImpl moneyConverter = new MoneyConverterImpl();
+
+        double result = moneyConverter.convertEURtoUSD(10);
+
+        assertEquals(11, result);
+    }
+}

patterns-modules/monkey-patching/src/test/java/com/baeldung/monkey/patching/decorator/MoneyConverterDecoratorUnitTest.java

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+package com.baeldung.monkey.patching.decorator;
+
+import com.baeldung.monkey.patching.converter.MoneyConverter;
+import com.baeldung.monkey.patching.converter.MoneyConverterImpl;
+import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
+import static org.junit.Assert.assertTrue;
+
+public class MoneyConverterDecoratorUnitTest {
+
+    @Test
+    public void whenMethodCalled_thenSurroundedByLogs() {
+        ByteArrayOutputStream logOutputStream = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(logOutputStream));
+        MoneyConverter moneyConverter = new MoneyConverterDecorator(new MoneyConverterImpl());
+
+        double result = moneyConverter.convertEURtoUSD(10);
+
+        Assertions.assertEquals(11, result);
+        String logOutput = logOutputStream.toString();
+        assertTrue(logOutput.contains("Before method: convertEURtoUSD"));
+        assertTrue(logOutput.contains("After method: convertEURtoUSD"));
+    }
+}

patterns-modules/monkey-patching/src/test/java/com/baeldung/monkey/patching/proxy/LoggingInvocationHandlerUnitTest.java

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+package com.baeldung.monkey.patching.proxy;
+
+import com.baeldung.monkey.patching.converter.MoneyConverter;
+import com.baeldung.monkey.patching.converter.MoneyConverterImpl;
+import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.lang.reflect.Proxy;
+
+import static org.junit.Assert.assertTrue;
+
+public class LoggingInvocationHandlerUnitTest {
+
+    @Test
+    public void whenMethodCalled_thenSurroundedByLogs() {
+        ByteArrayOutputStream logOutputStream = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(logOutputStream));
+        MoneyConverter moneyConverter = new MoneyConverterImpl();
+        MoneyConverter proxy = (MoneyConverter) Proxy.newProxyInstance(
+                MoneyConverter.class.getClassLoader(),
+                new Class[]{MoneyConverter.class},
+                new LoggingInvocationHandler(moneyConverter)
+        );
+
+        double result = proxy.convertEURtoUSD(10);
+
+        Assertions.assertEquals(11, result);
+        String logOutput = logOutputStream.toString();
+        assertTrue(logOutput.contains("Before method: convertEURtoUSD"));
+        assertTrue(logOutput.contains("After method: convertEURtoUSD"));
+    }
+}

patterns-modules/monkey-patching/src/test/java/com/baeldung/monkey/patching/reflection/ReflectionUnitTest.java

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+package com.baeldung.monkey.patching.reflection;
+
+import com.baeldung.monkey.patching.converter.MoneyConverter;
+import com.baeldung.monkey.patching.converter.MoneyConverterImpl;
+import org.junit.Test;
+
+import java.lang.reflect.Field;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ReflectionUnitTest {
+
+@Test
+public void givenPrivateField_whenUsingReflection_thenBehaviorCanBeChanged() throws IllegalAccessException, NoSuchFieldException {
+    MoneyConverter moneyConvertor = new MoneyConverterImpl();
+
+    Field conversionRate = MoneyConverterImpl.class.getDeclaredField("conversionRate");
+    conversionRate.setAccessible(true);
+    conversionRate.set(moneyConvertor, 1.2);
+    double result = moneyConvertor.convertEURtoUSD(10);
+
+    assertEquals(12, result);
+}
+}

patterns-modules/pom.xml

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         <module>idd</module>
         <module>intercepting-filter</module>
         <module>solid</module>
+        <module>monkey-patching</module>
     </modules>
 
 </project>