BAEL-4463: What is com.sun.proxy.$Proxy

core-java-modules/core-java-reflection-2/src/main/java/com/baeldung/reflection/proxy/AdvancedOperation.java

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+package com.baeldung.reflection.proxy;
+
+public interface AdvancedOperation {
+    int multiple(int a, int b);
+
+    int divide(int a, int b);
+}

core-java-modules/core-java-reflection-2/src/main/java/com/baeldung/reflection/proxy/BasicOperation.java

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+package com.baeldung.reflection.proxy;
+
+public interface BasicOperation {
+    int add(int a, int b);
+
+    int sub(int a, int b);
+}

core-java-modules/core-java-reflection-2/src/test/java/com/baeldung/reflection/proxy/DollarProxyUnitTest.java

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+package com.baeldung.reflection.proxy;
+
+import org.junit.Test;
+
+import java.lang.reflect.Proxy;
+import java.util.function.Consumer;
+
+import static org.junit.Assert.assertTrue;
+
+public class DollarProxyUnitTest {
+    @Test
+    public void givenProxy_whenInvokingGetProxyClass_thenGeneratingProxyClass() {
+        // Java 8: -Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
+        // Java 9 or later: -Djdk.proxy.ProxyGenerator.saveGeneratedFiles=true
+        // Note: System.setProperty() doesn't work here
+        // because ProxyGenerator.saveGeneratedFiles read its property only once.
+        // The @Test annotation in this method will generate a $Proxy class.
+
+        ClassLoader classLoader = ClassLoader.getSystemClassLoader();
+        Class<?>[] interfaces = {BasicOperation.class, AdvancedOperation.class};
+        Class<?> proxyClass = Proxy.getProxyClass(classLoader, interfaces);
+
+        boolean isProxyClass = Proxy.isProxyClass(proxyClass);
+        assertTrue(isProxyClass);
+    }
+
+    @Test
+    public void givenReflection_whenReadingAnnotation_thenGeneratingProxyClass() {
+        FunctionalInterface instance = Consumer.class.getDeclaredAnnotation(FunctionalInterface.class);
+        Class<?> clazz = instance.getClass();
+
+        boolean isProxyClass = Proxy.isProxyClass(clazz);
+        assertTrue(isProxyClass);
+    }
+}