[BAEL-5077] Hidden Classes (eugenp#11663)

Author: tirumani03

core-java-modules/core-java-15/pom.xml

@@ -21,6 +21,11 @@
             <artifactId>commons-lang3</artifactId>
             <version>${commons-lang3.version}</version>
         </dependency>
+        <dependency>
+            <groupId>commons-io</groupId>
+            <artifactId>commons-io</artifactId>
+            <version>${commons-io.version}</version>
+        </dependency>
     </dependencies>
 
     <build>

core-java-modules/core-java-15/src/main/java/com/baeldung/hidden/classes/HiddenClass.java

@@ -0,0 +1,7 @@
+package com.baeldung.hidden.classes;
+
+public class HiddenClass {
+    public String convertToUpperCase(String s) {
+        return s.toUpperCase();
+    }
+}

core-java-modules/core-java-15/src/test/java/com/baeldung/hidden/classes/HiddenClassUnitTest.java

@@ -0,0 +1,60 @@
+package com.baeldung.hidden.classes;
+
+
+import java.io.InputStream;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodHandles.Lookup.ClassOption;
+import java.lang.reflect.Method;
+
+import org.apache.commons.io.IOUtils;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class HiddenClassUnitTest {
+
+    private static final Logger LOG = LoggerFactory.getLogger(HiddenClassUnitTest.class);
+
+    @Test
+    void whenInvokeMethodOnHiddenClass_thenSuccess() {
+        // Initiate lookup class
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+
+        // Create a byte code of a class
+
+        Class<?> clazz = HiddenClass.class;
+        String className = clazz.getName();
+        String classAsPath = className.replace('.', '/') + ".class";
+        InputStream stream = clazz.getClassLoader()
+            .getResourceAsStream(classAsPath);
+
+        try {
+            // Define hidden class with byte code
+            Class<?> hiddenClass = lookup.defineHiddenClass(IOUtils.toByteArray(stream), true, ClassOption.NESTMATE)
+                .lookupClass();
+            Object hiddenClassObject = hiddenClass.getConstructor()
+                .newInstance();
+
+            Method method = hiddenClassObject.getClass()
+                .getDeclaredMethod("convertToUpperCase", String.class);
+
+            Assertions.assertEquals(true, hiddenClass.isHidden());
+
+            Assertions.assertEquals("HELLO", method.invoke(hiddenClassObject, "Hello"));
+
+            Assertions.assertEquals(this.getClass()
+                .getClassLoader(), hiddenClass.getClassLoader());
+
+            Assertions.assertEquals(null, hiddenClass.getCanonicalName());      
+            
+            Assertions.assertThrows(ClassNotFoundException.class, () -> Class.forName(hiddenClass.getName()));
+            
+            Assertions.assertThrows(ClassNotFoundException.class, () -> lookup.findClass(hiddenClass.getName()));
+
+        } catch (Exception e) {
+            LOG.error("Couldn't instantiate hidden class" + e);
+        }
+    }
+
+}