Welcome to this article about Hidden Class in Java! If you’re a Java developer, you might have heard of Hidden Classes but aren’t entirely sure what they are or how to use them. In this article, we’ll dive into the concept of Hidden Classes, their creation and usage, and how they differ from Anonymous Classes.
1. Introduction
In Java, classes are fundamental building blocks that encapsulate data and behavior. They define the blueprint for objects, allowing you to create instances and manipulate their state. In general, classes in Java are either public or package-private, which means they’re visible to other classes in the same package.
However, there are cases where you might want to create a class that’s hidden from other classes. This is where Hidden Classes come into play. Hidden Classes, also known as “non-discoverable” classes, are classes that are not visible to the class loader and cannot be accessed directly by the application. Instead, they’re only accessible through reflection.
Hidden Classes were introduced in Java 9 as a part of the Java Platform Module System (JPMS). They provide a way to create classes that are used internally by the JVM and aren’t part of the application’s public API.
2. Creating Hidden Classes with Java Code
Creating a Hidden Class in Java involves three steps:
- Define a class using the
Lookupclass. TheLookupclass is a factory class that allows you to create hidden classes. - Define the class’s bytecode. You can either generate the bytecode yourself or use a bytecode generation library like ASM.
- Use the
defineHiddenClassmethod of theLookupclass to create the hidden class.
Here’s an example of creating a Hidden Class using the Lookup class:
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodHandles.Lookup;
class HiddenClassDemo {
public static void main(String[] args) throws Throwable {
Lookup lookup = MethodHandles.lookup();
byte[] bytecode = generateBytecode(); // generate bytecode
Class<?> hiddenClass = lookup.defineHiddenClass(bytecode, true).lookupClass();
System.out.println(hiddenClass.getName());
}
private static byte[] generateBytecode() {
// generate bytecode
}
}
In this example, we’re using the Lookup class to define a hidden class. We first generate the class’s bytecode using a hypothetical generateBytecode() method. Then, we use the defineHiddenClass() method of the Lookup class to create the hidden class. Finally, we print the name of the hidden class using the getName() method.
3. Using Hidden Classes
Once you’ve created a Hidden Class, you can use it like any other class through reflection. For example, you can create an instance of the hidden class using the newInstance() method:
Class<?> hiddenClass = ... // create hidden class Object instance = hiddenClass.newInstance();
You can also access the class’s methods and fields using reflection:
Class<?> hiddenClass = ... // create hidden class
Method method = hiddenClass.getDeclaredMethod("myMethod");
method.setAccessible(true);
method.invoke(instance);
4. Differences between Hidden and Anonymous Classes
Hidden classes are similar to anonymous classes, in that they allow you to create classes on-the-fly. However, there are some key differences between the two.
One difference is that hidden classes are not visible at compile time, whereas anonymous classes are. This means that you can use hidden classes to create classes that are truly dynamic, and that can’t be predicted at compile time.
Another difference is that anonymous classes are always inner classes, whereas hidden classes can be top-level classes or inner classes. This gives you more flexibility in how you structure your code.
Finally, hidden classes are created using the MethodHandles.Lookup API, whereas anonymous classes are created using the new keyword. This means that hidden classes are more powerful and flexible than anonymous classes, but they’re also more complex to use.
5. Conclusion
In summary, hidden classes are a powerful new feature in Java that allows you to create classes dynamically at runtime. They’re not visible at compile time, but can be used just like any other class at runtime. While they’re similar to anonymous classes, they offer more flexibility and power, at the cost of increased complexity. If you need to create truly dynamic classes in your Java code, hidden classes are definitely worth exploring!
