Mastering Java Hidden Classes: A Step-by-Step Guide with Code

What are Hidden Classes

Hidden classes are classes that cannot be accessed directly by other classes; they are intended for frameworks to generate and use at runtime via reflection.

Hidden Class Example

Step 1: Create a normal Java class.

public class JEP371HiddenClasses {
    public static String hello() {
        return "https://www.didispace.com";
    }
}

Step 2: Compile the class, read the .class file and encode it with Base64.

String filePath = "JEP371HiddenClasses.class";
byte[] b = Files.readAllBytes(Paths.get(filePath));
log.info(Base64.getEncoder().encodeToString(b));

The resulting Base64 string represents the compiled class.

Step 3: Load the hidden class via MethodHandles.defineHiddenClass and invoke its hello method.

@Test
void testHiddenClasses() throws Throwable {
    // 1. Load the encoded hidden class
    String CLASS_INFO = "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";
    byte[] classInBytes = getDecoder().decode(CLASS_INFO);
    Class<?> proxy = MethodHandles.lookup()
        .defineHiddenClass(classInBytes, true, MethodHandles.Lookup.ClassOption.NESTMATE)
        .lookupClass();

    // Output class name and methods
    log.info(proxy.getName());
    for (Method method : proxy.getDeclaredMethods()) {
        log.info(method.getName());
    }

    // 2. Invoke hello()
    MethodHandle mh = MethodHandles.lookup().findStatic(proxy, "hello",
        MethodType.methodType(String.class));
    String result = (String) mh.invokeExact();
    log.info(result);
}

Running the test prints the hidden class name, its method name, and the string returned by hello():

17:20:50.360 [main] INFO com.didispace.debug.java15.JEP371Test - com.didispace.debug.java15.JEP371HiddenClasses/0x0000000800cb0c00
17:20:50.361 [main] INFO com.didispace.debug.java15.JEP371Test - hello
17:20:50.361 [main] INFO com.didispace.debug.java15.JEP371Test - https://www.didispace.com

This demonstrates how hidden classes provide an additional dynamic capability for framework developers.