Demo: Loading classes in memory

中文版

jdk8

use ClassLoader#defineClass

1. define a class loader, extends URLClassLoader
2. define a new public method, call defineClass indirectly

    public Class<?> loadClassFromBytes(String className, byte[] bytes) {
        return this.defineClass(className, bytes, 0, bytes.length);
    }

use Unsafe#defineAnonymousClass

need to use sun.misc.Unsafe, which is not recommended

    public Class<?> loadClass(String className, byte[] classBytes) {
        return UNSAFE.defineAnonymousClass(SimTarget.class, classBytes, EMPTY_OBJECTS);
    }

jdk11

use ClassLoader#defineClass

same as jdk8

use Unsafe#defineAnonymousClass

need to use jvm argument: --add-exports java.base/jdk.internal.misc=<module>

    public Class<?> loadClass(String className, byte[] classBytes) {
        return UNSAFE.defineAnonymousClass(AnonClassHolder.class, classBytes, EMPTY_OBJECTS);
    }

jdk17

use ClassLoader#defineClass

same as jdk8

use MethodLookup#defineHiddenClass

need to let the hidden class and the holder class in the same package!

    public static Class<?> loadClass(String className, byte[] classBytes) {
        if (!className.startsWith(PACKAGE_NAME)) {
            throw new RuntimeException("not same package, should failed");
        }
        try {
            MethodHandles.Lookup lookup = MethodHandles.lookup();
            return lookup.defineHiddenClass(classBytes, true,
                                            MethodHandles.Lookup.ClassOption.NESTMATE,
                                            MethodHandles.Lookup.ClassOption.STRONG)
                         .lookupClass();
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

HowTo

1. (optional) cd demo-inmem-base

2. (optional) compile

   mvn package -Dmaven.test.skip=true
   # or
   mvnd package -Dmaven.test.skip=true

3. (optional) call class2bytes.sh, generate the Java file, which is hold bytes.

4. test in demo-inmem-jdk8/demo-inmem-jdk11/demo-inmem-jdk17