http://www.javassist.org/: Javassist (JAVA programming ASSISTant) makes Java bytecode manipulation simple. It is a class library for editing bytecodes in Java. (Shigeru Chiba, www.javassist.org) 
/*
 * Javassist, a Java-bytecode translator toolkit.
 * Copyright (C) 1999- Shigeru Chiba. All Rights Reserved.
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License.  Alternatively, the contents of this file may be used under
 * the terms of the GNU Lesser General Public License Version 2.1 or later,
 * or the Apache License Version 2.0.
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 */

package javassist.util.proxy;

import java.lang.invoke.MethodHandle;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;

import javassist.CannotCompileException;
import javassist.CtClass;
import javassist.bytecode.ClassFile;


Since:3.22
/**
 * Helper class for invoking {@link ClassLoader#defineClass(String,byte[],int,int)}.
 *
 * @since 3.22
 */
public class DefinePackageHelper
{
    private static abstract class Helper {
        abstract Package definePackage(ClassLoader loader, String name, String specTitle,
                String specVersion, String specVendor, String implTitle, String implVersion,
                String implVendor, URL sealBase)
            throws IllegalArgumentException;
    }

    private static class Java9 extends Helper {
        // definePackage has been discontinued for JAVA 9
        @Override
        Package definePackage(ClassLoader loader, String name, String specTitle,
                    String specVersion, String specVendor, String implTitle, String implVersion,
                    String implVendor, URL sealBase)
            throws IllegalArgumentException
        {
            throw new RuntimeException("define package has been disabled for jigsaw");
        }
    };

    private static class Java7 extends Helper {
        private final SecurityActions stack = SecurityActions.stack;
        private final MethodHandle definePackage = getDefinePackageMethodHandle();

        private MethodHandle getDefinePackageMethodHandle() {
            if (stack.getCallerClass() != this.getClass())
                throw new IllegalAccessError("Access denied for caller.");
            try {
                return SecurityActions.getMethodHandle(ClassLoader.class, 
                            "definePackage", new Class[] {
                                String.class, String.class, String.class, String.class,
                                String.class, String.class, String.class, URL.class 
                            });
            } catch (NoSuchMethodException e) {
                throw new RuntimeException("cannot initialize", e);
            }
        }
            
        @Override
        Package definePackage(ClassLoader loader, String name, String specTitle,
                    String specVersion, String specVendor, String implTitle, String implVersion,
                    String implVendor, URL sealBase)
            throws IllegalArgumentException
        {
            if (stack.getCallerClass() != DefinePackageHelper.class)
                throw new IllegalAccessError("Access denied for caller.");
            try {
                return (Package) definePackage.invokeWithArguments(loader, name, specTitle,
                        specVersion, specVendor, implTitle, implVersion, implVendor, sealBase);
            } catch (Throwable e) {
                if (e instanceof IllegalArgumentException) throw (IllegalArgumentException) e;
                if (e instanceof RuntimeException) throw (RuntimeException) e;
            }
            return null;
        }
    }

    private static class JavaOther extends Helper {
        private final SecurityActions stack = SecurityActions.stack;
        private final Method definePackage = getDefinePackageMethod();

        private Method getDefinePackageMethod() {
            if (stack.getCallerClass() != this.getClass())
                throw new IllegalAccessError("Access denied for caller.");
            try {
                return SecurityActions.getDeclaredMethod(ClassLoader.class, 
                            "definePackage", new Class[] {
                                String.class, String.class, String.class, String.class,
                                String.class, String.class, String.class, URL.class 
                            });
            } catch (NoSuchMethodException e) {
                throw new RuntimeException("cannot initialize", e);
            }
        }
            
        @Override
        Package definePackage(ClassLoader loader, String name, String specTitle,
                    String specVersion, String specVendor, String implTitle, String implVersion,
                    String implVendor, URL sealBase)
            throws IllegalArgumentException
        {
            if (stack.getCallerClass() != DefinePackageHelper.class)
                throw new IllegalAccessError("Access denied for caller.");
            try {
                definePackage.setAccessible(true);
                return (Package) definePackage.invoke(loader, new Object[] {
                        name, specTitle, specVersion, specVendor, implTitle,
                        implVersion, implVendor, sealBase                                
                    });
            } catch (Throwable e) {
                if (e instanceof InvocationTargetException) {
                    Throwable t = ((InvocationTargetException) e).getTargetException();
                    if (t instanceof IllegalArgumentException)
                        throw (IllegalArgumentException) t;
                }
                if (e instanceof RuntimeException) throw (RuntimeException) e;
            }
            return null;
        }
    };

    private static final Helper privileged
        = ClassFile.MAJOR_VERSION >= ClassFile.JAVA_9
          ? new Java9() : ClassFile.MAJOR_VERSION >= ClassFile.JAVA_7
                          ? new Java7() : new JavaOther();

    
Defines a new package.  If the package is already defined, this method
performs nothing.

You do not necessarily need to
call this method.  If this method is called, then  
getPackage() on the Class object returned 
by toClass() will return a non-null object.


The jigsaw module introduced by Java 9 has broken this method.
In Java 9 or later, the VM argument
--add-opens java.base/java.lang=ALL-UNNAMED
has to be given to the JVM so that this method can run.



Params:
  • loader –        the class loader passed to toClass() or
                     the default one obtained by getClassLoader().
  • className –     the package name.
See Also:
/**
     * Defines a new package.  If the package is already defined, this method
     * performs nothing.
     *
     * <p>You do not necessarily need to
     * call this method.  If this method is called, then  
     * <code>getPackage()</code> on the <code>Class</code> object returned 
     * by <code>toClass()</code> will return a non-null object.</p>
     *
     * <p>The jigsaw module introduced by Java 9 has broken this method.
     * In Java 9 or later, the VM argument
     * <code>--add-opens java.base/java.lang=ALL-UNNAMED</code>
     * has to be given to the JVM so that this method can run.
     * </p>
     *
     * @param loader        the class loader passed to <code>toClass()</code> or
     *                      the default one obtained by <code>getClassLoader()</code>.
     * @param className     the package name.
     * @see Class#getClassLoader()
     * @see CtClass#toClass()
     */
    public static void definePackage(String className, ClassLoader loader)
        throws CannotCompileException
    {
        try {
            privileged.definePackage(loader, className, 
                    null, null, null, null, null, null, null);
        }
        catch (IllegalArgumentException e) {
            // if the package is already defined, an IllegalArgumentException
            // is thrown.
            return;
        }
        catch (Exception e) {
            throw new CannotCompileException(e);
        }
    }
    
    private DefinePackageHelper() {}
}