在Java中是否可以使用自定义类隐藏java.lang中的类？

Question

Is it possible in Java to hide classes from the platform with custom classes? For instance to create a custom java.lang.String or custom java.lang.StringBuffer? I am asking because I recently discovered there is an option in C# /nostdlib that allows you to redefine the entire System namespace and I was wondering if Java had something similar.

Note, C#'s /nostdlib is a compiler option, not something that gets taken into account at runtime.

Answer 1

You can modify the standard library or substitute your own for personal use, but Oracle doesn't support this and doesn't allow you to distribute the modified version to others. There are various commandline and configuration option to tell the JVM where to look for classes. Replacing bootstrap classes is harder but it is possible.

Note that creating your own version entirely from scratch will be difficult because several classes ( Object and Class at the very least) are necessarily tied very strongly to the JVM implementation. You'll probably want to keep all the classes that use native code the same for simplicity.

Note that this is different from class Shadowing, which is a purely compile time concept that occurs when multiple identifiers conflict, usually due to imports ( java.lang.* is implicitly imported and there's nothing you can do about that). These conflicts are generally resolved by either silently using one of the possibilities or raising a compile error depending on the situation (see the JLS for details).

Answer 2

In order to replace a java.lang class, or some other "system" class you must modify the boot classpath. If you run Java from a command line this is easily accomplished, but if you run it in some sort of "environment" you may not be allowed to make that change.

Additionally, many java.lang et al classes have native method implementations, and you must either supply your own replacement or somehow arrange to reuse the existing implementation.

Answer 3

You can use your own classes for the library classes. When starting you can define a -bootclasspath to define your own class library.

This will not an easy task because some classes are tighgtly bound to the VM running, eg the class String . You have to look at the JLS for such cases.

Answer 4

Sun/Oracle does not allow that by the license of the JSE/JDK. However, I recently learned, that Groovy can do amazing things, like adding methods or overwriting methods of the java.lang.* package.

Groovy is based on Java/JVM, so you can easily combine them within one application.