A custom String class creation

Question

I tried to create a custom class String in java.lang package in my eclipse workspace. initially I suspected that a same class in same package can not be created but to my utter surprise I was able to create a class (String) in same package i.e. java.lang

Now I am confused
1) why is it possible and
2) what can be the reason if that is allowed.
3) what will be the use if this type of creation of java classes is allowed in Java.

Answer 1

This is called class shadowing.

1.) It is possible because java classes are not statically linked, but linked at class load time.

2.) If it was not allowed, then the whole class loading would be a lot more difficult to achieve. Then for example you would also have to build your project against a specific Java version. because Java classes may change from version to version. This isn't going to be a maintainable approach.

3.) osgi makes use of this to be able to load different versions of the same bundle. Another common use case is to replace buggy classes in frameworks, where no other workaround is possible. However this technique should be used carefully, because errors might be hard to debug.

Please note that however shadowing classes in the java.* packages is not allowed, as this would break the Java security sandbox. So you are going to have problems at runtime.

Answer 2

You can create a new class in java.lang package. If it was forbidden how Oracle developers would be able to develop Java at all? I am sure they use the same javac as we do.

But you will not be able to load it, because java.lang.ClassLoader (that any classloader extends) does not allow it, every class being loaded goes through this check

...
        if ((name != null) && name.startsWith("java.")) {
            throw new SecurityException
                ("Prohibited package name: " + name.substring(0, name.lastIndexOf('.')));
        }
...

so you will end up in something like

Exception in thread "main" java.lang.SecurityException: Prohibited package name: java.lang
    at java.lang.ClassLoader.preDefineClass(ClassLoader.java:649)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:785)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
    at Test1.main(Test1.java:11)

As for classes that shadow existing classes like your java.lang.String they cannot be loaded because the System ClassLoader (default one) uses "parent first" strategy, so java.lang classes will be loaded from rt.jar with bootstrap classloader. So you will need to replace String.class in rt.jar with your version. Or override it using -Xbootclasspath/p: java option which prepends paths to bootstrap class loader search paths. So you can

1) copypaste real String.java content into your String.java

2) change a method, eg

public static String valueOf(double d) {
    return "Hi";
}

and compile your String.java

3) create a test class

public class Test1 {

    public static void main(String[] args) throws Exception {
        System.out.println(String.valueOf(1.0d));
    }
}

4) run it as

java -Xbootclasspath/p:path_to_your_classes Test1

and you will see

Hi

Answer 3

Yes you can create the package with name java.lang and also Class named as String.

But you wont be able to run your String class.

1) why is it possible : Compiler will compile your class successfully.

2) what can be the reason if that is allowed : You have a valid name for your package and class so compiler doesn't complain.

3) what will be the use if this type of creation of java classes is allowed in Java : But there is not much use of this String class. Bootstrap class loader will load class from the sun's java.lang package. So your custom String class will not get loaded and hence it fails during run.

Bootstrap class loader is part of JVM implementation and it loads the classes of Java API (which includes java.lang.String). Also for each class which gets loaded, JVM keeps track of which class loader whether bootstrap or user-defined - loaded the class. So any attempt to load a custom String class will fail as String class is already loaded.