Re: Why is String immutable?

"Chris Uppal" <>
Wed, 13 Sep 2006 12:28:13 +0100
"Chris Smith" <> wrote in message

The getfield opcode is checked by the bytecode verifier when the class
is loaded. If it tries to access a field to which it doesn't have
access permission, the verifier will throw a java.lang.VerifyError, and
the class will fail to load.

The picture is more complicated than that. For a start, I don't think that
it's the verifier's job to enforce access checks -- they shouldn't fail until
an illegal access is actually executed.

Secondly, the rules for when the JVM will actually enforce access seem to be
somewhat obscure. I know that it will /sometimes/ do checks, but not always.
And what "sometimes" means has changed over the years.

For instance. With JDK 1.5.0_06, compile these two files

========== =================
public class A
    // NB: deliberately public, despite the name
    public String privateField = "Ooops!";
=========== ================
public class B
    public static void
    main(String[] args)
        A a = new A();
        System.out.println("OK, here we go...");

Then change the declaration of A.privateField to private, and recompile only (All this messing around is just a way to generate bytecode containing
an "illegal" read -- people with convenient access to bytecode generation will
have more straightforward ways of doing it).

Then execute:

    java -cp . B

and it prints:

    OK, here we go...

OTOH, running java with the "future" argument:

    java -Xfuture -cp . B

and it prints:

    OK, here we go...
    Exception in thread "main" java.lang.IllegalAccessError: tried to access
    field A.privateField from class B
        at B.main(

On the third hand, even without the -Xfuture argument, the current JVM will
(iirc) stop you accessing the internal fields of a String object. I'm not sure
on what basis it determines that some accesses should be checked and others

As far as I know, /all/ such access should result in the JVM throwing runtime
exceptions, but Sun's JVM's seem to interpret that aspect of the spec a little

(And anyway, there's always JNI ;-)

    -- chris

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