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Java Forum / Virtual Machine / March 2007modifying java.lang.String.java
Hi, I'm trying to modify java.lang.String.java and add the modified String.class to rt.jar [THIS IS FOR MYSELF ONLY AND WILL NOT BE DEPLOYED]. I cannot add "private final boolean tainted[] = new boolean[5];" to String.java. If I do, it still compiles and I can add it to rt.jar and compile a test program against it. However, the JVM crashes with a strange message: java.lang.IllegalArgumentException: name can't be empty at java.security.BasicPermission.init(Unknown Source) at java.security.BasicPermission.<init>(Unknown Source) at java.lang.RuntimePermission.<init>(Unknown Source) at java.lang.Thread.<clinit>(Unknown Source) I tried various ways of rewriting the above such as adding "private final boolean tainted[];" and initializing it in each constructor. Nothing works. I can add methods to String.java and even the member variable "private boolean isTainted;" and it works just fine. Do you have any ideas for why adding a boolean array does not work (or better yet how to get around this issue)? Thank you, Daniel > I'm trying to modify java.lang.String.java and add the modified >String.class to >rt.jar [THIS IS FOR MYSELF ONLY AND WILL NOT BE DEPLOYED]. Try running the VM with -Xshare:off If this works re-create the shared archives (JAVA_HOME/bin/(client|server)/classes.jsa). HTH cu > danwi...@gmail.com wrote: > > I'm trying to modify java.lang.String.java and add the modified > >String.class to > >rt.jar [THIS IS FOR MYSELF ONLY AND WILL NOT BE DEPLOYED]. > > Try running the VM with > -Xshare:off > [quoted text clipped - 4 lines] > > cu That still does not work. I get the same error. >>danwi...@gmail.com wrote: >>> I'm trying to modify java.lang.String.java and add the modified [quoted text clipped - 13 lines] > >That still does not work. I get the same error. Well, the offending code seems to be this line in java.lang.Thread: private static final RuntimePermission SUBCLASS_IMPLEMENTATION_PERMISSION = new RuntimePermission("enableContextClassLoaderOverride"); where "new RuntimePermission(...)" results in your observed error if the supplied string returns 0 for length(). So I would assume, that after you modified java.lang.String the magic trickery within the VM that creates String objects from String literals in class files is broken with regard to your modified String class. I'd try to look there for the problem. Sorry, that I cannot be of more help. cu Thanks. I suspect you are right. I just wish that I knew exactly what the problem is. Unfortunatley since the error happens as the JVM is loading, I cannot attach a debugger. It is strange that adding "private final boolean tainted[] = new boolean[5];" causes a crash bud adding: "private final boolean tainted=false;" works fine. If anyone knows more about the internals of how the JVM makes strings out of string literals I would appreciate a lesson. > danwi...@gmail.com wrote: > >>danwi...@gmail.com wrote: [quoted text clipped - 35 lines] > > - Show quoted text - I think the error is caused by the intern() function in String.java. This is implemented in native code. I bet it somehow messes up. On Mar 25, 9:34 pm, "danwi...@gmail.com" <danwi...@gmail.com> wrote: > Thanks. I suspect you are right. I just wish that I knew exactly what > the problem is. Unfortunatley since the error happens as the JVM is [quoted text clipped - 51 lines] > > - Show quoted text - > I think the error is caused by the intern() function in String.java. > This is implemented in native code. I bet it somehow messes up. Have you considered AspectJ or some other kind of bytecode instrumentation? I cannot predict whether it will work, but it might be worth a try, and your project would become much more publishable. But apart from technical realizability, I think Tom is right; your approach should not be dependent on the ability to modify the String class. Regards, Moritz On Mar 26, 7:43 am, Moritz Hammer <newsgroup...@hammer-tour.com> wrote: > > I think the error is caused by the intern() function in String.java. > > This is implemented in native code. I bet it somehow messes up. [quoted text clipped - 8 lines] > Regards, > Moritz I tried instrumentation using JavaAssist and got the same problem. The reasons for why I want to modify the string class are rather complex so I'd rather not get into them. > I'm trying to modify java.lang.String.java and add the modified > String.class to > rt.jar [THIS IS FOR MYSELF ONLY AND WILL NOT BE DEPLOYED]. > I cannot add "private final boolean tainted[] = new Wouldn't it be easier to use WeakHashMap<String,boolean[]> (or something similar with the map value as a meaningful class type)? > boolean[5];" to String.java. If I do, it still compiles and I can add > it to rt.jar and compile a test program against it. However, the JVM [quoted text clipped - 5 lines] > at java.lang.RuntimePermission.<init>(Unknown Source) > at java.lang.Thread.<clinit>(Unknown Source) So if you look at the source, it appears that the literal string "enableContextClassLoaderOverride" has a length of zero. As someone else mentioned it will be in the native code loading strings from class files. The exact details I don't know. Moving your field after the existing fields might help. It might be that reference fields are treated differently to primitive fields for object layout. I suggest your approach is probably poor. Tom Hawtin Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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