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Java Forum / General / March 2007Maximum Length for a Java Class Name ?
Can anyone tell me the maximum length of a Java class name? Thanks, MattC > Can anyone tell me the maximum length of a Java class name? Have you tried the JLS (Java Language Specification), Google, and Google groups yet? > Can anyone tell me the maximum length of a Java class name? Public classes needs to be in a file with the same name as the class, so the file system will probably limit it to 255 or something like that. BTW, you other programmers on the team will murder you if you use classnames longer than about 20 anyway ... Arne > Can anyone tell me the maximum length of a Java class name? For current JVMs it is a maximum of 65536 characters for fully qualified class name. However, not all such long names are allowed. The limit is 65536 bytes for fully qualified class name encoded using modified UTF-8 as described in "The class File Format" spec. It comes from 'u2' capacity (16 bits) -- the type of 'length' from constant pool 'CONSTANT_Utf8_info' structure used to store class names in class files. piotr > For current JVMs it is a maximum of 65536 characters for fully qualified > class name. Occurs to me that there's a nice little obfuscation technique there -- rename all classes to so that their names start with, say, about 10 thousand spaces, and there aren't too mamy bytecode tools that will be usuable with the resulting classfiles ;-) -- chris > Occurs to me that there's a nice little obfuscation technique there -- rename > all classes to so that their names start with, say, about 10 thousand spaces, > and there aren't too mamy bytecode tools that will be usuable with the > resulting classfiles ;-) Well, I think the bytecode verifier will prevent against spaces in names, but with other char, say 'x', or simply with randomly generated long name, why not?... ;-) Hopeless, it is very easy to rename classes/methods/fields in class-files, so in a case, when you really need good obfuscation, let more sophisticated tools do theirs job... Well, I can't recommend any tool of such type, but I've heard they exist... :) (In fact, I don't recall any need for such a tool from my past... Of course, I see reasons for it, but I know also, what such a tool can, and what it can't do... So I probably will never "destroy" my classes with it, there are lot of nicer things to do with Java classes than obfuscating... :-) ). piotr Me: > Well, I think the bytecode verifier will prevent against spaces in > names, but with other char, say 'x', or simply with randomly generated > long name, why not?... ;-) Checked that, and it appears to me now, that, as Chris suggested, spaces are also allowed by recent class verifiers (even with -Xfuture option classes with only spaces in names are accepted). I tried to find any formal specification on verification which states that, but I haven't found anything stating it directly, neither in JLS, nor in JVMS, except a single non-normative fragment from the latter concerning verifications performed by the verifier: "• Checking that all field references and method references in the constant pool have valid names, valid classes, and a valid type descriptor." It is not about class names, it's on method and field references names only, and even for them, it is not clear to me what a "valid names" means here?... So, it's better to assume, that on JVM level, any class name (excluding maybe empty, and incorrectly encoded one) is allowed. P.S. There is also a mistake in my advice to the OP, the maximum class name length is 65535 (not 65536) characters, sorry for that. piotr > Can anyone tell me the maximum length of a Java class name? If you have to ask, your class name is probably too long. :) 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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