Geertjan's Blog

  • October 20, 2013

ct.sym steals the ASM class

Geertjan Wielenga
Product Manager

Some mild consternation on the Twittersphere yesterday. Marcus Lagergren not being able to find the ASM classes in JDK 8 in NetBeans IDE:

And there's no such problem in Eclipse (and apparently in IntelliJ IDEA). Help, does NetBeans (despite being incredibly awesome) suck, after all?

The truth of the matter is that there's something called "ct.sym" in the JDK. When javac is compiling code, it doesn't link against rt.jar. Instead, it uses a special symbol file lib/ct.sym with class stubs. Internal JDK classes are not put in that symbol file, since those are internal classes. You shouldn't want to use them, at all.

However, what if you're Marcus Lagergren who DOES need these classes? I.e., he's working on the internal JDK classes and hence needs to have access to them. Fair enough that the general Java population can't access those classes, since they're internal implementation classes that could be changed anytime and one wouldn't want all unknown clients of those classes to start breaking once changes are made to the implementation, i.e., this is the rt.jar's internal class protection mechanism.

But, again, we're now Marcus Lagergen and not the general Java population. For the solution, read Jan Lahoda, NetBeans Java Editor guru, here:


In particular, take note of this:

AFAIK, the ct.sym is new in JDK6. It contains stubs for all classes that existed in JDK5 (for compatibility with existing programs that would use private JDK classes), but does not contain implementation classes that were introduced in JDK6 (only API classes). This is to prevent application developers to accidentally use JDK's private classes (as such applications would be unportable and may not run on future versions of JDK). Note that this is not really a NB thing - this is the behavior of javac from the JDK. I do not know about any way to disable this except deleting ct.sym or the option mentioned above.

Regarding loading the classes: JVM uses two classpath's: classpath and bootclasspath. rt.jar is on the bootclasspath and has precedence over anything on the "custom" classpath, which is used by the application. The usual way to override classes on bootclasspath is to start the JVM with "-Xbootclasspath/p:" option, which prepends the given jars (and presumably also directories) to bootclasspath.

Hence, let's take the first option, the simpler one, and simply delete the "ct.sym" file. Again, only because we need to work with those internal classes as developers of the JDK, not because we want to hack our way around "ct.sym", which would mean you'd not have portable code at the end of the day. Go to the JDK 8 lib folder and you'll find the file:

Delete it. Start NetBeans IDE again, either on JDK 7 or JDK 8, doesn't make a difference for these purposes, create a new Java application (or use an existing one), make sure you have set the JDK above as the JDK of the application, and hey presto:

The above obviously assumes you have a build of JDK 8 that actually includes the ASM package.

And below you can see that not only are the classes found but my build succeeded, even though I'm using internal JDK classes. The yellow markings in the sidebar mean that the classes are imported but not used in the code, where normally, if I hadn't removed "ct.sym", I would have seen red error marking instead, and the code wouldn't have compiled.

Note: I've tried setting "-XDignore.symbol.file" in "netbeans.conf" and in other places, but so far haven't got that to work. Simply deleting the "ct.sym" file (or back it up somewhere and put it back when needed) is quite clearly the most straightforward solution.

Ultimately, if you want to be able to use those internal classes while still having portable code, do you know what you need to do? You need to create a JDK bug report stating that you need an internal class to be added to "ct.sym". Probably you'll get a motivation back stating WHY that internal class isn't supposed to be used externally. There must be a reason why those classes aren't available for external usage, otherwise they would have been added to "ct.sym".

So, now the only remaining question is why the Eclipse compiler doesn't hide the internal JDK classes. Apparently the Eclipse compiler ignores the "ct.sym" file. In other words, at the end of the day, far from being a bug in NetBeans... we have now found a (pretty enormous, I reckon) bug in Eclipse. The Eclipse compiler does not protect you from using internal JDK classes and the code that you create in Eclipse may not work with future releases of the JDK, since the JDK team is simply going to be changing those classes that are not found in the "ct.sym" file while assuming (correctly, thanks to the presence of "ct.sym" mechanism) that no code in the world, other than JDK code, is tied to those classes.

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