Environment variables truth source of the JVM (and how to mutate it)
pierre at 2bst.fr
Wed May 10 11:48:05 UTC 2017
Hi, I've been trying to understand how the JVM accesses environment variables and how they can be mutated.
For this I've made some assumptions and I would like to know if they're correct, could you help me on this? :-)
* It appears that the JVM gets a copy of its process environment variables and store them in static final fields theUnmodifiableEnvironment and theEnvironment of class java.lang.ProcessEnvironment.
* My assumption is: these fields are the "truth source" about environment variables inside the JVM and any attempt to access some of them will end up in a lookup of this fields. * I have a question about this: why two final fields instead of only one? Perhaps theUnmodifiableEnvironment stands for base JVM env whilst theEnvironment is for env of current process (which could be changed with Process.exec(String cmdarray, String envp, File dir))? * There is a subtle way to mutate them in Sun JDK (see http://www.javaspecialists.eu/archive/Issue161.html).
* My assumption is: These fields are passed to all new JVM threads, so mutating them (as ugly as it can sound) will be JVM-wide and will result in all thread getting mutated env as their environment variables. * Sensitive question: is this enforced? System.getenv() appears to correctly returns mutated env, can I deduce all new threads in the JVM will get mutated values? * Another sensitive question: as these fields are static final, can I deduce all threads in the JVM will get mutated values, not only new ones?It would be my pleasure to provide further details ifneedsbe. Just let me know if some of the above assumptions are incorrect!
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