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Would it be possible to implement a custom Thread class in Java (using JNI) in a safe / correct way?
Suppose I write my own NewThread class, with a native start() method, which forks the execution, calls run() in the forked thread and returns…
Is that possible? Would the JVM complain? Is it “legal” according to the specs? Would this break anything, like, in the memory-model? Does it depend on the particular JVM?
Your questions is answered in the Java Native Interface
Programmer’s Guide and Specification, section 8.1.5.
The important issue is that the VM has to use the same thread model as you are in your native code. Some of the first Java VMs used so called “green threads” on some operating systems (Linux) to emulate thread context switching, since the operating system itself didn’t offer native threading support. These “green threads” would not be able to interact with native threads, if you would use one of these old VMs on a newer operating system version with native thread support.
Since Sun’s JRE 1.3, I think all “normal” VMs are using native threads directly, which means that you can use native threads yourself in JNI code and expect everything to work as you expect.