What's the point of the JNI?

Question

I'm new to Java, and was told to use the Java Native Interface to run some code I wrote in C.

Now, this might be a stupid question, but what's the point of the JNI ? Can't I simply execute my process from a Java UI program and get its stdout to parse ?

Also, I've read that the use of JNI might cause security issues. Do these issues directly depend on the quality of the invoked code ? Or is this something deeper ?

Thanks.

Answer 1

what's the point of the JNI ?

It enables you to mix C and Java code within the same process.

Can't I simply execute my process from a Java UI program and get its stdout to parse ?

A lot of things that can be achieved by using JNI can also be achieved by using inter-process communication (IPC). However, you'd have to ship all the input data to the other process, and then ship all the results back. This can be pretty expensive, which makes IPC impractical for many situations where JNI can be used (e.g. wrapping existing C libraries).

Also, I've read that the use of JNI might cause security issues. Do these issues directly depend on the quality of the invoked code ? Or is this something deeper ?

The point here is that the JVM does a lot of work to ensure that whatever Java code is thrown at it, things like buffer overruns, stack smashing attacks etc can't occur. For example, it performs bounds checking on all array accesses (which C doesn't).

On the other hand, JNI code is a black box to the JVM. If there's a problem with the C code (e.g. a buffer overrun), all bets are off.

Answer 2

Can't I simply execute my process from a Java UI program and get its stdout to parse ?

That would depend on what you are calling.

Note that you cannot just call programs via JNI, but library code.

In addition to that, spawning new processes is relatively expensive and managing multiple processes is complicated.

Answer 3

Can't I simply execute my process from a Java UI program and get its stdout to parse ?

Do you think it's always appropriate to start a new process every time you want to execute any native code? Do you really want to be transferring potentially large amounts of data between processes? (Imagine a native image transformation.)

Also, I've read that the use of JNI might cause security issues. Do these issues directly depend on the quality of the invoked code?

Yes. Basically native code has less security sandboxing than Java running in a JVM. If the code has security bugs (e.g. buffer overflows) then clearly that will affect the security of your overall app.

I should say that it's relatively rare for Java developers to need to worry about JNI - I've certainly only touched it a couple of times in my career. You may also want to look at SWIG if the need arises.