Capture native STDOUT writes in Java code

Question

I'm writing a JUnit test case which invokes code that ultimately results in a native function call being invoked. Here's pseudocode of the test I want to write:

@Test
public void testNoWritesToStdOut() {
    x = new StdOutTrap();
    x.startTrappingStdOut();
    try {
        callTheFunctionUnderTest(); // Nothing should be written to stdout
        assertEquals("", x.capturedOutput());
    } finally {
        x.stopTrappingStdOut();
    }
}

Noting that callTheFuntionUnderTest() invokes native code which might be C code, as in:

...
printf("fail\n");

Or C++ code, as in:

...
std::cout << "fail" << std::endl;

For clarity, I don't want to redirect calls to System.out.X to a custom PrintStream via System.setOut. I want to get a copy of the bytes, if any, which were written to the stdout standard stream during the execution of a native call over which I have no control.

Ideas?

Answer 1

If you want to capture output from native code, you cannot do this in the JVM alone but need the operating system to help you.

You must invoke your test in a separate JVM and you must ask the operating system to capture the output for you. The usual way of running a process in a shell might be what you need.