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Why is my test leaking file descriptors?
Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.11:test (default-test) on project guitest: Failure or timeout
...
Process leaked file descriptors

File scrFile = ((TakesScreenshot) driver).getScreenshotAs(OutputType.FILE);
File destFile = new File(...);
FileUtils.copyFile(scrFile, destFile);

Runtime.getRuntime().addShutdownHook(new Thread(){
    @Override
    public void run(){
        for (WebDriver driver : drivers.values())
            driver.close();

        if (!driversInUse.isEmpty())
            throw new IllegalStateException("There are still drivers in use (" + driversInUse.size() + ")");
    }
});