Test System.out message printed in catch block

Question

I would like to create a test case to test this catch block

I want to test this message "File does't exit, please put the file inside resources folder." and FileNotFoundException exception using testNG.

try { //something here 

} catch (FileNotFoundException ex) { 
    System.out.println("File does't exit, please put the file inside resources folder.");
    ex.printStackTrace(); 
}

You may find some good ideas on [this question](https://stackoverflow.com/q/3677271/1081110), especially harunurhan's answer. — Dawood ibn Kareem, Sep 20 '22 at 00:27
thank you Dawood, how about the body of the test case I already did the same but no good results — Jack, Sep 20 '22 at 00:39
All you need to do in the body of your test case is run the method that throws the exception. If you've set up the `@Test` annotation properly, with the right `expectedExceptions` and `expectedExceptionsMessageRegExp` values, you don't need any other assertions. — Dawood ibn Kareem, Sep 20 '22 at 00:42
I did the same but still fail @Test(expectedExceptions = FileNotFoundException.class) public void testCatchBlockInTask() { //method that throws the exception } — Jack, Sep 20 '22 at 00:50
Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Sep 20 '22 at 02:32
Use `ex.getMessage()` and then use `testNG` `assertion` to verify. — Nandan A, Sep 20 '22 at 05:43
@OneCricketeer I think the question is not clear. He wants to do something like this `Assert.assertEquals(e.getMessage(), "File doesn't..");` — Nandan A, Sep 20 '22 at 05:52
@DawoodibnKareem Those suggestions will only work if the method under test actually throws, no? — OneCricketeer, Sep 20 '22 at 06:01

AbiSaran · Answer 1 · 2022-09-20T07:38:33.350

-2

Try this code, you will get the FileNotFoundException, the file 'test.txt' should not be available in the path:

    @Test
    public void fileSetup() {    
        try {
            File file = new File("C:\\Users\\test\\Desktop\\test.txt");
            Scanner sc = new Scanner(file);

            System.out.println(sc.next());
            sc.close();
        } catch (FileNotFoundException e) {
            System.out.println("In catch block");
            e.printStackTrace();
        }
    }

edited Sep 20 '22 at 07:38

answered Sep 20 '22 at 02:17

AbiSaran

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What does this have to do with TestNG? – OneCricketeer Sep 20 '22 at 05:48
@OneCricketeer, the question is, he wants to test the catch block with the exception - 'FileNotFoundException'. What is it for you? why are you intefering with other's answers – AbiSaran Sep 20 '22 at 06:54
The post literally says **using TestNG**. Plus, you didn't answer how to test (i.e. assert) the printed output using an assertion method. – OneCricketeer Sep 20 '22 at 16:30
@OneCricketeer, that can be answered by the questioner, you dont need to do that. – AbiSaran Sep 20 '22 at 17:01

OneCricketeer · Answer 2 · 2022-09-20T16:44:30.247

Ideally, you don't use System output. You'd rather inject a logger and test against that.

But, it can still be done

// Store the original standard out before changing it.
private final PrintStream originalStdOut = System.out;
private ByteArrayOutputStream consoleContent = new ByteArrayOutputStream();
 
@BeforeMethod
public void beforeTest() {
    // Redirect all System.out to consoleContent.
    System.setOut(new PrintStream(this.consoleContent));
}

You may need to do something similar for System.err for ex.printStackTrace() output...

Then run your function in a standard test, and assert against the consoleContent contents.

@Test
public void testFileExistence() {    
    doSomething("missing-file.txt");  // call a method of your service code that prints an exception
    
    int n = this.consoleContent.available();
    byte[] bytes = new byte[n];
    this.consoleContent.read(bytes, 0, n);
    String s = new String(bytes, StandardCharsets.UTF_8);
    // TODO: split on line-breaks, and test printed content
}

After each test, you'll need to reset the stream, and if you still want to print the data, then do that too

@AfterMethod
public void afterTest() {
    // Put back the standard out.
    System.setOut(this.originalStdOut);
 
    // Print what has been captured.
    System.out.println(this.consoleContent.toString());
    System.out.println(String.format("              ====>Captured Console length=%d",
            this.consoleContent.toString().length()));
 
    // Clear the consoleContent.
    this.consoleContent = new ByteArrayOutputStream();
}

Or, more ideally, you define your own RuntimeException / Exception subclasses, then use assertThrows methods, where you can properly verify exception object content, including the message body.

Yes, it is. This is using TestNG framework, as compared to your answer, which does not — OneCricketeer, Sep 20 '22 at 16:31

Test System.out message printed in catch block

2 Answers2