在 JUnit 捕获 IOException

Question

I would like to ask for help on how am I be able to catch IOException on my JUnit testing just to test if my code will throw the right exception once error occured.

Here is my main code:

public void LogHostStream(String v_strText) {
    
    Date dtToday = new Date();
    SimpleDateFormat DATE_FORMAT = new SimpleDateFormat("MM-dd-yyyy HH:mm:ss");
    String date = DATE_FORMAT.format(dtToday);
    
    SimpleDateFormat FILE_DATE_FORMAT = new SimpleDateFormat("yyyyMMdd");
    String filedate = FILE_DATE_FORMAT.format(dtToday);
    
    IConfiguration cfgHandler = ConfigurationFactory.getConfigHandle();
    String logFileName = cfgHandler.GetXMLValue("config.xml", "/config/log/host_logs");
    
    String outText = String.format("%s \n %s \n", date, v_strText);
    String fileName = logFileName + "_" + filedate + ".txt";

    try (FileWriter fw = new FileWriter(fileName, true);
         BufferedWriter bw = new BufferedWriter(fw);) {
        bw.write(outText);
    } catch (IOException e) {
        e.printStackTrace();
    }

And here is my JUnit:

 @Test(expected = IOException.class)
public void testLogHostStreamThrowsIOException() throws IOException {
    logger.LogHostStream("");
    String logFileName = cfgHandler.GetXMLValue("ocomw_config.xml", "/config/log/host_logs");
    FileWriter fWriter = new FileWriter(logFileName, true);
    BufferedWriter bw = new BufferedWriter(fWriter);
           
    Assertions.assertThrows(IOException.class, () -> new BufferedWriter(bw));    
}

I think I have to create a wrong path on my JUnit test to throw that exception, but I was not able to get the result I want. I appreciate any help for this. Thanks.

Answer 1

FileWriter will throw an exception when it cannot create or open the file for writing. One way to make this happen is making sure the directory the file is supposed to be in does not exist. This test succeeds:

@Test
void testThrows() throws IOException {
    assertThrows(IOException.class, () -> new FileWriter("/no/such/place"));
}

Creating a BufferedWriter will never throw an IOException , which means this assertion will always fail:

assertThrows(IOException.class, () -> new BufferedWriter(bw));

You shouldn't write tests for FileWriter and BufferedWriter though: they are well tested standard library classes and you can trust that they work as specified.

Answer 2

It is meaningless to call Assertions.assertThrows(IOException.class, () -> new BufferedWriter(bw)); to test standard library. In fact you should call Assertions.assertThrows(IOException.class, () -> logger.LogHostStream("logText")); to test your own method.

The reason you can't get the exception is that () -> new BufferedWriter(bw)); will never throw any exception. The place where exception is thrown is FileWriter fWriter = new FileWriter(logFileName, true);

If you want to test your method to checkout whether it throws the excpeted exception, you should remove your try-catch block like this:

FileWriter fw = new FileWriter(fileName, true);
BufferedWriter bw = new BufferedWriter(fw);
bw.write(outText);

Because once you catch your exception, you can't check the exception with Assertions.assertThrows

After that you should change the value of /config/log/host_logs in ocomw_config.xml to a invalid path value. Then call Assertions.assertThrows(IOException.class, () -> logger.LogHostStream("someLogText")); , and you will see the excepted exception.