java - How do you assert that a certain exception is thrown in JUnit 4 tests?

Update: JUnit5 has an improvement for exceptions testing: assertThrows.

following example is from: Junit 5 User Guide

 @Test

void exceptionTesting() {
    Throwable exception = assertThrows(IllegalArgumentException.class, () -> 
    {
        throw new IllegalArgumentException("a message");
    });
    assertEquals("a message", exception.getMessage());
}

Original answer using JUnit 4.

There are several ways to test that an exception is thrown. I have also discussed the below options in my post How to write great unit tests with JUnit

Set the expected parameter @Test(expected = FileNotFoundException.class).

@Test(expected = FileNotFoundException.class) 
public void testReadFile() { 
    myClass.readFile("test.txt");
}

Using try catch

public void testReadFile() { 
    try {
        myClass.readFile("test.txt");
        fail("Expected a FileNotFoundException to be thrown");
    } catch (FileNotFoundException e) {
        assertThat(e.getMessage(), is("The file test.txt does not exist!"));
    }


}

Testing with ExpectedException Rule.

@Rule
public ExpectedException thrown = ExpectedException.none();

@Test

public void testReadFile() throws FileNotFoundException {

    thrown.expect(FileNotFoundException.class);
    thrown.expectMessage(startsWith("The file test.txt"));
    myClass.readFile("test.txt");
}

You could read more about exceptions testing in JUnit4 wiki for Exception testing and bad.robot - Expecting Exceptions JUnit Rule.