How to write JUnit for a method that throws IOException and also takes in ResourceRequest as an argument

Following is the code for a method that takes ResourceRequest as the argument

JSONObject updateDetail(ResourceRequest request) throws IOException {
	
 	BufferedReader resourceReqBr = new BufferedReader(
			new InputStreamReader(request.getPortletInputStream()));
	String resourceReqStr = resourceReqBr.readLine();
	JSONObject updateCaDetailJsn = JSONObject.parse(resourceReqStr);
	
}

Here request.getPortletInputStream(), resourceReqBr.readLine(), and JSONObject.parse(resourceReqStr) are the lines of code that throw IOException. How do i setup request object such that it throws IOException.

My test code:

@Mock
	private ResourceRequest request;

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

@Test
    public void testUpdatDetailIOException() throws Exception
    {
        expectedException.expect(IOException.class);
        foo.updateDetail(request);
    }

This is causing nullpointer error (which is obvious).

Mocking a method call of ResourceRequest just to throw a IOException is in my opinion a not the only test case.

I would also try to make the JSONParser throw the IOException. For example expect an IOException when json string is faulty.

@Test
public void if_IOException_is_thrown_when_json_is_faulty() throws Exception {

  expectedException.expect(IOException.class);
  ResourceRequest mockResourceRequest = Mockito.mock(ResourceRequest.class);
  Mockito.when(mockResourceRequest.getPortletInputStream()).thenReturn(
      new ByteArrayInputStream("{[broken".getBytes(UTF_8)));
  foo.updateDetail(mockResourceRequest);
}

try below mocking in ur test code

ResourceRequest mockResourceRequest = Mockito.mock(ResourceRequest.class);
Mockito.when(mockResourceRequest.getPortletInputStream()).thenThrow(new IOException);
foo.updateDetail(mockResourceRequest);