InvoiceServiceExceptionUnitTest.java

    @Test
    public void testWhenInternalServerError_thenReturnHttp500() throws Exception {

        String mockPhoneNum = "542*******";

        this.mockServer.expect(ExpectedCount.once(), requestTo(invoiceUrl
                + mockPhoneNum))
                .andExpect(MockRestRequestMatchers.method(HttpMethod.GET))
                .andRespond((response) -> { throw new RuntimeException(); });

        responseInvoiceResponseEntity = mockService.getInvoiceInfo(mockPhoneNum);
        mockServer.verify();

        Assert.assertEquals("500", responseInvoiceResponseEntity.
                getStatusCode());
    }