Unit Testing StreamingOuput as Response entity Jersey - unit-testing

Question

I am doing something similar to mentioned in
Example of using StreamingOutput as Response entity in Jersey
#GET
#Produces(MediaType.APPLICATION_OCTET_STREAM)
public Response streamExample(#Context UriInfo uriInfo) {
StreamingOutput stream = new StreamingOutput() {
#Override
public void write(OutputStream os) throws IOException,WebApplicationException {
try{
Writer writer = new BufferedWriter(new OutputStreamWriter(os));
//Read resource from jar
InputStream inputStream = getClass().getClassLoader().getResourceAsStream("public/" + uriInfo.getPath());
...//manipulate the inputstream and build string with StringBuilder here//.......
String inputData = builder.toString();
Writer writer = new BufferedWriter(new OutputStreamWriter(os));
writer.write(inputData);
writer.flush();
} catch (ExceptionE1) {
throw new WebApplicationException();
}
}
};
return Response.ok(stream,MediaType.APPLICATION_OCTET_STREAM).build();
}
I am trying to unit test this by mocking URIInfo like mentioned in How to get instance of javax.ws.rs.core.UriInfo
public void testStreamExample() throws IOException, URISyntaxException {
UriInfo mockUriInfo = mock(UriInfo.class);
Mockito.when(mockUriInfo.getPath()).thenReturn("unusal-path");
Response response = myresource.streamExample(mockUriInfo);}
I want to be able to check that I get an Exception when I switch the path to jar to something else.But, when I run/debug the test, I never enter the
public void write(OutputStream os) throws IOException,
WebApplicationException {...}
part and I only always hit the return Response.ok(stream,MediaType.APPLICATION_OCTET_STREAM).build();
Am I missing something very obvious here??

Answer 1

Because the stream is not written to until it hits the MessageBodyWriter (which is the component that ends up calling the StreamingOutput#write).
What you can do, is just get the Response from the return and call Response#getEntity() (which returns an Object) and cast it to StreamingOutput. Then call the write method yourself, passing an OutputStream, maybe a ByteArrayOutputStream so you can get the contents as a byte[] to check it. It all would look something like
UriInfo mockInfo = mockUriInfo();
Response response = resource.streamExample(mockInfo);
StreamingOutput output = (StreamingOutput) response.getEntity();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
output.write(baos);
byte[] data = baos.toByteArray();
String s = new String(data, StandardCharsets.UTF_8);
assertThat(s, is("SomeCharacterData"));