Add new line at the end of Jersey generated JSON - json

Question

I have a Jersey (1.x) based REST service. It uses Jackson 2.4.4 to generate JSON responses. I need to add a newline character at the end of response (cURL users complain that there's no new line in responses). I am using Jersey pretty-print feature (SerializationFeature.INDENT_OUTPUT).
current: {\n "prop" : "value"\n}
wanted: {\n "prop" : "value"\n}\n
I tried using a custom serializer. I need to add \n only at the end of the root object. Serializer is defined per data type, which means, if an instance of such class is nested in a response, I will get \n in the middle of my JSON.
I thought of subclassing com.fasterxml.jackson.core.JsonGenerator.java, overriding close() where i'd add writeRaw('\n'), but that feels very hacky.
Another idea would be to add Servlet filter which would re-write the response from Jersey Filter, adding the \n and incrementing the contentLenght by 1. Seems not only hacky, but also inefficient.
I could also give up Jersey taking care of serializing the content and do ObjectMapper.writeValue() + "\n", but this is quite intrusive to my code (need to change many places).
What is the clean solution for that problem?
I have found these threads for the same problem, but none of them provides solution:
http://markmail.org/message/nj4aqheqobmt4o5c
http://jackson-users.ning.com/forum/topics/add-newline-after-object-serialization-in-jersey
Update
Finally I went for #arachnid's solution with NewlineAddingPrettyPrinter (also bumper Jackson version to 2.6.2). Sadly, it does not work out of the box with Jaskson as JAX-RS Json provider. Changed PrettyPrinter in ObjectMapper does not get propagated to JsonGenerator (see here why). To make it work, I had to add ResponseFilter which adds ObjectWriterModifier (now I can easily toggle between pretty-print and minimal, based on input param ):
#Provider
public class PrettyPrintFilter extends BaseResponseFilter {
public ContainerResponse filter(ContainerRequest request, ContainerResponse response) {
ObjectWriterInjector.set(new PrettyPrintToggler(true));
return response;
}
final class PrettyPrintToggler extends ObjectWriterModifier {
private static final PrettyPrinter NO_PRETTY_PRINT = new MinimalPrettyPrinter();
private final boolean usePrettyPrint;
public PrettyPrintToggler(boolean usePrettyPrint) {
this.usePrettyPrint = usePrettyPrint;
}
#Override
public ObjectWriter modify(EndpointConfigBase<?> endpoint, MultivaluedMap<String, Object> responseHeaders,
Object valueToWrite, ObjectWriter w, JsonGenerator g) throws IOException {
if (usePrettyPrint) g.setPrettyPrinter(new NewlineAddingPrettyPrinter());
else g.setPrettyPrinter(NO_PRETTY_PRINT);
return w;
}
}
}

Answer 1

Actually, wrapping up (not subclassing) JsonGenerator isn't too bad:
public static final class NewlineAddingJsonFactory extends JsonFactory {
#Override
protected JsonGenerator _createGenerator(Writer out, IOContext ctxt) throws IOException {
return new NewlineAddingJsonGenerator(super._createGenerator(out, ctxt));
}
#Override
protected JsonGenerator _createUTF8Generator(OutputStream out, IOContext ctxt) throws IOException {
return new NewlineAddingJsonGenerator(super._createUTF8Generator(out, ctxt));
}
}
public static final class NewlineAddingJsonGenerator extends JsonGenerator {
private final JsonGenerator underlying;
private int depth = 0;
public NewlineAddingJsonGenerator(JsonGenerator underlying) {
this.underlying = underlying;
}
#Override
public void writeStartObject() throws IOException {
underlying.writeStartObject();
++depth;
}
#Override
public void writeEndObject() throws IOException {
underlying.writeEndObject();
if (--depth == 0) {
underlying.writeRaw('\n');
}
}
// ... and delegate all the other methods of JsonGenerator (CGLIB can hide this if you put in some time)
}
#Test
public void append_newline_after_end_of_json() throws Exception {
ObjectWriter writer = new ObjectMapper(new NewlineAddingJsonFactory()).writer();
assertThat(writer.writeValueAsString(ImmutableMap.of()), equalTo("{}\n"));
assertThat(writer.writeValueAsString(ImmutableMap.of("foo", "bar")), equalTo("{\"foo\":\"bar\"}\n"));
}
A servlet filter isn't necessarily too bad either, although recently the ServletOutputStream interface has been more involved to intercept properly.
I found doing this via PrettyPrinter problematic on earlier Jackson versions (such as your 2.4.4), in part because of the need to go through an ObjectWriter to configure it properly: only fixed in Jackson 2.6. For completeness, this is a working 2.5 solution:
#Test
public void append_newline_after_end_of_json() throws Exception {
// Jackson 2.6:
// ObjectMapper mapper = new ObjectMapper()
// .setDefaultPrettyPrinter(new NewlineAddingPrettyPrinter())
// .enable(SerializationFeature.INDENT_OUTPUT);
// ObjectWriter writer = mapper.writer();
ObjectMapper mapper = new ObjectMapper();
ObjectWriter writer = mapper.writer().with(new NewlineAddingPrettyPrinter());
assertThat(writer.writeValueAsString(ImmutableMap.of()), equalTo("{}\n"));
assertThat(writer.writeValueAsString(ImmutableMap.of("foo", "bar")),
equalTo("{\"foo\":\"bar\"}\n"));
}
public static final class NewlineAddingPrettyPrinter
extends MinimalPrettyPrinter
implements Instantiatable<PrettyPrinter> {
private int depth = 0;
#Override
public void writeStartObject(JsonGenerator jg) throws IOException, JsonGenerationException {
super.writeStartObject(jg);
++depth;
}
#Override
public void writeEndObject(JsonGenerator jg, int nrOfEntries) throws IOException, JsonGenerationException {
super.writeEndObject(jg, nrOfEntries);
if (--depth == 0) {
jg.writeRaw('\n');
}
}
#Override
public PrettyPrinter createInstance() {
return new NewlineAddingPrettyPrinter();
}
}

Answer 2

Not yet tested but the following should work:
public class MyObjectMapper extends ObjectMapper {
_defaultPrettyPrinter = com.fasterxml.jackson.core.util.MinimalPrettyPrinter("\n");
// AND/OR
#Override
protected PrettyPrinter _defaultPrettyPrinter() {
return new com.fasterxml.jackson.core.util.MinimalPrettyPrinter("\n");
}
}
public class JerseyConfiguration extends ResourceConfig {
...
MyObjectMapper mapper = new MyObjectMapper();
mapper.enable(SerializationFeature.INDENT_OUTPUT); //enables pretty printing
// create JsonProvider to provide custom ObjectMapper
JacksonJaxbJsonProvider provider = new JacksonJaxbJsonProvider();
provider.setMapper(mapper);
register(provider); //register so that jersey use it
}
Do not know if this is the "cleanest" solution but it feels less hacky than the others.
Should produce something like
{\n "root" : "1"\n}\n{\n "root2" : "2"\n}
But it seems that does not work if there is only one root element.
Idea is from https://gist.github.com/deverton/7743979