用于方法多参数注入的自定义InjectableProvider，解析时出错

Question

I have a hard time having a nice and clean way to implement this JsonParamInjectable. I searched on this forum and elsewhere but found nowhere an hint that would tell me how to implement it nice and clean.

For a jaxrs method :

public Object add(
        @JsonParam("a") int a,
        @JsonParam("b") int b
)

It parse a json {"a":1, "b":2} to param a and b

For this to work i implement an InjectableProvider, which create one JsonInjectable instance by method param.

@Provider
public class JsonParamProvider implements InjectableProvider<JsonParam, Type> {

private Gson gson;

public JsonParamProvider(@Context ServletConfig sc) throws Exception {
    super();
    this.gson = GsonFactory.newGson(sc);
}

@Override
public Injectable<Object> getInjectable(ComponentContext cc, JsonParam a, Type type) {
    if (a.value() != null) {
        String signature = cc.getAccesibleObject().toString();
        return new JsonInjectable(signature, a, type, gson);
    }
    return null;
}

@Override
public ComponentScope getScope() {
    return ComponentScope.Singleton;
}

The magic is done in this JsonInjectable, and its where id did a dirty trick :

public class JsonInjectable extends AbstractHttpContextInjectable<Object> {

private final JsonParam param;
private final Type type;
private final String signature;
private final Gson gson;

private static ThreadLocal<WeakReference<HttpContext>> contextCache = new ThreadLocal<WeakReference<HttpContext>>(){
    @Override
    protected WeakReference<HttpContext> initialValue() {
        return new WeakReference<HttpContext>(null);
    }
};
private static ThreadLocal<WeakReference<JsonElement>> entityCache = new ThreadLocal<WeakReference<JsonElement>>(){
    @Override
    protected WeakReference<JsonElement> initialValue() {
        return new WeakReference<JsonElement>(null);
    }
};

public JsonInjectable(String signature, JsonParam param, Type type, Gson gson) {
    this.signature = signature;
    this.param = param;
    this.type = type;
    this.gson = gson;
}

@Override
public Object getValue(HttpContext context) {
    try {
        JsonElement methodJsonElement = entityCache.get().get();
        HttpContext context2 = contextCache.get().get();
        if (context != context2) {
            contextCache.set(new WeakReference<HttpContext>(context));
            String entity = context.getRequest().getEntity(String.class);
            System.out.println("entity:"+entity);
            JsonParser parser = new JsonParser();
            methodJsonElement = parser.parse(entity);
            entityCache.set(new WeakReference<JsonElement>(methodJsonElement));
        }
        if (methodJsonElement == null || methodJsonElement.isJsonNull()) {
            return null;
        }
        final JsonElement valueJsonElement = ((JsonObject)methodJsonElement).get(this.param.value());
        if (valueJsonElement == null || valueJsonElement.isJsonNull()) {
            return null;
        }
        if (this.type.equals(java.lang.Integer.class)) {
            Number number = valueJsonElement.getAsNumber();
            return number.intValue();
        }
        if (this.type.equals(java.lang.String.class)) {
            return valueJsonElement.getAsString();
        }
        Class<?> c = ((Class<?>) this.type);
        if (int.class.equals(c)) {
            return valueJsonElement.getAsInt();
        }

                    //other parsing code...

        //try with gson
        return this.gson.fromJson(valueJsonElement, this.type);

    } catch (RuntimeException e) {
        throw e;
    }
}

The problem is, in some case entity is empty for, i suspect, a valid http request. Resulting in java.io.EOFException: End of input at line 1 column 2. This problem arise in production, but i am unable to reproduce it in testing env.

If there is a problem, it is surely related with "context != context2". For each injectable is binded to a param and injectables are called in an order i dont control, and each injectable work on the same data : parsed json from request entity. So to avoid re-parsing entity each time, i use context != context2 to detect if its a new request.

What is the nice and clean way to detect a new request so json parsing can only occur 1 time per request.

Answer 1

It seems to me like your approach is way too complicated. All is required is to define bean with two fields and have that as a single param with two fields populated from json.

public class TwoParams {
public int a;
public int b;
// instead of public fields you could do proper bean 
// or even make immutable with final
}

public Object add(@JsonParam TwoParams params)
....

Mapping like that is trivial using Jackson so don't even need to hand roll the mapping code. If you use Hibernate Validator you could add annotations to validate input without writing any extra code.

Answer 2

Ok this solution seem to work and is cleaner than the first one. Thanks for helping.

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public abstract @interface JsonParam {

    public abstract String value();

}

@Provider
public class JsonParamProvider implements InjectableProvider<JsonParam, Type> {

    private Gson gson;

    public JsonParamProvider(@Context ServletConfig sc) throws Exception {
        super();
        this.gson = GsonFactory.newGson(sc);
    }

    @Override
    public Injectable<Object> getInjectable(ComponentContext cc, JsonParam a, Type type) {
        if (a.value() != null) {
            String signature = cc.getAccesibleObject().toString();
            return new JsonParamInjectable(signature, a, type, gson);
        }
        return null;
    }

    @Override
    public ComponentScope getScope() {
        return ComponentScope.Singleton;
    }

}

@Provider
public class JsonParamDispatchProvider extends AbstractResourceMethodDispatchProvider {

    @Override
    protected InjectableValuesProvider getInjectableValuesProvider(AbstractResourceMethod abstractResourceMethod) {
        return new JsonParamInjectableValuesProvider(processParameters(abstractResourceMethod));
    }

    //copied from EntityParamDispatchProvider source code
    private List<Injectable> processParameters(AbstractResourceMethod method) {

        if ((null == method.getParameters()) || (0 == method.getParameters().size())) {
            return Collections.emptyList();
        }

        boolean hasEntity = false;
        final List<Injectable> is = new ArrayList<Injectable>(method.getParameters().size());
        for (int i = 0; i < method.getParameters().size(); i++) {
            final Parameter parameter = method.getParameters().get(i);

            if (Parameter.Source.ENTITY == parameter.getSource()) {
                hasEntity = true;
                is.add(processEntityParameter(
                        parameter,
                        method.getMethod().getParameterAnnotations()[i]));
            } else {
                is.add(getInjectableProviderContext().
                        getInjectable(method.getMethod(), parameter, ComponentScope.PerRequest));
            }
        }

        if (hasEntity)
            return is;

        // Try to find entity if there is one unresolved parameter and
        // the annotations are unknown
        if (Collections.frequency(is, null) == 1) {
            final int i = is.lastIndexOf(null);
            final Parameter parameter = method.getParameters().get(i);
            if (Parameter.Source.UNKNOWN == parameter.getSource()) {
                if (!parameter.isQualified()) {
                    final Injectable ij = processEntityParameter(
                        parameter,
                        method.getMethod().getParameterAnnotations()[i]);
                    is.set(i, ij);
                }
            }
        }

        return is;
    }

    //copied from EntityParamDispatchProvider source code
    static final class EntityInjectable extends AbstractHttpContextInjectable<Object> {
        final Class<?> c;
        final Type t;
        final Annotation[] as;

        EntityInjectable(Class c, Type t, Annotation[] as) {
            this.c = c;
            this.t = t;
            this.as = as;
        }

        @Override
        public Object getValue(HttpContext context) {
            return context.getRequest().getEntity(c, t, as);
        }
    }

    //copied from EntityParamDispatchProvider source code
    private Injectable processEntityParameter(
            Parameter parameter,
            Annotation[] annotations) {
        return new EntityInjectable(parameter.getParameterClass(),
                parameter.getParameterType(), annotations);
    }

}

@SuppressWarnings("rawtypes")
public class JsonParamInjectableValuesProvider extends InjectableValuesProvider {

    public static final String JSON_ELEMENT_CONTEXT_PROPERTY_KEY = "JsonParamInjectableValuesProvider.jsonElementContextPropertyKey";

    public JsonParamInjectableValuesProvider(List<Injectable> is) {
        super(is);
    }

    /**
     * Get the injectable values.
     *
     * @param context the http contest.
     * @return the injectable values. Each element in the object array
     *         is a value obtained from the injectable at the list index
     *         that is the element index.
     */
    @Override
    public Object[] getInjectableValues(HttpContext context) {
        List<AbstractHttpContextInjectable> is = getInjectables();
        final Object[] params = new Object[is.size()];
        try {
            //parse json element and add it to context
            context.getProperties().put(JSON_ELEMENT_CONTEXT_PROPERTY_KEY, parseJsonElement(context));
            //map jsonElement with injectable
            int index = 0;
            for (AbstractHttpContextInjectable i : is) {
                params[index++] = i.getValue(context);
            }
            return params;
        } catch (WebApplicationException e) {
            throw e;
        } catch (ContainerException e) {
            throw e;
        } catch (RuntimeException e) {
            throw new ContainerException("Exception obtaining parameters", e);
        } finally {
            context.getProperties().remove(JSON_ELEMENT_CONTEXT_PROPERTY_KEY);
        }
    }

    private static JsonElement parseJsonElement(HttpContext context) {
        String entity = context.getRequest().getEntity(String.class);
        if (StringUtils.isBlank(entity)) {
            throw new ContainerException("entity is blank for request " + context.getRequest());
        }
        JsonParser parser = new JsonParser();
        return parser.parse(entity);
    }
}

public class JsonParamInjectable extends AbstractHttpContextInjectable<Object> {

    private final JsonParam param;
    private final Type type;
    private final String signature;
    private final Gson gson;

    public JsonParamInjectable(String signature, JsonParam param, Type type, Gson gson) {
        this.signature = signature;
        this.param = param;
        this.type = type;
        this.gson = gson;
    }

    @Override
    public Object getValue(HttpContext context) {
        try {
            JsonElement jsonElement = (JsonElement) context.getProperties().get(JsonParamInjectableValuesProvider.JSON_ELEMENT_CONTEXT_PROPERTY_KEY);
            if (jsonElement == null) {
                throw new ContainerException("invalid json element in context. " + context.getRequest());
            }
            if (jsonElement == null || jsonElement.isJsonNull()) {
                return null;
            }
            final JsonElement valueJsonElement = ((JsonObject)jsonElement).get(this.param.value());
            if (valueJsonElement == null || valueJsonElement.isJsonNull()) {
                return null;
            }
            if (this.type.equals(java.lang.Integer.class)) {
                Number number = valueJsonElement.getAsNumber();
                return number.intValue();
            }
            if (this.type.equals(java.lang.String.class)) {
                return valueJsonElement.getAsString();
            }
            Class<?> c = ((Class<?>) this.type);
            if (int.class.equals(c)) {
                return valueJsonElement.getAsInt();
            }
            if (long.class.equals(c)) {
                return valueJsonElement.getAsLong();
            }
            if (boolean.class.equals(c)) {
                return valueJsonElement.getAsBoolean();
            }
            //other parsing code...

            //try with gson
            return this.gson.fromJson(valueJsonElement, this.type);

        } catch (RuntimeException e) {
            throw e;
        }
    }

    @Override
    public String toString() {
        return "JsonParamInjectable " + this.hashCode() + " [param=" + this.param + ", type=" + this.type + ", signature=" + this.signature + "]";  }

}