I want to replicate the following Java code in Golang in the most idiomatic way possible:
public class Handler {
private Storage storage;
private Mapper mapper;
public Handler(Storage storage, Mapper mapper) {
this.storage = storage;
this.mapper = mapper;
}
public void handleKey(String k) {
storage.put(k, mapper.map(k));
}
}
interface Storage {
public void put(String k, String v);
public String get(String k);
}
#FunctionalInterface
interface Mapper {
public String map(String a);
}
private static class InMemoryStorage implements Storage {
private Map<String, String> table;
public InMemoryStorage() {
table = new HashMap<>();
}
public void put(String k, String v) {
table.put(k, v);
}
public String get(String k) {
return table.get(k);
}
}
Handler class only exposes the method handleKey. The behaviour of this class is parametrized by passing different concrete Storage and Mapper implementations.
After reading Effective Go - Embedding, I thought this would be a good use of embedding interfaces intro structs. But I can't figure out how to avoid exposing the embedded interfaces' methods to Handler users. I can do something like
type Handler struct {
store Storage
mapper Mapper
}
func (h Handler) Handle(k string) {
h.store.Put(k, h.mapper.Map(k))
}
type Storage interface {
Put(k string, v string)
Get(k string) string
}
type Mapper interface {
Map(k string) string
}
type inMemoryStorage {
table map[string]string
}
func NewInMemoryStorage() Storage {
return &inMemoryStore{table: make(map[string]string)}
}
but then I cannot pass concrete implementations to the Handler (creating struct literal) because store and mapper are unexported. And I do not want to create factory methods for each possible combination... Any suggestions?
Those are not embedded; embedding has a specific meaning in Go, as outlined in the spec, and as explained in the Effective Go section you linked to. It refers to unnamed fields, whose fields and methods are accessible implicitly from their containing type. Your fields are named, not embedded.
That said, your two struct fields, store and mapper, are not exported, so they will not be exposed to any user outside the package in which Handler is defined; you seem to already have your desired behavior in that regard.
I'm not sure what you mean when you say you would have to "create factory methods for each possible combination" - I don't see any reason that would be necessary. You need only one factory method:
func NewHandler(s Storage, m Mapper) Handler {
return Handler{store: s, mapper: m}
}
Which could be used with any combination of implementations of Storage and Mapper by passing appropriate values to the function.
Related
I have a number of Java bean interfaces like this:
public interface Dog
{
String getName();
void setName( final String value );
}
I also auto-generate bean implementations like this:
public final class DogImpl implements Dog
{
public String getName()
{
return m_name;
}
public void setName( final String value )
{
m_value = value;
}
private volatile String m_value;
}
ObjectMapper works perfectly except when I start nesting these beans like this:
public interface Dog
{
String getName();
void setName( final String value );
Dog getParent();
void setParent( final Dog value );
}
I get this error:
abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
It's complaining because the bean definition is an interface and not the concrete type. My question is if there is a way for me to define the mapping of interface types to concrete types for the ObjectMapper via a module or something?
Specifically, I can get a Map< Class< ? >, Class< ? > > of api type to implementation concrete type, but have no idea how to "give this" to the ObjectMapper so it understands how to look up the concrete types from the api types so it can instantiate them. How do I accomplish this?
This can be done using a SimpleAbstractTypeResolver.
This link shows you how to add the mappings to the resolver: Jackson - How to specify a single implementation for interface-referenced deserialization?
And this is how you add the resolver to an ObjectMapper:
final SimpleAbstractTypeResolver resolver = new SimpleAbstractTypeResolver();
for ( final Class< ? > api : apis )
{
resolver.addMapping( api, getConcreteImpl( api ) );
}
final SimpleModule module = new SimpleModule();
module.setAbstractTypes( resolver );
mapper.registerModule( module );
This is my client side code to get the string "get-image-data" through RPC calls and getting byte[] from the server.
CommandMessage msg = new CommandMessage(itemId, "get-image-data");
cmain.ivClient.execute(msg, new AsyncCallback<ResponseMessage>() {
#Override
public void onFailure(Throwable caught) {
}
#Override
public void onSuccess(ResponseMessage result) {
if (result.result) {
result.data is byte[].
}
}
});
From the server side I got the length of the data is 241336.
But I could not get the value in onSuccess method. It is always goes to onFailure method.
And I got log on Apache:
com.google.gwt.user.client.rpc.SerializationException: Type '[B' was
not included in the set of types which can be serialized by this
SerializationPolicy or its Class object could not be loaded.
How can I do serialisation in GWT?
1) Create a pojo which implements Serializable interface
Let this pojo has all the data you want in the response of RPC service, in this case image-data
2) Pass this pojo in the response for your RPC service.
The below tutorial has enough information for creating RPC service
http://www.gwtproject.org/doc/latest/tutorial/RPC.html
The objects you transfer to and from the server has to implement IsSerializable.
All your custom Objects within the Object you are transferring also needs to implement IsSerializable.
Your objects cannot have final fields and needs an no argument constructor.
You need getters and setters.
A common serialize object in GWT:
public class MyClass implements IsSerializable {
private String txt;
private MyOtherClass myOtherClass; // Also implements IsSerializable
public MyClass() {
}
public String getTxt() {
return this.txt;
}
public void setTxt(String txt) {
return this.txt = txt;
}
public String getMyOtherClass() {
return this.myOtherClass;
}
public void setMyOtherClass(MyOtherClass myOtherClass) {
return this.myOtherClass = myOtherClass;
}
}
i have a question regarding design patterns.
suppose i want to design pig killing factory
so the ways will be
1) catch pig
2)clean pig
3) kill pig
now since these pigs are supplied to me by a truck driver
now if want to design an application how should i proceed
what i have done is
public class killer{
private Pig pig ;
public void catchPig(){ //do something };
public void cleanPig(){ };
public void killPig(){};
}
now iam thing since i know that the steps will be called in catchPig--->cleanPig---->KillPig manner so i should have an abstract class containing these methods and an execute method calling all these 3 methods.
but i can not have instance of abstract class so i am confused how to implement this.
remenber i have to execute this process for all the pigs that comes in truck.
so my question is what design should i select and which design pattern is best to solve such problems .
I would suggest a different approach than what was suggested here before.
I would do something like this:
public abstract class Killer {
protected Pig pig;
protected abstract void catchPig();
protected abstract void cleanPig();
protected abstract void killPig();
public void executeKillPig {
catchPig();
cleanPig();
killPig();
}
}
Each kill will extend Killer class and will have to implement the abstract methods. The executeKillPig() is the same for every sub-class and will always be performed in the order you wanted catch->clean->kill. The abstract methods are protected because they're the inner implementation of the public executeKillPig.
This extends Avi's answer and addresses the comments.
The points of the code:
abstract base class to emphasize IS A relationships
Template pattern to ensure the steps are in the right order
Strategy Pattern - an abstract class is as much a interface (little "i") as much as a Interface (capital "I") is.
Extend the base and not use an interface.
No coupling of concrete classes. Coupling is not an issue of abstract vs interface but rather good design.
public abstract Animal {
public abstract bool Escape(){}
public abstract string SaySomething(){}
}
public Wabbit : Animal {
public override bool Escape() {//wabbit hopping frantically }
public override string SaySomething() { return #"What's Up Doc?"; }
}
public abstract class Killer {
protected Animal food;
protected abstract void Catch(){}
protected abstract void Kill(){}
protected abstract void Clean(){}
protected abstract string Lure(){}
// this method defines the process: the methods and the order of
// those calls. Exactly how to do each individual step is left up to sub classes.
// Even if you define a "PigKiller" interface we need this method
// ** in the base class ** to make sure all Killer's do it right.
// This method is the template (pattern) for subclasses.
protected void FeedTheFamily(Animal somethingTasty) {
food = somethingTasty;
Catch();
Kill();
Clean();
}
}
public class WabbitHunter : Killer {
protected override Catch() { //wabbit catching technique }
protected override Kill() { //wabbit killing technique }
protected override Clean() { //wabbit cleaning technique }
protected override Lure() { return "Come here you wascuhwy wabbit!"; }
}
// client code ********************
public class AHuntingWeWillGo {
Killer hunter;
Animal prey;
public AHuntingWeWillGo (Killer aHunter, Animal aAnimal) {
hunter = aHunter;
prey = aAnimal;
}
public void Hunt() {
if ( !prey.Escape() ) hunter.FeedTheFamily(prey)
}
}
public static void main () {
// look, ma! no coupling. Because we pass in our objects vice
// new them up inside the using classes
Killer ElmerFudd = new WabbitHunter();
Animal BugsBunny = new Wabbit();
AHuntingWeWillGo safari = new AHuntingWeWillGo( ElmerFudd, BugsBunny );
safari.Hunt();
}
The problem you are facing refer to part of OOP called polymorphism
Instead of abstract class i will be using a interface, the difference between interface an abstract class is that interface have only method descriptors, a abstract class can have also method with implementation.
public interface InterfaceOfPigKiller {
void catchPig();
void cleanPig();
void killPig();
}
In the abstract class we implement two of three available methods, because we assume that those operation are common for every future type that will inherit form our class.
public abstract class AbstractPigKiller implements InterfaceOfPigKiller{
private Ping pig;
public void catchPig() {
//the logic of catching pigs.
}
public void cleanPig() {
// the logic of pig cleaning.
}
}
Now we will create two new classes:
AnimalKiller - The person responsible for pig death.
AnimalSaver - The person responsible for pig release.
public class AnimalKiller extends AbstractPigKiller {
public void killPig() {
// The killing operation
}
}
public class AnimalSaver extends AbstractPigKiller {
public void killPing() {
// The operation that will make pig free
}
}
As we have our structure lets see how it will work.
First the method that will execute the sequence:
public void doTheRequiredOperation(InterfaceOfPigKiller killer) {
killer.catchPig();
killer.cleanPig();
killer.killPig();
}
As we see in the parameter we do not use class AnimalKiller or AnimalSever. Instead of that we have the interface. Thank to this operation we can operate on any class that implement used interface.
Example 1:
public void test() {
AnimalKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `AnimalKilleraKiller `
AnimalSaver aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
Example 2:
public void test() {
InterfaceOfPigKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `InterfaceOfPigKiller `
InterfaceOfPigKiller aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
The code example 1 and 2 are equally in scope of method doTheRequiredOperation. The difference is that in we assign once type to type and in the second we assign type to interface.
Conclusion
We can not create new object of abstract class or interface but we can assign object to interface or class type.
I have an object: X, that can be saved or loaded in various formats: TXT, PDF, HTML, etc..
What is the best way to manage this situation? Add a pair of method to X for each format, create a new Class for each format, or exists (as I trust) a better solution?
I'd choose the strategy pattern. For example:
interface XStartegy {
X load();
void save(X x);
}
class TxtStrategy implements XStartegy {
//...implementation...
}
class PdfStrategy implements XStartegy {
//...implementation...
}
class HtmlStrategy implements XStartegy {
//...implementation...
}
class XContext {
private XStartegy strategy;
public XContext(XStartegy strategy) {
this.strategy = strategy;
}
public X load() {
return strategy.load();
}
public void save(X x) {
strategy.save(x);
}
}
I agree with #DarthVader , though in Java you'd better write
public class XDocument implements IDocument { ...
You could also use an abstract class, if much behavior is common to the documents, and in the common methods of base class call an abstract save(), which is only implemented in the subclasses.
I would go with Factory pattern. It looks like you can use inheritance/polymorphism with generics. You can even do dependency injection if you go with the similar design as follows.
public interface IDocument
{
void Save();
}
public class Document : IDocument
{
}
public class PdfDocument: IDocument
{
public void Save(){//...}
}
public class TxtDocument: IDocument
{
public void Save(){//...}
}
public class HtmlDocument : IDocument
{
public void Save(){//...}
}
then in another class you can do this:
public void SaveDocument(T document) where T : IDocument
{
document.save();
}
It depends on your objects, but it is possible, that visitor pattern (http://en.wikipedia.org/wiki/Visitor_pattern) can be used here.
There are different visitors (PDFVisitor, HHTMLVisitor etc) that knows how to serialize parts of your objects that they visit.
I would instead suggest the Strategy pattern. You're always saving and restoring, the only difference is how you do it (your strategy). So you have save() and restore() methods that defer to various FormatStrategy objects you can plug and play with at run time.
I think this falls under the concept of contextual binding, but the Ninject documentation, while very thorough, does not have any examples close enough to my current situation for me to really be certain. I'm still pretty confused.
I basically have classes that represent parameter structures for queries. For instance..
class CurrentUser {
string Email { get; set; }
}
And then an interface that represents its database retrieval (in the data layer)
class CurrentUserQuery : IQueryFor<CurrentUser> {
public CurrentUserQuery(ISession session) {
this.session = session;
}
public Member ExecuteQuery(CurrentUser parameters) {
var member = session.Query<Member>().Where(n => n.Email == CurrentUser.Email);
// validation logic
return member;
}
}
Now then, what I want to do is to establish a simple class that can take a given object and from it get the IQueryFor<T> class, construct it from my Ninject.IKernel (constructor parameter), and perform the ExecuteQuery method on it, passing through the given object.
The only way I have been able to do this was to basically do the following...
Bind<IQueryFor<CurrentUser>>().To<CurrentUserQuery>();
This solves the problem for that one query. But I anticipate there will be a great number of queries... so this method will become not only tedious, but also very prone to redundancy.
I was wondering if there is an inherit way in Ninject to incorporate this kind of behavior.
:-
In the end, my (ideal) way of using this would be ...
class HomeController : Controller {
public HomeController(ITransit transit) {
// injection of the transit service
}
public ActionResult CurrentMember() {
var member = transit.Send(new CurrentUser{ Email = User.Identity.Name });
}
}
Obviously that's not going to work right, since the Send method has no way of knowing the return type.
I've been dissecting Rhino Service Bus extensively and project Alexandria to try and make my light, light, lightweight implementation.
Update
I have been able to get a fairly desired result using .NET 4.0 dynamic objects, such as the following...
dynamic Send<T>(object message);
And then declaring my interface...
public interface IQueryFor<T,K>
{
K Execute(T message);
}
And then its use ...
public class TestCurrentMember
{
public string Email { get; set; }
}
public class TestCurrentMemberQuery : IConsumerFor<TestCurrentMember, Member>
{
private readonly ISession session;
public TestCurrentMemberQuery(ISession session) {
this.session = session;
}
public Member Execute(TestCurrentMember user)
{
// query the session for the current member
var member = session.Query<Member>()
.Where(n => n.Email == user.Email).SingleOrDefault();
return member;
}
}
And then in my Controller...
var member = Transit.Send<TestCurrentMemberQuery>(
new TestCurrentMember {
Email = User.Identity.Name
}
);
effectively using the <T> as my 'Hey, This is what implements the query parameters!'. It does work, but I feel pretty uncomfortable with it. Is this an inappropriate use of the dynamic function of .NET 4.0? Or is this more the reason why it exists in the first place?
Update (2)
For the sake of consistency and keeping this post relative to just the initial question, I'm opening up a different question for the dynamic issue.
Yes, you should be able to handle this with Ninject Conventions. I am just learning the Conventions part of Ninject, and the documentation is sparse; however, the source code for the Conventions extension is quite light and easy to read/navigate, also Remo Gloor is very helpful both here and on the mailing list.
The first thing I would try is a GenericBindingGenerator (changing the filters and scope as needed for your application):
internal class YourModule : NinjectModule
{
public override void Load()
{
Kernel.Scan(a => {
a.From(System.Reflection.Assembly.GetExecutingAssembly());
a.InTransientScope();
a.BindWith(new GenericBindingGenerator(typeof(IQueryFor<>)));
});
}
}
The heart of any BindingGenerator is this interface:
public interface IBindingGenerator
{
void Process(Type type, Func<IContext, object> scopeCallback, IKernel kernel);
}
The Default Binding Generator simply checks if the name of the class matches the name of the interface:
public void Process(Type type, Func<IContext, object> scopeCallback, IKernel kernel)
{
if (!type.IsInterface && !type.IsAbstract)
{
Type service = type.GetInterface("I" + type.Name, false);
if (service != null)
{
kernel.Bind(service).To(type).InScope(scopeCallback);
}
}
}
The GenericBindingGenerator takes a type as a constructor argument, and checks interfaces on classes scanned to see if the Generic definitions of those interfaces match the type passed into the constructor:
public GenericBindingGenerator(Type contractType)
{
if (!contractType.IsGenericType && !contractType.ContainsGenericParameters)
{
throw new ArgumentException("The contract must be an open generic type.", "contractType");
}
this._contractType = contractType;
}
public void Process(Type type, Func<IContext, object> scopeCallback, IKernel kernel)
{
Type service = this.ResolveClosingInterface(type);
if (service != null)
{
kernel.Bind(service).To(type).InScope(scopeCallback);
}
}
public Type ResolveClosingInterface(Type targetType)
{
if (!targetType.IsInterface && !targetType.IsAbstract)
{
do
{
foreach (Type type in targetType.GetInterfaces())
{
if (type.IsGenericType && (type.GetGenericTypeDefinition() == this._contractType))
{
return type;
}
}
targetType = targetType.BaseType;
}
while (targetType != TypeOfObject);
}
return null;
}
So, when the Conventions extension scans the class CurrentUserQuery it will see the interface IQueryFor<CurrentUser>. The generic definition of that interface is IQueryFor<>, so it will match and that type should get registered for that interface.
Lastly, there is a RegexBindingGenerator. It tries to match interfaces of the classes scanned to a Regex given as a constructor argument. If you want to see the details of how that operates, you should be able to peruse the source code for it now.
Also, you should be able to write any implementation of IBindingGenerator that you may need, as the contract is quite simple.