Here is my problem, I have a Client interface that has got a <T> Prop<T> getProp(Class<T>) method. A PropKey may be constructed with PropKey.of(Class).
I inject an instance of Client in a module like this bind(Client.class).to(ClientImpl.class).in(Scopes.SINGLETON); and I wanna be able to inject Prop like this :
public class MyService implements Service {
#Inject Client client;
#Inject Prop<User> user;
}
How can I tell Guice that injection of Prop<User> will lead to client.getProp(User.class). I primarily searched how to do this via SPI but I didn't find any way to get unknown binding. I also reviewed Jukito which is known to be a quality project using SPI.
There's no way to get Guice to bind Prop<T> for all types T unless Prop is an injectable concrete class itself. In that case it will "just work."
But if the set of types T is small you can bind to providers explicitly like this:
class PropProvider<T> implements Provider<Prop<T>> {
private final Class<T> type;
#Inject Client client;
PropProvider(Class<T> type) {
this.type = type;
}
#Override
public void get() {
return client.getProp(type);
}
}
class MyModule extends AbstractModule {
#override
protected void configure() {
bind(new TypeLiteral<Prop<User>>() { })
.toProvider(new PropProvider(User.class));
// More similar statments...
}
}
Alternatively you can use custom injections to be able to write something like
class MyService implements Service {
#Inject Client client;
#InjectProp Prop<User> user;
}
but you'll need to use a custom annotation.
I have a class with a constructor, e.g.:
#Inject
public ClassTest(ITestInterface testInterface, Class<?> clazz){
...
}
The problem is how do I bind a class to an implementation which can be injected in this constructor and will the ClassTest binding pick the right class?
I want to inject different classes at different point of time. When I attempted to solve it Guice gives an error that it cannot find any suitable constructor on java.lang.Class.
I think you have to use assisted inject extension of Guice.
Basically, you define your ClassTest as it is, but mark 'varying' dependencies as #Assisted:
#Inject
public ClassTest(ITestInterface testInterface, #Assisted Class<?> clazz){
...
}
Then you create a factory interface for ClassTest objects which will accept Class argument and return ClassTests:
public interface ClassTestFactory {
ClassTest create(Class<?> clazz);
}
Then you install special kind of module which will create factories for you:
// Inside your module
install(new FactoryModuleBuilder().build(ClassTestFactory.class));
Then wherever you need ClassTest instances you should inject ClassTestFactory interface instead:
#Inject
YourLogicClass(ClassTestFactory ctFactory) {
this.ctFactory = ctFactory;
}
And finally you use it to create ClassTests for every class object you want:
ClassTest ct1 = ctFactory.create(SomeClass.class);
ClassTest ct2 = ctFactory.create(AnotherClass.class);
But if I were you, I would really reconsider the whole class architecture to avoid the need in such things.
This can be solved even without assisted inject, simply by using a TypeLiteral when creating the binding:
import javax.inject.Inject;
import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Injector;
import com.google.inject.Module;
import com.google.inject.TypeLiteral;
public class ClassTest
{
static interface ITestInterface {}
#Inject
public ClassTest(ITestInterface testInterface, Class<?> clazz)
{
System.err.println("testInterface=" + testInterface);
System.err.println("clazz=" + clazz);
}
public static void main(String... argument)
{
ITestInterface testObject = new ITestInterface() {};
Module module = new AbstractModule()
{
#Override
protected void configure()
{
binder().bind(ITestInterface.class).toInstance(testObject);
binder().bind(new TypeLiteral<Class<?>>() {}).toInstance(testObject.getClass());
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
}
};
Injector injector = Guice.createInjector(module);
injector.getInstance(ClassTest.class);
}
}
The output when running this code is something like:
testInterface=ClassTest$1#3d921e20
clazz=class ClassTest$1
I have to agree with #VladimirMatveev though, that this is a somewhat unusual use case, and that the need for injection of java.lang.Class objects might be indicative of a design flaw. The only seemingly valid case of this type of injection that I've come across is for type checking, where an injected class needs the Class object to check the type of some other object (via Class.isInstance(...)) but it is not desirable to inject an instance (!) of that class (e.g., because it is not a singleton and might spawn all sorts of other undesired object creations). Still, even that scenario is somewhat hokey and might be solvable in a better way.
At the very least, I would use a more specific type argument, like Class<? extends ITestInterface> (which, I suspect, is what's intended by the OP).
To change injected value over time you could use the Provider bindings. And then it could look like this:
The module configuration:
public class SomeModule extends AbstractModule{
#Override
protected void configure() {
bind(Class.class).toProvider(SomeProvider.class);
}
}
The provider(not very elegant but may be for a start...):
public class SomeProvider implements Provider<Class<?>>{
private static Class<?> myClazz;
public static void setClass(Class<?> clazz){
myClazz = clazz;
}
#Override
public Class<?> get() {
return myClazz;
}
}
Some different classes:
public class SomeClass{
public int itWorks;
}
public class SomeOtherClass{
public int itWorksGreat;
}
Example client code:
public static void main(String[] args){
SomeProvider.setClass(SomeClass.class);
Injector injector = Guice.createInjector(new SomeModule());
printFields(injector.getInstance(Class.class));
SomeProvider.setClass(SomeOtherClass.class);
printFields(injector.getInstance(Class.class));
}
private static void printFields(Class clazz) {
Field[] declaredFields = clazz.getDeclaredFields();
for(Field field : declaredFields){
System.out.println(field.getName());
}
}
And finally the result:
itWorks
itWorksGreat
When I try to mock a Dao using Jukito I get the following exception:
java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
at org.jukito.JukitoModule.addKeyDependency(JukitoModule.java:338)
at org.jukito.JukitoModule.addInjectionPointDependencies(JukitoModule.java:330)
at org.jukito.JukitoModule.addDependencies(JukitoModule.java:313)
The object I try to mock is a ConcreteDao.
public class ConcreteDao extends AbstractDao<MyDomain> {
}
public abstract class AbstractDao<T extends DatastoreObject> {
}
I read several posts on SO about this binding generics but I can't figure out a way to use TypeLiteral for my binding.
This is what I tried:
bind(new TypeLiteral<AbstractDao<MyDomain>>(){}).to(ConcreteDao.class);
You need to bind like this:
bind(new TypeLiteral<AbstractDao<MyDomain>>(){}).to(new TypeLiteral<ConcreteDao<MyDomain>>(){});
This is how you can retrieve the generic class:
class AbstractDao {
protected final Class<T> clazz;
#Inject
public AbstractDao(TypeLiteral<T> type) {
clazz = (Class<T>) type.getRawType();
}
}
Subclasses of AbstractDao will need to pass entity specific TypeLiterals to the parent class (AbstractDao):
class ConcreteDao extends AbstractDao<MyDomain> {
#Inject
public ConcreteDao(TypeLiteral<MyDomain> type) {
super(type);
}
}
Note that you can make your AbstractDao class non-abstract and implement basic CRUD operations, so that you can use it without the need to extend AbstractDao for each entity. You will just need a binding for each entity like this:
bind(new TypeLiteral<GenericDao<User>>(){}).in(Scopes.SINGLETON);
See my blog-post for more information.
Using the generic dao pattern, I define the generic interface:
public interface GenericDao<T extends DataObject, ID extends Serializable> {
T save(T t);
void delete(ID id);
T findById(ID id);
Class<T> getPersistentClass();
}
I then implemented an default GenericDaoImpl implementation to perform these functions with the following constructor:
public GenericDaoImpl(Class<T> clazz) {
this.persistentClass = clazz;
DaoRegistry.getInstance().register(clazz, this);
}
The point of the DaoRegistry is to look up a Dao by the class associating to it. This allows me to extend GenericDaoImpl and overwrite methods for objects that requires special handling:
DaoRegistry.getInstance().getDao(someClass.getClass()).save(someClass);
While it works, there are a few things that I don't like about it:
DaoRegistry is an singleton
The logic of calling save is complicated
Is there a better way to do this?
Edit
I am not looking to debate whether Singleton is an anti-pattern or not.
First of all, what is your problem with DaoRegistry being singleton?
Anyway, you could have an abstract base class for your entities that'd implement save like this
public T save(){
DaoRegistry.getInstance().getDao(this.getClass()).save(this);
}
then you could simply call someEntity.save()
Or it may be more straightforward if the entity classes itself implemented the whole GenericDao interface (save, delete and find methods), so the contents of your GenericDaoImpl would be in the base class of your entities.
It could be better to use instance of DaoRegistry instead of static methods. It would make it more manageable for test configurations. You could implement it as
#Component("daoRegistry")
public class DaoRegistry {
#Autowired
private List<GenericDao> customDaos;
private GenericDao defaultDao = new GenericDaoImpl();
public <T> T getDao(Class<T> clazz) {
// search customDaos for matching clazz, return default dao otherwise
}
}
Also you could add save method to it and rename accordingly. All customised daos should be available as beans.
If I have the following class:
public class ObjectDAOMongoDBImpl<T> extends GenericDAOMongoDBImpl<T, ObjectId> implements ObjectDAO<T> {
public ObjectDAOMongoDBImpl(Class<T> entityClass, Mongo mongo, Morphia morphia, String dbName) {
super(entityClass, mongo, morphia, dbName);
}
}
Where, entityClass is provided at run-time - how can I use guice to bind the said type to an interface?
public class RunnerModule extends AbstractModule {
#Override
protected void configure() {
bind(GenericDAO.class).to(ObjectDAOMongoDBImpl.class);
}
}
public class Runner<T, V> {
GenericDAO<T, V> dao;
#Inject
public Runner(GenericDAO<T, V> dao) {
this.dao = dao;
}
public static void main(String[] args) {
Injector injector = Guice.createInjector(new RunnerModule());
injector.getInstance(Runner.class);
}
}
It's fine to define mongo, morphia, and dbName as literals to RunnerModule (is there a cleaner way?), but I have no way of knowing what entityClass is until runtime.
This isn't doable with Guice idiomatically, and it isn't its primary focus either.
jfpoilpret have said everything that can be said, but I would like to approach the problem from another direction, where you have the option to (possibly) solve your problem by losing type-safety.
So, in your code, you ask Guice to get an instance of your Runner<T, V> class like this
injector.getInstance(Runner.class);
but this can't be resolved by Guice, because Runner<T, V> has a dependency on GenericDAO<T, V>, but you didn't bind an exact implementation for it. So as jfpoilpret has said, you have to bind some concrete implementations for it in your module.
I'm guessing that you want to determine the exact GenericDAO<T, V> implementation that you pass to your Runner<T, V> based on some input data, which data's type isn't known at compile time. Now, let's assume you have two implementations.
bind(new TypeLiteral<GenericDAO<String, ObjectID>>(){}).to(StringDAO.class);
bind(new TypeLiteral<GenericDAO<Double, ObjectID>>(){}).to(IntegerDAO.class);
Based on different type of inputs you can do this
Injector injector = Guice.createInjector(new RunnerModule());
// possible input which you get from *somewhere* dynamically
Object object = 1.0;
TypeLiteral<?> matchedTypeLiteral = null;
for (Key<?> key : injector.getAllBindings().keySet()) {
TypeLiteral<?> typeLiteral = key.getTypeLiteral();
Type type = typeLiteral.getType();
if (type instanceof ParameterizedType) {
ParameterizedType parameterizedType = (ParameterizedType) type;
if (parameterizedType.getRawType() == GenericDAO.class) {
List<Type> actualTypeArguments = Arrays.asList(parameterizedType.getActualTypeArguments());
if (actualTypeArguments.get(0) == object.getClass())
matchedTypeLiteral = typeLiteral;
}
}
};
Runner<?, ?> runner = new Runner<>((GenericDAO<?, ?>) injector.getInstance(Key.get(matchedTypeLiteral)));
System.out.println(runner.dao.getClass()); // IntegerDAO.class
If Object object = "string";, then the other implementation will be found. This is of course rather ugly and can be improved with checking for sub-classes and stuff, but I think you get the idea. The bottom-line is that you can't get around this.
If you manage to do it (getting around it), please drop me an e-mail because I would like to know about it! I had faced the same problem as you're facing not too long ago. I've written a simple BSON codec where I wanted to load specific implementations of a generic interface based on the type of some arbitrary input. This worked well with Java-to-BSON mappings, but I couldn't do it the other way around in any sensible way, so I've opted for a simpler solution.
The way you wrote it, entityClass can only be Object.class (== Class<Object>), and nothing else.
Hence, first of all, your ObjectDAOMongoDBImpl should be generic:
public class ObjectDAOMongoDBImpl<T>
extends GenericDAOMongoDBImpl<T, ObjectId> ...
That part of the problem is related to java, not Guice.
Now for Guice part, you need to define a binding including the generic types, ie by using Guice TypeLiteral:
bind(new TypeLiteral<GenericDAO<T, V>>(){}).to(...);
where T and V must be known in the code above (can't just be generic parameters there).
Looking at this question may also give you further details related to your situation.
This question is a bit old, but I've recently ran into a similar problem, and managed to solve it quite elegantly by adding a tiny extra layer, a factory.
Consider the following repository:
public interface Repository<T extends Model<T>> {
void save(T t);
T load(long key);
}
class SomeDbRepositoryImpl<T extends Model<T>> implements Repository<T> {
private final SomeDbConnection db;
private final Class<T> type;
RepositoryImpl(final Class<T> type, final SomeDbConnection db) {
this.db = db;
this.type = type;
}
...
}
Then, suppose I have a service that needs an instance of Repository<User>. My first try was to try to make Guice pass an instance of Repository<User> in the constructor, and then I'd bind it somehow. The problem is that I really don't want to have to add repository bindings and providers for each model. If I were to do that, the code would look like this:
// Won't work.
class MyService {
private final Repository<User> userRepository;
#Inject MyService(final Repository<User> userRepository) {
this.userRepository = userRepository;
}
...
}
What I ended up doing is creating a RepositoryFactory class, which is not generic by itself, but it contains a generic method.
public interface RepositoryFactory {
<T extends Model<T>> Repository<T> getRepository(Class<T> type);
}
class SomeDbRepositoryFactoryImpl implements RepositoryFactory {
private final SomeDbConnection db;
#Inject SomeDbRepositoryFactoryImpl(final SomeDbConnection db) {
this.db = db;
#Override <T extends Model<T>> Repository<T> getRepository(Class<T> type) {
return new SomeDbRepositoryImpl(type, db);
}
}
So, this is completely type-safe, and I don't have to add a binding for each module. The service that uses a Repository would then look like:
class MyService {
private final Repository<User> userRepository;
#Inject MyService(final RepositoryFactory f) {
this.userRepository = f.getRepository(User.class);
}
...
}
You could also keep the instance of RepositoryFactory instead of already getting the Repository instance.
I hope this can be useful to someone.
Beyond what Kohányi has said, you could load the DAO or entity classes reflectively by name, and then bind only the specific types asked for in the command-line arguments:
package com.example;
public class App
{
public static void main(final String[] args)
{
final Injector appleInjector = Guice.createInjector(new DynamicDaoModule(getClass("com.example.AppleDao")));
appleInjector.getInstance(Runner.class);
final Injector orangeInjector = Guice.createInjector(new DynamicDaoModule( getClass("com.example.OrangeDao")));
orangeInjector.getInstance(Runner.class);
// final Injector commandLineInjector = Guice.createInjector(new DynamicDaoModule(getClass(args[0])));
// commandLineInjector.getInstance(Runner.class);
}
private static Class getClass(final String className)
{
try
{
return Class.forName(className);
}
catch (final ClassNotFoundException e)
{
throw new RuntimeException(e);
}
}
}
class DynamicDaoModule extends AbstractModule
{
private final Class<? extends GenericDao<? extends Entity>> daoClass;
public DynamicDaoModule(final Class<? extends GenericDao<? extends Entity>> daoClass)
{
this.daoClass = daoClass;
}
#Override
protected void configure()
{
// bind GenericDao<? extends Entity> to daoClass
final TypeLiteral<GenericDao<? extends Entity>> daoOfEntity = (TypeLiteral) TypeLiteral.get(Types.newParameterizedType(GenericDao.class, Types.subtypeOf(Entity.class)));
bind(daoOfEntity).to(daoClass);
}
}
interface Entity
{
}
class Apple implements Entity
{
}
class Orange implements Entity
{
}
class Runner
{
#Inject
public Runner(final GenericDao<? extends Entity> dao)
{
System.out.println("This runner has an " + dao);
}
}
class GenericDao<T extends Entity>
{
private final Class<? extends Entity> entityClass;
protected GenericDao(final Class<? extends Entity> entityClass)
{
this.entityClass = entityClass;
}
#Override
public String toString()
{
return String.format("%s constructed with entityClass %s", getClass().getSimpleName(), entityClass.getSimpleName());
}
}
class AppleDao extends GenericDao<Apple>
{
#Inject
public AppleDao()
{
super(Apple.class);
}
}
class OrangeDao extends GenericDao<Orange>
{
#Inject
public OrangeDao()
{
super(Orange.class);
}
}
And the output would be
This runner has an AppleDao constructed with entityClass Apple
This runner has an OrangeDao constructed with entityClass Orange
I have changed the example to have entity classes implement an interface, in case they share some functionality that would be useful to Runner or GenericDao. If in fact you don't have such an interface, the technique also works with entity classes like String and Double if you remove the extends Entity upper bounds (e.g. GenericDao<T>).
I also removed the <T> parameter on Runner, as that was providing no benefit due to type erasure. If you were to subclass Runner<T>, then you might have Guice provide AppleRunner extends Runner<Apple> or OrangeRunner extends Runner<Orange>. But the type parameter provides nothing if Runner itself is the only concrete class Guice will be providing.
EDIT Oops, I left the class injections in. They are removed now. Of course, if you have a concrete GenericDao subclass for each entity, then perhaps you don't need to inject the entity classes themselves.
I suppose it's not clear to me whether you can provide concrete GenericDao subclasses for all the entity types ahead of time or not. If not, and you were only using the GenericDao class itself for each different kind of entity class, then you would want to inject concrete entity classes and not concrete DAO classes.