Develop Reference

CDI: Dynamical injection of a group of classes how to?

I need to dynamically Inject a variable group of classes in my application. The purpose is, as the application grows, only have to add more classes inheriting the same interface. This is easy to do with tradicional java as I just need to search for all classes in a package and perform a loop to instantiate them. I want to do it in CDI. For example:
public MyValidatorInterface {
public boolean validate();
}
#Named
MyValidator1 implements MyValidatorInterface
...
#Named
MyValidator2 implements MyValidatorInterface
...
Now the ugly non real java code just to get the idea of what I want to do:
public MyValidatorFactory {
for (String className: classNames) {
#Inject
MyValidatorInterface<className> myValidatorInstance;
myValidatorInstance.validate();
}
}
I want to loop over all implementations found in classNames list (all will be in the same package BTW) and Inject them dynamically so if next week I add a new validator, MyValidator3, I just have to code the new class and add it to the project. The loop in MyValidatorFactory will find it, inject it and execute the validate() method on the new class too.
I have read about dynamic injection but I can't find a way to loop over a group of class names and inject them just like I used to Instantiate them the old way.
Thanks

What you are describing is what Instance<T> does.
For your sample above, you would do:
`#Inject Instance<MyValidatorInterface> allInstances`
Now, allInstances variable contains all your beans which have the given Type (MyValidatorInterface). You can further narrow down the set by calling select(..) based on qualifiers and/or class of bean. This will again return an Instance but with only a subset of previously fitting beans. Finally, you call get() which retrieves the bean instance for you.
NOTE: if you call get() straight away (without select) in the above case, you will get an exception because you have two beans of given type and CDI cannot determine which one should be used. This is implied by rules of type-safe resolution.
What you most likely want to know is that Instance<T> also implements Iterable so that's how you get to iterate over the beans. You will want to do something like this:
#Inject
Instance<MyValidatorInterface> allInstances;
public void validateAll() {
Iterator<MyValidatorInterface> iterator = allInstances.iterator();
while (iterator.hasNext()) {
iterator.next().callYourValidationMethod();
}}
}

GsonRequest class is a List

I am currently making a GsonRequest as follows:
final GsonRequest gsonRequest =
new GsonRequest(url, People.class, null, new Response.Listener<People>() { ... }
But my People class has only one member object: List of Person(s).
public class People {
private List<Person> people;
}
I did it like because the argument for the GsonRequest called for a class (i.e., People.class). To me it seems strange and silly to make a class that only has one member object which is just a list of another objects. But the request I am making will return multiple Person(s). So, is there a better way? Can I pass a List of objects instead of a made up class like I did? My way is working, but I can't help but think there is a better way???

You can define it like this:
public class People extends List<Person> {
}
And that will work, at least you aren't defining a one-member class (you also need to make API change to match).
If you want to avoid the class completely, you can also use TypeToken<E>:
new TypeToken<List<Person>>(){}.getType()
However your GsonRequest class must be able to accept a Type instead of a class parameter.

Java - how can I loop methods with same name but different parameters

I have class named: ComplexValidator that extends absract class Validator which have two methods:
Validate(Part part);
getAnswer():
I also have validators, lets name them A, B, C and D.
So
AValidator extends Validator
BValidator extends Validator
CValidator extends Validator
DValidator extends Validator
I am not in front of my code right not so I will use pseudo-code.
CValidator takes different parameter than rest of it, A B and D uses part to get filename etc, but CValidator uses numberOfFiles (which are increased in loop (for part p: multipart) so after every time loop is repeated numberoffiles is increased so I can compare it with maxNumberOfFiles field).
Sadly I didnt know how to make abstract class that will take any parameter to method so all methods must take Part part. Cvalidator doesnt use it and I had to make field numberOfFiles static so I can get access to it.
Is there any way to make those validators takes no parameters but no using static?
Is there any way to make abstract class the way that child classes will be able to change arguments it take?
And if it takes other arguments HOW can I loop it all when I have:
List <Validator> validators = new ArrayList<>();
in my ComplexValidator.
and then I add all child validators to it and loop over them like that:
for (Validator v: validators){
validate(part);
}

The types of the parameters of an overriden method must be the same as the original method.
To face your problem I would create a custom class that wraps all the different parameters that you might want to pass to the validate function.
Something like that
class Wrapper{
Part param1;
File param2;
File param3;
}
class Validator{
void validate (Wrapper wrapper);
}
class ValidatorA extends Validate{
void validate (Wrapper wrapper){
//use wrapper.part...
}
}
class ValidatorC extends Validate{
void validate (Wrapper wrapper){
//use wrapper.file...
}
}

You may want to use java reflection. With a Class you can either getMethods and loop throught the methods and getParameterTypes of each method or if you know in advance the types of the method you wish you can getMethod (without s) and provide an array of type.
In your case I would go to the first method and depending on the presence of the second parameter (number of files), invoke the method the good way (with all the parameters needed).

Best way to pass arguments in constructor

I have a class that create rows in table layout. The row creation depend upon data and metadata. As metadata is same for each row like show/hide visibility properties etc. so I have created metadata property as a static and initialize once using initWidget of RowWidget.
just example:
class RowWidget extends FlexTable{
public static void initWidget(Form form,
HashMap<Long, ContractorPermissionEnum> formModePermissionMap,
GridMode gridMode,
boolean isApplied,
boolean isChildExist,
boolean isChildAttachment)
{
// ...
}
}
Then I called below constructor for each record data.
public RowWidget(DataRawType dataRawType, Data data, Data parentData) {
// ...
}
As I thought this is not right approach. because as pattern when anyone see this class then understand it will create one row. I don't want to call initially initWidget. I want to pass each required parameter in constructor only like
public RowWidget(DataRawType dataRawType,
Data data,
Data parentData,
Form form,
HashMap<Long, ContractorPermissionEnum> formModePermissionMap,
GridMode gridMode,
boolean isApplied,
boolean isChildExist,
boolean isChildAttachment) {
// ...
}
But due to this, constructor have no of arguments. and I think it's also bad pattern to have 5+ parameter in constructor.
Is Anyone suggest me:
How to construct class which have same property required in another
instance?
Note:I know this is possible through static only but don't want to use static.
What is best way to construct class with having some default fix
property for all instances?
Note: I don't want to create another class to achieve it. or any getter/setter method.
Thanks In advance.

I would suggest builder pattern. You would need one extra class to create RowWidget objects. So the call would look like that:
RowWidget widget = new RowWidget.Builder().withData(data).withParentData(parentData).withDataRawType(dataRawType).build();
Here is neat explanation of the pattern:https://stackoverflow.com/a/1953567/991164

Why not create method which will accept the newValues for the properties you want to change & return a new instance of the classes with all other properties copied from the instance on which you invoked this method.

You could separate/extract the parameters from the RowWidget-class fro example in a RowWidgetConfig-class.
class RowWidgetConfig {
// put here all your parameters that you need to initialize only once
// init using setters
}
Now create once instance of that class and pass it among the other parameters to RowWidget constructor.
Another alternative would be to have factory for creating RowWidget instances. The factory would also contain all the parameters you need for a row instance plus a factory method createNewRowWidget witch creates an instance base on the parameters contained in the factory.
class RowWidgetFactory {
// put here all your parameters that you need to initialize only once
// init using setters
public RowWidget createNewRowWidget() {
// create
return ...
}
}

How to construct class which have same property required in another instance?
To achive this you can have a super class with all the properties you want. So any class extending this super class will be have these properties. This way you don't need to use static keyword.
What is best way to construct class with having some default fix property for all instances?
For this one you can have an interface with some constant properties. This way any class implementing this interface will be having the fixed properties.

The static initWidget() thing just doesn't seem right for me. Though probably now you will only have one set of RowWidgets which share some properties, it is also reasonable to have 2 sets of RowWidgets, each set will have its own "shared" properties. Things will be much more fluent and you have much more choices in building more reasonable APIs if you refactor your code to make a more reasonable design
Assume now I introduce something like a RowGroup (which kind of represents the "shared" thing you mentioned)
(Honestly I don't quite get the meaning for your design, I am just making it up base on your code);
public class RowGroup {
public RowGroup(Form form,
HashMap<Long, ContractorPermissionEnum> formModePermissionMap,
GridMode gridMode,
boolean isApplied,
boolean isChildExist,
boolean isChildAttachment) { .... }
public void addRow(DataRawType dataRawType, Data data, Data parentData) {...}
}
When people use, it looks something like:
RowGroup rowGroup = new RowGroup(form, permissionMap, gridMode, isApplied, isChildExist, isChildAttach);
rowGroup.addRow(DataRawType.A, dataA, parentA);
rowGroup.addRow(DataRawType.B, dataB, parentB);
You may even provide builder-like syntax or a lot other choices.
RowGroup rowGroup
= new RowGroup(.....)
.addRow(DataRawType.A, dataA, parentA)
.addRow(DataRawType.B, dataB, parentB);
Even more important, the design now make more sense to me.

If you did not want to create another class, I'd suggest what A4L suggested.
Without creating another class, I would create constructor that takes all parameters and factory method that uses current instance as template and pass its own parameters to constructor parameter.
example (with obvious parts ommited)
class A{
public A(int p1, int p2){...}
public A create(int p2) {
return new A(this.p1,p2);
}

how to read object attribute dynamically in java?

Is there any way to read and print the object attribute dynamically(Java) ? for example if I have following object
public class A{
int age ;
String name;
float income;
}
public class B{
int age;
String name;
}
public class mainA{
A obj1 = new A();
method(A);
method(B);
}
the output should be like
While running method(A):
Attribute of Object are age,name,income;
While executing method(B):
Attribute of Objects are age,name;
My question is I can pass various object in method(), is there any way I can access the attribute of the differnt object in general.

You want to use The Reflection API. Specifically, take a look at discovering class members.
You could do something like the following:
public void showFields(Object o) {
Class<?> clazz = o.getClass();
for(Field field : clazz.getDeclaredFields()) {
//you can also use .toGenericString() instead of .getName(). This will
//give you the type information as well.
System.out.println(field.getName());
}
}
I just wanted to add a cautionary note that you normally don't need to do anything like this and for most things you probably shouldn't. Reflection can make the code hard to maintain and read. Of course there are specific cases when you would want to use Reflection, but those relatively rare.

Using org.apache.commons.beanutils.PropertyUtils we can do this. If the proper getters and setters are defined for the bean we can also dynamically set the value:
import org.apache.commons.beanutils.PropertyUtils;
import java.beans.PropertyDescriptor;
public class PropertyDescriptorTest {
public static void main(String[] args) {
// Declaring and setting values on the object
AnyObject anObject = new AnyObject();
anObject.setIntProperty(1);
anObject.setLongProperty(234L);
anObject.setStrProperty("string value");
// Getting the PropertyDescriptors for the object
PropertyDescriptor[] objDescriptors = PropertyUtils.getPropertyDescriptors(anObject);
// Iterating through each of the PropertyDescriptors
for (PropertyDescriptor objDescriptor : objDescriptors) {
try {
String propertyName = objDescriptor.getName();
Object propType = PropertyUtils.getPropertyType(anObject, propertyName);
Object propValue = PropertyUtils.getProperty(anObject, propertyName);
// Printing the details
System.out.println("Property="+propertyName+", Type="+propType+", Value="+propValue);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
To set the value of a particular property:
// Here we have to make sure the value is
// of the same type as propertyName
PropertyUtils.setProperty(anObject, propertyName, value);
Output will be:
Property=class, Type=class java.lang.Class, Value=class genericTester.AnyObject
Property=intProperty, Type=int, Value=1
Property=longProperty, Type=class java.lang.Long, Value=234
Property=strProperty, Type=class java.lang.String, Value=string value

You can use reflection to get every field from your object (if security configuration allows you).
If you need it not for the sake of self-education, then it may be worth using ReflectionUtils from Apache Commons.

You can use reflection, but the API is not very nice to use. But what you are trying to do is not at all object-oriented. The A and B should have method "print yourself" which would output their values (you should specify the method in superclass/interface to call the method using polymorphism).

I think I would consider a different approach.
If you really want to treat these like data is there any reason they couldn't be hashtables (Do they have associated code)?
Reflection will do it but it's a last resort--you should always seriously consider different approaches before dropping to reflection.
Cases where you must access variables like that exist--like database mapping (Hibernate) and injection (Spring). You might want to consider if a packaged solution like that fits your need so that future programmers can understand what you did without learning everything about your specific solution.
Also, Spring injection can do things that might fit your needs.
Also also if you are going to use reflection, seriously consider annotations so that you aren't tying your functionality to what should be simple arbitrary attribute names.