Skip to main content

Getting a Java Type Parameter class at runtime

Certain use cases like deserializing a collection requires knowing the type of the declared field. As the common understanding goes, Type Parameters are erased at build time. So How do we get them?

Its a common misconception that "Type Parameters are erased at build time" - Instead what is actually happening is - Generic collections are converted to raw types. The class definition is NOT modified at build/run time. The Type Parameter(s) can be accessed from the class definition through reflection.

For example:

Lets say here is a Bean

public class Document {
 private List<Model> models;
        ..
}

Given doc, an instance of Document, at runtime, getting the type of doc.models by traditional reflection, can only get that its type is java.util.List

//somewhere in runtime
Document doc = new Document();

 Field f = bean.getClass().getDeclaredField(field);
 f.setAccessible(true);
 assert f.getClass() == List.class

To get the type from parameter, Assuming we know that it is a ParameterizedType (like List<T>)

private <T> Class getListType(T bean, String field) throws NoSuchFieldException {
 Class childType;
 Field f = bean.getClass().getDeclaredField(field);
 f.setAccessible(true);
 childType = (Class) ((ParameterizedType) f.getGenericType()).getActualTypeArguments()[0];
 return childType;
}

//somewhere in runtime
Document doc = new Document();
assert getListType(doc, "models") == Model.class


The same logic can be applied to get multi parameter types (like Map)

 keyType = (Class) ((ParameterizedType) f.getGenericType()).getActualTypeArguments()[0];
 valueType = (Class) ((ParameterizedType) f.getGenericType()).getActualTypeArguments()[1];

Popular posts from this blog

Powered By

As it goes, We ought to give thanks to people who power us. This page will be updated, like the version page , to show all the tools, and people this site is Powered By! Ubuntu GIMP Firebug Blogger Google [AppEngine, Ajax and other Apis] AddtoAny Project Fondue jQuery

One page Stock

Alright.. That was a long absence. The whole last week I dint blog. I dint go away. I was "occupied". I was learning stock trading. Its very fascinating. I have a good weeeked blog for you all. Here is my experience. I can literally hyper-link every word from the following paragraphs, but I am writing it as simple as I can so you can look up the italicised words in wikipedia . I got a paper trading account from a brokerage firm . You need one brokerage account first. Then it can be an Equity account where all your money is yours or a Margin account , where some of the money is lent by the brokerage firm. Then I get Buying power , which is the dollor value of how much stocks you can buy. I can make profit by simple rules. Buy when Price is low. Sell when price is high. There is another more intersting way of earning money. Selling short . Thats when price is not high, per say, but when are confident that the price WILL go down. then buy back when its lowest. This is what

classpath*: making your Modular Spring Resources

Spring gives multiple options to load XML resources for building contexts. the reference documentation does explain this feature quite well. However, I am taking my shot at explaining the different practical scenarios ( by order of growing modularisation) For Example, A simplest Spring based web Context Loader can be configured with resources like this <context-param> <param-name>contextConfigLocation</param-name> <param-value>applicationContext.xml</param-value> </context-param> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> You just need to put applicationContext.xml in WEB-INF/ folder of your webapp. However, Typically an application is n-tiered. You can also have multiple files setup and in relative paths. like <param-value> context-files/applicationContext.xml context-files/dao.xml context-files/service.xml </param-value>