Question

I’m developing some kind of plugin which should be called by external java app.
my Plugin is using Spring and of cause I tried to simplify my as I can:

Let’s consider that this is 3d party app and it’s calling my plugin in its main function.

public class ThirdPartyClass {

    public static void main(String[] args) {
        GeneralPlugin plugin  = new MyPlugin();
        plugin.init();
        //calling ext. plugin functionality.
        plugin.method1();
    } 
}

Now this is my plugin

package com.vanilla.spring;

@Component 
    public class MyPlugin implements GeneralPlugin{

        @Autowired
        Dao mydao;

        public void init(){
            //some initiation logic goes here...
        }

        public void method1(){
            dao.delete();
        }
    }

Now my Dao

package com.vanilla.spring;

Component(value="dao")
public class MyDao {

    public void delete(){
//SOME DATABASE LOGIC GOES HERE!!!  
}
}

now my XML:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns:p="http://www.springframework.org/schema/p"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.0.xsd
<context:annotation-config />
<context:component-scan base-package="com.vanilla.spring"></context:component-scan>
</beans>

My problem is that my dao is null and I’m getting NullPointerException when accessing dao object.

I
m believe it happens because I’m initiation bean out of Application context and as the result my Autowiring is not working.

Is there any other way to make autowiring work?

Answer 1

“Spring beans” are just that: Java beans. They have no intrinsic abilities other than those given to them by inheritance or object instantiation.

The Spring Application Context is responsible for creating the bean and “wiring” it, which is the process of creating other beans in the context and calling the bean’s setters (and constructors) with the results to configure them. To do this is uses the XML configuration file and annotations to decide what to create and where to put it.

If you aren’t going to use an actual Application Context, then you have to do all of that work yourself, manually. That is, create the DAO with the proper data source, create the plugin bean, and set the DAO on the plugin bean.

In this specific example, since the 3rd party application controls the instantiation of your plugin bean, you will likely have to either a) create the DAO in the plugin constructor (which is what you’re using Spring to avoid in the first place), or b) create an Application Context in the plugin constructor and reference the beans the plugin needs by querying the context. This isn’t quite as useful as letting the context do everything, but at least you don’t have to configure the rest of the beans your application uses manually (with user names, connection URLs, etc).

If you go the second route you would then need the Spring configuration file somewhere in the classpath or somehow otherwise able to be referenced by the plugin bean.