Serializing Java Objects

Juli 18th, 2015

There are various ways to serialize Java Objects. This be came an issue for me in the context of the save game feature for OpenPatrician. So it was clear from the start that the output format should be XML. As The project already uses JAXB to generate some classes from XSD this seemed to be an obvious choice. Another option turned out to be XStream.
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Mountain Range in Gimp

März 23rd, 2014

Maps contain usually some mountains and are a projection from above. One way to distinguish the height level is by using colors as this is typically done by relief maps. The map that I’m creating uses color for different kinds of vegetation. Therefore there needs to be another approach. This image tutorial follows the same path as this video tutorial.

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Package private beans

Februar 16th, 2014

Thinking about separation in my code the question of package private beans popped up. The general pattern here is to have interfaces defined. The implementing classes however are package private. Instances are generated through a factory. I looked around to find some documentation how this can be done with a Spring context, but found none, so here it is. Special interest is the Spring configuration with Java.

The package private beans are annotated and the classpath is scanned. These classes are discovered, as this basically involves reflection. The beans however cannot be instantiated in the configuration, as the beans are not visible in the package of the configuration class. This is where the factory comes in. The factory resides in the same package as the beans however is a public @Service and so is available in the application context.

I tried this out with two beans:

package ch.sahits.spring.bean.internal;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

import java.util.Random;

@Component
class PackagePrivateBean {
    @Autowired
    private Random rnd;

    public Random getRnd() {
        return rnd;
    }
}
package ch.sahits.spring.bean.internal;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Component;

@Component
@Scope(value = "prototype")
class PackagePrivatePrototype {
    private final int a;

    public PackagePrivatePrototype(int a) {
        this.a = a;
    }

    public int getA() {
        return a;
    }
}

The configuration looks like this:

 

package ch.sahits.spring;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

import java.util.Random;

@Configuration
@ComponentScan(basePackages = {"ch.sahits.spring.bean.internal"})
public class PackagePrivateSpringConfiguration {
    @Bean
    public Random rnd() {
        return new Random();
    }

}

The factory:

package ch.sahits.spring.bean.internal;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Service;

@Service
public class PackagePrivateBeanFactory {

    @Autowired
    private ApplicationContext context;

    public PackagePrivateBean getBean() {
        return context.getBean(PackagePrivateBean.class);
    }

    public PackagePrivatePrototype getBean(int i) {
        Object[] args = new Object[]{Integer.valueOf(i)};
        return (PackagePrivatePrototype)context.getBean("packagePrivatePrototype", args);
    }
}

I tested this with a Test-Case:

package ch.sahits.spring;

import ch.sahits.spring.bean.internal.PackagePrivateBeanFactory;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
import org.springframework.test.context.support.AnnotationConfigContextLoader;

import static org.junit.Assert.assertNotNull;

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes=PackagePrivateSpringConfiguration.class, loader=AnnotationConfigContextLoader.class)
public class PrivatePackageConfigurationTest {
    @Autowired
    private PackagePrivateBeanFactory factory;

    @Test
    public void factoryShouldNotBeNull() {
        assertNotNull(factory);
    }

    @Test
    public void packagePrivateComponentShouldNotBeNull() {
        Object o = factory.getBean();
        assertNotNull(o);
    }

    @Test
    public void packagePrivatePrototypeShouldNotBeNull() {
        Object o = factory.getBean(47);
        assertNotNull(o);
    }
}

The curious thing is that the PackagePrivateBean is by default of scope Singleton and thereby has to be created on application context startup. As this happens through reflection of the default constructor this is even possible even though the bean itself is not visible and can therefore not be defined through the configuration class. This is a case, where you get an instance through the package scan that cannot be done through explicit definition. This applies in the same manner for XML configuration. Defining a package private bean in XML would cause a runtime exception when reading the XML application context.

Shutting down Spring backed JavaFX application

Februar 8th, 2014

This is just another short tip on how to combine JavaFX and the Spring Framework. These two technologies do not work perfectly together, but they can be made to cooperate. One of the two is the lead, meaning one is initialized first from the main method and therefore controls the lifecycle of the application. In my case this is the Spring framework as the user interface is optional. However I still want to handle the case where the user interface is closed (or the menu point quit is selected) and this should also shut down the application context of Spring.

There are three steps that should be taken:

  • Register the shutdown hooks for your application context. This has nothing to do with canceling the application in a controlled manner. This case covers things like Ctrl+C or kill -9.
((AbstractApplicationContext)context).registerShutdownHook();
  • Override the stop method of Application. When closing the application window, the stop method is called. You will want to hook into this method to signal the spring context that his end has come.
    @Override
    public void stop() throws Exception {
        System.out.println("Stopping the UI Application");

        stopUIApplicationContext();
        super.stop();
    }

In my case I used Guava’s EventBus to signal the context.

  • Finally in your code handling the context end it.
((AbstractApplicationContext)context).close();

That’s it, folks!

Custom Spring scope done right

Januar 31st, 2014

You can define your own Spring Scope. One thing to consider is that the beans that are defined in a scope must somehow be backed by a store. For the singlton scope this is the application context itself, for a session scope it is the session, for a request scope it is the request. With this setup the beans lifecycle is bound to the lifecycle of the backing store, e.g. when a session terminates it is deleted and with it all the corresponding beans.

There are other examples out there that implement a custom scope by implementing the store in the scope. This then requires that the lifecycle has to be managed from the outside. So let’s do this the right way:

  • We want a scope that matches the lifecycle of a dialog. The dialog is opened some content is displayed, some work is done and finally the dialog is closed.
  • The application code should not be polluted with logic to manage the bean life cycle of the scope.

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