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How to Generate Customized Java 8 Code with Plugins

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How to Generate Customized Java 8 Code with Plugins

Learn how you can plug in your own Java classes into Speedment to control the code generation. You can map to virtually any type using custom modules.

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One thing most programmers hate is to write boilerplate code. Endless hours are spent setting up entity classes and configuring database connections. To avoid this you can let a program like Speedment Open Source generate all this code for you. This makes it easy to get a database project up and running with minimal manual labour, but how do you maintain control of the written code when large parts of it is handed over to a machine?

Say that you have a database with a table "user" which has a column "gender", and you want that implemented as an enum in java. If you run Speedment and use it to generate code, the "gender" field will be represented as a String. The reason for this is that there isn’t any built-in mappers for converting between database ENUMs and custom java classes. This is one of those cases when you might feel that the generator is taking away control for you. Well, fear not, for since the 2.3 Hamilton release, you can get this same control by creating your own plugin for Speedment!

The Goal of this Article

In this example we have a database schema with a table called "Person". A person has an id, a name and a gender. The gender is declared as an ENUM with three possible values: "Male", "Female" and "Other". If we use the default settings in Speedment to generate this class, Speedment will consider the ENUM a String. There are some issues with this however. For an example, if you want to persist a new person into the database, there is nothing that prevents you from spelling a gender wrong and getting an exception when you do the insert. Instead, we want to define a java enum with the specified alternatives as constants. What would make the generated code more secure and easier to use.

We can achieve this using a plugin for Speedment!

Creating the Plugin Project

To do any custom modifications to the Speedment platform we will need to define a plugin. A plugin is a piece of software that can be plugged into the Speedment runtime from the pom.xml-file. The plugin resides in its own maven project and can be shared between projects.

Begin by creating a new Maven Project and declare Speedment as a dependency. You will not need the speedment-maven-plugin in this project.

<dependency>
    <groupId>com.speedment</groupId>
    <artifactId>speedment</artifactId>
    <version>${speedment.version}</version>
</dependency>


The plugin system revolves around two interfaces; Component and ComponentConstructor. A Component is a pluggable piece of software that can be executed as part of the Speedment lifecycle. Every component has a number of stages in which it is allowed to execute. These are "initialize", "load", "resolve" and "start".

The ComponentConstructor is a lightweight type that has a default constructor and a method for initializing new instances of the custom component. This is used by the maven plugin to setup the new code.

Here is how our two implementations will look:

CustomMappingComponent.java

public final class CustomMappingComponent 
extends AbstractComponent {

    CustomMappingComponent(Speedment speedment) {
        super(speedment);
    }

    @Override
    public void onResolve() {
        // Resolve logic here...
    }

    @Override
    public Class<CustomMappingComponent> getComponentClass() {
        return CustomMappingComponent.class;
    }

    @Override
    public Software asSoftware() {
        return AbstractSoftware.with(
            "Custom Mapping Component", 
            "1.0", 
            APACHE_2
        );
    }

    @Override
    public Component defaultCopy(Speedment speedment) {
        return new CustomMappingComponent(speedment);
    }
}


CustomMappingComponentInstaller.java

public final class CustomMappingComponentInstaller 
implements ComponentConstructor<CustomMappingComponent> {

    @Override
    public Component create(Speedment speedment) {
        return new CustomMappingComponent(speedment);
    }
}


We now have a bare-bone plugin that can be added to a Speedment project. The next step is to define the logic that maps between strings and genders. To this this, first we need the Gender enum.

Gender.java

public enum Gender {
    MALE   ("Male"), 
    FEMALE ("Female"),
    OTHER  ("Other");

    private final String databaseName;

    Gender(String databaseName) {
        this.databaseName = databaseName;
    }

    public String getDatabaseName() {
        return databaseName;
    }
}


If you store the enum values in upper-case in the database, this class could be much shorter since you could simply use the Enum.name()-method to get the database name, but this approach is better if you want flexibility in naming the constants.

Now for the final piece. We need to declare a type that implements the TypeMapper-interface in Speedment. A type mapper is really simple. It contains two methods for mapping to and from the database type as well as methods for retrieving the java class of both types.

StringToGenderMapper.java

public final class StringToGenderMapper implements TypeMapper<String, Gender> {

    @Override
    public Class<Gender> getJavaType() {
        return Gender.class;
    }

    @Override
    public Class<String> getDatabaseType() {
        return String.class;
    }

    @Override
    public Gender toJavaType(String value) {
        if (value == null) {
            return null;
        } else {
            return Stream.of(Gender.values())
                .filter(g -> g.getDatabaseName().equals(value))
                .findAny()
                .orElseThrow(() -> 
                    new UnsupportedOperationException(
                        "Unknown gender '" + value + "'."
                    )
                );
        }
    }

    @Override
    public String toDatabaseType(Gender value) {
        if (value == null) return null;
        else return value.getDatabaseName();
    }

    @Override
    public boolean isIdentityMapper() {
        return false;
    }
}


This new mapper also need to be installed in the Speedment platform. We can do that from the component we created earlier by modifying the onResolve()-method:

CustomMappingComponent.java

@Override
public void onResolve() {
    // Resolve logic here...
    getSpeedment().getTypeMapperComponent()
        .install(StringToGenderMapper::new);
}


Our new plugin is now done! Build the project and you are set to go!

Using the Plugin

To use a plugin in a project, you only need to modify the pom.xml-file of that project. Open up an existing Speedment project and locate the pom.xml-file. In it, you should be able to find the speedment-maven-plugin. To make your own plugin accessible for the maven plugin, you need to add it as a dependency inside the <plugin>-tag and add the ComponentInstaller to the configuration. Here is an example of how it can look:

pom.xml

<plugin>
    <groupId>com.speedment</groupId>
    <artifactId>speedment-maven-plugin</artifactId>
    <version>${speedment.version}</version>

    <dependencies>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>${mysql.version}</version>
        </dependency>

        <!-- Our plugin project -->
        <dependency>
            <groupId>com.github.pyknic</groupId>
            <artifactId>custom-mapping-component</artifactId>
            <version>1.0.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

    <configuration>
        <components>
            <!-- Path to the component installer -->
            <component implementation="
com.github.pyknic.salesinfo.plugin.CustomMappingComponentInstaller
            " />
        </components>
    </configuration>
</plugin>


You also need to add the project as a runtime dependency since the new Gender-enum must be accessible from the generated code.

<dependencies>
    ...
    <dependency>
        <groupId>com.github.pyknic</groupId>
        <artifactId>custom-mapping-component</artifactId>
        <version>1.0.0-SNAPSHOT</version>
    </dependency>
    ...
</dependencies>


Trying it out

That’s it! The plugin is installed! If you want to a particular column to be mapped to a Gender instead of a String, you can go into the User Interface, navigate to the particular column in the "Project Tree" and select your new Type Mapper in the dropdown list.

screenshot of the Speedment user interface

If you want to see a list of all the components and/or type mappers loaded into the platform, you can also go to "About" → "Components..." in the UI. There you should see the new component.

screenshot of the components dialog in the Speedment user interface

Summary

In this article you have learned how to create a custom plugin for Speedment that integrates a new Type Mapper from a String to a Gender enum. You have also learned how you can see which components are loaded into the platform and select which type mapper you want to use for each column.

PS: If you create some cool new mappers for your Speedment project, consider sharing them with the community in our Gitter chat!

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Topics:
java 8 ,code generation ,stream ,enum

Published at DZone with permission of Emil Forslund, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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