Extending Your Application with Custom Java

Last modified: April 18, 2024

1 Introduction

Most application logic can be developed using microflows and nanoflows. These are very powerful and contain most of the features you need in every application. However, If there is a feature missing you can extend Mendix microflows with the use of Java actions.

This document teaches you how to do the following:

Extend your application with custom Java code you can call from a microflow
Use the Mendix Runtime API to retrieve data from the database using an XPath in a Java action

2 Prerequisites

This how-to assumes that you are familiar with using Studio Pro to create a simple app with a Domain Model, Pages, and Microflows.

Before starting this how-to, Mendix recommends that you install Eclipse. Eclipse is a Java editor which can be downloaded from the Eclipse Foundation.

Mendix recommends using Eclipse to edit your Java code, although you can use any text editor. Studio Pro contains a Deploy for Eclipse feature which automatically configures everything so you only have to import the app into your Eclipse working environment.

3 Setting Up a Simple App

Before you start, you need to create a simple app in which to create and use your Java actions. The app will store a number of products, each of which is assigned to a category. Each product has an indication as to whether it is available or not.

To create the app, follow these steps:

Open Studio Pro and Create New App using the Blank Web App template.
Right-click the module MyFirstModule and rename it to Products.
Create a Domain model in the Products module consisting of three entities:
- Category – with attribute
  - Name (string)
- Product – with attributes
  - Name (string)
  - Description (string)
- ProductState - with attributes
  - Available (boolean)
  - Description (string)
- Association Product_Category – one Category has many Products
- Association Product_ProductState - one ProductState has many Products
The Domain Model should look like this:
Right-click one of the entities and Generate overview pages for all three entities.
Add the Product_Overview page to the navigation or home page.
Run your app, go to the product overview page, and add some categories, products, and the two product states for availability true or false.

Now you have some data to use when building your app.

The examples below use categories which include “book” and products with the name “The Lord of the Rings”.

4 Adding a Java Action to Reverse a Product Name

To demonstrate how to add a Java action, you will use Java to reverse the name of a product and display it.

4.1 Adding a Java Action to the Products Module

In this section, you will create a new Java action and deploy the app for Eclipse, which creates a file containing the Java class for the action.

Right-click the Products module and select Add other > Java action.
Enter ReverseProductName for the Name of the new Java action and click OK.
In the Java Action pane, click Add to add a parameter and do the following:
1. Enter productParameter for the Name of the new parameter.
2. Select Object for the Type.
3. Click Select for Entity and select Products.Product as the object type.
4. Click OK.
Change the Type in the Return section of the Java action to String, change the Variable name to ReversedName.
Click ctrl + s to save the Java action.
Select App > Deploy for Eclipse from the Studio Pro menu.

By deploying for Eclipse, Studio Pro generates Java code for all your Java actions. This Java code has three places where you can customize your Java actions:

You can import additional libraries
You can add code between //BEGIN USER CODE and //END USER CODE
You can add code between //BEGIN EXTRA CODE and //END EXTRA CODE

Any other changes you make will be overwritten when you deploy your app.

4.2 Editing the Java Action in Eclipse

To edit the Java action in Eclipse, open Eclipse and follow these steps:

Select File > Import… from the Eclipse menu.
Select Existing Projects into Workspace and click Next >.
In Select root directory browse to the root directory of your app.
Click Finish.

If you don’t know what the app directory is, select App > Show App Directory in Explorer in Studio Pro.

Search in Eclipse for ReverseProductName.java and open it. You can also find it in the Package Explorer pane.

In the Java code, there is some placeholder code between the //BEGIN USER CODE and //END USER CODE comment statements. This is where you can add your own Java code.

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	@java.lang.Override
	public java.lang.String executeAction() throws Exception
	{
		// BEGIN USER CODE
		throw new com.mendix.systemwideinterfaces.MendixRuntimeException("Java action was not implemented");
		// END USER CODE
	}

Studio Pro uses your Java action configuration to generate a Product object called productParameter. You can use this object to get the name of the product and reverse it.

Replace the existing line throw new com.mendix.systemwideinterfaces.MendixRuntimeException("Java action was not implemented"); between //BEGIN USER CODE and //END USER CODE, with the code:
```
String productName = this.productParameter.getName(this.getContext());
return new StringBuilder(productName).reverse().toString();
```
This reverses the name of the product.
Select File > Save to save the Java action in Eclipse.

4.3 Calling the Java Action from a Microflow

Now you will add a button to the Product_NewEdit page which uses a microflow to display the product name backwards when pressed.

Back in Studio Pro, open the Product_NewEdit page.
Add a Call microflow button to the page, next to the Save and Cancel buttons.
Select a New microflow and call it ReverseName. Studio Pro automatically makes the current Product object a parameter to the microflow.
Open the ReverseName microflow.
Drag the ReverseProductName Java action from the App Explorer into the microflow.
In the Call Java Action properties editor, select $Product from the Product parameter drop-down.
Click OK to save the properties.
Drag a Show message activity into the microflow.
In the Show Message properties editor enter Reversed name: {1} for Template.
In the Parameters section, add a New parameter with the value $ReversedName. This is the output variable of the Java action.
Click OK to save the parameter. The Show Message properties should now look like this:
Click OK to save the Show message activity. The microflow will be similar to this this:

4.4 Deploying and Seeing the Results

Click Run Locally () to deploy the application locally, and click View App to open the application in your browser.
Navigate to the Product overview, and add a new Product, or edit an existing one.
Ensure your product has a name, then click Reverse name.

You will see the name of the product in the dialog box, but displayed in reverse.

5 XPath Retrieval in Java

Using the Mendix Runtime API, your Java actions can interact with many parts of your app. One thing that many developers want to do is to retrieve a list of objects using an XPath Constraint. This section describes how to implement an XPath retrieval in a Java action.

Using the Domain Model you set up in Setting Up a Simple App, above, you want to retrieve a list of Products which meet the following criteria:

Have a specified name
Belong to a given category
Are available

The example skips many features you would want to add to a real app, but is designed for simplicity.

5.1 XPath in Microflow

Firstly, a reminder of how this XPath could be implemented in a Retrieve activity. You can use this XPath in a Retrieve activity in a microflow using the following steps:

Create a new microflow in the Products module and name it ListWithRetrieve.
Add a Retrieve activity and configure it as follows:
- Source: From database
- Entity: Products.Category
- XPath constraint: Select a single category you know exists, for example [(Name = 'Book')]
- Range: First
- Output > Type: Products.Category
- Output > Object Name: Category
This will retrieve a category for us to use in the XPath.
Add a second Retrieve activity and configure it as follows:
- Source: From database
- Entity: Products.Product
- XPath constraint:
```
[Products.Product_ProductState/Products.ProductState[
Available = true ()
]]
[Products.Product_Category = $Category]
[Name = 'The Lord of the Rings']
```
- Range: All
- Output > Type: List of Products.Product
- Output > Object Name: ProductList
Set the End Event to return $ProductList as a Return type List of Products.Product.
Now add a Responsive (Web) page to the Products module with the following characteristics:
- Page name: MicroflowProductList
- Layout: Grids > Grid
Set the Data source to be the Microflow Products.ListWithRetrieve and allow the grid to be filled automatically (paging controls are not necessary).
Configure the New Product and Edit () buttons to open the Product_NewEdit page. Alternatively, you can just delete these two buttons.
Go to the Home_Web page and drag the MicroflowProductList page onto it, so that a button is added to the page.
Run your app locally.

Clicking the Microflow product list button will display all Products named “The Lord of the Rings” with the Category “Book” which are Available. You may have to add some examples using the Product Overview if none are displayed.

5.2 XPath in a Java Action

To use this same XPath in a Java action called from a microflow, perform the following steps:

Add a new Java action using the instructions in Adding a Java Action to the Products Module. The Java action has the following configuration:
- Name – findProductByNameAndCategory
- Parameters:
  - name – a String
  - category - an Object of type Products.Category
- Return > Type – List of Products.Product with the Variable name ProductList
Deploy the app for Eclipse using the menu item App > Deploy for Eclipse.
Search in Eclipse for findProductByNameAndCategory.java and open it. If the package was already open in Eclipse, you may need to refresh the editor using F5.

You will now add the Java code to retrieve the product list using an XPath.

Using raw strings in XPath is generally recognized as a bad practice. Using strings you will not get compile errors if the domain model changes. You can avoid this by using format strings and the proxy classes generated by Mendix Studio Pro. As soon as the domain model changes, the proxy classes are regenerated, and the identifiers used in this code will become unresolvable, resulting in compiler errors.

First you need to import the classes you will use.

Add the following import statements after the generated import statements in the Java file:

import java.util.List;
import com.mendix.core.Core;
import products.proxies.Product;
import products.proxies.ProductState;

And now add the XPath code.

Add the following instructions between //BEGIN USER CODE and //END USER CODE to build the XPath and execute it. This replaces the placeholder command to throw a runtime exception.
```
//create the xpath
String xpath = String.format("//%s[%s/%s[%s = true()]][%s = $Category][%s = '%s']",
		Product.entityName,
		Product.MemberNames.Product_ProductState, ProductState.entityName,
		ProductState.MemberNames.Available,
		Product.MemberNames.Product_Category, 
		Product.MemberNames.Name, name
        );
//execute the xpath and return the list of products found
return Core.createXPathQuery(xpath)
		.setVariable("Category", __category)
		.execute(getContext());
```
This, simplified, approach should be made more robust in a real app by checking and sanitizing the inputs to prevent exceptions or attempts at hacking if the end-user can formulate the search.

Alternatively, you can use the XPath class from the Community Commons module, which provides a powerful way of working with XPaths and performs checks automatically.
Select File > Save to save the Java action in Eclipse.
In Studio Pro, Duplicate the ListWithRetrieve microflow you created in the previous section and Rename it ListWithJavaAction.
Edit the ListWithJavaAction microflow.
Delete the second Retrieve activity.
Drag the findProductByNameAndCategory Java action to the end of the microflow
Configure the inputs of the findProductByNameAndCategory Java action as follows:
- Name: a product name which exists in your data, for example ‘The Lord of the Rings’
- Category: $Category
Duplicate the MicroflowProductList page and Rename it JavaActionProductList.
Open the JavaActionProductList page.
Set the Data source for the data grid to be to be the Microflow Products.ListWithJavaAction.
Go to the Home_Web page and drag the JavaActionProductList page onto it, so that a button is added to the page.
Run your app locally.

Clicking the Java action product list button will display all Products named “The Lord of the Rings” with the Category “Book” which are Available. You may have to add some examples using the Product Overview if none are displayed.

6 Troubleshooting

6.1 Unsupported Class Version Error

If you get an UnsupportedClassVersionError when running your app, follow these steps:

Clean your app’s deployment folder by selecting App > Clean Deployment Directory.
Add the same JDK version to Eclipse as that which you are using in Studio Pro (this is the recommended version correlation). For details on JDK requirements, see the Mendix Studio Pro section of System Requirements.

6.2 Compile Errors

When you deploy your app, Studio Pro will compile all the Java actions. If you have made a mistake in the Java, the action will not compile and you will be shown the error which you will need to correct using Eclipse.

6.3 Runtime Errors

When you use your app, you may encounter an error. You can look in the Studio Pro console to see if this is caused by an error in your Java.

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