Sling Models

🔍 What Is It?

A Sling Model is a Java POJO (Plain Old Java Object) annotated with @Model. It:

• →Adapts a Resource or SlingHttpServletRequest to a typed Java object
• →Automatically injects JCR property values using @ValueMapValue
• →Can inject OSGi services using @OSGiService
• →Can inject child resources, parent pages, and request parameters
• →Is used by HTL templates via data-sly-use

Sling Models replaced the older WCMUsePojo and are now the standard way to write AEM component logic.

🎯 Why Do We Need It?

HTL is intentionally limited — it can't do complex logic, call services, or format data. That's by design. Sling Models fill that gap:

• →Separation of concerns — HTL handles rendering, Sling Models handle logic
• →Testability — Java classes are easy to unit test
• →Type safety — You get compile-time checking instead of runtime errors
• →Clean HTL — Your templates stay readable and maintainable
• →Service access — You can call OSGi services, APIs, and databases from a model

Without Sling Models, you'd either put logic in HTL (bad) or use scriptlets (very bad).

🏗️ What Happens in Real Projects?

Here's how Sling Models are used in a real project:

Scenario 1: Reading component properties

An author fills in a dialog with a heading, description, and link. Your Sling Model reads those JCR properties and exposes them as typed Java getters to HTL.

Scenario 2: Calling an external API

A "Latest News" component needs to fetch articles from a CMS API. The Sling Model injects a NewsService (OSGi service) and returns a list of NewsItem objects to HTL.

Scenario 3: Page-level data

A breadcrumb component needs the current page's ancestors. The Sling Model uses @ScriptVariable to get the currentPage and traverses the page tree.

Scenario 4: Exporter for headless

A Sling Model with @Exporter annotation exposes component data as JSON for headless delivery via Content Services.

⚙️ How It Works

Injection Strategy

When AEM instantiates your Sling Model, it goes through each field annotated with @Inject, @ValueMapValue, @ChildResource, etc., and tries to resolve the value from the adaptable (Resource or Request).

Adaptables

• →Resource.class — Use when you only need JCR properties. Faster, simpler.
• →SlingHttpServletRequest.class — Use when you need request parameters, selectors, or suffixes. Can also access the resource through the request.

Key Annotations

DefaultInjectionStrategy

Always use DefaultInjectionStrategy.OPTIONAL unless a field is truly required. With REQUIRED (the default), if any injection fails, the entire model fails to instantiate — causing a blank component.

💻 Example

A Complete Sling Model Example — A News Card component that reads JCR properties, calls an OSGi service, and exposes data to HTL.

package com.myproject.core.models;

import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.models.annotations.Model;
import org.apache.sling.models.annotations.DefaultInjectionStrategy;
import org.apache.sling.models.annotations.injectorspecific.ValueMapValue;
import org.apache.sling.models.annotations.injectorspecific.OSGiService;
import org.apache.sling.models.annotations.injectorspecific.ScriptVariable;
import com.day.cq.wcm.api.Page;
import javax.annotation.PostConstruct;
import java.util.List;

@Model(
    adaptables = SlingHttpServletRequest.class,
    adapters = NewsCardModel.class,
    defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL,
    resourceType = "myproject/components/newscard"
)
public class NewsCardModel {

    // Injects the JCR property "heading" from the component's node
    @ValueMapValue
    private String heading;

    @ValueMapValue
    private String description;

    @ValueMapValue
    private String linkPath;

    // Injects an OSGi service
    @OSGiService
    private NewsService newsService;

    // Injects the current page from HTL context
    @ScriptVariable
    private Page currentPage;

    private List<NewsItem> latestNews;

    // Runs after all injections are complete
    @PostConstruct
    protected void init() {
        if (newsService != null) {
            latestNews = newsService.getLatestNews(3);
        }
    }

    public String getHeading() {
        return heading != null ? heading : "Latest News";
    }

    public String getDescription() { return description; }

    public String getLinkPath() { return linkPath; }

    public List<NewsItem> getLatestNews() { return latestNews; }

    public String getCurrentPageTitle() {
        return currentPage != null ? currentPage.getTitle() : "";
    }
}

🤔 Common Confusions

🚨 Common Production Issues

✅ Best Practices

🏛️ Architect's Note

💬 Frequently Asked Questions

🎯 Key Takeaways

🔗 Related Topics

🎯 Practice this topic in interviews

↗ Where this topic fits