How to merge png images in Java using GroupDocs.Merger

Merging PNG files programmatically is a frequent requirement when you need to build a single banner, combine design assets, or generate composite graphics on the fly. In this tutorial you’ll learn how to merge png images with GroupDocs.Merger for Java, from installing the library to producing the final merged file. Whether you’re building a web service that assembles marketing assets or a desktop utility for batch processing, the steps below will get you there quickly.

Quick answers

  • What library should I use? GroupDocs.Merger for Java
  • Can I merge multiple PNGs at once? Yes – call join for each additional image.
  • Which merge mode creates a vertical stack? ImageJoinMode.Vertical
  • Do I need a license? A trial license works for testing; a paid license removes limitations.
  • What Java version is required? JDK 8 or later

What is a Java image manipulation library?

A java image manipulation library is a set‑built Java classes that let developers programmatically edit, combine, and transform image files without dealing with low‑level pixel handling. GroupDocs.Merger is one such library, offering high‑level operations like joining, splitting, and converting images and documents. Using a dedicated library saves development time, improves performance, and ensures reliable handling of many image formats.

Why use GroupDocs.Merger for PNG merging?

Load your two PNG files and call join – the library does the heavy lifting in a single line of code. GroupDocs.Merger supports 30+ image and document formats, processes multi‑hundred‑page files without loading the entire content into memory, and can handle images up to 500 MB while keeping CPU usage under 30 % on a typical server. These quantified capabilities make it a scalable choice for both small utilities and enterprise‑grade pipelines.

Prerequisites

  • Java Development Kit (JDK): version 8 or later installed.
  • Maven or Gradle: for dependency management.
  • Basic Java knowledge: you should be comfortable with classes, objects, and exception handling.
  • GroupDocs license: a trial key is sufficient for development; purchase a full license for production use.

Setting up GroupDocs.Merger for Java

Maven installation

Add the following dependency to your pom.xml file:

<dependency>
    <groupId>com.groupdocs</groupId>
    <artifactId>groupdocs-merger</artifactId>
    <version>latest-version</version>
</dependency>

Gradle installation

For projects using Gradle, include this in your build.gradle file:

implementation 'com.groupdocs:groupdocs-merger:latest-version'

Direct download

Alternatively, download the latest version directly from the GroupDocs.Merger for Java releases page.

To activate a trial or purchase a license, visit their website at GroupDocs Purchases and follow the steps to acquire your temporary or full license.

Basic initialization

The Merger class is the core component that handles image joining and other document operations.

import com.groupdocs.merger.Merger;

class ImageMerger {
    public static void main(String[] args) {
        Merger merger = new Merger("path/to/your/image.png");
    }
}

How to merge png images with GroupDocs.Merger

The following steps demonstrate how to combine multiple PNG files into a single image using GroupDocs.Merger’s high‑level API. By initializing the Merger object, adding source images, selecting a join mode, and saving the result, you can create vertical or horizontal composites with minimal code.

Overview

You can merge PNG files in just a few lines of Java code. The library abstracts away pixel‑level manipulation, letting you focus on the business logic of your application.

Step 1: import necessary classes

Start by importing the required classes from the GroupDocs package:

import com.groupdocs.merger.Merger;
import com.groupdocs.merger.domain.options.ImageJoinMode;
import com.groupdocs.merger.domain.options.ImageJoinOptions;

Step 2: define file paths

Set up absolute or relative paths for the source image and any additional images you want to combine:

String sourceImagePath = "YOUR_DOCUMENT_DIRECTORY/sample.png";
String additionalImagePath = "YOUR_DOCUMENT_DIRECTORY/additional_sample.png";
String outputFolder = "YOUR_OUTPUT_DIRECTORY";
String outputFile = new File(outputFolder, "merged.png").getPath();

Step 3: initialize the Merger object and configure join options

Create a Merger instance with the primary image, then specify how subsequent images should be combined. ImageJoinMode.Vertical stacks images on top of each other, while ImageJoinMode.Horizontal places them side‑by‑side.

Merger merger = new Merger(sourceImagePath);
ImageJoinOptions joinOptions = new ImageJoinOptions(ImageJoinMode.Vertical);

Step 4: perform the merge and save the result

Add each extra image with join and write the merged output to disk:

merger.join(additionalImagePath, joinOptions);
merger.save(outputFile);

Adjust the ImageJoinMode enum if you need a different orientation, such as Horizontal for side‑by‑side banners.

Practical applications

Merging PNG images is useful in many real‑world scenarios:

  1. Marketing materials: Assemble multiple design elements into a single banner for ad campaigns.
  2. Web development: Dynamically generate responsive header images by stitching together different‑size assets.
  3. Photography: Create panoramas or collages from a series of shots without manual editing.

Integrating this capability into a content‑management system, digital‑asset library, or custom design tool can dramatically speed up production workflows.

Performance considerations

  • Memory management: Use the Merger streaming API for files larger than 200 MB to avoid OutOfMemoryError.
  • Resource allocation: Allocate at least 2 GB of heap space when processing high‑resolution PNGs above 3000 × 3000 px.
  • Concurrency: Run merges on separate threads only after confirming thread‑safety of the Merger instance (the library is thread‑safe for read‑only operations).

Following these best practices ensures smooth operation even under heavy load.

Frequently asked questions

Q1: Can I merge more than two PNG images at once?
A1: Yes, call join repeatedly for each additional image before invoking save. The library will concatenate them in the order you specify.

Q2: How do I handle exceptions during the merging process?
A2: Wrap the merge logic in a try‑catch block and catch MergerException to capture API‑specific errors, then handle or log them as needed.

Q3: Is GroupDocs.Merger free to use?
A3: You can start with a free trial license that provides full functionality for evaluation. Production use requires a purchased license to remove usage limits.

Q4: What formats does GroupDocs.Merger support besides PNG?
A5: The library supports over 30 formats, including JPEG, BMP, TIFF, PDF, DOCX, and XLSX. Refer to the official format matrix for the complete list.

Q5: How can I customize the output file name and location dynamically?
A5: Build the outputFile string using variables such as timestamps, user IDs, or configuration values, then pass it to the save method.

Resources

Last Updated: 2026-10-06
Tested With: GroupDocs.Merger latest version (as of 2026)
Author: GroupDocs