Blog Post

How to Watermark a PDF Using Java

Vyom Srivastava
Illustration: How to Watermark a PDF Using Java

In this post, you’ll learn how to watermark a PDF using our Watermark PDF Java API. With our API, you can process up to 100 PDF files per month for free. You’ll just need to create a free account to access your API key.

This post will be especially helpful for developers working with Java in document-heavy workflows who need to programmatically watermark PDFs.

Watermarks are used to identify and mark proprietary documents and discourage unauthorized use. In the context of a PDF, a watermark is usually found in the form of text or an image that appears on a document like a stamp. The most common example is applying a CONFIDENTIAL watermark to sensitive documents.

PSPDFKit API

Watermarking PDFs is just one of the operations possible with our 30+ PDF API tools. You can combine our deletion tool with other tools to create complex document processing workflows, such as:

  • Converting MS Office files and images into PDFs and then watermarking them

  • Duplicating or deleting PDF pages before watermarking a PDF

  • Merging or flattening PDFs and then watermarking the resulting document

Once you create your account, you’ll be able to access all our PDF API tools.

Step 1 — Creating a Free Account on PSPDFKit

Go to our website, where you’ll see the page below, prompting you to create your free account.

Free account PSPDFKit API

Once you’ve created your account, you’ll be welcomed by the page below, which shows an overview of your plan details.

Free plan PSPDFKit API

As you can see in the bottom-left corner, you’ll start with 100 documents to process, and you’ll be able to access all our PDF API tools.

Step 2 — Obtaining the API Key

After you’ve verified your email, you can get your API key from the dashboard. In the menu on the left, click API Keys. You’ll see the following page, which is an overview of your keys:

Watermark PDF Java API Key

Copy the Live API Key, because you’ll need this for the Watermark PDF API.

Step 3 — Setting Up Folders and Files

For this tutorial, you’ll use IntelliJ IDEA as your primary code editor. Next, create a new project called watermark_pdf. You can choose any location, but make sure to select Java as the language, Gradle as the build system, and Groovy as the Gradle DSL.

Watermark PDFs Java API Project Setup

Create a new directory in your project. Right-click on your project’s name and select New > Directory. From there, choose the src/main/java option. Once done, create a class file inside the src/main/java folder called processor.java, and create two folders called input_documents and processed_documents in the root folder.

Place both an image that you want to use as a watermark and the PDF in the input_documents folder. Name them logo.png and document.pdf.

Your folder structure will look like this:

watermark_pdf
├── input_documents
|    └── document.pdf
|    └── logo.png
├── processed_documents
├── src
|    └── main
|        └── java
|            └── processor.java

Step 4 — Installing Dependencies

Next, you’ll install two libraries:

  • OkHttp — This library makes API requests.

  • JSON — This library will parse the JSON payload.

Open the build.gradle file and add the following dependencies to your project:

dependencies {
    implementation 'com.squareup.okhttp3:okhttp:4.9.2'
    implementation 'org.json:json:20210307'
}

Once done, click the Add Configuration button in IntelliJ IDEA. This will open a dropdown menu.

Watermark PDF Java API

Next, select Application from the menu.

Watermark PDF Java API

Now, fill the form with the required details. Most of the fields will be prefilled, but you need to select java 18 in the module field and add -cp watermark_pdf.main in the main class.

Watermark PDF Java API

To apply settings, click the Apply button.

Step 5 — Writing the Code

Now, open the processor.java file and paste the code below into it:

package com.example.pspdfkit;

import java.io.File;
import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Files;
import java.nio.file.StandardCopyOption;

import org.json.JSONArray;
import org.json.JSONObject;

import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public final class Processor {
    public static void main(final String[] args) throws IOException {
        final RequestBody body = new MultipartBody.Builder()
                .setType(MultipartBody.FORM)
                .addFormDataPart(
                        "document",
                        "document.pdf",
                        RequestBody.create(
                                new File("input_documents/document.pdf"),
                                MediaType.parse("application/pdf")
                        )
                )
                .addFormDataPart(
                        "logo",
                        "logo.png",
                        RequestBody.create(
                                new File("input_documents/logo.png"),
                                MediaType.parse("image/png")
                        )
                )
                .addFormDataPart(
                        "instructions",
                        new JSONObject()
                                .put("parts", new JSONArray()
                                        .put(new JSONObject()
                                                .put("file", "document")
                                        )
                                )
                                .put("actions", new JSONArray()
                                        .put(new JSONObject()
                                                .put("type", "watermark")
                                                .put("image", "logo")
                                                .put("width", "25%")
                                        )
                                ).toString()
                )
                .build();

        final Request request = new Request.Builder()
                .url("https://api.pspdfkit.com/build")
                .method("POST", body)
                .addHeader("Authorization", "Bearer  YOUR_API_KEY_HERE' ")
                .build();

        final OkHttpClient client = new OkHttpClient()
                .newBuilder()
                .build();

        final Response response = client.newCall(request).execute();

        if (response.isSuccessful()) {
            Files.copy(
                    response.body().byteStream(),
                    FileSystems.getDefault().getPath("processed_documents/result.pdf"),
                    StandardCopyOption.REPLACE_EXISTING
            );
        } else {
            // Handle the error.
            throw new IOException(response.body().string());
        }
    }
}

ℹ️ Note: Make sure to replace YOUR_API_KEY_HERE with your API key.

Code Explanation

In the code above, you’re importing all the packages required to run the code and creating a class called processor. In the main function, you’re first creating the request body for the API call that contains all the instructions for watermarking the PDF. After that, you’re calling the API to process the instructions.

You’re then calling the execute() function and passing the FormData variable. The response of the API is then stored in the folder called processed_documents.

Output

To execute the code, click the Run button (which is a little green arrow). This is next to the field that says Processor, which is where you set the configuration.

Watermark PDF Java API

On the successful execution of the code, you’ll see a new processed file named result.pdf in the processed_documents folder.

The folder structure will look like this:

watermark_pdf
├── input_documents
|    └── document.pdf
|    └── logo.png
├── processed_documents
|    └── result.pdf
├── src
|    └── main
|        └── java
|            └── processor.java

Final Words

In this post, you learned how to easily and automatically watermark PDF documents for your Java application using our Watermark PDF API.

You can integrate these functions into your existing applications to watermark PDF pages. With the same API token, you can also perform other operations, such as merging documents into a single PDF, running OCR, duplicating pages, and more. To get started with a free trial, sign up here.

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