How to Merge HTML Documents in Java
Merging HTML content in Java is more challenging than it sounds. This article covers some major difficulties of merging HTML and suggests a few API solutions.
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Join For FreeJava developers are often handed the challenge of consolidating documents in efficient file processing workflows. With this prompt, HTML might not be the first document format that comes to mind — we might think of “file processing” as pertaining to robust, “business-y” formats like PDF or Excel first and foremost — but HTML’s importance in many modern enterprise environments can’t be understated. Whether it's a question of processing data pulled together from multiple online sources, piecing scraped web pages together, or consolidating custom web-based reports, programmatically combining and packaging HTML content is often highly relevant.
In this article, we’ll take a closer look at what it means to merge HTML content programmatically, and we’ll point out some of the specific challenges Java developers can expect to encounter in this endeavor. Towards the end, we’ll touch on some open-source libraries and third-party APIs we can use to build HTML merging capabilities into a file processing workflow, carefully weighing the benefits of each approach.
Common Scenarios for HTML Merging in Java Projects
We’ll start with some context, much like merging Excel documents (which, being composed of compressed XML-based files, share a surprisingly similar structure to vanilla HTML content), merging HTML is often a critical part of automated report bundling workflows. HTML5 is a fantastic solution for custom data reporting involving unique and attractive visualization; Java systems are often designed to handle the backend dirty work, which prepares those files on their journey to various frontend applications.
In a similar vein, HTML merging can serve as data consolidation for web scraping — a practice which tends to yield a series of separate HTML fragments, all of which need to be merged into a single document to make the data useful and presentable. Documentation portals — especially large ones — also like to break HTML content into fragments, leaving Java applications to piece them back together with robust logic.
HTML merging can also be a preferential pre-processing step before converting HTML to formats like PDF. Merging HTML before a conversion to PDF (rather than merging converted PDFs in a post-processing step) can save some memory in large-scale programs.
What Does It Mean to Merge HTML Files in Java?
It’s easy to underestimate the challenges involved in programmatically combining HTML content. Merging HTML isn’t just about stringing lines of HTML code together — it’s about combining web pages in a way that preserves their functionality, styling, and content integrity. It requires a strategy for handling structure, styling, and behavior.
There are a few core issues to consider when we’re merging HTML files in Java.
Head Tags
For starters, each valid HTML document has its own <head> tag. These tags can carry the document’s styles, scripts, and meta tags — all crucial information for preserving the intended functionality of the file. Inadvertently overwriting or duplicating key elements is easy to do if we don’t handle each file’s <head> correctly in our file merging workflow.
Manually parsing and merging <head> sections in Java means writing brittle logic to identify and de-duplicate tags. That often involves using DOM libraries or regex, neither of which handles edge cases well out of the box.
CSS and JavaScript
Different HTML documents carrying their own CSS and JavaScript code can also “collide” with one another, which can have a negative impact on the intended functionality of each page. If, for example, two HTML files have the same CSS selectors or JavaScript functions, a sloppy merge operation can break them.
There’s a noticeable lack of built-in Java tools for handling CSS/JavaScript conflict resolution; addressing that means writing custom preprocessing steps or relying on third-party parsers to isolate and rewrite scripts safely. This is doable, of course, but it’s tedious and tangential.
Relative Paths
Any backend programmer who’s dabbled in HTML development will remember that images, stylesheets, and scripts often rely on relative paths. All of these can wind up as a broken mess when HTML content is merged. References need to be adjusted in complex merge operations to ensure all the various resources in each file are loaded correctly.
When we handle relative paths in Java, we often need to walk the DOM to rewrite each individual SRC, HREF, or URL() reference. This can be extremely tedious, especially when we’re dealing with deeply nested or dynamically generated HTML content.
Malformed Content
Finally, of course, there’s the perennial issue of malformed content — a problem we face in most document processing workflows, not just those that involve HTML file merging. It’s common to encounter incomplete tags, improperly closed elements, or mismatched nesting in HTML files when we merge web content from multiple sources. We need to make sure our application identifies and resolves issues like these before merging files to avoid display errors — and to avoid sinister security vulnerabilities like cross-site scripting (XSS), which can occur when dangerous attributes or embedded scripts aren’t properly sanitized.
The default HTML parsing libraries we get in Java (like JSoup, for example) can help clean up malformed content, but they still require careful configuration to avoid stripping important elements or missing dangerous ones.
Merging HTML With Open-Source Java Libraries
Building efficient Java programs is all about finding the right tools for the job, and open-source tools are the preferred option for many. Below, we’ll look at a few popular open-source APIs for handling HTML documents in Java.
One option we alluded to in the prior section — JSoup — offers a popular, powerful, and easy-to-use API. Methods like Jsoup.parse(), Jsoup.clean(), and Jsoup.select(), Jsoup.text(), etc. let us parse HTML, clean up dirty markup, and manipulate the DOM in complex ways. This still requires custom code for merging documents, however, so while it’s great for parsing and cleaning individual files, it won’t do the whole job for us. It also won’t directly resolve conflicts between styles and scripts on our behalf.
The Jericho HTML Parser is another worthy option; a particularly good choice if we need fine-grained control over how HTML content is merged. It most notably provides the option to modify HTML documents without losing formatting (e.g., Source source = new Source(<html here>) parses HTML while preserving the original formatting and spacing), and it reproduces unrecognized or invalid HTML verbatim. Some may find it verbose, though, and it’s certainly going to require more additional coding than utilizing JSoup will.
Lastly, if we’re most focused on cleaning up malformed HTML and transforming it into a consistent, well-formed structure, HTMLCleaner is worth mentioning. Like Jericho, it doesn’t offer a full out-of-the-box merging solution, but a method like clean(<string html>) works well for sanitizing input.
Merging HTML With a Web API
In some cases, the control that open-source tools provide is outweighed by the cumbersome amount of code required to be written along with it. Fully realized conversion APIs are worth considering if we’re actively avoiding piling more upfront coding and downstream debugging time onto our plate.
To that end, we can take advantage of a third-party API for merging HTML in Java using some example code provided below. This is one example of a solution that’s free to use, bearing in mind that it requires API key authorization and utilization of external server resources to get the job done.
We can use this API in two different ways: 1) for merging exactly two HTML files together, and 2) for merging 2+ HTML files together. We can structure our API call slightly differently depending on which iteration we need.
In either case, we’ll install the Java SDK with Maven by adding a reference to the repository in our pom.xml:
<repositories>
<repository>
<id>jitpack.io</id>
<url>https://jitpack.io</url>
</repository>
</repositories>And then adding a reference to the dependency in our pom.xml:
<dependencies>
<dependency>
<groupId>com.github.Cloudmersive</groupId>
<artifactId>Cloudmersive.APIClient.Java</artifactId>
<version>v4.25</version>
</dependency>
</dependencies>For Gradle projects, we can add it in our root build.gradle at the end of repositories:
allprojects {
repositories {
...
maven { url 'https://jitpack.io' }
}
}And then add the dependency in build.gradle:
dependencies {
implementation 'com.github.Cloudmersive:Cloudmersive.APIClient.Java:v4.25'
}With installation out of the way, the import classes go at the top of our file (commented out for now to avoid issues):
// Import classes:
//import com.cloudmersive.client.invoker.ApiClient;
//import com.cloudmersive.client.invoker.ApiException;
//import com.cloudmersive.client.invoker.Configuration;
//import com.cloudmersive.client.invoker.auth.*;
//import com.cloudmersive.client.MergeDocumentApi;We can then configure the API client with our API key:
ApiClient defaultClient = Configuration.getDefaultApiClient();
// Configure API key authorization: Apikey
ApiKeyAuth Apikey = (ApiKeyAuth) defaultClient.getAuthentication("Apikey");
Apikey.setApiKey("YOUR API KEY");
// Uncomment the following line to set a prefix for the API key, e.g. "Token" (defaults to null)
//Apikey.setApiKeyPrefix("Token");After that, we can structure requests to merge 2 or 2+ files. For 2 files only, we can use the below code:
MergeDocumentApi apiInstance = new MergeDocumentApi();
File inputFile1 = new File("/path/to/inputfile"); // File | First input file to perform the operation on.
File inputFile2 = new File("/path/to/inputfile"); // File | Second input file to perform the operation on (more than 2 can be supplied).
try {
byte[] result = apiInstance.mergeDocumentHtml(inputFile1, inputFile2);
System.out.println(result);
} catch (ApiException e) {
System.err.println("Exception when calling MergeDocumentApi#mergeDocumentHtml");
e.printStackTrace();
}And for 2+ files, we can use the below code instead:
MergeDocumentApi apiInstance = new MergeDocumentApi();
File inputFile1 = new File("/path/to/inputfile"); // File | First input file to perform the operation on.
File inputFile2 = new File("/path/to/inputfile"); // File | Second input file to perform the operation on.
File inputFile3 = new File("/path/to/inputfile"); // File | Third input file to perform the operation on.
File inputFile4 = new File("/path/to/inputfile"); // File | Fourth input file to perform the operation on.
File inputFile5 = new File("/path/to/inputfile"); // File | Fifth input file to perform the operation on.
File inputFile6 = new File("/path/to/inputfile"); // File | Sixth input file to perform the operation on.
File inputFile7 = new File("/path/to/inputfile"); // File | Seventh input file to perform the operation on.
File inputFile8 = new File("/path/to/inputfile"); // File | Eighth input file to perform the operation on.
File inputFile9 = new File("/path/to/inputfile"); // File | Ninth input file to perform the operation on.
File inputFile10 = new File("/path/to/inputfile"); // File | Tenth input file to perform the operation on.
try {
byte[] result = apiInstance.mergeDocumentHtmlMulti(inputFile1, inputFile2, inputFile3, inputFile4, inputFile5, inputFile6, inputFile7, inputFile8, inputFile9, inputFile10);
System.out.println(result);
} catch (ApiException e) {
System.err.println("Exception when calling MergeDocumentApi#mergeDocumentHtmlMulti");
e.printStackTrace();
}We can write the resulting byte[] array from our conversion to a new HTML document, and we’re all done. The benefit here is simplicity; we don’t have to worry about maintaining a bunch of code to handle simple HTML merge operations now, and we can rely on organized error reporting to call out any problems with files that might make merging impossible or dangerous to do.
Conclusion
In this article, we covered some examples of programmatic HTML merging use-cases in the real world and discussed the challenges associated with building HTML merging applications in Java. We covered popular open-source Java APIs for handling HTML content in multiple different capacities, and we looked at code examples for calling APIs designed specifically to merge valid HTML and nothing else.
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