How to Determine the Size of XML Serialization with JAXB Before Writing to a File

Discover how to efficiently estimate the size of your `JAXB` serialization before writing it to a file. This guide provides practical solutions using `CountingOutputStream` and `ByteArrayOutputStream`.
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How to Determine the Size of XML Serialization with JAXB Before Writing to a File

Understanding the size of an object when marshalling it to XML with JAXB can be quite essential, especially if you need to ensure that the total size of serialized objects stays within certain limits. In this guide, we'll address a specific challenge: figuring out how much space it will take to write an object to a file before actually writing it.

The Problem

Imagine you have an existing marshalled object and you want to marshal a new object to XML. You want to make sure that the total size of these marshalled objects does not exceed a specific threshold, such as 40 KB.

Here's a simplified version of the code you might be thinking of:

[[See Video to Reveal this Text or Code Snippet]]

The real challenge arises in determining the size of the new object (toBeMarshalled), including all necessary tags and prefixes, before you attempt to write it out. Is there a library or method that can help solve this dilemma?

The Solution

The simplest and most effective way to determine the size of your marshalled XML content is to marshal the XML itself. This approach can be implemented using a couple of methods:

Using CountingOutputStream

One approach involves using Apache CountingOutputStream in conjunction with JAXB. Here’s a step-by-step breakdown of how you can achieve this:

Set Up Streams: Create a NullOutputStream which will discard the data and a CountingOutputStream that will count the bytes that would be written to the output stream.

Marshal the Object: Use an OutputStreamWriter on the CountingOutputStream to marshal your object to get the byte count.

Here’s a sample code snippet demonstrating this approach:

[[See Video to Reveal this Text or Code Snippet]]

Count and Write: By using CountingOutputStream, you can determine the byte size through cos.getByteCount(). You will need to run the marshalling process twice—once to get the count and then again to actually write the output. This is the only deterministic method to accurately measure the size without consuming substantial memory.

Alternative with ByteArrayOutputStream

If memory consumption is not a concern, an alternative method involves using ByteArrayOutputStream:

Marshal to ByteArrayOutputStream: Instead of counting with CountingOutputStream, you'll marshal the object directly into a ByteArrayOutputStream.

Determine Size: After marshalling, you can check the size of the resulting byte array.

Here’s how that might look:

[[See Video to Reveal this Text or Code Snippet]]

Weighing the Options

CountingOutputStream:

Pros: Low memory usage while counting.

Cons: Requires two passes to marshal, which can be less efficient.

ByteArrayOutputStream:

Pros: Simple implementation, size can be retrieved directly after the operation.

Cons: Higher memory consumption, especially with large objects.

Conclusion

By employing either the CountingOutputStream or the ByteArrayOutputStream, you can successfully estimate the size of your JAXB marshalling operations prior to writing to a file. This way, you're able to manage your memory usage effectively and ensure that you stay within your defined limits.

Experiment with these solutions to find out which one best suits your needs. Happy coding!