Скачать или смотреть Understanding the Java Apache POI Bug: Resolving Issues with SXSSFWorkbook

Explore the intricacies of Java Apache POI `SXSSFWorkbook` and how to fix the bug when reading values. Learn about inline strings vs shared strings table issues.
---
This video is based on the question https://stackoverflow.com/q/71395373/ asked by the user 'Matze' ( https://stackoverflow.com/u/3355628/ ) and on the answer https://stackoverflow.com/a/71395807/ provided by the user 'Axel Richter' ( https://stackoverflow.com/u/3915431/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Java Apache POI Bug

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Java Apache POI Bug: Resolving Issues with SXSSFWorkbook

When working with Java’s Apache POI library for manipulating Excel files, developers may encounter perplexing bugs. One such bug arises in the handling of SXSSFWorkbook, a streaming version of XSSFWorkbook. As curious programmers dive into the details, they often wonder why their expectations don't meet reality. In this guide, we will walk through a specific case, identify the root of the problem, and explore an effective solution.

The Problem at Hand

Imagine you're using SXSSFWorkbook to create an Excel workbook with a single cell containing the value "TEST". You write this to an .xlsx file, and when you read it back to print its value, the output comes up empty.

Here's a brief overview of the situation:

You create a cell in SXSSFWorkbook with the text "TEST".

When reopening the workbook and explicitly setting the cell type to CellType.STRING, you expect to see "TEST" printed in the console.

Instead, you encounter an empty output.

Observations

If you skip the line that sets the cell type, the expected "TEST" is printed.

Switching to XSSFWorkbook eliminates the issue.

Opening and saving the workbook with Excel corrects the problem.

What could be causing this unintended behavior?

The Root Cause Explained

The issue occurs due to the differences between how SXSSFWorkbook and XSSFWorkbook handle string data. Understanding the internal workings of these classes is key.

Inline Strings vs. Shared Strings Table

SXSSFWorkbook uses inline strings by default. This means the cell directly contains the string value. In XML representation, it appears as:

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

On the other hand, XSSFWorkbook expects string values to be stored in a shared strings table when the cell type is set to CellType.STRING. This means the cell doesn’t contain the string directly, but rather an index that refers to the string stored separately.

What Happens When You Set Cell Type?

When you call Cell.setCellType(CellType.STRING), the cell type is changed from inlineStr to s. This shift implies that the cell now expects a reference to the shared strings table. Since there is no index corresponding to "TEST" in the shared strings table, the output fails to return any value, resulting in an empty console output.

The Solution

To rectify this issue while working with SXSSFWorkbook, you have a couple of options:

Option 1: Force Shared Strings Table

You can create your SXSSFWorkbook with a constructor parameter that utilizes a shared strings table:

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

Important Notes

While this approach will ensure that strings are stored in a shared strings table, it may impact the performance benefits offered by the streaming approach of SXSSFWorkbook. This is crucial to consider especially if your applications require handling large sets of data.

Option 2: Avoid Setting Cell Type

Alternatively, if you do not change the cell type after creation (i.e., do not use setCellType(CellType.STRING)), the data will print correctly. This approach, however, may require careful handling and checks to ensure that no unintended data type conversions occur.

Why Does Saving in Excel Fix the Issue?

When you open and save the file in Excel, it converts inline strings to their respective indices in the shared strings table. Consequently, when you read the file afterward, the cell types are adjusted correctly, resolving the issue.

Conclusion

Understanding the distinction between inline strings and the shared strings table is vital when working with Apache POI's SXSSFWorkbook. By properly managing these properties, you can effectively manipulate Excel files without running into unexpecte