Скачать или смотреть Understanding Why encoding/json Marshals Slices as Base64 in Go

Learn why the Go `encoding/json` package encodes certain slice types as Base64 and how to work around it in your projects.
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Understanding Why encoding/json Marshals Slices as Base64 in Go

When working with JSON in Go, developers often run into peculiarities that can be both confusing and frustrating. One such occurrence is when the encoding/json package marshals slices of uint8 as Base64 strings instead of standard JSON arrays. In this guide, we'll explore why this behavior occurs and how you can manage it in your applications.

The Problem: Observing Base64 Output

Consider the following struct definitions that are commonly used while interacting with APIs:

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

When attempting to marshal an instance of TTLOption, you might notice that the ValueList field is output as a Base64-encoded string, such as:

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

This can be perplexing, especially when all you expect is a traditional JSON array representation.

The Cause: Go's JSON Marshaling Behavior

According to the Go documentation for the encoding/json package, arrays and slices are typically marshaled as JSON arrays. However, there are exceptions:

The []byte type (which is an alias for uint8 slices) encodes as a Base64-encoded string.

A nil slice will simply encode as a null JSON value.

So, when you use []uint8, it is treated as []byte, leading to this Base64 encoding during marshaling.

Why Does This Happen?

Type Aliasing: A byte in Go is just an alias for uint8. When the Go JSON encoder sees a slice of uint8, it maps it to []byte for encoding purposes.

Base64 Encoding: []byte is encoded as a Base64 string to ensure that binary data can be safely represented in a JSON format, which is inherently a text-based representation.

Solution: How to Marshal as a JSON Array

If you prefer to marshal your ValueList as a standard JSON array instead of a Base64 string, consider the following approach:

Using a Custom Struct Tag

Instead of using a slice of uint8, you could define ValueList as a slice of a more generic numeric type, such as int:

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

This change will ensure that the ValueList is treated differently during marshaling, and the output will reflect a standard JSON array as you intend it to.

Custom MarshalJSON Interface

Another approach is to implement a custom MarshalJSON method for your struct, where you can control how each field is marshaled. Here’s an example of how that could look:

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

Closing Thoughts

Understanding how Go handles marshaling, especially with slices of uint8, can save you time and help you avoid unnecessary pitfalls. By opting for a different type for JSON representation or implementing a custom marshaling function, you can control the output and meet your application requirements. Happy coding!