Why Doesn't Go's `json.Marshal()` Serialize Struct Fields with Lowercase Prefixes?

Exploring JSON Generation Restrictions for Struct Fields with Lowercase Prefixes in Go

In Go, developers frequently encounter an issue while attempting to generate JSON from structs that possess fields with lowercase prefix characters. Understanding the underlying reason is critical for effectively resolving this problem.

The issue arises due to Go's convention for identifying public identifiers within a package. Identifiers with an uppercase initial letter are considered public and can be accessed outside the package. Conversely, identifiers with a lowercase initial letter are deemed private and are only visible within the current package.

Consider the following code snippet:

type Machine struct {
  m_ip string
  m_type string
  m_serial string
}

When attempting to generate JSON from an instance of this struct using json.Marshal(), you might encounter an unexpected result of just "{}." This is because the fields m_ip, m_type, and m_serial are not visible to json.Marshal() as they are considered private (indicated by the lowercase prefixes).

However, changing the fields to uppercase, as seen below, will resolve the issue:

type Machine struct {
  MachIp string
  MachType string
  MachSerial string
}

By utilizing uppercase prefixes, you make these fields publicly accessible, allowing json.Marshal() to successfully generate the desired JSON output.

If you require lowercase identifiers in your JSON output, you can employ field tagging. By annotating each field with the corresponding JSON name, you can override the default behavior. For instance:

type Machine struct {
    MachIp     string `json:"m_ip"`
    MachType   string `json:"m_type"`
    MachSerial string `json:"m_serial"`
}

Through this tagging mechanism, you can explicitly specify the desired JSON field names, regardless of their initial character case, thus enabling you to create JSON outputs that align with your application's requirements.

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