單例模式在Go中通過(guò)包級(jí)變量和函數(shù)實(shí)現(xiàn)，確保一個(gè)類型只有一個(gè)實(shí)例並提供全局訪問(wèn)點(diǎn)。 1. 使用私有變量和公共訪問(wèn)函數(shù)實(shí)現(xiàn)懶加載；2. 通過(guò)sync.Once確保並發(fā)安全；3. 常用於配置管理、數(shù)據(jù)庫(kù)連接等場(chǎng)景。儘管單例因引入全局狀態(tài)、隱藏依賴等問(wèn)題常被視為反模式，但在小型項(xiàng)目或性能敏感場(chǎng)景下仍適用，如需維護(hù)多個(gè)實(shí)例則應(yīng)考慮依賴注入。

Singleton pattern in Go is a design pattern that ensures a type has only one instance and provides a global point of access to it. It's commonly used for things like configuration managers, database connections, or logging systems — places where having multiple instances could lead to inconsistency or unnecessary overhead.

In Go, since there's no class structure like in traditional object-oriented languages, the singleton pattern is usually implemented using package-level variables and functions. The most straightforward way is to use a private variable combined with a public function to access it.

How to implement singleton in Go

Go doesn't enforce access modifiers like private or public , but by convention, identifiers starting with lowercase are considered unexported (private), while uppercase ones are exported (public). This allows us to control instantiation.

Here's a basic example:

 package singleton

type Config struct {
    Data string
}

var config *Config

func GetConfig() *Config {
    if config == nil {
        config = &Config{
            Data: "default",
        }
    }
    return config
}

This implementation uses lazy initialization — the instance is created only when GetConfig() is first called.

Note: In concurrent environments, you should protect this with synchronization. You can use sync.Once to make sure the initialization happens exactly once:

 var (
    config *Config
    once sync.Once
)

func GetConfig() *Config {
    once.Do(func() {
        config = &Config{
            Data: "default",
        }
    })
    return config
}

This ensures thread safety without overcomplicating the logic.

Why some consider singleton an anti-pattern

While the singleton pattern is useful, it often gets criticized for these reasons:

• Global state : Like global variables, singletons introduce shared state across your application, which can make testing and debugging harder.
• Hidden dependencies : When a function relies on a singleton, it's not immediately clear from its signature what dependencies it needs.
• Harder to mock : In unit tests, replacing a singleton with a mock or stub is more complicated than passing in a dependency.
• Limits flexibility : If later you decide you need multiple instances (eg, different configs for different tenants), the singleton becomes a hurdle.

For example, imagine a logger singleton used throughout your app. If you want to test how your code behaves with different log levels or outputs, it's easier if each component receives a logger interface rather than pulling from a global instance.

When it's okay to use singleton in Go

Despite the downsides, there are situations where the singleton pattern makes sense in Go:

• Package-level utilities : For types that truly need only one instance, like a connection pool or a configuration loader.
• Performance-sensitive scenarios : Avoiding repeated creation of expensive objects (like DB connections) can be a valid use case.
• Simple applications : In smaller projects, strict dependency injection might feel like overkill.

Still, even in those cases, it's worth considering whether the singleton can be wrapped behind an interface so it's easier to replace later.

If you're building something small or performance-critical, go ahead and use a singleton. But for larger systems, especially ones that aim to be maintainable and testable, prefer dependency injection over global access points.

基本上就這些。

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