这个回调处理器类拥有两个线程本地变量，用于存储来自客户端实例的用户ID和密码。
The callback handler class has two thread local variables that can store the user ID and password from the client instance.
可以跨客户端和服务器无缝地使用它，这就意味着您可以在多个执行中使用相同的变量。
Its ability to be used seamlessly across client and server means you can use the same variable across multiple executions.
不同于绑定变量，替代变量在客户端加以处理，且其值会在提交执行命令之前被替换。
Unlike bind variables, a substitution variable is processed on the client side, and its value will be substituted before submitting the command for execution.
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