defer语句会在以下几种情况下被执行：

Each time a "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, if the surrounding function returns through an explicit return statement, deferred functions are executed after any result parameters are set by that return statement but before the function returns to its caller. If a deferred function value evaluates to nil, execution panics when the function is invoked, not when the "defer" statement is executed.

每次执行defer语句时，会将其定义的函数和参数保存在一个栈中，但是定义的函数并不会被执行。但是，在外层函数退出之前，defer函数会按照定义的顺序逆序执行。如果外层函数通过显示return语句返回，则defer定义的函数是在设置结果参数之后，返回调用方之前执行。如果defer要执行的函数为nil，则会在定义的函数调用时产生panic，而不是defer语句执行时。