Kotlin异常处理(5)补充：典型异常面试题

• finally不被执行情况【仅此一种情况下不执行】
• finally中的代码块先于return语句执行
• finally代码块虽然先于return执行，但不能修改return返回的局部变量值
• finally代码块中建议不要是用return语句。

一、finally不被执行情况【仅此一种情况下不执行】

fun tryCatch1() {
    try {
        exitProcess(0)
    } catch (e: Exception) {
    } finally {
        println("Finally is running.")
    }
}

由于先执行了exitProcess(0)终止了程序，自然finally语句块不会被执行。

二、finally中的代码块先于return语句执行

fun tryCatch1(): Int {
    try {
        return 0
    } catch (e: Exception) {
        return 1
    } finally {
        println("Finally is running.")
    }
}

fun main() { tryCatch1().also(::print) }

// 运行结果：
Finally is running.
0
Process finished with exit code 0

fun tryCatch1(): Int {
    try {
        throw IllegalStateException()
    } catch (e: Exception) {
        return 1
    } finally {
        println("Finally is running.")
    }
}

fun main() { tryCatch1().also(::println) }

// 运行结果：
Finally is running.
1
Process finished with exit code 0

finally代码块先于 return语句 和 catch代码块 执行。

三、finally代码块虽然先于return执行，但不能修改return返回的局部变量值

fun tryCatch1(): Int {
    var a = 0
    val b = 3
    try {
        return a + b
    } catch (e: Exception) {
        return a + b
    } finally {
        println("Finally is running.")
        a = 2
    }
}

fun main() { tryCatch1().also(::print) }

// 运行结果：
Finally is running.
3
Process finished with exit code 0

  finally代码块虽然在return语句之前执行【这里是指在return返回之前执行，即先执行 a+b，再执行finally代码块，再返回。】，但此时finally代码块不能通过重新给变量赋值的方式改变return语句的返回值。

• 下面是一个易错理解案例。

data class Person(var name: String)

fun tryCatch1(): Person {
    val a = Person("张三")
    try {
        return a
    } catch (e: Exception) {
        return a
    } finally {
        println("Finally is running.")
        a.name = "李四"
    }
}

fun main() { tryCatch1().also(::print) }

// 运行结果：
Finally is running.
Person1(name=李四)
Process finished with exit code 0

  这里看似finally中的代码影响了return的返回值，但我们要理解return实际返回的是什么!!!（地址）

四、finally代码块中建议不要是用return语句。

fun tryCatch1() {
    try {
        throw IllegalArgumentException()
    } catch (e: Exception) {
        println("Catch is running.")
        throw IllegalArgumentException("Catch throw exception.")
    } finally {
        return
    }
}

fun main() = Unit.also { try { tryCatch1() } catch (e: Exception) { e.message.also(::print) } }

这段代码我们期望打印出的结果是：

Catch is running.
Catch throw exception.

但实际运行结果是这样的：

Catch is running.

Process finished with exit code 0

这是因为finally代码块先于throw执行，而finally中已经return，自然导致throw没有执行的机会。