try catch finally的底層原理

來源：https://blog.csdn.net/lwd512768098/article/details/114728720

finally終究執(zhí)行

我們先來驗(yàn)證一下finally終究執(zhí)行

????public?static?int?test()?{?
????????int?i?=?1;
????????try?{?
????????????i++;
????????}?finally?{?
????????????System.out.println("finally?yeah!");
????????}
????????return?i;
????}

????public?static?void?main(String[]?args)?{?
????????System.out.println(test());
????}

輸出為

????public?static?int?test()?{?
????????int?i?=?1;
????????try?{?
????????????i++;
????????????throw?new?Exception();
????????}?catch?(Exception?e)?{?
????????????System.out.println("Exception?yeah!");
????????}?finally?{?
????????????System.out.println("finally?yeah!");
????????}
????????return?i;
????}

????public?static?void?main(String[]?args)?{?
????????System.out.println(test());
????}

帶有return的情況

finally不帶return的情況

對(duì)于return情況的話，只要記住一句話就行，如果finally里面有return, 那么就會(huì)覆蓋try塊或者catch里面的return內(nèi)容，否則的話，在執(zhí)行finally的內(nèi)容之前(try塊或者catch里面有return語句)，會(huì)計(jì)算好try或者catch里面的return表達(dá)式的值，然后保存到另一個(gè)局部變量，當(dāng)執(zhí)行fianlly的時(shí)候，會(huì)重新加載這個(gè)局部變量作為返回值，因此在finally的操作不會(huì)影響返回值，下面我們字節(jié)碼來看看內(nèi)部的工作原理。

????public?static?int?test()?{?
????????int?i?=?1;
????????try?{?
????????????i++;
????????????return?i;
????????}?finally?{?
????????????i++;
????????????System.out.println("finally?yeah!");
????????}
????}

????public?static?void?main(String[]?args)?{?
????????System.out.println(test());
????}

總結(jié)一下，總體的意思就是，當(dāng)執(zhí)行完try塊的時(shí)候，會(huì)計(jì)算return表達(dá)式的值，然后把這個(gè)返回值存在另一個(gè)臨時(shí)變量里面，最后返回的時(shí)候會(huì)重新讀取存放的變量，因此在finally后面無論如何修改，都不會(huì)影響返回值（除非finally里面含有return,這個(gè)我們?cè)谙旅嬗懻摚?/strong>

finally帶有return

????public?static?int?test()?{?
????????int?i?=?1;
????????try?{?
????????????i++;
????????????return?i;
????????}?finally?{?
????????????i++;
????????????System.out.println("finally?yeah!");
????????????return?i;
????????}
????}

????public?static?void?main(String[]?args)?{?
????????System.out.println(test());
????}

相信各位能夠正常猜出結(jié)果了

沒錯(cuò)，返回了3，說明finally對(duì)i的修過是其效果的，我們從字節(jié)碼看看原因：

try, catch, finally均沒有return語句

這種情況比較簡單，跟finally含有return類似

????public?static?int?test()?{?
????????int?i?=?1;
????????try?{?
????????????i++;
????????}?finally?{?
????????????i++;
????????????System.out.println("finally?yeah!");
????????}
????????return?i;
????}

????public?static?void?main(String[]?args)?{?
????????System.out.println(test());
????}

總結(jié)

這里總結(jié)一下，只要finally里面沒有return語句，那么返回值就由try或者catch的return語句決定，否則由finally的return語句決定