Java finally 和 return 的關系

finally 和 return 關系的總結

• try塊中有System.exit(0)這樣的語句，由于它是終止Java虛拟機JVM的，連JVM都停止了，所有都結束了，當然finally語句也不會被執行到。
• 其它情況下，finally語句都必然會被執行。是以可以在這裡執行一些資源的釋放操作。
  • finally中的return會覆寫try/catch中的renturn。
  • 在finally中寫return語句會有警告，因為它會阻止函數抛出異常，而改為正常傳回。

package com.demo.test;

public class FinallyAndReturn {

    private static void finallyAndTryReturn() {
        try {
            System.out.println("finallyAndTryReturn -> try");
            return;
        } catch (Exception e) {
            System.out.println("finallyAndTryReturn -> catch");
        } finally {
            System.out.println("finallyAndTryReturn -> finally");
        }
    }

    private static void finallyAndCatchReturn() {
        try {
            System.out.println("finallyAndCatchReturn -> try");
            throw new Exception();
        } catch (Exception e) {
            System.out.println("finallyAndCatchReturn -> catch");
            return;
        } finally {
            System.out.println("finallyAndCatchReturn -> finally");
        }
    }

    // finally語句是在try的return語句執行之後，return傳回之前執行。
    private static String tryReturn() {
        String str = "initialized";
        try {
            System.out.println("tryReturn -> try");
            str = "try";
            return str;
        } catch (Exception e) {
            System.out.println("tryReturn -> catch");
            str = "catch";
        } finally {
            System.out.println("tryReturn -> finally");
            str = "finally";
        }
        return null;
    }

    private static String tryReturnAndFinallyReturn() {
        String str = "initialized";
        try {
            System.out.println("tryReturnAndFinallyReturn -> try");
            str = "try";
            return str;
        } catch (Exception e) {
            System.out.println("tryReturnAndFinallyReturn -> catch");
            str = "catch";
        } finally {
            System.out.println("tryReturnAndFinallyReturn -> finally");
            /*
             *  Warning: finally block does not complete normally
             *  如果finally塊中包含了return語句，即使前面的catch塊重新抛出了異常，則調用該方法的語句也不會獲得catch塊重新抛出的異常，而是會得到finally塊的傳回值，并且不會捕獲異常。
             */
            str = "finally";
            return str;
        }
    }

    private static String tryThrowAndFinallyReturn() throws Exception {
        String str = "initialized";
        try {
            System.out.println("tryThrowAndFinallyReturn -> try");
            str = "try";
            throw new Exception();
        } catch (Exception e) {
            System.out.println("tryThrowAndFinallyReturn -> catch");
            str = "catch";
            throw new Exception();
        } finally {
            System.out.println("tryThrowAndFinallyReturn -> finally");
            /*
             *  Warning: finally block does not complete normally
             *  如果finally塊中包含了return語句，即使前面的catch塊重新抛出了異常，則調用該方法的語句也不會獲得catch塊重新抛出的異常，而是會得到finally塊的傳回值，并且不會捕獲異常。
             */
            str = "finally";
            return str;
        }
    }

    private static void finallyAndRuntimeException() {
        try {
            System.out.println("finallyAndRuntimeException -> try");
            throw new RuntimeException();
        } catch (Exception e) {
            System.out.println("finallyAndRuntimeException -> catch");
        } finally {
            System.out.println("finallyAndRuntimeException -> finally");
        }
    }

    private static void finallyAndExit() {
        try {
            System.out.println("finallyAndExit -> try");
            // System.exit(0);是終止Java虛拟機JVM的，連JVM都停止了，所有都結束了，當然finally語句也不會被執行到。
            System.exit();
        } catch (Exception e) {
            System.out.println("finallyAndExit -> catch");
        } finally {
            System.out.println("finallyAndExit -> finally");
        }
    }

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

        finallyAndCatchReturn();
        System.out.println();

        System.out.println("tryReturn return -> " + tryReturn());
        System.out.println();

        System.out.println("tryReturnAndFinallyReturn return -> " + tryReturnAndFinallyReturn());
        System.out.println();

        try {
            System.out.println("tryThrowAndFinallyReturn return -> " + tryThrowAndFinallyReturn());
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println();

        finallyAndRuntimeException();
        System.out.println();

        finallyAndExit();
    }
}