Java finally 和 return 的关系

finally 和 return 关系的总结

• try块中有System.exit(0)这样的语句，由于它是终止Java虚拟机JVM的，连JVM都停止了，所有都结束了，当然finally语句也不会被执行到。
• 其它情况下，finally语句都必然会被执行。因此可以在这里执行一些资源的释放操作。
  • finally中的return会覆盖try/catch中的renturn。
  • 在finally中写return语句会有警告，因为它会阻止函数抛出异常，而改为正常返回。

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语句是在try的return语句执行之后，return返回之前执行。
    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块中包含了return语句，即使前面的catch块重新抛出了异常，则调用该方法的语句也不会获得catch块重新抛出的异常，而是会得到finally块的返回值，并且不会捕获异常。
             */
            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块中包含了return语句，即使前面的catch块重新抛出了异常，则调用该方法的语句也不会获得catch块重新抛出的异常，而是会得到finally块的返回值，并且不会捕获异常。
             */
            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);是终止Java虚拟机JVM的，连JVM都停止了，所有都结束了，当然finally语句也不会被执行到。
            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();
    }
}