Java 的 finally 代码块的代码一定会执行吗？

明明如月学长

发布于 2021-08-31 14:59:50

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发布于 2021-08-31 14:59:50

一、背景

对于很多初学者而言，会想当然地认为 “finally 代码块一定会被执行”，因此我们可以看下面这个案例：

public class Demo {

    public static void main(String[] args) {
        try {
            BufferedReader br = new BufferedReader(new FileReader("file.txt"));
            System.out.println(br.readLine());
            br.close();
        } catch (IOException e) {
            // 省略一些代码
        } finally {
            System.out.println("Exiting the program");
        }
    }
}

问题是：该段代码 finally 的代码块一定会被执行吗？为什么？

二、分析

通常实际编码时，捕获异常后会记录日志或者将异常抛出等，此时 finally 代码块一般肯定会被执行到。

那么如何才能不执行finally呢？

于是我们想到，如果让虚拟机退出，问题不就解决了吗？ (就是这么暴力)

因此填充代码：

public class Demo {

    public static void main(String[] args) {
        try {
            BufferedReader br = new BufferedReader(new FileReader("file.txt"));
            System.out.println(br.readLine());
            br.close();
        } catch (IOException e) {
           System.exit(2);
        } finally {
            System.out.println("Exiting the program");
        }
    }
}

如果捕获到 IO异常，则会执行虚拟机退出指令，则不会执行finally 代码块。

System#exit 的源码如下：

    /**
     * Terminates the currently running Java Virtual Machine. The
     * argument serves as a status code; by convention, a nonzero status
     * code indicates abnormal termination.
     * 
     * This method calls the exit method in class
     * Runtime. This method never returns normally.
     * 
     * The call System.exit(n) is effectively equivalent to
     * the call:
     *      * Runtime.getRuntime().exit(n)
     * 
     *
     * @param      status   exit status.
     * @throws  SecurityException
     *        if a security manager exists and its checkExit
     *        method doesn't allow exit with the specified status.
     * @see        java.lang.Runtime#exit(int)
     */
    public static void exit(int status) {
        Runtime.getRuntime().exit(status);
    }

通过注释我们可以了解到，当 status 为非0 时，表示异常退出。

底层调用到 Runtime#exit：

    /**
     * Terminates the currently running Java virtual machine by initiating its
     * shutdown sequence.  This method never returns normally.  The argument
     * serves as a status code; by convention, a nonzero status code indicates
     * abnormal termination.
     *
     *  The virtual machine's shutdown sequence consists of two phases.  In
     * the first phase all registered {@link #addShutdownHook shutdown hooks},
     * if any, are started in some unspecified order and allowed to run
     * concurrently until they finish.  In the second phase all uninvoked
     * finalizers are run if {@link #runFinalizersOnExit finalization-on-exit}
     * has been enabled.  Once this is done the virtual machine {@link #halt
     * halts}.
     *
     *  If this method is invoked after the virtual machine has begun its
     * shutdown sequence then if shutdown hooks are being run this method will
     * block indefinitely.  If shutdown hooks have already been run and on-exit
     * finalization has been enabled then this method halts the virtual machine
     * with the given status code if the status is nonzero; otherwise, it
     * blocks indefinitely.
     *
     *  The {@link System#exit(int) System.exit} method is the
     * conventional and convenient means of invoking this method. 
     *
     * @param  status
     *         Termination status.  By convention, a nonzero status code
     *         indicates abnormal termination.
     *
     * @throws SecurityException
     *         If a security manager is present and its {@link
     *         SecurityManager#checkExit checkExit} method does not permit
     *         exiting with the specified status
     *
     * @see java.lang.SecurityException
     * @see java.lang.SecurityManager#checkExit(int)
     * @see #addShutdownHook
     * @see #removeShutdownHook
     * @see #runFinalizersOnExit
     * @see #halt(int)
     */
    public void exit(int status) {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkExit(status);
        }
        Shutdown.exit(status);
    }

三、延伸

同样的问题，请看下面代码片段：

public class Demo {

    public static void main(String[] args) {

      // 一些代码
        try {
            BufferedReader br = new BufferedReader(new FileReader("file.txt"));
            System.out.println(br.readLine());
            br.close();
        } catch (IOException e) {
           System.exit(2);
        } finally {
            System.out.println("Exiting the program");
        }
    }
}

问题是：如果try 代码块部分发生IO异常，是否一定不会执行到 finally 代码块呢？

what? 上面不是说不会执行吗？

我们再仔细看上面给出的 Runtime#exit 源码，可以发现，如果 SecurityManager 不为 null ，则会进行安全检查。

public void exit(int status) {

// 如果有securityManager , 则调用 checkExit函数
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkExit(status);
        }

// 检查通过后退出
        Shutdown.exit(status);
    }

安全检查通过才会执行 Shutdown#exit 执行最终的虚拟机退出。

因此如果我们可以修改 SecurityManager 如果检查退出时抛出异常，那么在执行 System.exit(2) 时就会发生异常，最终依然会执行到 finally代码块。

public class Demo {

    public static void main(String[] args) {
        // 修改 SecurityManager
        System.setSecurityManager(new SecurityManager() {
            @Override
            public void checkExit(int status) {
                throw new SecurityException("不允许退出");
            }
        });

        try {
            BufferedReader br = new BufferedReader(new FileReader("file.txt"));
            System.out.println(br.readLine());
            br.close();
        } catch (IOException e) {
            System.exit(2);
        } finally {
            System.out.println("Exiting the program");
        }
    }
}

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原始发表：2019/10/08 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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