# 踩坑的点：

我删除文件的方式，一开始，为了方便，使用了

file.deleteOnExit();

这种方式，还顺便判断了一下，只有文件存在是时候才删除，省的多写一步 判空操作，但是，闲来无事的时候，做了个测试，因为发现cpu 有的时候占用率比较高

# 翻翻源码

    /**
     * Requests that the file or directory denoted by this abstract
     * pathname be deleted when the virtual machine terminates.
     * Files (or directories) are deleted in the reverse order that
     * they are registered. Invoking this method to delete a file or
     * directory that is already registered for deletion has no effect.
     * Deletion will be attempted only for normal termination of the
     * virtual machine, as defined by the Java Language Specification.
     *
     * <p> Once deletion has been requested, it is not possible to cancel the
     * request.  This method should therefore be used with care.
     *
     * <P>
     * Note: this method should <i>not</i> be used for file-locking, as
     * the resulting protocol cannot be made to work reliably. The
     * {@link java.nio.channels.FileLock FileLock}
     * facility should be used instead.
     *
     * @throws  SecurityException
     *          If a security manager exists and its <code>{@link
     *          java.lang.SecurityManager#checkDelete}</code> method denies
     *          delete access to the file
     *
     * @see #delete
     *
     * @since 1.2
     */
    public void deleteOnExit() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkDelete(path);
        }
        if (isInvalid()) {
            return;
        }
        DeleteOnExitHook.add(path);
    }

注释的意思是说，这个文件只有在虚拟机停止的时候，才会被删除，那正常来说我的虚拟机肯定是一直活在的，挂了那就得加班改bug 了呀，所以，这个方法只是相当于把文件加到了一个队列（DeleteOnExitHook.add(path)） ，需要被删除，但是并没有被立即删除

/**
 * This class holds a set of filenames to be deleted on VM exit through a shutdown hook.
 * A set is used both to prevent double-insertion of the same file as well as offer
 * quick removal.
 */

class DeleteOnExitHook {}

这个类的注释说得清清楚楚，这类持有一些文件名，需要在虚拟机退出的时候，通过运行钩子函数删除

就是指的runHooks 这个方法，所以，如果是想立即删除文件，请使用下面的方式

common io 包里面的

FileUtils.forceDelete(file);

或者是

final boolean delete = file.delete();

都可以立即删除对应的文件