删除文件踩的坑

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.
 *
 * 
 Once deletion has been requested, it is not possible to cancel the
 * request.  This method should therefore be used with care.
 *
 * 

 * Note: this method should not 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 {@link
 *          java.lang.SecurityManager#checkDelete} 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 {...}

这个类的注释说的清清楚楚，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图片:

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