目录
- 一、背景
- 二、源码分析
- 2.1 问题1:为什么需要 NonNul 检查 ?
- 2.1.1 怎么检查的?
- 2.1.2 为什么要做这个检查呢?
- 2.2 问题2: 为什么不根据当前系统类型来获取分隔符?
- 三、Zoom Out
- 3.1 代码健壮性
- 3.2 代码严谨性
- 3.3 如何写注释
- 四、总结
一、背景
最近用到了 org.apache.commons.io.FilenameUtils#getName
这个方法,该方法可以传入文件路径,获取文件名。 简单看了下源码,虽然并不复杂,但和自己设想略有区别,值得学习,本文简单分析下。
二、源码分析
org.apache.commons.io.FilenameUtils#getName
/** * Gets the name minus the path from a full fileName. * <p> * This method will handle a file in either Unix or Windows format. * The text after the last forward or backslash is returned. * <pre> * a/b/c.txt --> c.txt * a.txt --> a.txt * a/b/c --> c * a/b/c/ --> "" * </pre> * <p> * The output will be the same irrespective of the machine that the code is running on. * * @param fileName the fileName to query, null returns null * @return the name of the file without the path, or an empty string if none exists. * Null bytes inside string will be removed */ public static String getName(final String fileName) { // 传入 null 直接返回 null if (fileName == null) { return null; } // NonNul 检查 requireNonNullChars(fileName); // 查找最后一个分隔符 final int index = indexOfLastSeparator(fileName); // 从最后一个分隔符窃到最后 return fileName.substring(index + 1); }
2.1 问题1:为什么需要 NonNul 检查 ?
2.1.1 怎么检查的?
org.apache.commons.io.FilenameUtils#requireNonNullChars
/** * Checks the input for null bytes, a sign of unsanitized data being passed to to file level functions. * * This may be used for poison byte attacks. * * @param path the path to check */ private static void requireNonNullChars(final String path) { if (path.indexOf(0) >= 0) { throw new IllegalArgumentException("Null byte present in file/path name. There are no " + "known legitimate use cases for such data, but several injection attacks may use it"); } }
java.lang.String#indexOf(int)
源码:
/** * Returns the index within this string of the first occurrence of * the specified character. If a character with value * {@code ch} occurs in the character sequence represented by * this {@code String} object, then the index (in Unicode * code units) of the first such occurrence is returned. For * values of {@code ch} in the range from 0 to 0xFFFF * (inclusive), this is the smallest value <i>k</i> such that: * <blockquote><pre> * this.charAt(<i>k</i>) == ch * </pre></blockquote> * is true. For other values of {@code ch}, it is the * smallest value <i>k</i> such that: * <blockquote><pre> * this.codePointAt(<i>k</i>) == ch * </pre></blockquote> * is true. In either case, if no such character occurs in this * string, then {@code -1} is returned. * * @param ch a character (Unicode code point). * @return the index of the first occurrence of the character in the * character sequence represented by this object, or * {@code -1} if the character does not occur. */ public int indexOf(int ch) { return indexOf(ch, 0); }
可知,indexOf(0)
目的是查找 ASCII 码为 0 的字符的位置,如果找到则抛出 IllegalArgumentException
异常。 搜索 ASCII 对照表,得知 ASCII 值为 0 代表控制字符 NUT,并不是常规的文件名所应该包含的字符。
2.1.2 为什么要做这个检查呢?
null 字节是一个值为 0 的字节,如十六进制中的 0x00。 存在与 null 字节有关的安全漏洞。 因为 C 语言中使用 null 字节作为字符串终结符,而其他语言(Java,PHP等)没有这个字符串终结符; 例如,Java Web 项目只允许用户上传 .jpg 格式的图片,但利用这个漏洞就可以上传 .jsp 文件。 如用户上传 hack.jsp<NUL>.jpg
文件, Java 会认为符合 .jpg 格式,实际调用 C 语言系统函数写入磁盘时讲 当做字符串分隔符,结果将文件保存为 hack.jsp
。 有些编程语言不允许在文件名中使用 ·· <NUL>
,如果你使用的编程语言没有对此处理,就需要自己去处理。 因此,这个检查很有必要。
代码示例:
package org.example; import org.apache.commons.io.FilenameUtils; public class FilenameDemo { public static void main(String[] args) { String filename= "hack.jsp\0.jpg"; System.out.println( FilenameUtils.getName(filename)); } }
报错信息:
Exception in thread "main" java.lang.IllegalArgumentException: Null byte present in file/path name. There are no known legitimate use cases for such data, but several injection attacks may use it
at org.apache.commons.io.FilenameUtils.requireNonNullChars(FilenameUtils.java:998)
at org.apache.commons.io.FilenameUtils.getName(FilenameUtils.java:984)
at org.example.FilenameDemo.main(FilenameDemo.java:8)
如果去掉校验:
package org.example; import org.apache.commons.io.FilenameUtils; public class FilenameDemo { public static void main(String[] args) { String filename= "hack.jsp\0.jpg"; // 不添加校验 String name = getName(filename); // 获取拓展名 String extension = FilenameUtils.getExtension(name); System.out.println(extension); } public static String getName(final String fileName) { if (fileName == null) { return null; } final int index = FilenameUtils.indexOfLastSeparator(fileName); return fileName.substring(index + 1); } }
Java 的确会将拓展名识别为 jpg
jpg
JDK 8 及其以上版本试图创建 hack.jsp\0.jpg
的文件时,底层也会做类似的校验,无法创建成功。
大家感兴趣可以试试使用 C 语言写入名为 hack.jsp\0.jpg
的文件,最终很可能文件名为 hack.jsp
。
2.2 问题2: 为什么不根据当前系统类型来获取分隔符?
查找最后一个分隔符 org.apache.commons.io.FilenameUtils#indexOfLastSeparator
/** * Returns the index of the last directory separator character. * <p> * This method will handle a file in either Unix or Windows format. * The position of the last forward or backslash is returned. * <p> * The output will be the same irrespective of the machine that the code is running on. * * @param fileName the fileName to find the last path separator in, null returns -1 * @return the index of the last separator character, or -1 if there * is no such character */ public static int indexOfLastSeparator(final String fileName) { if (fileName == null) { return NOT_FOUND; } final int lastUnixPos = fileName.lastIndexOf(UNIX_SEPARATOR); final int lastWindowsPos = fileName.lastIndexOf(WINDOWS_SEPARATOR); return Math.max(lastUnixPos, lastWindowsPos); }
该方法的语义是获取文件名,那么从函数的语义层面上来说,不管是啥系统的文件分隔符都必须要保证得到正确的文件名。 试想一下,在 Windows 系统上调用该函数,传入一个 Unix 文件路径,得不到正确的文件名合理吗? 函数设计本身就应该考虑兼容性。 因此不能获取当前系统的分隔符来截取文件名。 源码中分别获取 Window 和 Unix 分隔符,有哪个用哪个,显然更加合理。
三、Zoom Out
3.1 代码健壮性
我们日常编码时,要做防御性编程,对于错误的、非法的输入都要做好预防。
3.2 代码严谨性
我们写代码一定不要想当然。 我们先想清楚这个函数究竟要实现怎样的功能,而且不是做一个 “CV 工程师”,无脑“拷贝”代码。 同时,我们也应该写好单测,充分考虑各种异常 Case ,保证正常和异常的 Case 都覆盖到。
3.3 如何写注释
org.apache.commons.io.FilenameUtils#requireNonNullChars
函数注释部分就给出了这么设计的原因:This may be used for poison byte attacks.
注释不应该“喃喃自语”讲一些显而易见的废话。 对于容易让人困惑的设计,一定要通过注释讲清楚设计原因。
此外,结合工作经验,推荐一些其他注释技巧: (1)对于稍微复杂或者重要的设计,可以通过注释给出核心的设计思路; 如: java.util.concurrent.ThreadPoolExecutor#execute
/** * Executes the given task sometime in the future. The task * may execute in a new thread or in an existing pooled thread. * * If the task cannot be submitted for execution, either because this * executor has been shutdown or because its capacity has been reached, * the task is handled by the current {@link RejectedExecutionHandler}. * * @param command the task to execute * @throws RejectedExecutionException at discretion of * {@code RejectedExecutionHandler}, if the task * cannot be accepted for execution * @throws NullPointerException if {@code command} is null */ public void execute(Runnable command) { if (command == null) throw new NullPointerException(); /* * Proceed in 3 steps: * * 1. If fewer than corePoolSize threads are running, try to * start a new thread with the given command as its first * task. The call to addWorker atomically checks runState and * workerCount, and so prevents false alarms that would add * threads when it shouldn't, by returning false. * * 2. If a task can be successfully queued, then we still need * to double-check whether we should have added a thread * (because existing ones died since last checking) or that * the pool shut down since entry into this method. So we * recheck state and if necessary roll back the enqueuing if * stopped, or start a new thread if there are none. * * 3. If we cannot queue task, then we try to add a new * thread. If it fails, we know we are shut down or saturated * and so reject the task. */ int c = ctl.get(); if (workerCountOf(c) < corePoolSize) { if (addWorker(command, true)) return; c = ctl.get(); } if (isRunning(c) && workQueue.offer(command)) { int recheck = ctl.get(); if (! isRunning(recheck) && remove(command)) reject(command); else if (workerCountOf(recheck) == 0) addWorker(null, false); } else if (!addWorker(command, false)) reject(command); }
(2)对于关联的代码,可以使用 @see 或者 {@link } 的方式,在代码中提供关联代码的快捷跳转方式。
/** * Sets the core number of threads. This overrides any value set * in the constructor. If the new value is smaller than the * current value, excess existing threads will be terminated when * they next become idle. If larger, new threads will, if needed, * be started to execute any queued tasks. * * @param corePoolSize the new core size * @throws IllegalArgumentException if {@code corePoolSize < 0} * or {@code corePoolSize} is greater than the {@linkplain * #getMaximumPoolSize() maximum pool size} * @see #getCorePoolSize */ public void setCorePoolSize(int corePoolSize) { if (corePoolSize < 0 || maximumPoolSize < corePoolSize) throw new IllegalArgumentException(); int delta = corePoolSize - this.corePoolSize; this.corePoolSize = corePoolSize; if (workerCountOf(ctl.get()) > corePoolSize) interruptIdleWorkers(); else if (delta > 0) { // We don't really know how many new threads are "needed". // As a heuristic, prestart enough new workers (up to new // core size) to handle the current number of tasks in // queue, but stop if queue becomes empty while doing so. int k = Math.min(delta, workQueue.size()); while (k-- > 0 && addWorker(null, true)) { if (workQueue.isEmpty()) break; } } }
(2)在日常业务开发中,非常推荐讲相关的文档、配置页面链接也放到注释中,极大方便后期维护。 如:
/** * 某某功能 * * 相关文档: * <a href="https://blog.csdn.net/w605283073" rel="external nofollow" rel="external nofollow" >设计文档</a> * <a href="https://blog.csdn.net/w605283073" rel="external nofollow" rel="external nofollow" >三方API地址</a> */ public void demo(){ // 省略 }
(4)对于工具类可以考虑讲给出常见的输入对应的输出。 如 org.apache.commons.lang3.StringUtils#center(java.lang.String, int, char)
/** * <p>Centers a String in a larger String of size {@code size}. * Uses a supplied character as the value to pad the String with.</p> * * <p>If the size is less than the String length, the String is returned. * A {@code null} String returns {@code null}. * A negative size is treated as zero.</p> * * <pre> * StringUtils.center(null, *, *) = null * StringUtils.center("", 4, ' ') = " " * StringUtils.center("ab", -1, ' ') = "ab" * StringUtils.center("ab", 4, ' ') = " ab " * StringUtils.center("abcd", 2, ' ') = "abcd" * StringUtils.center("a", 4, ' ') = " a " * StringUtils.center("a", 4, 'y') = "yayy" * </pre> * * @param str the String to center, may be null * @param size the int size of new String, negative treated as zero * @param padChar the character to pad the new String with * @return centered String, {@code null} if null String input * @since 2.0 */ public static String center(String str, final int size, final char padChar) { if (str == null || size <= 0) { return str; } final int strLen = str.length(); final int pads = size - strLen; if (pads <= 0) { return str; } str = leftPad(str, strLen + pads / 2, padChar); str = rightPad(str, size, padChar); return str; }
(5) 对于废弃的方法,一定要注明废弃的原因,给出替代方案。 如:java.security.Signature#setParameter(java.lang.String, java.lang.Object)
/** * 省略部分 * * @see #getParameter * * @deprecated Use * {@link #setParameter(java.security.spec.AlgorithmParameterSpec) * setParameter}. */ @Deprecated public final void setParameter(String param, Object value) throws InvalidParameterException { engineSetParameter(param, value); }
四、总结
很多优秀的开源项目的代码设计都非常严谨,往往简单的代码中也蕴藏着缜密的思考。 我们有时间可以看看一些优秀的开源项目,可以从简单的入手,可以先想想如果自己写大概该如何实现,然后和作者的实现思路对比,会有更大收获。 平时看源码时,不仅要知道源码长这样,更要了解为什么这么设计。
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