首页 \ 问答 \ ClassLoader.getResource（）上的NullPointerException(NullPointerException on ClassLoader.getResource())

ClassLoader.getResource（）上的NullPointerException(NullPointerException on ClassLoader.getResource())

 我正在开发一个小项目，我需要将文件从应用程序JAR移动到JAR之外的另一个位置。 
 

 这就是我的JAR文件的样子： 
 

MyApp.JAR
   |- META-INF
       |- MANIFEST.MF
   |- files
       |- myFile.exe
   |- MainClass.class

 

 我想做的是将myFile.exe移动到C:\\ （例如）。 所以在MainClass我试过： 
 

23. Path path = Paths.get(getClass().getClassLoader().getResource("/files/myFile.exe").toURI());
24. byte[] bytes = Files.readAllBytes(path);

25. FileOutputStream fos = new FileOutputStream("C:\\myFile.exe");
26. fos.write(b);
27. fos.close();

 

 运行我的应用程序后，我得到一个NullPointerException ，这是它： 
 

java.lang.NullPointerException
    at MainClass.<init>(MainClass.java:23)
    at MainClass.main(MainClass.java:66)

 

 （第66行是用new MainClass();调用构造函数的地方new MainClass(); ） 
 

 我知道NPE可能被抛出，因为无法找到myFile.exe ，但我无法弄清楚为什么......当我打开JAR时，我可以看到带有exe的files夹。 也许我正在以错误的方式访问该文件？ 我在StackOverflow上尝试了许多其他方法，但没有任何作用...... 
 

 提前致谢，对不起我的英语。 

i'm working on a little project and i need to move a file from the application JAR to another location outside the JAR.
 

This is how my JAR file looks like:
 

MyApp.JAR
   |- META-INF
       |- MANIFEST.MF
   |- files
       |- myFile.exe
   |- MainClass.class

 

What i'd like to do is move myFile.exe to C:\\ (for example). So in the MainClass i tried with:
 

23. Path path = Paths.get(getClass().getClassLoader().getResource("/files/myFile.exe").toURI());
24. byte[] bytes = Files.readAllBytes(path);

25. FileOutputStream fos = new FileOutputStream("C:\\myFile.exe");
26. fos.write(b);
27. fos.close();

 

After running my application i get a NullPointerException, here's it:
 

java.lang.NullPointerException
    at MainClass.<init>(MainClass.java:23)
    at MainClass.main(MainClass.java:66)

 

(Line 66 is where i call the constructor with new MainClass();)
 

I know the NPE is probably thrown because myFile.exe cannot be found, but i can't figure out why... When i open the JAR i can see the files folder with the exe inside. Maybe i'm accessing the file in a wrong way? I tried many other ways that i found here on StackOverflow but nothing works...
 

Thanks in advance and sorry for my english.

原文：https://stackoverflow.com/questions/34373172
更新时间：2023-11-08 21:11

最满意答案

 值得阅读R FAQ 7.31并考虑浮点表示的准确性。 
 

 我无法在当前的版本（1.9.2）中重现这一点。 运用 
 

R version 3.0.3 (2014-03-06)
Platform: x86_64-w64-mingw32/x64 (64-bit)

 

 我猜测行为的变化将与此新闻项目有关。 
 

 
 
 o数值数据仍然像以前一样在公差范围内连接和分组，但不是公差为sqrt（.Machine $ double.eps）== 1.490116e-08（与base :: all.equal的默认值相同）现在已将有效数字舍入为最后2个字节，apx 11 sf这更适用于大型（1.23e20）和小型（1.23e-20）数字，并且通过简单的位旋转更快。 一些函数提供了一个“容差”参数但是没有被传递，所以已被删除。 我们的目标是在将来的版本中添加一个全局选项（例如，2个，1个或0个字节的舍入）。 
 

 

 

 马特更新 
 

 是的，这是v1.9.2中的故意更改，data.table现在将0.0000000000000000000000000001与0区分开来（正如user3340145正确认为的那样），因为上面从NEWS突出显示了改进的舍入方法。 
 

 我还添加了Rick对测试套件的回答的for循环测试。 
 

 顺便说一句，＃5369现在已在v1.9.3中实现（尽管这个问题都不需要这些）： 
 

 
 
 o bit64 :: integer64现在可用于分组和连接，＃5369。 感谢James Sams突出UPC。 
 
 
 o新函数setNumericRounding（）可用于在加入或分组“numeric”类型的列＃5369时减少1个字节或0个字节的舍入。 请参阅v1.9.2中的setNumericRounding和NEWS项中的示例。 getNumericRounding（）返回当前设置。 
 

 

 请注意，现在（从v1.9.2开始）舍入有关有效数的准确性; 即有效数字的数量。 0.0000000000000000000000000001 == 1.0e-28精确到1平方英尺，因此新的舍入方法不会将其与0.0组合在一起。 
 

 简而言之，问题的答案是：从v1.8.10升级到v1.9.2或更高版本。 

It is worth reading R FAQ 7.31 and thinking about the accuracy of floating point represenations.
 

I can't reproduce this in the current cran version (1.9.2). using 
 

R version 3.0.3 (2014-03-06)
Platform: x86_64-w64-mingw32/x64 (64-bit)

 

My guess that the change in behaivour will be related to this news item.
 

 
 
o Numeric data is still joined and grouped within tolerance as before but instead of tolerance being sqrt(.Machine$double.eps) == 1.490116e-08 (the same as base::all.equal's default) the significand is now rounded to the last 2 bytes, apx 11 s.f. This is more appropriate for large (1.23e20) and small (1.23e-20) numerics and is faster via a simple bit twiddle. A few functions provided a 'tolerance' argument but this wasn't being passed through so has been removed. We aim to add a global option (e.g. 2, 1 or 0 byte rounding) in a future release.
 

 

 

Update from Matt
 

Yes this was a deliberate change in v1.9.2 and data.table now distinguishes 0.0000000000000000000000000001 from 0 (as user3340145 rightly thought it should) due to the improved rounding method highlighted above from NEWS.
 

I've also added the for loop test from Rick's answer to the test suite.
 

Btw, #5369 is now implemented in v1.9.3 (although neither of these are needed for this question) :
 

 
 
o bit64::integer64 now works in grouping and joins, #5369. Thanks to James Sams for highlighting UPCs.
 
 
o New function setNumericRounding() may be used to reduce to 1 byte or 0 byte rounding when joining to or grouping columns of type 'numeric', #5369. See example in ?setNumericRounding and NEWS item from v1.9.2. getNumericRounding() returns the current setting.
 

 

Notice that rounding is now (as from v1.9.2) about the accuracy of the significand; i.e. the number of significant figures. 0.0000000000000000000000000001 == 1.0e-28 is accurate to just 1 s.f., so the new rounding method doesn't group this together with 0.0.
 

In short, the answer to the question is : upgrade from v1.8.10 to v1.9.2 or greater.

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