Java 使用 ClassLoader 加载文件
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Loading files with ClassLoader
提问by twolfe18
This problem has been bugging me for a while. I have to load a couple files in my java app, and the only way I got working so far looks like this:
这个问题困扰了我一段时间。我必须在我的 java 应用程序中加载几个文件，到目前为止我工作的唯一方法是这样的：
URL hsURL;
if(System.getProperty("os.name").toLowerCase().contains("windows")) {
hsURL = new URL("file:/" + System.getProperty("user.dir") + "/helpsets/helpset.hs");
}
else {
hsURL = new URL("file://" + System.getProperty("user.dir") + "/helpsets/helpset.hs");
}
But this is ugly and terrible. For a while I thought I had this working:
但这既丑陋又可怕。有一段时间我以为我有这个工作：
hsURL = ClassLoader.getSystemResource("helpsets/helpset.hs");
But that no longer works for some reason (I must have changed something and not noticed. It returns null.
但这由于某种原因不再有效（我一定是改变了一些东西而没有注意到。它返回空值。
Should I be using getResource() instead of getSystemResource() (if so, why is getSystemResource() static but not getResource())?
我应该使用 getResource() 而不是 getSystemResource() （如果是这样，为什么 getSystemResource() 是静态的而不是 getResource() ）？
I am using eclipse and I have tried including the folder in the build path (classpath) and not including it, it doesn't seem to make a difference.
我正在使用 eclipse 并且我尝试在构建路径（类路径）中包含该文件夹而不包含它，它似乎没有什么区别。
采纳答案by Jon Skeet
getSystemResourceis static because it will use the systemclassloader, which is available statically. (ClassLoader.getSystemClassLoader)
getSystemResource是静态的，因为它将使用静态可用的系统类加载器。( ClassLoader.getSystemClassLoader)
If your resource is available in the classpath, I would suggest using ClassLoader.getResource()or Class.getResourcefrom an appropriate class, e.g.
如果您的资源在类路径中可用，我建议使用ClassLoader.getResource()或Class.getResource来自适当的类，例如
Foo.class.getResource("/helpsets/helpset.hs");
(ClassLoader.getResourceis "absolute"; Class.getResourceis relative to the package of the class unless you prefix it with a '/'.)
（ClassLoader.getResource是“绝对的”；Class.getResource是相对于类的包的，除非你在它前面加上一个“/”。）
If this doesn't work, please post how your app is configured in terms of the classpath, and where your file is.
如果这不起作用，请发布您的应用程序是如何根据类路径配置的，以及您的文件在哪里。
EDIT: I usually find the URL less useful than an InputStream, so I use getResourceAsStreaminstead of getResource. YMMV
编辑：我通常发现 URL 不如 有用InputStream，所以我使用getResourceAsStream代替getResource。青年会
回答by kdgregory
You've mentioned several different things here, so let's sort them out.
你在这里提到了几个不同的事情，所以让我们把它们整理一下。
1) Creating a "file:" URL based on "user.dir"
1) 基于“user.dir”创建一个“file:” URL
The "user.dir" property refers to the current working directory -- wherever the user might have been when s/he started the app. Chances are good that files written here will disappear between two runs (because the user might run from a different directory).
“user.dir”属性指的是当前工作目录——用户启动应用程序时可能所在的位置。在两次运行之间，这里写的文件很可能会消失（因为用户可能从不同的目录运行）。
The "user.home" property refers to the user's home directory -- which should remain the same between runs.
“user.home”属性指的是用户的主目录——在两次运行之间应该保持不变。
In either case, use a File object to open files, don't muck around with creating a "file:" URL. You get no benefit, and as you can see, you have to write messy code to access it.
在任何一种情况下，使用 File 对象打开文件，不要乱创建“file:” URL。你没有得到任何好处，正如你所看到的，你必须编写凌乱的代码才能访问它。
2) Retrieving a resource via the classloader
2) 通过类加载器获取资源
This is meant to retrieve files that are packaged with your application -- read-only files. As you have seen, there are multiple variants. I prefer using the following, because I assume that a class will want to load a file that's packaged with it.
这是为了检索与您的应用程序打包在一起的文件——只读文件。如您所见，有多种变体。我更喜欢使用以下内容，因为我假设一个类将要加载一个与它打包在一起的文件。
InputStream in = this.getClass().getClassLoader().getResourceAsStream(fileName);
