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前言：JavaEE程序有一大路径陷阱，那就是ServletContext的getRealPath方法。我们常常使用getRealPath(“/”)来获得Web应用程序根目录的绝对路径。这是绝对要不得的！

绝对不要使用ServletContext的getRealPath方法获取Web应用的路径！应该使用ServletContext的getResource()方法，直接使用相对于Web应用根目录的相对路径来获取资源。

getResource

java.net.URL getResource(java.lang.String path)

throws java.net.MalformedURLException

Returns a URL to the resource that is mapped to a specified path. The path must begin with a "/" and is interpreted as relative to the current context root.

This method allows the servlet container to make a resource available to servlets from any source. Resources can be located on a local or remote file system, in a database, or in a .war file.

The servlet container must implement the URL handlers and URLConnection objects that are necessary to access the resource.

This method returns null if no resource is mapped to the pathname.

Some containers may allow writing to the URL returned by this method using the methods of the URL class.

The resource content is returned directly, so be aware that requesting a .jsp page returns the JSP source code. Use a RequestDispatcher instead to include results of an execution.

This method has a different purpose than java.lang.Class.getResource, which looks up resources based on a class loader. This method does not use class loaders.

Parameters:

path - a String specifying the path to the resource

Returns:

the resource located at the named path, or null if there is no resource at that path

Throws:

java.net.MalformedURLException - if the pathname is not given in the correct form

getResourceAsStream

java.io.InputStream getResourceAsStream(java.lang.String path)

Returns the resource located at the named path as an InputStream object.

The data in the InputStream can be of any type or length. The path must be specified according to the rules given in getResource. This method returns null if no resource exists at the specified path.

Meta-information such as content length and content type that is available via getResource method is lost when using this method.

The servlet container must implement the URL handlers and URLConnection objects necessary to access the resource.

This method is different from java.lang.Class.getResourceAsStream, which uses a class loader. This method allows servlet containers to make a resource available to a servlet from any location, without using a class loader.

Parameters:

path - a String specifying the path to the resource

Returns:

the InputStream returned to the servlet, or null if no resource exists at the specified path

getRealPath

java.lang.String getRealPath(java.lang.String path)

Returns a String containing the real path for a given virtual path. For example, the path "/index.html" returns the absolute file path on the server's filesystem would be served by a request for "http://host/contextPath/index.html", where contextPath is the context path of this ServletContext..

The real path returned will be in a form appropriate to the computer and operating system on which the servlet container is running, including the proper path separators. This method returns null if the servlet container cannot translate the virtual path to a real path for any reason (such as when the content is being made available from a .war archive).

Parameters:

path - a String specifying a virtual path

Returns:

a String specifying the real path, or null if the translation cannot be performed

总结

可以看到，ServletContext接口中的getResource()等方法，可以找到任何从应用程序的根目录开始的资源。包括在.war包这样的压缩文件中。参数必须以/开头。

而我们常用的getRealPath(“/”)方法，在.war包发布时，就会失效。会返回null。

因此，我们应该避免使用getRealPath(“/”)这样的方法来获取应用程序的绝对路径。

参考：JavaEE路径陷阱之getRealPath