Sunday, February 2, 2014

Better approach to load resources using relative paths in Java

FileInputStream (Absolute path)

To load a resource file such as for program use, first thing that we would consider will be specifying the absolute file path as given below:

InputStream input = new FileInputStream("/Users/jwithanawasam/some_dir/src/main/resources/”);

However, when ever we moved the project to another location, this path has to be changed, which is not acceptable.

FileInputStream (Relative path)

So, the next option would be to use the relative file path as given below, instead of giving absolute file path:

InputStream input = new FileInputStream("src/main/resources/”);

This approach seems to solve the above mentioned concern.

However, problem with this is the relative path is depending on the current working directory, which JVM is started. In this scenario, it is "/Users/jwithanawasam/some_dir". But, in a different deployment setting this may change, which leads to change the given relative path accordingly. Moreover, we, as developers do not have much control over JVMs current working directory.

In any of the above cases, we will get error, which is a familiar exception for most java developers.


At runtime, JVM checks the class path to locate any user defined  classes and packages. (In Maven, build artifacts and dependancies are stored under path given for M2_REPO class path variable. E.g., /Users/jwithanawasam/.m2/repository) The .jar file which is the deployable unit of the project will be located here.

JVM uses class loader to load java libraries specified in class path.

So, best thing we can do is load the resource specifying a path relative to its class path using class loader. Here, specified relative path will work  irrespective of the actual disk location the package is deployed.

Following methods reads the file using class loader.

InputStream input = Test.class.getResourceAsStream("/");

Usually, in Java projects resources such as configuration files, images etc. are located in src/main/resources/ path. So, if we add a resource immediately inside this folder, during packaging, the file will be located in the immediate folder in .jar file.

We can verify this using the following command to extract content of jar file:

jar xf someproject.jar

If you place the resources in another sub folder, then you have to specify the path relative to src/main/resources/ path.

So, using this approach we can load resources using relative paths in a hard disk location independent manner. Once we package the application, it is ready to be deployed anywhere, as it it is, without the overhead of having to validate resource file paths, thus improving the portability of the application.

ServletContext.getResourceAsStream for web applications

For web applications, use the following method:

ServletContext context = getServletContext();
    InputStream is = context.getResourceAsStream("/filename.txt");
Here, file path is taken relative to your web application folder. (The unzipped version of the .war file)
E.g., mywebapplication.war (unzipped) will have a hierarchy similar to the following.  
So, "/" means the root of this web application folder.  
This method allows servlet containers to make a resource available to a servlet from any location, without using a class loader. 


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