How to Properly Get a File from resources Folder

Question

Many tutorials for reading a file from a resources folder use class loader. However, using that method I cannot get past problems with static warnings and the result is always a null pointer exception.

public class Test {
    public static void main(String[] args) {
        StringBuilder contentBuilder=new StringBuilder();
        ClassLoader classLoader=Test.class.getClassLoader();
        File file=new File(classLoader.getSystemResource("test.html").getFile());
        try {
            BufferedReader buffer=new BufferedReader(new FileReader(file));
            String sCurrentLine="";
            while ((sCurrentLine=buffer.readLine())!=null) {
                contentBuilder.append(sCurrentLine);
            }
            buffer.close();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
        String content=contentBuilder.toString();
        System.out.println("content="+content);
    }
}

The warning from my IDE on the "File" line is:

The static method getSystemResource(String) from the type ClassLoader should be accessed in a static way

I cannot figure out how to eliminate the warning and if I just ignore it and run the code I get a null pointer exception. Why am I getting this and how do I fix it? TIA.

Answer 1

Test.class.getClassLoader(); obtains a reference to the ClassLoader class from the Class' method public ClassLoader getClassLoader() - see private final ClassLoader classLoader below.

Since you are accessing the ClassLoader class from an object of that class, you're not accessing it in a static way.

From Class.java, Java SE 1.7:

@CallerSensitive
public ClassLoader getClassLoader() {
    ClassLoader cl = getClassLoader0();
    if (cl == null)
        return null;
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        ClassLoader.checkClassLoaderPermission(cl, Reflection.getCallerClass());
    }
    return cl;
}

// Package-private to allow ClassLoader access
ClassLoader getClassLoader0() { return classLoader; }

// Initialized in JVM not by private constructor
// This field is filtered from reflection access, i.e. getDeclaredField
// will throw NoSuchFieldException
private final ClassLoader classLoader;

In order to access the method in a static way, it has to be called from the class which declares it as static if you want to get rid of the warning:

ClassLoader.getSystemResource("test.html").getFile()

To avoid the NPE the test.html file should be under your source folder.

---

To respond to your comment, using a method which returns other than a URL solves your problem - see Reading a resource file from within jar.

public class Test {

    public static void main(String[] args) {
        StringBuilder contentBuilder = new StringBuilder();

        InputStream is = Test.class.getResourceAsStream("/test.html");
        try {
            String sCurrentLine;

            BufferedReader buffer = new BufferedReader(new InputStreamReader(is));
            while ((sCurrentLine = buffer.readLine())!=null)
                contentBuilder.append(sCurrentLine);
            buffer.close();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }

        System.out.println("content=" + contentBuilder.toString());
    }
}