Getting fields from another Jar File

I try to read fields from an external jar. We name the external File: 'Data.jar'.

Data.jar contains a class called Constants with some public, static, final fields. For example:

public static final String string1 = "Test"; 
public static final int integer1 = 1;

How can I access the external file in order to get the value of the fields string1 and integer1? Is it possible to use reflections? I know the external jar and the structure.

Edit:

The external jars structure is an older version of my current project. So I use the URLClassLoader and call the class Constants, I will get the value of my current project, and not the jars one. So is there a way to only call the external jars classes?

Solution:

    public Object getValueFromExternalJar(String className, String fieldName) throws MalformedURLException
    {
       Object val = null;
       // calling the external jar
       URLClassLoader cL = new URLClassLoader(new URL[] { jarURL }, null);
//the null is very important, if the jar is structural identical to this project
       try
       {
          // define the class(within the package)
          Class<?> clazz = cL.loadClass(className);
          // defining the field by its name
          Field field = clazz.getField(fieldName);
          field.setAccessible(true);
          // get the Target datatype
          Class<?> targetType = field.getType();
          Object objectValue = targetType.newInstance();
          // read the value
          val = field.get(objectValue);
       }
       catch (ClassNotFoundException | NoSuchFieldException | SecurityException | IllegalAccessException
             | InstantiationException e)
       {
          LOGGER.log(Level.SEVERE, "An error while accessing an external jar appears", e);
       }
       finally
       {
          try
          {
             cL.close();
          }
          catch (Exception e)
          {
          }
       }
       return val;
    }

Some example code using an URLClassLoader based on your description, as dan1st suggested:

URLClassLoader cl = new URLClassLoader(new URL[] {new File("Data.jar").toURI().toURL()}, ClassLoader.getPlatformClassLoader());
Class<?> clazz = cl.loadClass("Constants");
String string1 = (String) clazz.getField("string1").get(null);
int integer1 = clazz.getField("integer1").getInt(null);

I guess you have to change the example to match your structure.