Accessing Methods and functions of a object whose class type is dynamically known

Question

Guys,

I have an object A1 of type A. I dynamically find that out , that object A1 is of type A. I now have a property say "Name" which I want to access from A1 , how do I do it ?

Now the biggest problem is that the object A1 can even be of type B. If it is of type B then I will have to obtain the value "Address". Now How I resolve this ?

Below code does the type check ,

public static void testing(Object A1, String s) s - Classtype
  {
      try{
          Class c = Class.forName(s);

          if( c.isInstance(A1)) // 
          {
              //Now I know that A1 is of the  type C. But I dont know what type 'c' is (whether type A or type B. Because Only then I can access the appropriate member.) Like I said, type A contain 'name' and type B contains address.
              // The access may not only be a member but also a method .
          }
      }catch (Exception e){ System.out.println(e);}
  }

Any pointers would help a lot . thanks

Answer 1

You can know the declared fields of class

 Class cls = Class.forName("MyClass");
 Field fieldlist[] = cls.getDeclaredFields();  

• Documentation

Answer 2

The fields are typically private. So, to access them you have to call

field.setAccessible(true);

BTW, are you sure you really wish to use reflection in this case? Did you probably think about declaring interface? The class (implementation) can be still loaded dynamically.

For example: NameAccessor and AddressAccessor are interfaces.

FirstClass and SecondClass are classes. Let's assume that FirstClass implements NameAccessor and SecondClass implements both interfaces.

Now you can say:

Class clazz = Class.forName("SecondClass");
Object obj = clazz.newInstance();
//......
String name = ((NameAccessor)obj).getName();
String address = ((AddressAccessor)obj).getAddress();

I think (IMHO) that this solution is better than accessing private fields using reflection.

Answer 3

This kind of thing is tricky and error-prone if you do it manually. You should use one of the many BeanUtils / BeanHelper classes that almost every major framework contains. Here is my own quick example implementation which you can use if you want to:

public final class BeanHelper{

    /**
     * Return a map of an object's properties (key: property name, value:
     * property type).
     * 
     * @exception NullPointerException
     *                if bean is null
     */
    public static Map<String, Class<?>> describeProperties(final Object bean){
        if(bean == null){
            throw new NullPointerException();
        }
        final Map<String, Class<?>> map;
        final Class<?> beanClass = bean.getClass();
        if(PROPERTIES_CACHE.containsKey(beanClass)){
            map = PROPERTIES_CACHE.get(beanClass);
        } else{
            final PropertyDescriptor[] propertyDescriptors =
                getBeanInfo(beanClass);
            if(propertyDescriptors.length == 0){
                map = Collections.emptyMap();
            } else{
                final Map<String, Class<?>> innerMap =
                    new TreeMap<String, Class<?>>();
                for(final PropertyDescriptor pd : propertyDescriptors){
                    innerMap.put(pd.getName(), pd.getPropertyType());
                }
                map = Collections.unmodifiableMap(innerMap);
            }
            PROPERTIES_CACHE.put(beanClass, map);
        }
        return map;
    }

    private static PropertyDescriptor[] getBeanInfo(final Class<?> beanClass){
        try{
            return Introspector.getBeanInfo(beanClass, Object.class)
                .getPropertyDescriptors();
        } catch(final IntrospectionException e){
            throw new IllegalStateException(
                MessageFormat.format(
                "Couldn''t access bean properties for class {0}",
                beanClass),
                e);
        }
    }

    /**
     * Retrieve a named property from a specified object.
     * 
     * @return the property
     * @exception NullPointerException
     *                if one of the arguments is null
     * @exception IllegalArgumentException
     *                if there is no such property
     */
    public static Object getBeanProperty(final Object bean,
        final String property){
        if(bean == null || property == null){
            throw new NullPointerException();
        }
        final Class<?> beanClass = bean.getClass();
        Map<String, PropertyDescriptor> propMap;
        if(PROPERTY_DESCRIPTOR_CACHE.containsKey(beanClass)){
            propMap = PROPERTY_DESCRIPTOR_CACHE.get(beanClass);
        } else{
            final PropertyDescriptor[] beanInfo = getBeanInfo(beanClass);
            if(beanInfo.length == 0){
                propMap = Collections.emptyMap();
            } else{
                propMap =
                    new HashMap<String, PropertyDescriptor>(beanInfo.length);
                for(final PropertyDescriptor pd : beanInfo){
                    propMap.put(pd.getName(), pd);
                }
            }
            PROPERTY_DESCRIPTOR_CACHE.put(beanClass, propMap);
        }
        if(!propMap.containsKey(property)){
            throw new IllegalArgumentException(
                MessageFormat.format(
                "Class {0} does not have a property ''{1}''",
                beanClass,
                property));
        }
        return invokeMethod(propMap.get(property).getReadMethod(), bean);

    }

    private static Object invokeMethod(final Method method,
        final Object bean,
        final Object... args){
        try{
            return method.invoke(bean, args);
        } catch(final IllegalArgumentException e){
            throw e;
        } catch(final IllegalAccessException e){
            throw new IllegalStateException(
                MessageFormat.format(
                "Method not accessible: {0}",
                method),
                e);
        } catch(final InvocationTargetException e){
            throw new IllegalStateException(
                MessageFormat.format(
                "Error in method: {0}",
                method),
                e);
        }
    }

    private static final Map<Class<?>, Map<String, Class<?>>>
        PROPERTIES_CACHE =
        new ConcurrentHashMap<Class<?>, Map<String, Class<?>>>();

    private static final Map<Class<?>, Map<String, PropertyDescriptor>>
        PROPERTY_DESCRIPTOR_CACHE =
        new ConcurrentHashMap<Class<?>, Map<String, PropertyDescriptor>>();

    private BeanHelper(){
    }

}

Test Code:

public static void main(final String[] args){
    class Dummy{
        private String foo = "bar";
        private String baz = "phleem";
        public String getFoo(){
            return foo;
        }
        public void setFoo(final String foo){
            this.foo = foo;
        }
        public String getBaz(){
            return baz;
        }
        public void setBaz(final String baz){
            this.baz = baz;
        }
    }
    final Object dummy = new Dummy();
    final Map<String, Class<?>> beanProperties =
        BeanHelper.describeProperties(dummy);
    System.out.println(beanProperties);
    for(final String key : beanProperties.keySet()){
        System.out.println(MessageFormat.format("{0}:{1}",
            key,
            BeanHelper.getBeanProperty(dummy, key)));
    }
}

Output:

{baz=class java.lang.String, foo=class java.lang.String}
baz:phleem
foo:bar

Answer 4

Look at this: BeanUtils

myUser.setName("Bob");
// can instead be written:
BeanUtils.setProperty(myUser, "name", "Bob");
// and then retrieve:
BeanUtils.getProperty(myUser, "name");