Hello everybody,
How can I loop over a class attributes in java dynamically.
For eg :
public class MyClass
{
private type1 att1;
private type2 att2;
...
public void function()
{
for(var in MyClass.Attributes)
{
System.out.println(var.class);
}
}
}
is this possible in Java?
Java has Reflection (java.reflection.*), but I would suggest looking into a library like Apache Beanutils, it will make the process much less hairy than using reflection directly.
There is no linguistic support to do what you're asking for.
You can reflectively access the members of a type at run-time using reflection (e.g. with Class.getDeclaredFields() to get an array of Field), but depending on what you're trying to do, this may not be the best solution.
Here's a simple example to show only some of what reflection is capable of doing.
import java.lang.reflect.*;
public class DumpFields {
public static void main(String[] args) {
inspect(String.class);
}
static <T> void inspect(Class<T> klazz) {
Field[] fields = klazz.getDeclaredFields();
System.out.printf("%d fields:%n", fields.length);
for (Field field : fields) {
System.out.printf("%s %s %s%n",
Modifier.toString(field.getModifiers()),
field.getType().getSimpleName(),
field.getName()
);
}
}
}
The above snippet uses reflection to inspect all the declared fields of class String; it produces the following output:
7 fields:
private final char[] value
private final int offset
private final int count
private int hash
private static final long serialVersionUID
private static final ObjectStreamField[] serialPersistentFields
public static final Comparator CASE_INSENSITIVE_ORDER
These are excerpts from the book:
Given a
Classobject, you can obtainConstructor,Method, andFieldinstances representing the constructors, methods and fields of the class. [They] let you manipulate their underlying counterparts reflectively. This power, however, comes at a price:
- You lose all the benefits of compile-time checking.
- The code required to perform reflective access is clumsy and verbose.
- Performance suffers.
As a rule, objects should not be accessed reflectively in normal applications at runtime.
There are a few sophisticated applications that require reflection. Examples include [...omitted on purpose...] If you have any doubts as to whether your application falls into one of these categories, it probably doesn't.
Accessing the fields directly is not really good style in java. I would suggest creating getter and setter methods for the fields of your bean and then using then Introspector and BeanInfo classes from the java.beans package.
MyBean bean = new MyBean();
BeanInfo beanInfo = Introspector.getBeanInfo(MyBean.class);
for (PropertyDescriptor propertyDesc : beanInfo.getPropertyDescriptors()) {
String propertyName = propertyDesc.getName();
Object value = propertyDesc.getReadMethod().invoke(bean);
}
While I agree with Jörn's answer, there is a more elegant possibility if you use spring.
Spring has a class named ReflectionUtils that offers some very powerful functionality, including doWithFields(class, callback), a visitor-style method that lets you iterate over a classes fields using a callback object like this:
public void analyze(Object obj){
ReflectionUtils.doWithFields(obj.getClass(), new FieldCallback(){
@Override
public void doWith(final Field field)
throws IllegalArgumentException, IllegalAccessException {
System.out.println("Field name: " + field.getName());
field.setAccessible(true);
System.out.println("Field value: "+ field.get(obj));
}
});
}
But here's a warning: the class is labeled as "for internal use only", which is a pity if you ask me