Java Reflection - Fields
Jakob Jenkov |
Using Java Reflection you can inspect the fields (member variables) of classes and get / set them at runtime. This is done via
the Java class java.lang.reflect.Field
. This text will get into more
detail about the Java Field
object. Remember to check the JavaDoc from Sun out too.
Obtaining Field Objects
The Field
class is obtained from the Class
object.
Here is an example:
Class aClass = ...//obtain class object Field[] fields = aClass.getFields();
The Field[]
array will have one Field
instance for each public field
declared in the class.
If you know the name of the field you want to access, you can access it like this:
Class aClass = MyObject.class Field field = aClass.getField("someField");
The example above will return the Field
instance corresponding to the field
someField
as declared in the MyObject
below:
public class MyObject{ public String someField = null; }
If no field exists with the name given as parameter to the getField()
method,
a NoSuchFieldException
is thrown.
Field Name
Once you have obtained a Field
instance, you can get its field name using the
Field.getName()
method, like this:
Field field = ... //obtain field object String fieldName = field.getName();
Field Type
You can determine the field type (String, int etc.) of a field using the Field.getType()
method:
Field field = aClass.getField("someField"); Object fieldType = field.getType();
Getting and Setting Field Values
Once you have obtained a Field
reference you can get and set its values using the
Field.get()
and Field.set()
methods, like this:
Class aClass = MyObject.class Field field = aClass.getField("someField"); MyObject objectInstance = new MyObject(); Object value = field.get(objectInstance); field.set(objetInstance, value);
The objectInstance
parameter passed to the get
and set
method should be an instance of the class that owns the field. In the above example an
instance of MyObject
is used, because the someField
is an instance
member of the MyObject
class.
It the field is a static field (public static ...) pass null
as parameter
to the get
and set
methods, instead of the objectInstance
parameter passed above.
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Jakob Jenkov |