Java Reflection - Constructors

http://tutorials.jenkov.com/java-reflection/constructors.html

Using Java Reflection you can inspect the constructors of classes and instantiate objects at runtime. This is done via the Java class

java.lang.reflect.Constructor

. This text will get into more detail
about the Java

Constructor

object.

Obtaining Constructor Objects

The

Constructor

class is obtained from the

Class

object. Here is an example:

Class aClass = ...//obtain class object
Constructor[] constructors = aClass.getConstructors();

The

Constructor[]

array will have one

Constructor

instance for each public constructor declared in the class.

If you know the precise parameter types of the constructor you want to access, you can do so rather than obtain the array all constructors. This example returns the public constructor of the given class which takes a

String

as
parameter:

Class aClass = ...//obtain class object
Constructor constructor =
        aClass.getConstructor(new Class[]{String.class});

If no constructor matches the given constructor arguments, in this case

String.class

, a

NoSuchMethodException

is thrown.

Constructor Parameters

You can read what parameters a given constructor takes like this:

Constructor constructor = ... // obtain constructor - see above
Class[] parameterTypes = constructor.getParameterTypes();

Instantiating Objects using Constructor Object

You can instantiate an object like this:

//get constructor that takes a String as argument
Constructor constructor = MyObject.class.getConstructor(String.class);

MyObject myObject = (MyObject)
        constructor.newInstance("constructor-arg1");

The

Constructor.newInstance()

method takes an optional amount of parameters, but you must supply exactly one parameter per argument in the constructor you are invoking. In this case it was a constructor
taking a

String

, so one

String

must be supplied.