New characteristics of 23--JDK 7-reflection 2-filed, construction Class (construction method)

1. The constructor class provides the following methods for creating objects for a class:

Public Object newinstance (Object...initargs)

Initargs used to specify parameters received by the constructor

Learn more:Sun Company creates an object for simplifying developers, and it also provides a newinstance method in the class object to create objects for the class. This allows developers to avoid having to reflect the constructor class each time to create objects.

However , it is important to note that the Class.newinstance method internally is an object created by a reflection class parameterless constructor, so when creating a class object in this way, the class must have an parameterless constructor.

Specific case Analysis

packageCom.java.reflect;

importjava.lang.reflect.Constructor;

importjava.lang.reflect.InvocationTargetException;

importorg.junit.Test;

Publicclass Demo2 {

@Test

Reflects the parameterless build function and creates the person object

publicvoid test1 ()throws classnotfoundexception,nosuchmethodexception, SecurityException, Instantiationexception,illegalaccessexception, IllegalArgumentException, invocationtargetexception {

Load class

Class C = class.forname ("Com.java.reflect.Person");

The method of reflection construction

Constructor cs = C.getconstructor (null);

Person P = (person) cs.newinstance (null);

System.out.println (P.getname ());

The output result is: parameterless constructor Zhangsan

}

@Test

Reflection out of a build function with one argument public person (String name)

publicvoid test2 ()throws ClassNotFoundException, Nosuchmethodexception,securityexception, Instantiationexception, Illegalaccessexception,illegalargumentexception, invocationtargetexception {

Load class

Class C = class.forname ("Com.java.reflect.Person");

The method of reflection construction

Constructor cs = C.getconstructor (String. Class);

Person P = (person) cs.newinstance ("Dick");

System.out.println (P.getname ());

The output is: Dick Zhangsan analysis: The first output Lee four is the calling of the constructor method, the first Zhangsan is the property value in the person class, because there is no this.name = name in the constructor for this parameter

}

@Test

Reflection out of a constructor with two arguments public person (String Name,int age)

publicvoid test3 ()throws classnotfoundexception,nosuchmethodexception, SecurityException, Instantiationexception,illegalaccessexception, IllegalArgumentException, invocationtargetexception {

Load class

Class C = class.forname ("Com.java.reflect.Person");

The method of reflection construction

Constructor cs = C.getconstructor (String. Class,int. class);

Person P = (person) cs.newinstance ("Dick", 20);

System.out.println (P.getname ());

The output is: Dick ... Zhangsan

}

@Test

Reflects private constructors (requires a getdeclaredconstructor () method)

publicvoid test4 ()throws classnotfoundexception,nosuchmethodexception, SecurityException, Instantiationexception,illegalaccessexception, IllegalArgumentException, invocationtargetexception {

Load class

Class C = class.forname ("Com.java.reflect.Person");

The method of reflection construction

Constructor cs = c.getdeclaredconstructor (int. Class);

Cs.setaccessible (true); If it is private, you must set access permissions

Person P = (person) cs.newinstance (60);

System.out.println (P.getname ());

The output is: 60zhangsan

}

@Test

Using the Newinstance () method of class classes to reflect the method of constructing without parameters

publicvoid test5 ()throws Classnotfoundexception,instantiationexception, illegalaccessexception {

Load class

Class C = class.forname ("Com.java.reflect.Person");

Person P = (person) c.newinstance ();

System.out.println (P.getname ());

The output result is: The parameterless construction method Zhangsan

}

}

2. The method object provides the following methods for executing the method it represents:

public Object Invoke(object Obj,object ... args)

This is essentially the same as when the construction class calls the constructor method:

Specific cases:

packageCom.java.reflect;

importjava.lang.reflect.InvocationTargetException;

importJava.lang.reflect.Method;

importorg.junit.Test;

Publicclass Demo3 {

/**

* @param args

* @throws classnotfoundexception

* @throws SecurityException

* @throws nosuchmethodexception

* @throws invocationtargetexception

* @throws illegalargumentexception

* @throws illegalaccessexception

*/

@Test

Call parameterless method public void run ()

publicvoid test1 ()throws Classnotfoundexception,nosuchmethodexception,

Securityexception,illegalaccessexception,

illegalargumentexception,invocationtargetexception {

Person p = new person ();

Class C = class.forname ("Com.java.reflect.Person");

Method m = C.getmethod ("Run",null);

M.invoke (p, null);

Output result: The parameterless constructor (because the parameterless constructor of the person class was called) run ....

}

@Test

Call method with one parameter public void run (String name)

publicvoid test2 ()throws ClassNotFoundException, Nosuchmethodexception,

Securityexception,illegalaccessexception,

illegalargumentexception,invocationtargetexception {

Person p = new person ();

Class C = class.forname ("Com.java.reflect.Person");

Method m = C.getmethod ("Run", String. ) Class);

M.invoke (P, "Yang");

Output Result: parameterless constructor (because the parameterless constructor of the person class was called) run ... Yang

}

@Test

Call method with two arguments public void run (int i,int j)

publicvoid test3 ()throws Classnotfoundexception,nosuchmethodexception,

Securityexception,illegalaccessexception,

illegalargumentexception,invocationtargetexception {

Person p = new person ();

Class C = class.forname ("Com.java.reflect.Person");

Method m = C.getmethod ("sum",int. Class,int. class);

System.out.println (M.invoke (p, 10, 20));

/*

* The following output results are as int value = (Integer) m.invoke (p,10,20);

*system.out.println (value);

*/

Output Result: parameterless constructor (because the parameterless constructor of the person class was called) 30

}

@Test

Call private method prvate void sum ()

publicvoid test4 ()throws Classnotfoundexception,nosuchmethodexception,

Securityexception,illegalaccessexception,

illegalargumentexception,invocationtargetexception {

Person p = new person ();

Class C = class.forname ("Com.java.reflect.Person");

Method m = C.getdeclaredmethod ("sum",null);

M.setaccessible (true);

M.invoke (p, null);

Output Result: parameterless constructor (because the parameterless constructor of the person class is called) private ....

}

@Test

Call static method public static void Run1 ()

publicvoid test5 ()throws Classnotfoundexception,nosuchmethodexception,

SecurityException, Illegalaccessexception,

illegalargumentexception,invocationtargetexception {

Person P = Newperson (); Calling a static method can not pass in an object

Class C = class.forname ("Com.java.reflect.Person"