vendredi 22 juin 2018

Instantiate a Class by Accessing Private Constructor

//Sample class Student.java

 public class Student {
        private Integer sudentId;
        private String studentName;
        private Student(){}
        private Student(Integer studentId, String studentName) {
            this.studentId = studentId;
            this.studentName = studentName;
        }
        public Integer getStudentId() {
            return studentId;
        }
        public String getStudentName() {
            return studentName;
        }
    }

In below code, There are two ways to instantiate the class
1- Find the private constructor using given constructor name and instantiate the class. 2- Find the private constructor for given number of arguments and types and instantiate the class

        import java.lang.reflect.Constructor;
        import java.lang.reflect.InvocationTargetException;
        import java.lang.reflect.Modifier;

        public class PrivateConstructorDemo {
            //Find the private constructor using given constructor name and instantiate the class.
            public void createObjectByConstructorName(int id, String name) throws NoSuchMethodException, SecurityException,
                    InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{

                Constructor<Student> constructor = Student.class.getDeclaredConstructor(Integer.class, String.class);
                if (Modifier.isPrivate(constructor.getModifiers())) {
                    constructor.setAccessible(true);
                    Student student = (Student)constructor.newInstance(id, name);
                    System.out.println("Student Id:"+ student.getStudentId());
                    System.out.println("Student Name:"+ student.getStudentName());
                }
            } 

            //For given number of arguments and types and instantiate the class. 
            public void createObject(int id, String name) throws InstantiationException, 
                                IllegalAccessException, IllegalArgumentException, InvocationTargetException {

                   Constructor<?>[] constructors = Student.class.getDeclaredConstructors();
                   for (Constructor<?> constructor : constructors) {
                     if (Modifier.isPrivate(constructor.getModifiers())) {
                        constructor.setAccessible(true);
                        Class<?>[] clazzs = constructor.getParameterTypes();
                        if (constructor.getParameterCount() == 2 && clazzs[0] == Integer.class && 
                                                             clazzs[1]  == String.class) {
                            Object ob = constructor.newInstance(id, name);
                            if (ob instanceof Student) {
                                Student student = (Student)ob;
                                System.out.println("Student Id:"+ student.getStudentId());
                                System.out.println("Student Name:"+ student.getStudentName());
                            }
                        }
                     }
                   }
            }

            public static void main(String[] args) throws InstantiationException, IllegalAccessException,
                    IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {

                PrivateConstructorDemo obj = new PrivateConstructorDemo();
                obj.createObject(10, "Sandeep");
                System.out.println("-------------------------");
                obj.createObjectByConstructorName(20,"Sandeep");
            }
        } 





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