Java Class New Instance newInstance(final Class clazz)

Description

new Instance

License

Open Source License

Declaration

public static <T> T newInstance(final Class<T> clazz) 

Method Source Code

//package com.java2s;
/*/*  w  ww  .j a va2s.c  om*/
 * Copyright 2012 Red Hat, Inc. and/or its affiliates.
 *
 * Licensed under the Eclipse Public License version 1.0, available at
 * http://www.eclipse.org/legal/epl-v10.html
 */

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

public class Main {
    public static <T> T newInstance(final Class<T> clazz) {
        if (clazz.isAnonymousClass()) {
            throw new RuntimeException("Can not create new instance of an anonymous class");
        } else if (clazz.isInterface())
            throw new RuntimeException("Can not create new instance of an interface");
        else if (clazz.isAnnotation())
            throw new RuntimeException("Can not create new instance of an annotation");

        for (Constructor<?> constructor : clazz.getConstructors()) {
            T object = (T) instantiateWithConstructor(constructor);
            if (object != null)
                return object;
        }

        for (Constructor<?> constructor : clazz.getDeclaredConstructors()) {
            T object = (T) instantiateWithConstructor(constructor);
            if (object != null)
                return object;
        }

        throw new RuntimeException("Could not instantiate class: " + clazz + ", no access to constructors.");
    }

    private static <T> T instantiateWithConstructor(Constructor<T> constructor) {
        if (constructor.getParameterTypes().length == 0) {
            if (Modifier.isPrivate(constructor.getModifiers())
                    || Modifier.isProtected(constructor.getModifiers())) {
                constructor.setAccessible(true);
            }
            try {
                return constructor.newInstance();
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }
        }
        //inner classes require parent class as parameter
        else if (constructor.getParameterTypes().length == 1) {
            if (Modifier.isPrivate(constructor.getModifiers())
                    || Modifier.isProtected(constructor.getModifiers())) {
                constructor.setAccessible(true);
            }

            Constructor paramConstructor = getConstructorWithNoParams(constructor.getParameterTypes()[0]);

            if (paramConstructor != null) {
                try {
                    return constructor.newInstance(paramConstructor.newInstance());
                } catch (InstantiationException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                } catch (InvocationTargetException e) {
                    e.printStackTrace();
                }
            }
        }
        return null;
    }

    private static Constructor getConstructorWithNoParams(Class clazz) {
        for (Constructor constructor : clazz.getConstructors()) {
            if (constructor.getParameterTypes().length == 0) {
                if (Modifier.isPrivate(constructor.getModifiers())
                        || Modifier.isProtected(constructor.getModifiers())) {
                    constructor.setAccessible(true);
                }
                return constructor;
            }
        }
        return null;
    }
}

Related

1. newInstance(Constructor ctor, Object... params)
2. newInstance(Field field)
3. newInstance(final Class clazz)
4. newInstance(final Class clazz, final Object[] parameters)
5. newInstance(final Class arrayClass, final int length)
6. newInstance(final Class clazz)
7. newInstance(final Class clazz)
8. newInstance(final Class clazz)
9. newInstance(final Class clazz)