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;
/*//from  w ww . j a v  a  2 s  . com
 * JBoss, Home of Professional Open Source
 * Copyright 2014 Red Hat Inc. and/or its affiliates and other contributors
 * as indicated by the @authors tag. All rights reserved.
 * See the copyright.txt in the distribution for a
 * full listing of individual contributors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

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() || clazz.isInterface() || clazz.isAnnotation()) {
            throw new RuntimeException("Can not build new instance of an " + clazz.getName());
        }

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

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

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

    private static <T> T instantiateWithConstructor(Constructor<T> constructor) {
        T instance = null;
        if (constructor.getParameterTypes().length == 0) {
            instance = newInstanceWithoutParameterTypes(constructor);
        }

        if (constructor.getParameterTypes().length == 1) {
            instance = newInstanceWithParameterTypes(constructor);
        }

        return isValidInstance(instance) ? instance : null;
    }

    private static <T> boolean isValidInstance(T instance) {
        if (instance != null) {
            return true;
        }
        return false;
    }

    private static <T> T newInstanceWithoutParameterTypes(Constructor<T> constructor) {
        try {
            setAccessible(constructor);
            return constructor.newInstance();
        } catch (InstantiationException | IllegalAccessException | InvocationTargetException e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }

    private static <T> T newInstanceWithParameterTypes(Constructor<T> constructor) {
        Constructor paramConstructor = getConstructorWithNoParams(constructor.getParameterTypes()[0]);
        if (paramConstructor == null)
            return null;
        setAccessible(constructor);
        try {
            return constructor.newInstance(paramConstructor.newInstance());
        } catch (InstantiationException | IllegalAccessException | InvocationTargetException e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }

    private static <T> void setAccessible(Constructor<T> constructor) {
        if (Modifier.isPrivate(constructor.getModifiers()) || Modifier.isProtected(constructor.getModifiers())) {
            constructor.setAccessible(true);
        }
    }

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

Related

1. newInstance(final Class clazz)
2. newInstance(final Class clazz, final Object[] parameters)
3. newInstance(final Class arrayClass, final int length)
4. newInstance(final Class clazz)
5. newInstance(final Class clazz)
6. newInstance(final Class clazz)
7. newInstance(final Class clazz)
8. newInstance(final Class type, final int length)
9. newInstance(final ClassLoader classLoader, final String className, final Object... constructorArgs)