Spring Framework IoC延遲依賴查找

本篇我們來介紹下Spring Framework IoC的延遲依賴查找。這裡我總結了以下幾種方式：

* Bean 延遲依賴查找接口
	* ObjectFactory
	* ObjectProvider
		* Spring 5 對Java 8特性擴充
			* 函數式接口
				* getIfAvailable(Supplier)
				* ifAvaiable(Consumer)
			* Stream擴充 - stream()           

接下來我們就通過實際例子一起來剖析一下。代碼如下：

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */
package org.eleven.thinking.in.spring.dependency.lookup;

import org.eleven.thinking.in.spring.ioc.overview.domain.User;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Primary;

/**
 * 通過 {@link ObjectProvider} 進行依賴查找
 *
 * @author <a href="mailto:[email protected]">eleven</a>
 * @since
 */
public class ObjectProviderDemo { // @Configuration 是非必須注解

    public static void main(String[] args) {
        // 建立 BeanFactory 容器
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
        // 将目前類 ObjectProviderDemo 作為配置類（Configuration Class）
        applicationContext.register(ObjectProviderDemo.class);
        // 啟動應用上下文
        applicationContext.refresh();
        // 依賴查找集合對象
        lookupByObjectProvider(applicationContext);
        lookupIfAvailable(applicationContext);
        lookupByStreamOps(applicationContext);

        // 關閉應用上下文
        applicationContext.close();

    }

    private static void lookupByStreamOps(AnnotationConfigApplicationContext applicationContext) {
        ObjectProvider<String> objectProvider = applicationContext.getBeanProvider(String.class);
//        Iterable<String> stringIterable = objectProvider;
//        for (String string : stringIterable) {
//            System.out.println(string);
//        }
        // Stream -> Method reference
        objectProvider.stream().forEach(System.out::println);
    }

    private static void lookupIfAvailable(AnnotationConfigApplicationContext applicationContext) {
        ObjectProvider<User> userObjectProvider = applicationContext.getBeanProvider(User.class);
        User user = userObjectProvider.getIfAvailable(User::createUser);
        System.out.println("目前 User 對象：" + user);
    }

    @Bean
    @Primary
    public String helloWorld() { // 方法名就是 Bean 名稱 = "helloWorld"
        return "Hello,World";
    }
eleven
    @Bean
    public String message() {
        return "Message";
    }

    private static void lookupByObjectProvider(AnnotationConfigApplicationContext applicationContext) {
        ObjectProvider<String> objectProvider = applicationContext.getBeanProvider(String.class);
        System.out.println(objectProvider.getObject());
    }
}