在设计模式中按照不同的处理方式共包含三大类：创建型模式、结构型模式和行为模式。
创建型模式
工厂方法模式(Factory Method Pattern)
抽象工厂模式（Abstract Factory Pattern）
建造者模式(Builder Pattern)
原型模式(Prototype Pattern)
单例模式(Singleton Pattern)
定义
创建者模式又叫建造者模式，是将一个复杂的对象的构建与它的表示分离，使得同样的构建过程可以创建不同的表示。创建者模式隐藏了复杂对象的创建过程，它把复杂对象的创建过程加以抽象，通过子类继承或者重载的方式，动态的创建具有复合属性的对象。
我的理解
我理解的的建造者模式为创建一个复杂的对象 对象有很多属性。有时候不需要很多属性都进行预置 只需要几个比较重要的属性初始化好就可以使用！建造者模式通过链式变成的方式为每个重要的属性初始化好 从而得到一个完整的对象
创建者模式代码
/**
* @author: yangqiang
* @create: 2021-02-22 11:35
*/
public class Course {
private String name;
private String means;
private String note;
private String homework;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getMeans() {
return means;
}
public void setMeans(String means) {
this.means = means;
}
public String getNote() {
return note;
}
public void setNote(String note) {
this.note = note;
}
public String getHomework() {
return homework;
}
public void setHomework(String homework) {
this.homework = homework;
}
public static class CourseBuilder {
private Course course = new Course();
public CourseBuilder addName(String name) {
course.setName(name);
return this;
}
public CourseBuilder addMeans(String means) {
course.setMeans(means);
return this;
}
public CourseBuilder addNote(String note) {
course.setNote(note);
return this;
}
public CourseBuilder addHomework(String homework) {
course.setHomework(homework);
return this;
}
public Course build() {
return course;
}
}
}
建造者模式在Spring中的应用
/*
* Copyright 2002-2020 the original author or authors.
*
* 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
*
* https://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.springframework.aop.aspectj.annotation;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import org.aspectj.lang.reflect.PerClauseKind;
import org.springframework.aop.Advisor;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.ListableBeanFactory;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
/**
* Helper for retrieving @AspectJ beans from a BeanFactory and building
* Spring Advisors based on them, for use with auto-proxying.
*
* @author Juergen Hoeller
* @since 2.0.2
* @see AnnotationAwareAspectJAutoProxyCreator
*/
public class BeanFactoryAspectJAdvisorsBuilder {
private final ListableBeanFactory beanFactory;
private final AspectJAdvisorFactory advisorFactory;
@Nullable
private volatile List<String> aspectBeanNames;
private final Map<String, List<Advisor>> advisorsCache = new ConcurrentHashMap<>();
private final Map<String, MetadataAwareAspectInstanceFactory> aspectFactoryCache = new ConcurrentHashMap<>();
/**
* Create a new BeanFactoryAspectJAdvisorsBuilder for the given BeanFactory.
* @param beanFactory the ListableBeanFactory to scan
*/
public BeanFactoryAspectJAdvisorsBuilder(ListableBeanFactory beanFactory) {
this(beanFactory, new ReflectiveAspectJAdvisorFactory(beanFactory));
}
/**
* Create a new BeanFactoryAspectJAdvisorsBuilder for the given BeanFactory.
* @param beanFactory the ListableBeanFactory to scan
* @param advisorFactory the AspectJAdvisorFactory to build each Advisor with
*/
public BeanFactoryAspectJAdvisorsBuilder(ListableBeanFactory beanFactory, AspectJAdvisorFactory advisorFactory) {
Assert.notNull(beanFactory, "ListableBeanFactory must not be null");
Assert.notNull(advisorFactory, "AspectJAdvisorFactory must not be null");
this.beanFactory = beanFactory;
this.advisorFactory = advisorFactory;
}
/**
* Look for AspectJ-annotated aspect beans in the current bean factory,
* and return to a list of Spring AOP Advisors representing them.
* <p>Creates a Spring Advisor for each AspectJ advice method.
* @return the list of {@link org.springframework.aop.Advisor} beans
* @see #isEligibleBean
*/
public List<Advisor> buildAspectJAdvisors() {
List<String> aspectNames = this.aspectBeanNames;
if (aspectNames == null) {
synchronized (this) {
aspectNames = this.aspectBeanNames;
if (aspectNames == null) {
List<Advisor> advisors = new ArrayList<>();
aspectNames = new ArrayList<>();
String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
this.beanFactory, Object.class, true, false);
for (String beanName : beanNames) {
if (!isEligibleBean(beanName)) {
continue;
}
// We must be careful not to instantiate beans eagerly as in this case they
// would be cached by the Spring container but would not have been weaved.
Class<?> beanType = this.beanFactory.getType(beanName, false);
if (beanType == null) {
continue;
}//是否带有@Aspect 和非ajc编译的类
if (this.advisorFactory.isAspect(beanType)) {
aspectNames.add(beanName);
AspectMetadata amd = new AspectMetadata(beanType, beanName);//创建切面元数据
if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
MetadataAwareAspectInstanceFactory factory =
new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
if (this.beanFactory.isSingleton(beanName)) {
this.advisorsCache.put(beanName, classAdvisors);
}
else {
this.aspectFactoryCache.put(beanName, factory);
}
advisors.addAll(classAdvisors);
}
else {
// Per target or per this.
if (this.beanFactory.isSingleton(beanName)) {
throw new IllegalArgumentException("Bean with name '" + beanName +
"' is a singleton, but aspect instantiation model is not singleton");
}
MetadataAwareAspectInstanceFactory factory =
new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
this.aspectFactoryCache.put(beanName, factory);
advisors.addAll(this.advisorFactory.getAdvisors(factory));
}
}
}
this.aspectBeanNames = aspectNames;
return advisors;
}
}
}
if (aspectNames.isEmpty()) {
return Collections.emptyList();
}
List<Advisor> advisors = new ArrayList<>();
for (String aspectName : aspectNames) {
List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
if (cachedAdvisors != null) {
advisors.addAll(cachedAdvisors);
}
else {
MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
advisors.addAll(this.advisorFactory.getAdvisors(factory));
}
}
return advisors;
}
/**
* Return whether the aspect bean with the given name is eligible.
* @param beanName the name of the aspect bean
* @return whether the bean is eligible
*/
protected boolean isEligibleBean(String beanName) {
return true;
}
}
优点
- 使用建造者模式可以使客户端不必知道产品内部组成的细节。
- 具体的建造者类之间是相互独立的，这有利于系统的扩展。
- 具体的建造者相互独立，因此可以对建造的过程逐步细化，而不会对其他模块产生任何影响。
缺点
- 建造者模式所创建的产品一般具有较多的共同点，其组成部分相似；如果产品之间的差异性很大，则不适合使用建造者模式，因此其使用范围受到一定的限制。
- 如果产品的内部变化复杂，可能会导致需要定义很多具体建造者类来实现这种变化，导致系统变得很庞大。
一般情况下，我们更习惯使用静态内部类的方式实现建造者模式，即一个产品类内部自动带有一个具体建造者，由它负责该产品的组装创建，不再需要 Builder 和 Director。这样做可以使产品表示与创建之间的联系更加紧密，结构更加紧凑，同时使得建造者模式更加简洁。如果采用静态内部类形式实现建造者模式，则《建造者模式实现链式赋值》一节的示例可以改写如下。
与抽象工厂模式对比
- 与抽象工厂模式相比，建造者模式返回一个组装好的完整产品，而抽象工厂模式返回一系列相关的产品，这些产品位于不同的产品等级结构，构成了一个产品族 。
- 在抽象工厂模式中，客户端实例化工厂类，然后调用工厂方法获取所需产品对象，而在建造者模式中，客户端可以不直接调用建造者的相关方法，而是通过指挥者类来指导如何生成对象，包括对象的组装过程和建造步骤，它侧重于一步步构造一个复杂对象，返回一个完整的对象 。
- 如果将抽象工厂模式看成汽车配件生产工厂，生产一个产品族的产品，那么建造者模式就是一个汽车组装工厂，通过对部件的组装可以返回一辆完整的汽车
