Spring创建bean实例的几种方式分享
作者:蓝黑2020
前言
Spring常见的创建bean实例的方式有:
1.通过bean的class属性创建实例
- 无参构造器
- 带参构造器
2.工厂方法
- 静态工厂方法
- 实例工厂方法
3.工厂bean
关于每种方式的Spring配置,详见文末总结。
环境
- Ubuntu 22.04
- IntelliJ IDEA 2022.1.3
- JDK 17.0.3
- Spring 5.3.21 准备
创建Maven项目 test0705
。
修改 pom.xml 文件,添加依赖:
...... <!-- https://mvnrepository.com/artifact/junit/junit --> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.13.2</version> <scope>test</scope> </dependency> <!-- https://mvnrepository.com/artifact/org.springframework/spring-webmvc --> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-webmvc</artifactId> <version>5.3.21</version> </dependency> ......
在 src/main/resources
目录下创建 applicationContext.xml
文件:
<?xml version="1.0" encoding="utf-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> </beans>
在 src/test/java
目录下创建测试:
public class Test0705 { }
通过bean的class属性创建实例(无参构造器)
注:这是最常见的创建bean的方式。我其它文档也用过相似代码。如果已熟悉可直接略过。
创建如下POJO:
Axe
:Axe接口;StoneAxe
:Axe实现类;SteelAxe
:Axe实现类;Person
:Person持有Axe;
package pojo; public interface Axe { public void chop(); }
package pojo; public class StoneAxe implements Axe{ public StoneAxe() { System.out.println("StoneAxe constructor"); } @Override public void chop() { System.out.println("Stone axe!"); } }
package pojo; public class SteelAxe implements Axe{ public SteelAxe() { System.out.println("SteelAxe constructor"); } @Override public void chop() { System.out.println("Steel axe!"); } }
package pojo; public class Person { private String name; private Axe axe; public void setAxe(Axe axe) { this.axe = axe; } public void setName(String name) { this.name = name; } public Person() { System.out.println("Person constructor"); } public void useAxe() { System.out.println("I am " + name); axe.chop(); } }
在 applicationContext.xml
中注册bean:
...... <bean id="stoneAxe" class="pojo.StoneAxe"/> <bean id="steelAxe" class="pojo.SteelAxe"/> <bean id="person" class="pojo.Person"> <property name="name" value="Tom"/> <property name="axe" ref="stoneAxe"/> </bean> ......
创建测试用例:
@Test public void test1() { var ctx = new ClassPathXmlApplicationContext("applicationContext.xml"); System.out.println("before getBean person"); var person = ctx.getBean("person", Person.class); person.useAxe(); }
运行测试,如下:
StoneAxe constructor SteelAxe constructor Person constructor before getBean person I am Tom Stone axe!
通过bean的class属性创建实例(带参构造器)
注:这是较为常见的创建bean的方式。我其它文档也用过相似代码。如果已熟悉可直接略过。
创建如下POJO:
Book
:Book接口;PlayBook
:Book实现类;StudyBook
:Book实现类;Student
:Student持有Book;
package pojo; public interface Book { public void show(); }
package pojo; public class PlayBook implements Book{ public PlayBook() { System.out.println("PlayBook constructor"); } @Override public void show() { System.out.println("Play book!"); } }
package pojo; public class StudyBook implements Book{ public StudyBook() { System.out.println("StudyBook constructor"); } @Override public void show() { System.out.println("Study book!"); } }
package pojo; public class Student { private String name; private Book book; public Student(String name, Book book) { System.out.println("Student constructor"); this.name = name; this.book = book; } public void readBook() { System.out.println("I am " + name); book.show(); } }
在 applicationContext.xml
中注册bean:
...... <bean id="playBook" class="pojo.PlayBook"/> <bean id="studyBook" class="pojo.StudyBook"/> <bean id="student" class="pojo.Student"> <constructor-arg index="0" value="Jerry"/> <constructor-arg index="1" ref="playBook"/> </bean> ......
创建测试用例:
@Test public void test2() { var ctx = new ClassPathXmlApplicationContext("applicationContext.xml"); System.out.println("before getBean student"); var student = ctx.getBean("student", Student.class); student.readBook(); }
运行测试,如下:
......
PlayBook constructor
StudyBook constructor
Student constructor
before getBean student
I am Jerry
Play book!
工厂方法(静态工厂方法)
配置:
class
属性指向静态工厂类factory-method
属性指向静态工厂方法
注:如果静态工厂方法需要参数,则通过 constructor-arg
来指定。
例如:
<bean id="dog" class="factory.AnimalFactory" factory-method="getAnimal"> <constructor-arg value="dog"/> <property name="name" value="Snoopy"/> </bean>
当调用 ctx.getBean("dog")
时,Spring会调用 AnimalFactory
类的静态方法 getAnimal()
,并传入参数 "dog"
,创建一个Animal的实例并返回。
注:同普通的bean一样,其默认scope是 singleton
,多次调用 ctx.getBean("dog")
返回的是同一个实例(在Spring初始化时生产bean)。
完整例子如下:
创建如下POJO:
Animal
:Animal接口;Dog
:Animal实现类;Cat
:Animal实现类;
package pojo; public interface Animal { public void cry(); }
package pojo; public class Dog implements Animal{ private String name; public void setName(String name) { this.name = name; } public Dog() { System.out.println("Dog constructor"); } @Override public void cry() { System.out.println("I am " + name); System.out.println("Wang wang..."); } }
package pojo; public class Cat implements Animal{ private String name; public void setName(String name) { this.name = name; } public Cat() { System.out.println("Cat constructor"); } @Override public void cry() { System.out.println("I am " + name); System.out.println("Miao Miao..."); } }
创建工厂类 AnimalFactory
:
package factory; import pojo.Animal; import pojo.Cat; import pojo.Dog; public class AnimalFactory { public static Animal getAnimal(String type) { System.out.println("creating new Animal object"); if (type.equalsIgnoreCase("dog")) { return new Dog(); } else if (type.equalsIgnoreCase("cat")) { return new Cat(); } else { return null; } } }
在 applicationContext.xml
中注册bean:
<bean id="dog" class="factory.AnimalFactory" factory-method="getAnimal"> <constructor-arg value="dog"/> <property name="name" value="Snoopy"/> </bean> <bean id="cat" class="factory.AnimalFactory" factory-method="getAnimal"> <constructor-arg value="cat"/> <property name="name" value="Kitty"/> </bean>
创建测试用例:
@Test public void test3() { var ctx = new ClassPathXmlApplicationContext("applicationContext.xml"); System.out.println("before getBean dog cat"); var animal1 = ctx.getBean("dog", Animal.class); animal1.cry(); var animal2 = ctx.getBean("cat", Animal.class); animal2.cry(); }
运行测试,如下:
creating new Animal object
Dog constructor
creating new Animal object
Cat constructor
before getBean dog cat
I am Snoopy
Wang wang...
I am Kitty
Miao Miao...
工厂方法(实例工厂方法)
实例工厂方法跟静态工厂方法很相像,主要区别是:
- 静态工厂方法:不用实例化工厂,只需直接调用工厂类的静态工厂方法;
- 实例工厂方法:工厂方法不是静态的,因此需要通过
factory-bean
来指定工厂类的实例。
比如:
<bean id="fruitFactory" class="factory.FruitFactory"/> <bean id="apple" factory-bean="fruitFactory" factory-method="getFruit"> <constructor-arg value="apple"/> <property name="name" value="Fuji"/> </bean>
其它都一样。
完整例子如下:
创建如下POJO:
Fruit
:Fruit接口;Apple
:Fruit实现类;Banana
:Fruit实现类;
package pojo; public interface Fruit { public void describe(); }
package pojo; public class Apple implements Fruit { private String name; public void setName(String name) { this.name = name; } public Apple() { System.out.println("Apple constructor"); } @Override public void describe() { System.out.println("I am " + name); System.out.println("Juicy!"); } }
package pojo; public class Banana implements Fruit { private String name; public void setName(String name) { this.name = name; } public Banana() { System.out.println("Banana constructor"); } @Override public void describe() { System.out.println("I am " + name); System.out.println("Sweet!"); } }
创建工厂类 FruitFactory
:
package factory; import pojo.Apple; import pojo.Banana; import pojo.Fruit; public class FruitFactory { public Fruit getFruit(String type) { System.out.println("creating new Fruit object"); if (type.equalsIgnoreCase("apple")) { return new Apple(); } else if (type.equalsIgnoreCase("banana")) { return new Banana(); } else { return null; } } }
在 applicationContext.xml
中注册bean:
...... <bean id="fruitFactory" class="factory.FruitFactory"/> <bean id="apple" factory-bean="fruitFactory" factory-method="getFruit"> <constructor-arg value="apple"/> <property name="name" value="Fuji"/> </bean> <bean id="banana" factory-bean="fruitFactory" factory-method="getFruit"> <constructor-arg value="banana"/> <property name="name" value="Pisang"/> </bean> ......
创建测试用例:
@Test public void test4() { var ctx = new ClassPathXmlApplicationContext("applicationContext.xml"); System.out.println("before getBean apple banana"); var fruit1 = ctx.getBean("apple", Fruit.class); fruit1.describe(); var fruit2 = ctx.getBean("banana", Fruit.class); fruit2.describe(); }
运行测试,如下:
creating new Fruit object
Apple constructor
creating new Fruit object
Banana constructor
before getBean apple banana
I am Fuji
Juicy!
I am Pisang
Sweet!
工厂bean
工厂bean是指实现了 FactoryBean
接口的类。
FactoryBean
接口有3个方法:
T getObject() throws
:创建产品实例;Class<?> getObjectType()
:获取产品的类;default boolean isSingleton() {return true;}
:是否单例,默认值为true;
实现 FactoryBean
接口,则无需再配置 factory-method
,Spring知道如何创建产品(通过 getObject()
方法)。
比如:
<bean id="benz" class="factory.CarFactoryBean"> <property name="type" value="benz"/> </bean>
注意:该配置看起来很像普通的bean,实际上因为 CarFactoryBean
实现了 FactoryBean
接口,当调用 ctx.getBean("benz")
方法时,返回的并不是 CarFactoryBean
的实例,而是其 getObject()
方法的返回值。
注:默认scope是 singleton
,多次调用 ctx.getBean("dog")
返回的是同一个实例。如果在 CarFactoryBean
中override isSingleton()
方法并返回false,则每次调用ctx.getBean("benz")
返回的是不同对象。
注:工厂bean跟其它bean有一点不同,即使是singleton,它也不是在Spring初始化时生产bean,而是在第一次调用 ctx.getBean()
时才生产bean。
完整例子如下:
创建如下POJO:
Car
:Car接口;Benz
:Car实现类;Audi
:Car实现类;
package pojo; public interface Car { public void run(); }
package pojo; public class Benz implements Car{ public Benz() { System.out.println("Benz constructor"); } @Override public void run() { System.out.println("Go go go!"); } }
package pojo; public class Audi implements Car{ public Audi() { System.out.println("Audi constructor"); } @Override public void run() { System.out.println("OOOO"); } }
创建工厂类 FactoryBean
:
package factory; import org.springframework.beans.factory.FactoryBean; import pojo.Audi; import pojo.Benz; import pojo.Car; public class CarFactoryBean implements FactoryBean { private String type; public void setType(String type) { this.type = type; } @Override public Object getObject() throws Exception { System.out.println("creating new Car object"); if (type.equalsIgnoreCase("benz")) { return new Benz(); } else if (type.equalsIgnoreCase("audi")) { return new Audi(); } else return null; } @Override public Class<?> getObjectType() { return Car.class; } }
在 applicationContext.xml
中注册bean:
...... <bean id="benz" class="factory.CarFactoryBean"> <property name="type" value="benz"/> </bean> <bean id="audi" class="factory.CarFactoryBean"> <property name="type" value="audi"/> </bean> ......
创建测试用例:
@Test public void test5() { var ctx = new ClassPathXmlApplicationContext("applicationContext.xml"); System.out.println("before getBean benz audi"); var car1 = ctx.getBean("benz", Car.class); car1.run(); var car2 = ctx.getBean("audi", Car.class); car2.run(); }
运行测试,如下:
before getBean benz audi
creating new Car object
Benz constructor
Go go go!
creating new Car object
Audi constructor
OOOO
总结
...... <!-- 通过bean的class属性创建bean实例(无参构造器) --> <bean id="stoneAxe" class="pojo.StoneAxe"/> <bean id="steelAxe" class="pojo.SteelAxe"/> <bean id="person" class="pojo.Person"> <property name="name" value="Tom"/> <property name="axe" ref="stoneAxe"/> </bean> <!-- 通过bean的class属性创建bean实例(带参构造器) --> <bean id="playBook" class="pojo.PlayBook"/> <bean id="studyBook" class="pojo.StudyBook"/> <bean id="student" class="pojo.Student"> <constructor-arg index="0" value="Jerry"/> <constructor-arg index="1" ref="playBook"/> </bean> <!-- 通过静态工厂方法创建bean实例 --> <bean id="dog" class="factory.AnimalFactory" factory-method="getAnimal"> <constructor-arg value="dog"/> <property name="name" value="Snoopy"/> </bean> <bean id="cat" class="factory.AnimalFactory" factory-method="getAnimal"> <constructor-arg value="cat"/> <property name="name" value="Kitty"/> </bean> <!-- 通过实例工厂方法创建bean实例 --> <bean id="fruitFactory" class="factory.FruitFactory"/> <bean id="apple" factory-bean="fruitFactory" factory-method="getFruit"> <constructor-arg value="apple"/> <property name="name" value="Fuji"/> </bean> <bean id="banana" factory-bean="fruitFactory" factory-method="getFruit"> <constructor-arg value="banana"/> <property name="name" value="Pisang"/> </bean> <!-- 通过工厂bean创建bean实例 --> <bean id="benz" class="factory.CarFactoryBean"> <property name="type" value="benz"/> </bean> <bean id="audi" class="factory.CarFactoryBean"> <property name="type" value="audi"/> </bean> ......
此外,有几点说明:
使用工厂bean来管理的bean,即使scope是 singleton
,也并不是在Spring初始化时生产对象的,而是在第一次调用 ctx.getBean()
方法时生产的。
Spring整合MyBatis,使用了 SqlSessionFactoryBean
、 MapperFactoryBean
,这两个就是工厂bean,它们实现了 FactoryBean
接口。前者生产 SqlSessionFactory
,后者生产 Mapper
。比如:
<!-- sqlSessionFactory --> <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean"> <property name="dataSource" ref="dataSource" /> <!-- <property name="configLocation" value="classpath:mybatis-config.xml"/>--> <property name="mapperLocations" value="classpath:dao/*.xml"/> <property name="transactionFactory"> <bean class="org.apache.ibatis.transaction.managed.ManagedTransactionFactory" /> </property> </bean> <bean id="myMapper" class="org.mybatis.spring.mapper.MapperFactoryBean"> <property name="mapperInterface" value="dao.MyMapper" /> <property name="sqlSessionFactory" ref="sqlSessionFactory" /> </bean>
到此这篇关于Spring创建bean实例的几种方式分享的文章就介绍到这了,更多相关Spring创建bean 内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!