Bean实例的创建通常有如下3中方式：
1、通过构造方法创建Bean
package com.test;
public class Person {
public Person() {
System.out.println("Person 构造方法");
}
private Hobby hobby;
public void setHobby(Hobby hobby) {
System.out.println("通过依赖关系注入");
this.hobby = hobby;
}
public void singASong() {
System.out.println(hobby.doIt());
}
}
interface Hobby {
public String doIt();
}
class Sing implements Hobby {
public Sing() {
System.out.println("Sing 构造方法");
}
public String doIt() {
return "I am singing.";
}
}
package com.test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class SpringTest {
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("bean.xml");
Person p = ctx.getBean("person", Person.class);
p.singASong();
}
}
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd"> <bean id="person" class="com.test.Person"> <property name="hobby" ref="hobbySing"></property> </bean> <bean id="hobbySing" class="com.test.Sing"></bean> </beans>
2、使用静态工厂方法创建Bean
package com.test2;
public class PersonCreator {
public static People createPerson(String type) {
if (type.equals("chn")) {
return new Chinese();
}
if (type.equals("jpn")) {
return new Japanese();
}
return null;
}
}
package com.test2;
public interface People {
public void say();
}
class Chinese implements People {
public void say() {
System.out.println("I am Chinese.");
}
}
class Japanese implements People {
public void say() {
System.out.println("I am Japanese.");
}
}
工厂方法一定要static
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd"> <!-- 静态工厂方法 --> <bean id="chinese" class="com.test2.PersonCreator" factory-method="createPerson"> <constructor-arg value="chn" /> </bean> <bean id="japanese" class="com.test2.PersonCreator" factory-method="createPerson"> <constructor-arg value="jpn" /> </bean> </beans>
package com.test2;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Test {
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("bean.xml");
People chn = ctx.getBean("chinese", People.class);
chn.say();
People jpn = ctx.getBean("japanese", People.class);
jpn.say();
}
}
输出：I am Chinese.
I am Japanese.
3、调用实例工厂方法创建Bean
注：实例工厂方法与静态工厂方法只有一点不同
静态工厂方法：只需使用工厂类即可，工厂方法必须static
实例工厂方法：必须使用工厂类的实例，工厂方法必须non-static
package com.test2;
public class PersonCreator {
public People createPerson(String type) {
if (type.equals("chn")) {
return new Chinese();
}
if (type.equals("jpn")) {
return new Japanese();
}
return null;
}
}
package com.test2;
public interface People {
public void say();
}
class Chinese implements People {
public void say() {
System.out.println("I am Chinese.");
}
}
class Japanese implements People {
public void say() {
System.out.println("I am Japanese.");
}
}
package com.test2;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Test {
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("bean.xml");
People chn = ctx.getBean("chinese", People.class);
chn.say();
People jpn = ctx.getBean("japanese", People.class);
jpn.say();
}
}
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd"> <!-- 实例工厂方法 --> <bean id="creator" class="com.test2.PersonCreator"/> <bean id="chinese" factory-bean="creator" factory-method="createPerson"> <constructor-arg value="chn" /> </bean> <bean id="japanese" factory-bean="creator" factory-method="createPerson"> <constructor-arg value="jpn" /> </bean> </beans>
输出：
I am Chinese.
I am Japanese.
总结：大多数情况下，BeanFactory直接通过new关键字调用构造器来创建Bean实例，而class属性指定了Bean实例的实现类。因此，<bean .../>元素必须指定Bean实例的class属性，但这并不是唯一的实例化Bean的方法。
使用实例工厂方法创建Bean实例，以及使用子Bean方法创建Bean实例时，都可以不指定class属性。