Spring学习心得（2）-- 创建对象的方式

一、通过构造函数创建对象。

2.1 利用无参构造函数+setter方法注入值

最基本的对象创建方式，只需要有一个无参构造函数（类中没有写任何的构造函数，默认就是有一个构造函数，如果写了任何一个构造函数，默认的无参构造函数就不会自动创建哦!!）和字段的setter方法。

Person类：

package com.mc.base.learn.spring.bean;

public class Person {

private String name;
private Integer id;

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Integer getId() {
return id;
}

public void setId(Integer id) {
this.id = id;
}

@Override
public String toString() {
return "Person [name=" + name + ", id=" + id + "]";
}

}
XML配置：
<?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">
<bean class="com.mc.base.learn.spring.bean.Person" id="person">
<property name="name" value="LiuChunfu"></property>
<property name="id" value="125"></property>
</bean>

</beans>
其本质为：

SpringContext利用无参的构造函数创建一个对象，然后利用setter方法赋值。所以如果无参构造函数不存在，Spring上下文创建对象的时候便会报错。

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'person' defined in class path resource [applicationContext.xml]: Instantiation of bean failed;
nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.mc.base.learn.spring.bean.Person]: No default constructor found;
nested exception is java.lang.NoSuchMethodException: com.mc.base.learn.spring.bean.Person.<init>()
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1105)
　　。。。。。

2.2 利用有参构造函数直接注入

Person类：

package com.mc.base.learn.spring.bean;

public class Person {

private String name;
private Integer id;

public Person(String name, Integer id) {
super();
this.name = name;
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Integer getId() {
return id;
}

public void setId(Integer id) {
this.id = id;
}

@Override
public String toString() {
return "Person [name=" + name + ", id=" + id + "]";
}

}

XML配置：
<?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">
<bean class="com.mc.base.learn.spring.bean.Person" id="person">
<constructor-arg name="id" value="123"></constructor-arg>
<constructor-arg name="name" value="LiuChunfu"></constructor-arg>
</bean>

</beans>

二、通过静态方法创建对象。

package com.mc.base.learn.spring.factory;

import com.mc.base.learn.spring.bean.Person;

public class PersonStaticFactory {

public static Person createPerson(){
return new Person();
}

/**
* 工厂方法带有参数如何处理？
* @Title: createPerson
* @Description: TODO(这里用一句话描述这个方法的作用)
* @param @param id
* @param @param name
* @param @return
* @return Person 返回类型
* @throws
*/
public static Person createPerson(Integer id,String name){
return new Person(name,id);
}
}

<!--静态的工厂方法核心是class+factory-method -->
<bean id="person" class="com.mc.base.learn.spring.factory.PersonStaticFactory" factory-method="createPerson"></bean>

三、通过工厂方法创建对象。

package com.mc.base.learn.spring.factory;

import com.mc.base.learn.spring.bean.Person;

public class PersonFactory {

public Person createInstance() {
return new Person();
}
}

<!-- 实例工程方法需要先创建工厂实例，然后在创建所需对象的时候，将其赋值为factory-bean -->
<bean id="personFactory" class="com.mc.base.learn.spring.factory.PersonFactory"></bean>
<bean id="person2" factory-bean="personFactory" factory-method="createInstance"></bean>