Spring 4学习——问题与注意事项（一）

1.Spring项目依赖的jar包有5个：

2.applicationContext.xml文件中，如下bean的property的name值对应的是HelloWorld类中的setter方法，即name对应的是getName()，如果setter方法写为setName1，则name应该写为name="name1"。

     <bean id = "helloWorld" class="com.swl.spring.beans.HelloWorld">
         <property name="name" value="long"></property>
     </bean>

 public class HelloWorld {
     private String name;
 
     public String getName() {
         return name;
     }
 
     public void setName(String name) {
         this.name = name;
     }
 
     public void hello(){
         System.out.println("hello "+name);
     }
 }

3.如果按照第11行的方式加载applicationContext.xml文件时，应该把applicationContext.xml文件放在文件夹src下，与包同一级别，而非包中，见图。

 package com.swl.spring.beans;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 
 public class Main {
     public static void main(String[] args){　　//这两步由Spring完成
 //        HelloWorld helloWorld = new HelloWorld();
 //        helloWorld.setName("song ");
 //加载applicationContext.xml文件，创建ApplicaitonContext，从中取出bean
         ApplicationContext ctx =          HelloWorld helloWorld = (HelloWorld) ctx.getBean("hello");
         helloWorld.hello();
     }
 }

如果像下图这样将applicationContext.xml文件放在包中，语句

 ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");

会报错

六月 11, 2017 8:59:46 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@4d405ef7: startup date [Sun Jun 11 20:59:46 CST 2017]; root of context hierarchy
六月 11, 2017 8:59:46 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [applicationContext.xml]
Exception in thread "main" org.springframework.beans.factory.BeanDefinitionStoreException: IOException parsing XML document from class path resource [applicationContext.xml]; nested exception is java.io.FileNotFoundException: class path resource [applicationContext.xml] cannot be opened because it does not exist
    at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:344)
    at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:304)
    at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:181)
    at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:217)
    at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:188)
    at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:252)
    at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:127)
    at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:93)
    at org.springframework.context.support.AbstractRefreshableApplicationContext.refreshBeanFactory(AbstractRefreshableApplicationContext.java:129)
    at org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:614)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:515)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at com.swl.spring.beans.Main.main(Main.java:11)
Caused by: java.io.FileNotFoundException: class path resource [applicationContext.xml] cannot be opened because it does not exist
    at org.springframework.core.io.ClassPathResource.getInputStream(ClassPathResource.java:172)
    at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:330)
    ... 13 more

4.我们观察下面代码11行做了什么工作

//Main类 1 package com.swl.spring.beans;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 
 public class Main {
     public static void main(String[] args){
 //        HelloWorld helloWorld = new HelloWorld();
 //        helloWorld.setName("song ");
         //创建Spring IOC容器对象
         ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
 //        HelloWorld helloWorld = (HelloWorld) ctx.getBean("hello");
 //        helloWorld.hello();
     }
 }

//HelloWorld类 1 package com.swl.spring.beans;
 
 public class HelloWorld {
     private String name;
 
     public HelloWorld() {
         System.out.println("HelloWorld Constructor");
     }
 
     public String getName() {
         return name;
     }
 
     public void setName(String name) {
         System.out.println("HelloWorld setName");
         this.name = name;
     }
 
     public void hello(){
         System.out.println("hello "+name);
     }
 }

//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">
    <bean id = "hello" class="com.swl.spring.beans.HelloWorld">
        <property name="name" value="long"></property>
    </bean>
</beans>

无参构造函数HelloWorld和setName()方法都输出标识语句，运行，在控制台可以看到如下输出

六月 11, 2017 9:14:30 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@4d405ef7: startup date [Sun Jun 11 21:14:30 CST 2017]; root of context hierarchy
六月 11, 2017 9:14:30 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [applicationContext.xml]
HelloWorld Constructor
HelloWorld setName

可以看到，语句

11         ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");

执行，创建IOC容器对象过程中，会调用构造函数构造bean，并调用setName()方法为name属性赋值。

5.通过上面全类名的方式来配置Bean，用到了Java的反射机制，这就要求Bean必须要有一个无参的构造器；如果没有，会报错。

我们将HelloWorld的构造器改成有参数的，其余代码和4中相同：

    public HelloWorld(String s) {
        System.out.println("HelloWorld Constructor");
    }

运行，会报出如下错误：

Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.swl.spring.beans.HelloWorld]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.swl.spring.beans.HelloWorld.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1147)
    ... 13 more

6.也可以利用类型来获取IOC中的Bean，如下

         ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
 //        HelloWorld helloWorld = (HelloWorld) ctx.getBean("hello");
         HelloWorld helloWorld = ctx.getBean(HelloWorld.class);

这种方式的缺点是在applicationContext.xml文件中配置两个类型相同的Bean时

    <bean id = "hello" class="com.swl.spring.beans.HelloWorld">
        <property name="name" value="long"></property>
    </bean>
 
    <bean id = "hello2" class="com.swl.spring.beans.HelloWorld">
        <property name="name" value="long"></property>
    </bean>

虽然可以运行，但是报错

HelloWorld Constructor
HelloWorld setName
HelloWorld Constructor
HelloWorld setName
Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: 　　　　No qualifying bean of type 'com.swl.spring.beans.HelloWorld' available: expected single matching bean but found 2: hello,hello2
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1041)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:345)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:340)
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1090)
    at com.swl.spring.beans.Main.main(Main.java:13)