spring新手
main方法报空指针异常，请大家帮忙看看什么原因。
public class Computer {
private Host host;
private Display display;
public void run(){
System.out.println("I am the computer, I am running.");
System.out.println(this.host.run());
System.out.println(this.display.run());
}
public Host getHost() {
return host;
}
public void setHost(Host host) {
this.host = host;
}
public Display getDisplay() {
return display;
}
public void setDisplay(Display display) {
this.display = display;
}
}
public class Host {
public String run(){
return "I am the host, I am running.";
}
}
public class Display {
public String run(){
return "I am the Displayer, I am running.";
}
}
package ioc.test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class TestIoc {
public static void main(String[] args){
ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
Computer computer1 = (Computer)ac.getBean("computer1");
System.out.println("byName");
computer1.run();
Computer computer2 = (Computer)ac.getBean("computer2");
System.out.println("byType");
computer2.run();
Computer computer3 = (Computer)ac.getBean("computer3");
System.out.println("default");
computer3.run();
}
}
配置文件
<?xml version="1.0" encoding="UTF-8"?>
<beans
xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
<bean id="computer1" class="ioc.test.Computer" autowire="byName"></bean>
<bean id="computer2" class="ioc.test.Computer" autowire="byType"></bean>
<bean id="computer3" class="ioc.test.Computer" autowire="default"></bean>
</beans>
就这些，大家帮忙看看，谢谢啦。
[最优解释]
当你指定bean标签的autowire="default"时，此时spring是根据beans标签中的default-autowire属性的值进行装配的，而default-autowire的默认值是no，所以你通过spring获取到的id="computer3"的对象实例中成员变量host和display都没有初始化，所以当你调用它们中的方法时就抛空指针异常了
[其他解释]
自己找到一些原因了，Host和Display的对象在调用run方法时候也需要初使化，所以也需要注入
<?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:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
<bean id="computer1" class="ioc.test.Computer" autowire="byName"></bean>
<bean id="computer2" class="ioc.test.Computer" autowire="byType"></bean>
<bean id="computer3" class="ioc.test.Computer" autowire="default"></bean>
<bean id="host" class="ioc.test.Host" autowire="byName"></bean>
<bean id="display" class="ioc.test.Display" autowire="byName"></bean>
</beans>
但是执行到
Computer computer3 = (Computer)ac.getBean("computer3");
System.out.println("default");
computer3.run();
注入方式是默认的时候还是会报空指针异常，暂时不知道怎么回事。
[其他解释]
