空指针异常怎么解决？

package cn.muke.ssh.action;

import com.opensymphony.xwork2.ActionSupport;
import com.opensymphony.xwork2.ModelDriven;

import cn.muke.ssh.domain.Produce;
import cn.muke.ssh.service.ProduceService;

public class Produceaction extends ActionSupport implements ModelDriven<Produce> {

	private Produce produce = new Produce();
	
	@Override
	public Produce getModel() {
		// TODO Auto-generated method stub
		return produce;
	}
	
	private ProduceService produceService;

	
	
	public void setProduceService(ProduceService produceService) {
		this.produceService = produceService;
	}



	public String save(){
		System.out.println("save");
		produceService.save(produce);
		return NONE;
	}
}

<?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:aop="http://www.springframework.org/schema/aop"
	xmlns:tx="http://www.springframework.org/schema/tx"
	xsi:schemaLocation="http://www.springframework.org/schema/beans 
    http://www.springframework.org/schema/beans/spring-beans.xsd  
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd
    http://www.springframework.org/schema/aop  
     http://www.springframework.org/schema/aop/spring-aop.xsd 
    http://www.springframework.org/schema/tx   
    http://www.springframework.org/schema/tx/spring-tx.xsd">

	<bean id="produceService" class="cn.muke.ssh.service.ProduceService">
		<property name="dao" ref="dao"></property>
	</bean>
	<bean id="dao" class="cn.muke.ssh.dao.ProduceDao">
	</bean>
</beans>