spring依赖注入原理

关于spring依赖注入原理的文章在网络上已经有很多，我要写的这篇文章原文出自http://taeky.iteye.com/blog/563450，只所以再一次写下来只是为了一为自己收藏，方便以后的复习；二是自己在用这个例子实践的时候遇到一些问题，自己记一下。另外这篇文章中用dom4j来解析spring的配置文件，我觉得也是个值得学习的地方。好了，不多说了，还是把代码再贴一下：
接口：PersonDao.java

package com.luojing.test.dao;

public interface PersonDao {
	
	public void add();

}

实现类：PersonDaoImp.java

package com.luojing.test.daoimp;

import com.luojing.test.dao.PersonDao;

public class PersonDaoImp implements PersonDao {

	public void add() {
		
		System.out.println("执行了add()方法!!!");
	}

}

服务接口：PersonService.java

package com.luojing.test.service;

public interface PersonService {
	
	public void save();

}

服务实现类：PersonServiceImp.java

package com.luojing.test.serviceimp;

import com.luojing.test.dao.PersonDao;
import com.luojing.test.service.PersonService;

public class PersonServiceImp implements PersonService {
	
	private PersonDao personDao;
	
	public PersonDao getPersonDao() {
		return personDao;
	}

    public void setPersonDao(PersonDao personDao) {
		this.personDao = personDao;
	}

    public void save() {
		
    	personDao.add();
    
	}

}

applicationContext.xml配置文件如下：

<?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-2.5.xsd" >
           
<bean id="personDao" class="com.luojing.test.daoimp.PersonDaoImp"/>

<bean id="service" class="com.luojing.test.serviceimp.PersonServiceImp">

<property name="personDao" ref="personDao"></property>

</bean>

</beans>

创建bean，然后其属性用一集合存储在bean里BeanDefinition.java：

package com.luojing.test.testioc;

import java.util.ArrayList;
import java.util.List;

public class BeanDefinition {
	
	private String id;
	
	private String classname;
	
	private List<PropertyDefinition> propertys = new ArrayList<PropertyDefinition>();

	public String getId() {
		return id;
	}

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

	public String getClassname() {
		return classname;
	}

	public void setClassname(String classname) {
		this.classname = classname;
	}

	public List<PropertyDefinition> getPropertys() {
		return propertys;
	}

	public void setPropertys(List<PropertyDefinition> propertys) {
		this.propertys = propertys;
	}
	
    public BeanDefinition(String id,String classname){
    	
    	this.id = id;
    	
    	this.classname = classname;
    }
}

属性bean PropertyDefinition.java：

package com.luojing.test.testioc;

public class PropertyDefinition {
	
	private String name;
	
	private String ref;

	public String getName() {
		return name;
	}

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

	public String getRef() {
		return ref;
	}

	public void setRef(String ref) {
		this.ref = ref;
	}
	
	public PropertyDefinition(String name,String ref){
		
		this.name = name;
		
		this.ref = ref;
	}

}

解析spring配置文件并实例话bean ItcastClassPathXMLApplicationContext.java:

package com.luojing.test.testioc;

import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.dom4j.Document;
import org.dom4j.Element;
import org.dom4j.XPath;
import org.dom4j.io.SAXReader;

public class ItcastClassPathXMLApplicationContext {
	
	 private List<BeanDefinition> beanDefines = new ArrayList<BeanDefinition>();  
	 
	 private Map<String, Object> sigletons = new HashMap<String, Object>();  
	
	 public ItcastClassPathXMLApplicationContext(String filename){
		
		this.readXML(filename);
		this.instanceBeans();
		this.injectObject();
	}
	
	 /**  
     * 实现bean的实例化  
     */  
    private void instanceBeans() {   
        for (BeanDefinition beanDefinition : beanDefines) {   
            try {   
                if (beanDefinition.getClassname() != null && !"".equals(beanDefinition.getClassname().trim()))   
                    sigletons.put(beanDefinition.getId(), Class.forName(   
                            beanDefinition.getClassname()).newInstance());   
            } catch (Exception e) {   
                // 通过反射技术把bean都创建出来   
                e.printStackTrace();   
            }   
        }   
  
    } 
    /**  
     * 为bean对象的属性注入值  
     */  
    private void injectObject() {   
        for (BeanDefinition beanDefinition : beanDefines) {   
            Object bean = sigletons.get(beanDefinition.getId());   
            if (bean != null) {   
                try {   
                    PropertyDescriptor[] ps = Introspector.getBeanInfo(   
                            bean.getClass()).getPropertyDescriptors();   
                    //Introspector通过这个类可以取得bean的定义信息   
                    for (PropertyDefinition propertyDefinition : beanDefinition   
                            .getPropertys()) {   
                        for (PropertyDescriptor properdesc : ps) {  
           
                            if (propertyDefinition.getName().equals(properdesc.getName())) {   
                                Method setter = properdesc.getWriteMethod();// 获取属性的setter方法   
                                                                            // ,private   
                                if (setter != null) {//属性可能没有set方法，所以这里要判断一下   
                                    Object value = sigletons   
                                            .get(propertyDefinition.getRef());   
                                    setter.setAccessible(true);//如果set方法是私有的话,要设置它允许被访问   
                                    setter.invoke(bean, value);// 把引用对象注入到属性   
                                }   
                                break;   
                            }   
                        }   
                    }   
                } catch (Exception e) {   
                }   
            }   
        }   
    }   

	/**  
     * 读取xml配置文件  
     */  
    private void readXML(String filename) {   
        SAXReader saxReader = new SAXReader();   
        Document document = null;   
        try {   
            URL xmlpath = this.getClass().getClassLoader()   
                    .getResource(filename);   
            document = saxReader.read(xmlpath);   
            Map<String, String> nsMap = new HashMap<String, String>();   
            nsMap.put("ns", "http://www.springframework.org/schema/beans");// 加入命名空间   
            XPath xsub = document.createXPath("//ns:beans/ns:bean");// 创建beans/bean查询路径   
            xsub.setNamespaceURIs(nsMap);// 设置命名空间   
            List<Element> beans = xsub.selectNodes(document);// 获取文档下所有bean节点   
            for (Element element : beans) {   
                String id = element.attributeValue("id");// 获取id属性值   
                String clazz = element.attributeValue("class"); // 获取class属性值   
                System.out.println(id + " = " + clazz);
                BeanDefinition beanDefine = new BeanDefinition(id, clazz);   
                XPath propertysub = element.createXPath("ns:property");   
                propertysub.setNamespaceURIs(nsMap);// 设置命名空间   
                List<Element> propertys = propertysub.selectNodes(element);   
                for (Element property : propertys) {   
                    String propertyName = property.attributeValue("name");   
                    String propertyref = property.attributeValue("ref");   
                    System.out.println(propertyName + " = " + propertyref);   
                    PropertyDefinition propertyDefinition = new PropertyDefinition(   
                            propertyName, propertyref);   
                    beanDefine.getPropertys().add(propertyDefinition);   
                }   
                beanDefines.add(beanDefine);   
            }   
        } catch (Exception e) {   
            e.printStackTrace();   
        }   
    }   
    
    /**  
     * 获取bean实例  
     */  
    public Object getBean(String beanName) {   
        return this.sigletons.get(beanName);   
    } 

}

测试类SpringTest.java:

package com.luojing.test.testioc;

import com.luojing.test.service.PersonService;

public class SpringTest {

	
	public static void main(String[] args) {
		
		 ItcastClassPathXMLApplicationContext ctx = new ItcastClassPathXMLApplicationContext("applicationContext.xml"); 
		 
		 PersonService ps = (PersonService)ctx.getBean("service");
			
		 ps.save();
	}

}

我在运行的过程中出现：java.lang.NoClassDefFoundError: org/jaxen/JaxenException弄了我好半天，后来在网上找出其原因是：除去必须有一个dom4j.jar外，还必须有一个jaxen-1.1.1.jar 文件，因为使用dom4j时调用了XPath,而没有在项目中加载jaxen-xx.xx.jar，jaxen是一个用Java开发的XPath 引擎，支持JDOM, dom4j。