从源码中懂得spring依赖注入和动态代理
使用DAO层对实体进行操作:对DAO实现类,日志功能实现类,事务类进行依赖注入。很多人初学者可能会问,为什么要使用AOP对日志和事物实现管理,下面例子会有说明。当然以下例子只是针对spring如何实现AOP作出的说明,实际运用上可以使用xml进行配置统一管理。
1:首先是实体类 User.java
public class User { private static final long serialVersionUID = -3061461829571439706L; private String username; private int age; public User() { super(); } public User(String username, int age) { super(); this.username = username; this.age = age; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } }
2:数据访问层分别有UserDao.java
public interface UserDao { public void create(User user); public void delete(User user); }
3:它的实现类UserDaoImpl.java
public class UserDaoImpl implements UserDao, UserDao2 { private static Logger logger = Logger.getLogger(UserDaoImpl.class); @Override public void create(User user) { logger.debug("invoke create(user): username=" + user.getUsername() + ", age=" + user.getAge()); } @Override public void delete(User user) { logger.debug("invoke delete(user): username=" + user.getUsername() + ", age=" + user.getAge()); } }
4:日志接口类Advice.java
public interface Advice { public void before(Method method, Object[] args); public void after(Method method, Object[] args); }
5:日志实现类LogAdvice.java(当然也可以是事务实现类,反正该类的目的就要能够在目标DAO方法前后发生。
public class LogAdvice implements Advice { private static Logger logger = Logger.getLogger(LogAdvice.class); @Override public void before(Method method, Object[] args) { logger.debug("LogAdvice before: current invoking method name:" + method.getName()); } @Override public void after(Method method, Object[] args) { logger.debug("LogAdvice after: current invoking method name:" + method.getName()); } }
6.写完需要拦截实现的方法后,就要进行动态代理类的编写了。要点:实现InvocationHandler接口的invoke方法,日志等实现接口的调用发生在代理对象前后。 InvocationHandlerImpl.java
class InvocationHandlerImpl implements InvocationHandler { private Object target; private List<advice> advices = new ArrayList<advice>(); public InvocationHandlerImpl() { super(); } public InvocationHandlerImpl(Object target) { super(); this.target = target; } public InvocationHandlerImpl(Object target, List<advice> advices) { super(); this.target = target; this.advices.addAll(advices); } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { for (Advice advice : advices) { advice.before(method, args); } Object result = method.invoke(target, args); for (Advice advice : advices) { advice.after(method, args); } return result; } public void setTarget(Object target) { this.target = target; } public void addAllAdvice(List<advice> advices) { this.advices.addAll(advices); } public void addAdvice(Advice advice) { this.advices.add(advice); } public void removeAdvice(Advice advice) { this.advices.remove(advice); } }
7.最关键的一步来了,编写Bean工厂,为什么spring能通过读取XML文件的BEAN动态实例化类呢,下面的类就体现了JAVA的反射技术,每个类都有共同的CLASS对象。 Object beanInstance = Class.forName(beanClass).newInstance(); 这句话就是通过BEAN的名字,即XML中对应的ID实例化改类的方法,其中这里生成了DAO和日志接口实现类的对象,并在最后一个方法中通过动态代理实现spring的AOP。
public class BeanFactory { private static final Logger logger = Logger.getLogger(BeanFactory.class); private Map<string object> beanFactory = new HashMap<string object>(); public BeanFactory() { this("beans.xml"); } public BeanFactory(String configFilePath) { try { parseConfigurationFile(configFilePath); }catch (Exception e) { logger.error("error happen.it's " + e.getMessage()); e.printStackTrace(); } } public Object getBean(String beanName) { return beanFactory.get(beanName); } public <t extends object> T getBean(Class<t> clazz) { for (Object bean : beanFactory.values()) { if (bean.getClass() == clazz) return (T) bean; } throw new RuntimeException("bean of " + clazz.getName() + " don't find."); } private void parseConfigurationFile(String configFilePath) throws Exception { InputStream inStream = this.getClass().getClassLoader().getResourceAsStream(configFilePath); SAXReader reader = new SAXReader(); Document document = null; try { logger.debug("start parse document."); document = reader.read(inStream); } catch (DocumentException e) { e.printStackTrace(); } if (document == null) { throw new RuntimeException(configFilePath + " parse failed."); } Element root = document.getRootElement(); logger.debug("root element is:" + root.getName()); List<element> beanEles = root.elements("bean"); Map<string list> beanAdvices = new HashMap<string list>(); getOriginalBeans(beanEles, beanAdvices); injectAdvice(beanAdvices); } private void getOriginalBeans(List<element> beanEles, Map<string list> beanAdvices) throws ClassNotFoundException, InstantiationException, IllegalAccessException { for (Element beanElement : beanEles) { String beanName = beanElement.attributeValue("id"); String beanClass = beanElement.attributeValue("class"); Object beanInstance = Class.forName(beanClass).newInstance(); beanFactory.put(beanName, beanInstance); List<element> adviceElements = beanElement.elements("advice"); if (adviceElements.isEmpty()) { continue; } List<string> adviceList = new ArrayList<string>(); for (Element adviceElement : adviceElements) { String adviceRef = adviceElement.attributeValue("ref"); adviceList.add(adviceRef); } beanAdvices.put(beanName, adviceList); } } private void injectAdvice(Map<string list>> beanAdvices) { for (Entry<string list> beanAdvice : beanAdvices.entrySet()) { String beanName = beanAdvice.getKey(); List<string> adviceNames = beanAdvice.getValue(); Object bean = beanFactory.get(beanName); Class[] superInterfaces = bean.getClass().getInterfaces(); List<advice> advices = new ArrayList<advice>(); for (String adviceName : adviceNames) { advices.add((Advice) beanFactory.get(adviceName)); } InvocationHandlerImpl invocationHandler = new InvocationHandlerImpl(bean, advices); Object beanProxy = Proxy.newProxyInstance(BeanFactory.class.getClassLoader(), superInterfaces, invocationHandler); beanFactory.put(beanName, beanProxy); } } }
8.XML文件,所需要依赖注入的类都在下面记录
<?xml version="1.0" encoding="UTF-8"?> <beans> <bean id="userDao" class="com.spring.framework.foundation.UserDaoImpl"> <advice id="logAdvice" ref="logAdvice"></advice> </bean> <bean id="userDao2" class="com.spring.framework.foundation.UserDaoImpl"></bean> <bean id="logAdvice" class="com.spring.framework.advice.LogAdvice"> </bean> </beans>
当去掉<advice id="logAdvice" ref="logAdvice"></advice>后,就不会产生日志文件,也不会注入日志实现类。
在此,已经简单的介绍了spring 的IOC和AOP的实现机制,其实我也是大致的了解,并没有参透其中的深层原理,待续。。。