对spring做java注解扩展

在开发Java程序，尤其是Java EE应用的时候，总是免不了与各种配置文件打交道。java注解则帮我们使之更整洁，不会配置文件满天飞了。
今天我写了一个简单的对spring做了扩展的注解，它的主要功能就是将配置文件里的属性赋值到你需要的地方。（建议在看该例子之前搜下注解的基础知识，熟悉下）
首先，写一个@PropertyConfig注解

package com.inigo.annotation.annotation.spring.exetend;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface PropertyConfig {
	String value() default "";

	boolean required() default true;
}

我们会发现，定义一个注解很像定义接口一样，里面的类似方法的东西就是声明了一个配置参数了。用的时候@PropertyConfig 或者@PropertyConfig("username")或者@PropertyConfig(value="username")
然后，要重写org.springframework.beans.factory.config.PropertyPlaceholderConfigurer这个类了，为的是取出spring读取到的配置文件里的属性

package com.inigo.annotation.annotation.spring.exetend;

import java.util.Properties;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.PropertyPlaceholderConfigurer;
import org.springframework.stereotype.Component;

@Component
public class ExtendedPropertyPlaceholderConfigurer extends PropertyPlaceholderConfigurer {
	private Properties props;

	@Override
	protected void processProperties(ConfigurableListableBeanFactory beanFactory, Properties props)
			throws BeansException {
		super.processProperties(beanFactory, props);
		this.props = props;
	}

	public Object getProperty(String key) {
		return props.get(key);
	}
}

这个类代码很简单，不必多说
注意配置spring的配置文件的时候要用这个类来取配置文件

<?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:jee="http://www.springframework.org/schema/jee" xmlns:tx="http://www.springframework.org/schema/tx"
	xmlns:util="http://www.springframework.org/schema/util" 
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd"
	default-autowire="byName" default-lazy-init="false">
	<!-- 属性文件读入-->
	<bean id="propertyConfigurer"
		class="com.inigo.annotation.annotation.spring.exetend.ExtendedPropertyPlaceholderConfigurer">
		<property name="ignoreResourceNotFound" value="true" />
		<property name="locations">
			<list>
				<value>classpath*:/application.properties</value>
			</list>
		</property>
	</bean>
	<context:component-scan base-package="com.inigo.annotation.annotation.spring.exetend" />
	<context:component-scan base-package="com.inigo.annotation.service" />
	<context:annotation-config/>
</beans>

再次，就是很重要的对注解进行处理的类了

package com.inigo.annotation.annotation.spring.exetend;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;

import org.springframework.beans.BeansException;
import org.springframework.beans.SimpleTypeConverter;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessorAdapter;
import org.springframework.stereotype.Component;
import org.springframework.util.ReflectionUtils;

@Component
public class ConfigAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter {
	@Autowired
	private ExtendedPropertyPlaceholderConfigurer propertyConfigurer;

	private SimpleTypeConverter typeConverter = new SimpleTypeConverter();

	/**
	 * <p>通过config配置变量,bean初始化以后设置properties文件里面的值</p>
	 * {@inheritDoc}
	 */
	@Override
	public boolean postProcessAfterInstantiation(final Object bean, String beanName) throws BeansException {
		ReflectionUtils.doWithFields(bean.getClass(), new ReflectionUtils.FieldCallback() {
			public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
				PropertyConfig cfg = field.getAnnotation(PropertyConfig.class);
				if (cfg != null) {
					if (Modifier.isStatic(field.getModifiers())) {
						throw new IllegalStateException("@PropertyConfig annotation is not supported on static fields");
					}

					String key = cfg.value().length() <= 0 ? field.getName() : cfg.value();
					Object value = propertyConfigurer.getProperty(key);
					if (cfg.required() && value == null) {
						throw new NullPointerException(bean.getClass().getSimpleName() + "." + field.getName()
								+ " is requred,but not been configured");
					} else if (value != null) {
						Object _value = typeConverter.convertIfNecessary(value, field.getType());
						ReflectionUtils.makeAccessible(field);
						field.set(bean, _value);
					}
				}
			}
		});
		return true;
	}
}

在这里面我们会有一个小处理，就是当注解没有写value的时候，默认用field.getName()来当做key去取properties了
接着，还要有一个简单类来用下这个注解

package com.inigo.annotation.service;

import org.springframework.stereotype.Component;

import com.inigo.annotation.annotation.spring.exetend.PropertyConfig;


@Component
public class TestService {
	@PropertyConfig
	private String username;
	@PropertyConfig
	private String password;
	
	public void testMethod() {
		System.out.println("Annotation begin");
		System.out.println("username:" + username);
		System.out.println("password:" + password);
	}
	
}

最后，就是用spring容器生成这个TestService类来测试下效果了