Spring(注解开发)
先来说说注解是什么:
注解顾名思义,就是类似于添加注释,但是又不跟注释完全一样,因此,我们可以将它理解为将类或者方法与特定的信息进行关联。
那么注解如何实现?
1、接口使用@interface定义。
2、通过继承以**解,实现功能:
元注解@Target,@Retention,@Documented,@Inherited
* 元注解@Target,@Retention,@Documented,@Inherited *
* @Target 表示该注解用于什么地方,可能的 ElemenetType 参数包括:
* ElemenetType.CONSTRUCTOR 构造器声明
* ElemenetType.FIELD 域声明(包括 enum 实例)
* ElemenetType.LOCAL_VARIABLE 局部变量声明
* ElemenetType.METHOD 方法声明
* ElemenetType.PACKAGE 包声明
* ElemenetType.PARAMETER 参数声明
* ElemenetType.TYPE 类,接口(包括注解类型)或enum声明
*
* @Retention 表示在什么级别保存该注解信息。可选的 RetentionPolicy 参数包括:
* RetentionPolicy.SOURCE 注解将被编译器丢弃
* RetentionPolicy.CLASS 注解在class文件中可用,但会被VM丢弃
* RetentionPolicy.RUNTIME VM将在运行期也保留注释,因此可以通过反射机制读取注解的信息。
*
* @Documented 将此注解包含在 javadoc 中
*
* @Inherited 允许子类继承父类中的注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
下面拿spring的controller来当做示例:
Controller类使用继承@Component注解的方法,将其以单例的形式放入spring容器,如果仔细看的话会发现每个注解里面都有一个默认的value()方法,它的作用是为当前的注解声明一个名字,一般默认为类名,然后spring会通过配置文件中的<context:component-scan>的配置,进行如下操作:
1、使用asm技术扫描.class文件,并将包含@Component及元注解为@Component的注解@Controller、@Service、@Repository或者其他自定义的的bean注册到beanFactory中,
2、然后spring在注册处理器
3、实例化处理器,然后将其放到beanPostFactory中,然后我们就可以在类中进行使用了。
4、创建bean时,会自动调用相应的处理器进行处理。
这里附上参考文章https://blog.csdn.net/jack_wang001/article/details/78781588
@Controller源码:
/*
* Copyright 2002-2007 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.stereotype;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* Indicates that an annotated class is a "Controller" (e.g. a web controller).
*
* <p>This annotation serves as a specialization of {@link Component @Component},
* allowing for implementation classes to be autodetected through classpath scanning.
* It is typically used in combination with annotated handler methods based on the
* {@link org.springframework.web.bind.annotation.RequestMapping} annotation.
*
* @author Arjen Poutsma
* @author Juergen Hoeller
* @since 2.5
* @see Component
* @see org.springframework.web.bind.annotation.RequestMapping
* @see org.springframework.context.annotation.ClassPathBeanDefinitionScanner
*/
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {
/**
* The value may indicate a suggestion for a logical component name,
* to be turned into a Spring bean in case of an autodetected component.
* @return the suggested component name, if any
*/
String value() default "";
}
下面是@Component的源码:
/*
* Copyright 2002-2007 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.stereotype;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* Indicates that an annotated class is a "component".
* Such classes are considered as candidates for auto-detection
* when using annotation-based configuration and classpath scanning.
*
* <p>Other class-level annotations may be considered as identifying
* a component as well, typically a special kind of component:
* e.g. the {@link Repository @Repository} annotation or AspectJ's
* {@link org.aspectj.lang.annotation.Aspect @Aspect} annotation.
*
* @author Mark Fisher
* @since 2.5
* @see Repository
* @see Service
* @see Controller
* @see org.springframework.context.annotation.ClassPathBeanDefinitionScanner
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Component {
/**
* The value may indicate a suggestion for a logical component name,
* to be turned into a Spring bean in case of an autodetected component.
* @return the suggested component name, if any
*/
String value() default "";
}
使用Java Web 注解开发可以不用不用任何xml配置文件来开发Java Web。
大致原理是:
实现ServletContainerInitializer接口
在META-INF/service 建立spi文件(spi机制),指定ServletContainerInitializer的实现类
在实现类上加@HandlesTypes 注解(Tomcat扫到该注解的指定的类型以及该类型的泛华类型)
实现类onStartup的方法参数Set<Class<?>> c 就是@HandlesTypes指定类型的泛化类型全部的类,由Tomcat容器扫描到的进来
项目启动Tomcat会扫描实现ServletContainerInitializer的类,并扫描实现类上@HandlesTypes指定的类型和泛化类型全部的类传到实现类的onStartup方法上
在实现类的onStartup里面注册filter,listener,servlet
@HandlesTypes({Handler.class})
public class SampleServletContainerInitializer implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
// c servlet 容器帮我们注入感兴趣的类型
ServletRegistration.Dynamic homeServlet = ctx.addServlet("home", HomeServlet.class);
homeServlet.addMapping("/home");
// 注册过滤器
FilterRegistration.Dynamic userFilter = ctx.addFilter("userFilter", UserFilter.class);
userFilter.addMappingForUrlPatterns(EnumSet.of(DispatcherType.REQUEST),true,"/*");
// 注册监听
ctx.addListener(UserListener.class);
}
}
上一篇: Session与cookie 心得
下一篇: 门面模式