# 6.Servlets, Filters, listeners

## 一.Web 开发使用 Controller 基本上可以完成大部分需求，但是我们还可能会用到 Servlet、 Filter、Listener 等等

## 二.在 spring boot 中的三种实现方式

### 方法一：通过注册 ServletRegistrationBean、 FilterRegistrationBean 和 ServletListenerRegistrationBean 获得控制

**自定义 servlet**

```java
/**

* 自定义 servlet

* */

public class CustomServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;


    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        System.out.println("servlet get method");
        doPost(request, response);
    }


    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        System.out.println("servlet post method");
        response.getWriter().write("hello world");

    }

}
```

**自定义 filter**

```java
/**

*自定义 filter

**/
public class CustomFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("init filter");
    }


    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("do filter");
        chain.doFilter(request, response);
    }

    @Override
    public void destroy() {
        System.out.println("destroy filter");
    }

}
```

**自定义 listener**

```java
/**
* 自定义 listener
**/

public class CustomListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("contextInitialized");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("contextDestroyed");
    }
}
```

**注册 bean**

```java
@Bean
public ServletRegistrationBean servletRegistrationBean() {
    return new ServletRegistrationBean(new CustomServlet(), "/test");
}

@Bean
public FilterRegistrationBean filterRegistrationBean() {
    return new FilterRegistrationBean(new CustomFilter(), servletRegistrationBean());
}

@Bean
public ServletListenerRegistrationBean<CustomListener> servletListenerRegistrationBean() {
    return new ServletListenerRegistrationBean<CustomListener>(new CustomListener());
}
```

具体实现：

![](/files/-LfnTI5jYrtQxd3nHn3r)

选中的文件就是需要实现的

CustomFilter.java

```java
package com.demotm.example;

import com.demotm.example.util.filter.CustomFilter;
import com.demotm.example.util.listener.CustomListener;
import com.demotm.example.util.servlet.CustomServlet;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.boot.web.servlet.ServletListenerRegistrationBean;
import org.springframework.boot.web.servlet.ServletRegistrationBean;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class DemotmApplication {

    @Bean
    public ServletRegistrationBean servletRegistrationBean() {
        return new ServletRegistrationBean(new CustomServlet(), "/test");
    }

    @Bean
    public FilterRegistrationBean filterRegistrationBean() {
        return new FilterRegistrationBean(new CustomFilter(), servletRegistrationBean());
    }

    @Bean
    public ServletListenerRegistrationBean<CustomListener> servletListenerRegistrationBean() { 
        return new ServletListenerRegistrationBean<CustomListener>(new CustomListener());
    }

    public static void main(String[] args) {
        SpringApplication.run(DemotmApplication.class, args);
    }
}
```

> 上面的注册也可以配置类方式
>
> ```java
> @Configuration
> public class WebConfig {
>     
>     @Bean
>     public ServletRegistrationBean servletRegistrationBean() {
>         return new ServletRegistrationBean(new CustomServlet(), "/test");
>     }
>
>     @Bean
>     public FilterRegistrationBean filterRegistrationBean() {
>         return new FilterRegistrationBean(new CustomFilter(), servletRegistrationBean());
>     }
>
>     @Bean
>     public ServletListenerRegistrationBean<CustomListener> servletListenerRegistrationBean() { 
>         return new ServletListenerRegistrationBean<CustomListener>(new CustomListener());
>     }
> }
> ```
>
> 示例使用已存在的过滤器，编码过滤器
>
> ```java
> @Configuration
> public class WebConfig {
>     
>     @Bean
>     public FilterRegistrationBean characterEncodingFilterRegister() {
>         
>         FilterRegistrationBean registrationBean = new FilterRegistrationBean();
>         CharacterEncodingFilter filter = new CharacterEncodingFilter("UTF-8");
>         filter.setForceEncoding(true);
>         registrationBean.setFilter(filter);
>         
>         List<String> urls = new ArrayList<>();
>         urls.add("/*"); // 限制使用filter的访问路径
>         registrationBean.setUrlPatterns(urls);
>         
>         return registrationBean;
>     }
>
> }
> ```

CustomListener.java

```java
package com.demotm.example.util.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

/**
* 自定义 listener
**/
public class CustomListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("contextInitialized");
    }


    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("contextDestroyed");
    }
}
```

CustomServlet.java

```java
package com.demotm.example.util.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
* 自定义 servlet
**/

public class CustomServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("servlet get method");
        doPost(request, response);
    }


    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("servlet post method");
        response.getWriter().write("hello world");
    }

}
```

DemotmApplication.java中的代码

```java
package com.demotm.example;

import com.demotm.example.util.filter.CustomFilter;
import com.demotm.example.util.listener.CustomListener;
import com.demotm.example.util.servlet.CustomServlet;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.boot.web.servlet.ServletListenerRegistrationBean;
import org.springframework.boot.web.servlet.ServletRegistrationBean;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class DemotmApplication {

    @Bean
    public ServletRegistrationBean servletRegistrationBean() {
        return new ServletRegistrationBean(new CustomServlet(), "/test");
    }

    @Bean
    public FilterRegistrationBean filterRegistrationBean() {
        return new FilterRegistrationBean(new CustomFilter(), servletRegistrationBean());
    }


    @Bean
    public ServletListenerRegistrationBean<CustomListener> servletListenerRegistrationBean() {
        return new ServletListenerRegistrationBean<CustomListener>(new CustomListener());
    }

    public static void main(String[] args) {
        SpringApplication.run(DemotmApplication.class, args);
    }
}
```

> 如果想对所有的访问执行filter，如下写，去掉第二个参数
>
> ```java
> @Bean
> public FilterRegistrationBean filterRegistrationBean() {
>     return new FilterRegistrationBean(new CustomFilter());
> }
> ```

访问：<http://localhost:8080/test>

![](/files/-LfnTI5sjvHAhnxvwOro)

控制台中的输出

![](/files/-LfnTI5uCP120P7MFYGI)

### 方法二：通过实现 ServletContextInitializer 接口直接注册

```java
implements ServletContextInitializer {

@Override
public void onStartup(ServletContext servletContext) throws ServletException {

    servletContext.addServlet("customServlet", new CustomServlet()).addMapping("/test");

    servletContext.addFilter("customFilter", new CustomFilter())

    .addMappingForServletNames(EnumSet.of(DispatcherType.REQUES T), true, "customServlet");
    servletContext.addListener(new CustomListener());
}
```

具体实现：

主要是改动方法二中的bean实现方式改成通过实现 ServletContextInitializer 接口直接注册

![](/files/-LfnTI5wWNkY9dbpeo9X)

DemotmApplication.java

```java
package com.demotm.example;

import com.demotm.example.util.filter.CustomFilter;
import com.demotm.example.util.listener.CustomListener;
import com.demotm.example.util.servlet.CustomServlet;
import org.springframework.boot.SpringApplication;


import java.util.EnumSet;

import javax.servlet.DispatcherType;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletContextInitializer;



@SpringBootApplication
public class DemotmApplication implements ServletContextInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {

        servletContext.addServlet("customServlet", new CustomServlet()).addMapping("/test");

        servletContext.addFilter("customFilter", new CustomFilter())
                .addMappingForServletNames(EnumSet.of(DispatcherType.REQUEST), true, "customServlet");
        servletContext.addListener(new CustomListener());
    }

    public static void main(String[] args) {
        SpringApplication.run(DemotmApplication.class, args);
    }
}
```

## 方法三 ：在 SpringBootApplication 上使用@ServletComponentScan 注解后，直接通过@WebServlet、@WebFilter、@WebListener 注解自动注册

具体实现

DemotmApplication.java 加入@ServletComponentScan

main的上面

```java
package com.demotm.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletComponentScan;

@ServletComponentScan
@SpringBootApplication
public class DemotmApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemotmApplication.class, args);
    }
}
```

相应的类加入注释

```java
@WebFilter(filterName = "customFilter", urlPatterns = "/*")
public class CustomFilter implements Filter {
 ....省略
}
```

```java
@WebListener
public class CustomListener implements ServletContextListener {
....省略
}
```

```java
@WebServlet(name = "customServlet", urlPatterns = "/test")
public class CustomServlet extends HttpServlet {
....省略
}
```


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