191122-SpringBoot系列教程web篇Servlet 註冊的四種姿勢

前面介紹了java web三要素中filter的使用指南與常見的易錯事項，接下來我們來看一下Servlet的使用姿勢，本篇主要帶來在SpringBoot環境下，註冊自定義的Servelt的四種姿勢

@WebServlet
ServletRegistrationBean
ServletContext

I. 環境配置

1. 項目搭建

首先我們需要搭建一個web工程，以方便後續的servelt註冊的實例演示，可以通過spring boot官網創建工程，也可以建立一個maven工程，在pom.xml中如下配置

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.1.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <java.version>1.8</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

<build>
    <pluginManagement>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </pluginManagement>
</build>
<repositories>
    <repository>
        <id>spring-snapshots</id>
        <name>Spring Snapshots</name>
        <url>https://repo.spring.io/libs-snapshot-local</url>
        <snapshots>
            <enabled>true</enabled>
        </snapshots>
    </repository>
    <repository>
        <id>spring-milestones</id>
        <name>Spring Milestones</name>
        <url>https://repo.spring.io/libs-milestone-local</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
    </repository>
    <repository>
        <id>spring-releases</id>
        <name>Spring Releases</name>
        <url>https://repo.spring.io/libs-release-local</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
    </repository>
</repositories>

特別說明：

爲了緊跟SpringBoot的最新版本，從本篇文章開始，博文對應的示例工程中SpringBoot版本升級到 2.2.1.RELEASE

II. Servlet註冊

自定義一個Servlet比較簡單，一般常見的操作是繼承 HttpServlet ，然後覆蓋 doGet , doPost 等方法即可；然而重點是我們自定義的這些Servlet如何才能被SpringBoot識別並使用纔是關鍵，下面介紹四種註冊方式

1. @WebServlet

在自定義的servlet上添加Servlet3+的註解 @WebServlet ，來聲明這個類是一個Servlet

和Fitler的註冊方式一樣，使用這個註解，需要配合Spring Boot的 @ServletComponentScan ，否則單純的添加上面的註解並不會生效

/**
 * 使用註解的方式來定義並註冊一個自定義Servlet
 * Created by @author yihui in 19:08 19/11/21.
 */
@WebServlet(urlPatterns = "/annotation")
public class AnnotationServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = req.getParameter("name");
        PrintWriter writer = resp.getWriter();
        writer.write("[AnnotationServlet] welcome " + name);
        writer.flush();
        writer.close();
    }
}

上面是一個簡單的測試Servlet，接收請求參數 name , 並返回 welcome xxx ；爲了讓上面的的註解生效，需要設置下啓動類

@ServletComponentScan
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class);
    }
}

然後啓動測試，輸出結果如:

➜  ~ curl http://localhost:8080/annotation\?name\=yihuihui
# 輸出結果
[AnnotationServlet] welcome yihuihui%

2. ServletRegistrationBean

在Filter的註冊中，我們知道有一種方式是定義一個Spring的Bean FilterRegistrationBean 來包裝我們的自定義Filter，從而讓Spring容器來管理我們的過濾器；同樣的在Servlet中，也有類似的包裝bean: ServletRegistrationBean

自定義的bean如下，注意類上沒有任何註解

/**
 * Created by @author yihui in 19:17 19/11/21.
 */
public class RegisterBeanServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = req.getParameter("name");
        PrintWriter writer = resp.getWriter();
        writer.write("[RegisterBeanServlet] welcome " + name);
        writer.flush();
        writer.close();
    }
}

接下來我們需要定義一個 ServletRegistrationBean ，讓它持有 RegisterBeanServlet 的實例

@Bean
public ServletRegistrationBean servletBean() {
    ServletRegistrationBean registrationBean = new ServletRegistrationBean();
    registrationBean.addUrlMappings("/register");
    registrationBean.setServlet(new RegisterBeanServlet());
    return registrationBean;
}

測試請求輸出如下:

➜  ~ curl 'http://localhost:8080/register?name=yihuihui'
# 輸出結果
[RegisterBeanServlet] welcome yihuihui%

3. ServletContext

這種姿勢，在實際的Servlet註冊中，其實用得並不太多，主要思路是在ServletContext初始化後，藉助 javax.servlet.ServletContext#addServlet(java.lang.String, java.lang.Class<? extends javax.servlet.Servlet>) 方法來主動添加一個Servlet

所以我們需要找一個合適的時機，獲取 ServletContext 實例，並註冊Servlet，在SpringBoot生態下，可以藉助 ServletContextInitializer

ServletContextInitializer主要被RegistrationBean實現用於往ServletContext容器中註冊Servlet,Filter或者EventListener。這些ServletContextInitializer的設計目的主要是用於這些實例被Spring IoC容器管理

/**
 * Created by @author yihui in 19:49 19/11/21.
 */
public class ContextServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = req.getParameter("name");
        PrintWriter writer = resp.getWriter();
        writer.write("[ContextServlet] welcome " + name);
        writer.flush();
        writer.close();
    }
}


/**
 * Created by @author yihui in 19:50 19/11/21.
 */
@Component
public class SelfServletConfig implements ServletContextInitializer {
    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        ServletRegistration initServlet = servletContext.addServlet("contextServlet", ContextServlet.class);
        initServlet.addMapping("/context");
    }
}

測試結果如下

➜  ~ curl 'http://localhost:8080/context?name=yihuihui'
# 輸出結果
[ContextServlet] welcome yihuihui%

4. bean

接下來的這種註冊方式，並不優雅，但是也可以實現Servlet的註冊目的，但是有坑，請各位大佬謹慎使用

看過我的前一篇博文 191016-SpringBoot系列教程web篇之過濾器Filter使用指南的同學，可能會有一點映象，可以在Filter上直接添加 @Component 註解，Spring容器掃描bean時，會查找所有實現Filter的子類，並主動將它包裝到 FilterRegistrationBean ，實現註冊的目的

我們的Servlet是否也可以這樣呢？接下來我們實測一下

@Component
public class BeanServlet1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = req.getParameter("name");
        PrintWriter writer = resp.getWriter();
        writer.write("[BeanServlet1] welcome " + name);
        writer.flush();
        writer.close();
    }
}

現在問題來了，上面這個Servlet沒有定義urlMapping規則，怎麼請求呢？

爲了確定上面的Servlet被註冊了，藉着前面Filter的源碼分析的關鍵鏈路，我們找到了實際註冊的地方 ServletContextInitializerBeans#addAsRegistrationBean

// org.springframework.boot.web.servlet.ServletContextInitializerBeans#addAsRegistrationBean(org.springframework.beans.factory.ListableBeanFactory, java.lang.Class<T>, java.lang.Class<B>, org.springframework.boot.web.servlet.ServletContextInitializerBeans.RegistrationBeanAdapter<T>)

@Override
public RegistrationBean createRegistrationBean(String name, Servlet source, int totalNumberOfSourceBeans) {
	String url = (totalNumberOfSourceBeans != 1) ? "/" + name + "/" : "/";
	if (name.equals(DISPATCHER_SERVLET_NAME)) {
		url = "/"; // always map the main dispatcherServlet to "/"
	}
	ServletRegistrationBean<Servlet> bean = new ServletRegistrationBean<>(source, url);
	bean.setName(name);
	bean.setMultipartConfig(this.multipartConfig);
	return bean;
}

從上面的源碼上可以看到，這個Servlet的url要麼是 / , 要麼是 /beanName/

接下來進行實測，全是404

➜  ~ curl 'http://localhost:8080/?name=yihuihui'
{"timestamp":"2019-11-22T00:52:00.448+0000","status":404,"error":"Not Found","message":"No message available","path":"/"}%

➜  ~ curl 'http://localhost:8080/beanServlet1?name=yihuihui'
{"timestamp":"2019-11-22T00:52:07.962+0000","status":404,"error":"Not Found","message":"No message available","path":"/beanServlet1"}%                                          

➜  ~ curl 'http://localhost:8080/beanServlet1/?name=yihuihui'
{"timestamp":"2019-11-22T00:52:11.202+0000","status":404,"error":"Not Found","message":"No message available","path":"/beanServlet1/"}%

然後再定義一個Servlet時

@Component
public class BeanServlet2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = req.getParameter("name");
        PrintWriter writer = resp.getWriter();
        writer.write("[BeanServlet2] welcome " + name);
        writer.flush();
        writer.close();
    }
}

再次測試

➜  ~ curl 'http://localhost:8080/beanServlet1?name=yihuihui'
{"timestamp":"2019-11-22T00:54:12.692+0000","status":404,"error":"Not Found","message":"No message available","path":"/beanServlet1"}%                                          

➜  ~ curl 'http://localhost:8080/beanServlet1/?name=yihuihui'
[BeanServlet1] welcome yihuihui%                                                                                                                                                

➜  ~ curl 'http://localhost:8080/beanServlet2/?name=yihuihui'
[BeanServlet2] welcome yihuihui%

從實際的測試結果可以看出，使用這種定義方式時，這個servlet相應的url爲 beanName + '/'

注意事項

然後問題來了，只定義一個Servlet的時候，根據前面的源碼分析，這個Servlet應該會相應 http://localhost:8080/ 的請求，然而測試的時候爲啥是404？

這個問題也好解答，主要就是Servlet的優先級問題，上面這種方式的Servlet的相應優先級低於Spring Web的Servelt優先級，相同的url請求先分配給Spring的Servlet了，爲了驗證這個也簡單，兩步

先註釋 BeanServlet2 類上的註解 @Component
在 BeanServlet1 的類上，添加註解 @Order(-10000)

然後再次啓動測試,輸出如下

➜  ~ curl 'http://localhost:8080/?name=yihuihui'
[BeanServlet1] welcome yihuihui%

➜  ~ curl 'http://localhost:8080?name=yihuihui'
[BeanServlet1] welcome yihuihui%