前言

叢集方式部署伺服器時，當高併發量的請求到達服務端時，服務端通過負載均衡演算法將請求分配到叢集中某個伺服器，那麼同一使用者的多個請求可能被分發到不同的伺服器，如果將session儲存到某個伺服器記憶體中，可能會出現session丟失的情況。

因此在叢集時存在session共享一致性的問題。session複製或者使用hash演算法反向代理存在不足，本篇利用spring-session框架把session儲存到第三方容器（database,redis等）

本地容器選擇redis，客戶端使用redisson。

整合Redisson

  jar包

        <!--redis客戶端 redisson-->
        <!-- JDK 1.8+ compatible -->
        <dependency>
            <groupId>org.redisson</groupId>
            <artifactId>redisson</artifactId>
            <version>3.7.5</version>
        </dependency>
        <!--spring session-->
       <dependency>
            <groupId>org.springframework.session</groupId>
            <artifactId>spring-session-data-redis</artifactId>
            <version>2.0.5.RELEASE</version>
        </dependency>
 

1.我們需要做的事很簡單，只需要配置redis客戶端並使用@EnableRedissonHttpSession註解。RedissonHttpSessionConfiguration類中實現了RedissonSessionRepository的注入，它用來操作session儲存值。

RedissonSessionConfig配置類。

@EnableRedissonHttpSession
public class RedissonSessionConfig {
    @Bean(destroyMethod="shutdown")
    RedissonClient redisson() throws IOException {
        Config config = new Config();
        config.useClusterServers()
                .addNodeAddress("redis://192.168.56.129:7000", "redis://192.168.56.129:7001","redis://192.168.56.129:7002");
        return Redisson.create(config);
    }
    
}
 

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觀察EnableRedissonHttpSession註解，通過@Import向容器匯入了有@Configuration註解的RedissonHttpSessionConfiguration配置類,其中包含與session儲存有關的配置。(4.2版本後@Import支援向spring容器匯入沒有@Configuration的類)

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Import({RedissonHttpSessionConfiguration.class})
@Configuration
public @interface EnableRedissonHttpSession {
    int maxInactiveIntervalInSeconds() default 1800;

    String keyPrefix() default "";
}

接著觀察RedissonHttpSessionConfiguration，實現了ImportWare介面，通過setImportMetadata() 方EnableRedissonHttpSession註解下的兩個屬性值（maxInactiveIntervalInSeconds，keyPrefix）注入RedissonHttpSessionConfiguration屬性，向容器注入了RedissonSessionRepository 一個與session儲存有關的容器類，它需要指定RedissonClient (redis客戶端)。

@Configuration
public class RedissonHttpSessionConfiguration extends SpringHttpSessionConfiguration implements ImportAware {
    private Integer maxInactiveIntervalInSeconds;
    private String keyPrefix;

    public RedissonHttpSessionConfiguration() {
    }

    @Bean
    public RedissonSessionRepository sessionRepository(RedissonClient redissonClient, ApplicationEventPublisher eventPublisher) {
        RedissonSessionRepository repository = new RedissonSessionRepository(redissonClient, eventPublisher);
        if (StringUtils.hasText(this.keyPrefix)) {
            repository.setKeyPrefix(this.keyPrefix);
        }

        repository.setDefaultMaxInactiveInterval(this.maxInactiveIntervalInSeconds.intValue());
        return repository;
    }

    public void setMaxInactiveIntervalInSeconds(Integer maxInactiveIntervalInSeconds) {
        this.maxInactiveIntervalInSeconds = maxInactiveIntervalInSeconds;
    }

    public void setKeyPrefix(String keyPrefix) {
        this.keyPrefix = keyPrefix;
    }

    public void setImportMetadata(AnnotationMetadata importMetadata) {
        Map<String, Object> map = importMetadata.getAnnotationAttributes(EnableRedissonHttpSession.class.getName());
        AnnotationAttributes attrs = AnnotationAttributes.fromMap(map);
        this.keyPrefix = attrs.getString("keyPrefix");
        this.maxInactiveIntervalInSeconds = (Integer)attrs.getNumber("maxInactiveIntervalInSeconds");
    }
}

觀察父類SpringHttpSessionConfiguration，其中向spring容器中注入了SessionRepositoryFilter的Bean。spring正是通過這個filter用來過濾包裝session

@Bean
    public <S extends Session> SessionRepositoryFilter<? extends Session> springSessionRepositoryFilter(SessionRepository<S> sessionRepository) {
        SessionRepositoryFilter<S> sessionRepositoryFilter = new SessionRepositoryFilter(sessionRepository);
        sessionRepositoryFilter.setServletContext(this.servletContext);
        sessionRepositoryFilter.setHttpSessionIdResolver(this.httpSessionIdResolver);
        return sessionRepositoryFilter;
    }

2.我們需要過濾request請求，對session進行包裝

web.xml 中新增過濾器 （spring boot中省略）

<!--spring session-->
  <filter>
    <filter-name>springSessionRepositoryFilter</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>springSessionRepositoryFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
    <dispatcher>ERROR</dispatcher>
  </filter-mapping>

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DelegatingFilterProxy 類是filter的代理類，交給spring去管理filter。如果未指定init-param引數的話，DelegatingFilterProxy就會把filter-name作為要查詢的Bean物件的name。這裡去spring容器中尋找名字為springSessionRepositoryFilter的bean，得知為SessionRespositoryFilter過濾器。

public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        if (request instanceof HttpServletRequest && response instanceof HttpServletResponse) {
            HttpServletRequest httpRequest = (HttpServletRequest)request;
            HttpServletResponse httpResponse = (HttpServletResponse)response;
            boolean hasAlreadyFilteredAttribute = request.getAttribute(this.alreadyFilteredAttributeName) != null;
            if (hasAlreadyFilteredAttribute) {
                filterChain.doFilter(request, response);
            } else {
                request.setAttribute(this.alreadyFilteredAttributeName, Boolean.TRUE);

                try {
                    this.doFilterInternal(httpRequest, httpResponse, filterChain);
                } finally {
                    request.removeAttribute(this.alreadyFilteredAttributeName);
                }
            }

        } else {
            throw new ServletException("OncePerRequestFilter just supports HTTP requests");
        }
    }

 觀察SessionRespositoryFilter的doFilter()方法，裡面呼叫了doFilterInternal()方法,在doFilterInternal中實現了對session的包裝

protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        request.setAttribute(SESSION_REPOSITORY_ATTR, this.sessionRepository);
        SessionRepositoryFilter<S>.SessionRepositoryRequestWrapper wrappedRequest = new SessionRepositoryFilter.SessionRepositoryRequestWrapper(request, response, this.servletContext);
        SessionRepositoryFilter.SessionRepositoryResponseWrapper wrappedResponse = new SessionRepositoryFilter.SessionRepositoryResponseWrapper(wrappedRequest, response);

        try {
            filterChain.doFilter(wrappedRequest, wrappedResponse);
        } finally {
            wrappedRequest.commitSession();
        }

    }

測試

寫一個controller類，獲得session並設值

@Controller
public class HelloController {
		
	@RequestMapping("/helloWorld")
	public String helloWorld(HttpServletRequest request,Model model) throws Exception {	
		request.getSession().setAttribute("value","this is a test");
		return "hello";
	}
	
	
}

觀察redis，以hash結構儲存。hash的key為redisson_spring_session:sessionId 構成。