1.共享Session問題

HttpSession是通過Servlet容器創建和管理的，像Tomcat/Jetty都是保存在內存中的。而如果我們把web服務器搭建成分佈式的集羣，然後利用LVS或Nginx做負載均衡，那麼來自同一用戶的Http請求將有可能被分發到兩個不同的web站點中去。那麼問題就來了，如何保證不同的web站點能夠共享同一份session數據呢？

最簡單的想法就是把session數據保存到內存以外的一個統一的地方，例如Memcached/Redis等數據庫中。那麼問題又來了，如何替換掉Servlet容器創建和管理HttpSession的實現呢？

（1）設計一個Filter，利用HttpServletRequestWrapper，實現自己的 getSession()方法，接管創建和管理Session數據的工作。spring-session就是通過這樣的思路實現的。

（2）利用Servlet容器提供的插件功能，自定義HttpSession的創建和管理策略，並通過配置的方式替換掉默認的策略。不過這種方式有個缺點，就是需要耦合Tomcat/Jetty等Servlet容器的代碼。這方面其實早就有開源項目了，例如memcached-session-manager，以及tomcat-redis-session-manager。暫時都只支持Tomcat6/Tomcat7。

2.Spring Session介紹

Spring Session是Spring的項目之一，GitHub地址：https://github.com/spring-projects/spring-session。

Spring Session提供了一套創建和管理Servlet HttpSession的方案。Spring Session提供了集羣Session（Clustered Sessions）功能，默認採用外置的Redis來存儲Session數據，以此來解決Session共享的問題。

下面是來自官網的特性介紹：

Features

Spring Session provides the following features:

API and implementations for managing a user's session
HttpSession - allows replacing the HttpSession in an application container (i.e. Tomcat) neutral way
- Clustered Sessions - Spring Session makes it trivial to support clustered sessions without being tied to an application container specific solution.
- Multiple Browser Sessions - Spring Session supports managing multiple users' sessions in a single browser instance (i.e. multiple authenticated accounts similar to Google).
- RESTful APIs - Spring Session allows providing session ids in headers to work with RESTful APIs
WebSocket - provides the ability to keep the HttpSession alive when receiving WebSocket messages

3.集成Spring Session的正確姿勢

下面是實際調試通過的例子，包含下面4個步驟：

（1）第一步，添加Maven依賴

根據官網Quick Start展示的依賴，在項目pom.xml中添加後各種找不到類引用。於是查看Spring Session項目的build.gradle文件，居然沒有配置依賴的項目，難道還要我自己去找它的依賴，太不專業了吧？！！！

<dependencies>
    <dependency>
        <groupId>org.springframework.session</groupId>
        <artifactId>spring-session</artifactId>
        <version>1.0.1.RELEASE</version>
    </dependency>
</dependencies>

終於在多番仔細研究Spring Session項目源碼之後，看到了spring-session-data-redis項目：

build.gradle文件裏配置了Spring Session編譯依賴的3個項目：

apply from: JAVA_GRADLE
apply from: MAVEN_GRADLE

apply plugin: 'spring-io'

description = "Aggregator for Spring Session and Spring Data Redis"

dependencies {
	compile project(':spring-session'),
			"org.springframework.data:spring-data-redis:$springDataRedisVersion",
			"redis.clients:jedis:$jedisVersion",
			"org.apache.commons:commons-pool2:$commonsPoolVersion"

	springIoVersions "io.spring.platform:platform-versions:${springIoVersion}@properties"
}

於是，真正的Maven依賴改成spring-session-data-redis就OK了：

<dependency>
    <groupId>org.springframework.session</groupId>
    <artifactId>spring-session-data-redis</artifactId>
    <version>1.0.1.RELEASE</version>
</dependency>

（2）第二步，編寫一個配置類，用來啓用RedisHttpSession功能，並向Spring容器中註冊一個RedisConnectionFactory。

import org.springframework.context.annotation.Bean;
import org.springframework.data.redis.connection.RedisConnectionFactory;
import org.springframework.data.redis.connection.jedis.JedisConnectionFactory;
import org.springframework.session.data.redis.config.annotation.web.http.EnableRedisHttpSession;

@EnableRedisHttpSession(maxInactiveIntervalInSeconds = 7200)
public class RedisHttpSessionConfig {

    @Bean
    public RedisConnectionFactory connectionFactory() {
        JedisConnectionFactory connectionFactory = new JedisConnectionFactory();
        connectionFactory.setPort(6379);
        connectionFactory.setHostName("10.18.15.190");
        return connectionFactory;
    }
}

（3）第三步，將RedisHttpSessionConfig加入到WebInitializer#getRootConfigClasses()中，讓Spring容器加載RedisHttpSessionConfig類。WebInitializer是一個自定義的AbstractAnnotationConfigDispatcherServletInitializer實現類，該類會在Servlet啓動時加載（當然也可以採用別的加載方法，比如採用掃描@Configuration註解類的方式等等）。

//該類採用Java Configuration，來代替web.xml   
public class WebInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
    
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class[]{Config1.class, Config2.class, RedisHttpSessionConfig.class};
    }
	
	//......
}

（4）第四步，編寫一個一個AbstractHttpSessionApplicationInitializer實現類，用於向Servlet容器中添加springSessionRepositoryFilter。

import org.springframework.session.web.context.AbstractHttpSessionApplicationInitializer;

public class SpringSessionInitializer extends AbstractHttpSessionApplicationInitializer {
}

4. Spring Session原理

（1）前面集成spring-sesion的第二步中，編寫了一個配置類RedisHttpSessionConfig，它包含註解@EnableRedisHttpSession，並通過@Bean註解註冊了一個RedisConnectionFactory到Spring容器中。

而@EnableRedisHttpSession註解通過Import，引入了RedisHttpSessionConfiguration配置類。該配置類通過@Bean註解，向Spring容器中註冊了一個SessionRepositoryFilter（SessionRepositoryFilter的依賴關係：SessionRepositoryFilter --> SessionRepository --> RedisTemplate --> RedisConnectionFactory）。

package org.springframework.session.data.redis.config.annotation.web.http;

@Configuration
@EnableScheduling
public class RedisHttpSessionConfiguration implements ImportAware, BeanClassLoaderAware {
	//......
	
	@Bean
	public RedisTemplate<String,ExpiringSession> sessionRedisTemplate(RedisConnectionFactory connectionFactory) {
		//......
		return template;
	}
	
	@Bean
	public RedisOperationsSessionRepository sessionRepository(RedisTemplate<String, ExpiringSession> sessionRedisTemplate) {
		//......
		return sessionRepository;
	}
	
	@Bean
	public <S extends ExpiringSession> SessionRepositoryFilter<? extends ExpiringSession> springSessionRepositoryFilter(SessionRepository<S> sessionRepository, ServletContext servletContext) {
		//......
		return sessionRepositoryFilter;
	}
	
	//......
}

（2）集成spring-sesion的第四步中，我們編寫了一個SpringSessionInitializer 類，它繼承自AbstractHttpSessionApplicationInitializer。該類不需要重載或實現任何方法，它的作用是在Servlet容器初始化時，從Spring容器中獲取一個默認名叫sessionRepositoryFilter的過濾器類（之前沒有註冊的話這裏找不到會報錯），並添加到Servlet過濾器鏈中。

package org.springframework.session.web.context;

/**
 * Registers the {@link DelegatingFilterProxy} to use the
 * springSessionRepositoryFilter before any other registered {@link Filter}. 
 *
 * ......
 */
@Order(100)
public abstract class AbstractHttpSessionApplicationInitializer implements WebApplicationInitializer {

	private static final String SERVLET_CONTEXT_PREFIX = "org.springframework.web.servlet.FrameworkServlet.CONTEXT.";

	public static final String DEFAULT_FILTER_NAME = "springSessionRepositoryFilter";

	//......

	public void onStartup(ServletContext servletContext)
			throws ServletException {
		beforeSessionRepositoryFilter(servletContext);
		if(configurationClasses != null) {
			AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext();
			rootAppContext.register(configurationClasses);
			servletContext.addListener(new ContextLoaderListener(rootAppContext));
		}
		insertSessionRepositoryFilter(servletContext);//註冊一個SessionRepositoryFilter
		afterSessionRepositoryFilter(servletContext);
	}

	/**
	 * Registers the springSessionRepositoryFilter
	 * @param servletContext the {@link ServletContext}
	 */
	private void insertSessionRepositoryFilter(ServletContext servletContext) {
		String filterName = DEFAULT_FILTER_NAME;//默認名字是springSessionRepositoryFilter
		DelegatingFilterProxy springSessionRepositoryFilter = new DelegatingFilterProxy(filterName);//該Filter代理會在初始化時從Spring容器中查找springSessionRepositoryFilter，之後實際會使用SessionRepositoryFilter進行doFilter操作 		
		String contextAttribute = getWebApplicationContextAttribute();
		if(contextAttribute != null) {
			springSessionRepositoryFilter.setContextAttribute(contextAttribute);
		}
		registerFilter(servletContext, true, filterName, springSessionRepositoryFilter);
	}
	
	//......
}

SessionRepositoryFilter是一個優先級最高的javax.servlet.Filter，它使用了一個SessionRepositoryRequestWrapper類接管了Http Session的創建和管理工作。

注意下面給出的是簡化過的示例代碼，與spring-session項目的源代碼有所差異。

@Order(SessionRepositoryFilter.DEFAULT_ORDER)
public class SessionRepositoryFilter implements Filter {

        public doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
                HttpServletRequest httpRequest = (HttpServletRequest) request;
                SessionRepositoryRequestWrapper customRequest =
                        new SessionRepositoryRequestWrapper(httpRequest);

                chain.doFilter(customRequest, response, chain);
        }

        // ...
}

public class SessionRepositoryRequestWrapper extends HttpServletRequestWrapper {

        public SessionRepositoryRequestWrapper(HttpServletRequest original) {
                super(original);
        }

        public HttpSession getSession() {
                return getSession(true);
        }

        public HttpSession getSession(boolean createNew) {
                // create an HttpSession implementation from Spring Session
        }

        // ... other methods delegate to the original HttpServletRequest ...
}

（3）好了，剩下的問題就是，如何在Servlet容器啓動時，加載下面兩個類。幸運的是，這兩個類由於都實現了WebApplicationInitializer接口，會被自動加載。

WebInitializer，負責加載配置類。它繼承自AbstractAnnotationConfigDispatcherServletInitializer，實現了WebApplicationInitializer接口
SpringSessionInitializer，負責添加sessionRepositoryFilter的過濾器類。它繼承自AbstractHttpSessionApplicationInitializer，實現了WebApplicationInitializer接口

在Servlet3.0規範中，Servlet容器啓動時會自動掃描javax.servlet.ServletContainerInitializer的實現類，在實現類中我們可以定製需要加載的類。在spring-web項目中，有一個ServletContainerInitializer實現類SpringServletContainerInitializer，它通過註解@HandlesTypes(WebApplicationInitializer.class)，讓Servlet容器在啓動該類時，會自動尋找所有的WebApplicationInitializer實現類。

package org.springframework.web;

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

	/**
	 * Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
	 * implementations present on the application classpath.
	 *
	 * <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
	 * Servlet 3.0+ containers will automatically scan the classpath for implementations
	 * of Spring's {@code WebApplicationInitializer} interface and provide the set of all
	 * such types to the {@code webAppInitializerClasses} parameter of this method.
	 *
	 * <p>If no {@code WebApplicationInitializer} implementations are found on the
	 * classpath, this method is effectively a no-op. An INFO-level log message will be
	 * issued notifying the user that the {@code ServletContainerInitializer} has indeed
	 * been invoked but that no {@code WebApplicationInitializer} implementations were
	 * found.
	 *
	 * <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,
	 * they will be instantiated (and <em>sorted</em> if the @{@link
	 * org.springframework.core.annotation.Order @Order} annotation is present or
	 * the {@link org.springframework.core.Ordered Ordered} interface has been
	 * implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
	 * method will be invoked on each instance, delegating the {@code ServletContext} such
	 * that each instance may register and configure servlets such as Spring's
	 * {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
	 * or any other Servlet API componentry such as filters.
	 *
	 * @param webAppInitializerClasses all implementations of
	 * {@link WebApplicationInitializer} found on the application classpath
	 * @param servletContext the servlet context to be initialized
	 * @see WebApplicationInitializer#onStartup(ServletContext)
	 * @see AnnotationAwareOrderComparator
	 */
	@Override
	public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {
		//......
	}

}

5. 如何在Redis中查看Session數據？

（1）Http Session數據在Redis中是以Hash結構存儲的。

（2）可以看到，還有一個key="spring:session:expirations:1431577740000"的數據，是以Set結構保存的。這個值記錄了所有session數據應該被刪除的時間（即最新的一個session數據過期的時間）。

127.0.0.1:6379> keys *
1) "spring:session:expirations:1431577740000"
2) "spring:session:sessions:e2cef3ae-c8ea-4346-ba6b-9b3b26eee578"
127.0.0.1:6379> type spring:session:sessions:e2cef3ae-c8ea-4346-ba6b-9b3b26eee578
hash
127.0.0.1:6379> type spring:session:expirations:1431577740000
set

127.0.0.1:6379> keys *
1) "spring:session:expirations:1431527520000"
2) "spring:session:sessions:59f3987c-d1e4-44b3-a83a-32079942888b"
3) "spring:session:sessions:11a69da6-138b-42bc-9916-60ae78aa55aa"
4) "spring:session:sessions:0a51e2c2-4a3b-4986-a754-d886d8a5d42d"
5) "spring:session:expirations:1431527460000"

127.0.0.1:6379> hkeys spring:session:sessions:59f3987c-d1e4-44b3-a83a-32079942888b
1) "maxInactiveInterval"
2) "creationTime"
3) "lastAccessedTime"
4) "sessionAttr:attr1"

127.0.0.1:6379> hget spring:session:sessions:59f3987c-d1e4-44b3-a83a-32079942888b sessionAttr:attr1
"\xac\xed\x00\x05sr\x00\x11java.lang.Integer\x12\xe2\xa0\xa4\xf7\x81\x878\x02\x00\x01I\x00\x05valuexr\x00\x10java.lang.Number\x86\xac\x95\x1d\x0b\x94\xe0\x8b\x02\x00\x00xp\x00\x00\x00\x03"

127.0.0.1:6379> hget spring:session:sessions:59f3987c-d1e4-44b3-a83a-32079942888b creationTime
"\xac\xed\x00\x05sr\x00\x0ejava.lang.Long;\x8b\xe4\x90\xcc\x8f#\xdf\x02\x00\x01J\x00\x05valuexr\x00\x10java.lang.Number\x86\xac\x95\x1d\x0b\x94\xe0\x8b\x02\x00\x00xp\x00\x00\x01MM\x94(\xec"

6.參考文章

Spring Session 1.01 Reference

spring session入門

集羣session共享機制

利用spring session解決共享Session問題

1.共享Session問題

2.Spring Session介紹

Features

3.集成Spring Session的正確姿勢

4. Spring Session原理

5. 如何在Redis中查看Session數據？

6.參考文章

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