1.共享Session问题

HttpSession是通过Servlet容器创建和管理的，像Tomcat/Jetty都是保存在内存中的。而如果我们把web服务器搭建成分布式的集群，然后利用LVS或Nginx做负载均衡，那么来自同一用户的Http请求将有可能被分发到两个不同的web站点中去。那么问题就来了，如何保证不同的web站点能够共享同一份session数据呢？

2.Spring Session介绍

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

Spring Session提供了一套创建和管理Servlet HttpSession的方案。Spring Session提供了集群Session（Clustered Sessions）功能，默认采用外置的Redis来存储Session数据，以此来解决Session共享的问题。

下面是来自官网的特性介绍：

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的正确姿势

下面是实际调试通过的例子，包含下面4个步骤：

（1）第一步，添加Maven依赖

根据官网Quick Start展示的依赖，在项目pom.xml中添加后各种找不到类引用。于是查看Spring Session项目的build.gradle文件，居然没有配置依赖的项目，难道还要我自己去找它的依赖，太不专业了吧？！！！

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

终于在多番仔细研究Spring Session项目源码之后，看到了spring-session-data-redis项目：

build.gradle文件里配置了Spring Session编译依赖的3个项目：

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"
}

于是，真正的Maven依赖改成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，编写一个配置类，用来启用RedisHttpSession功能，并向Spring容器中注册一个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，将RedisHttpSessionConfig加入到WebInitializer#getRootConfigClasses()中，让Spring容器加载RedisHttpSessionConfig类。WebInitializer是一个自定义的AbstractAnnotationConfigDispatcherServletInitializer实现类，该类会在Servlet启动时加载（当然也可以采用别的加载方法，比如采用扫描@Configuration注解类的方式等等）。

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

（4）第四步，编写一个一个AbstractHttpSessionApplicationInitializer实现类，用于向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，编写了一个配置类RedisHttpSessionConfig，它包含注解@EnableRedisHttpSession，并通过@Bean注解注册了一个RedisConnectionFactory到Spring容器中。

而@EnableRedisHttpSession注解通过Import，引入了RedisHttpSessionConfiguration配置类。该配置类通过@Bean注解，向Spring容器中注册了一个SessionRepositoryFilter（SessionRepositoryFilter的依赖关系：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，我们编写了一个SpringSessionInitializer 类，它继承自AbstractHttpSessionApplicationInitializer。该类不需要重载或实现任何方法，它的作用是在Servlet容器初始化时，从Spring容器中获取一个默认名叫sessionRepositoryFilter的过滤器类（之前没有注册的话这里找不到会报错），并添加到Servlet过滤器链中。

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);//注册一个SessionRepositoryFilter
		afterSessionRepositoryFilter(servletContext);
	}

	/**
	 * Registers the springSessionRepositoryFilter
	 * @param servletContext the {@link ServletContext}
	 */
	private void insertSessionRepositoryFilter(ServletContext servletContext) {
		String filterName = DEFAULT_FILTER_NAME;//默认名字是springSessionRepositoryFilter
		DelegatingFilterProxy springSessionRepositoryFilter = new DelegatingFilterProxy(filterName);//该Filter代理会在初始化时从Spring容器中查找springSessionRepositoryFilter，之后实际会使用SessionRepositoryFilter进行doFilter操作 		
		String contextAttribute = getWebApplicationContextAttribute();
		if(contextAttribute != null) {
			springSessionRepositoryFilter.setContextAttribute(contextAttribute);
		}
		registerFilter(servletContext, true, filterName, springSessionRepositoryFilter);
	}
	
	//......
}

注意下面给出的是简化过的示例代码，与spring-session项目的源代码有所差异。

@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 ...
}

• WebInitializer，负责加载配置类。它继承自AbstractAnnotationConfigDispatcherServletInitializer，实现了WebApplicationInitializer接口
• SpringSessionInitializer，负责添加sessionRepositoryFilter的过滤器类。它继承自AbstractHttpSessionApplicationInitializer，实现了WebApplicationInitializer接口

在Servlet3.0规范中，Servlet容器启动时会自动扫描javax.servlet.ServletContainerInitializer的实现类，在实现类中我们可以定制需要加载的类。在spring-web项目中，有一个ServletContainerInitializer实现类SpringServletContainerInitializer，它通过注解@HandlesTypes(WebApplicationInitializer.class)，让Servlet容器在启动该类时，会自动寻找所有的WebApplicationInitializer实现类。

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数据？

（1）Http Session数据在Redis中是以Hash结构存储的。

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
"xacxedx00x05srx00x11java.lang.Integerx12xe2xa0xa4xf7x81x878x02x00x01Ix00x05valuexrx00x10java.lang.Numberx86xacx95x1dx0bx94xe0x8bx02x00x00xpx00x00x00x03"

127.0.0.1:6379> hget spring:session:sessions:59f3987c-d1e4-44b3-a83a-32079942888b creationTime
"xacxedx00x05srx00x0ejava.lang.Long;x8bxe4x90xccx8f#xdfx02x00x01Jx00x05valuexrx00x10java.lang.Numberx86xacx95x1dx0bx94xe0x8bx02x00x00xpx00x00x01MMx94(xec"

6.参考文章

Spring Session 1.01 Reference

spring session入门

集群session共享机制