利用tomcat-redis-session-manager做session同步时自定义类对象属性保存不上的解决方法

在利用tomcat-redis-session-manager做session同步时，遇到了在session保存一个自定义对象时，修改该对象中的某个属性，session未进行序列化，属性没有被存储到redis中。

在 tomcat-redis-session-manager的github上有如下说明：

Session Change Tracking

As noted in the "Overview" section above, in order to prevent colliding writes, the Redis Session Manager only serializes the session object into Redis if the session object has changed (it always updates the expiration separately however.) This dirty tracking marks the session as needing serialization according to the following rules:

• Calling session.removeAttribute(key) always marks the session as dirty (needing serialization.)
• Calling session.setAttribute(key, newAttributeValue) marks the session as dirty if any of the following are true:
  • previousAttributeValue == null && newAttributeValue != null
  • previousAttributeValue != null && newAttributeValue == null
  • !newAttributeValue.getClass().isInstance(previousAttributeValue)
  • !newAttributeValue.equals(previousAttributeValue)

This feature can have the unintended consequence of hiding writes if you implicitly change a key in the session or if the object's equality does not change even though the key is updated. For example, assuming the session already contains the key "myArray" with an Array instance as its corresponding value, and has been previously serialized, the following code would not cause the session to be serialized again:

List myArray = session.getAttribute("myArray");
myArray.add(additionalArrayValue);

If your code makes these kind of changes, then the RedisSession provides a mechanism by which you can mark the session as dirty in order to guarantee serialization at the end of the request. For example:

List myArray = session.getAttribute("myArray");
myArray.add(additionalArrayValue);
session.setAttribute("__changed__");

In order to not cause issues with an application that may already use the key "__changed__", this feature is disabled by default. To enable this feature, simple call the following code in your application's initialization:

RedisSession.setManualDirtyTrackingSupportEnabled(true);

This feature also allows the attribute key used to mark the session as dirty to be changed. For example, if you executed the following:

RedisSession.setManualDirtyTrackingAttributeKey("customDirtyFlag");

Then the example above would look like this:

List myArray = session.getAttribute("myArray");
myArray.add(additionalArrayValue);
session.setAttribute("customDirtyFlag");

简单说就是tomcat-redis-session-manager判断一个session是否是dirty时，如果自定义对象所在类未覆盖equals方法，针对每个属性做判断的话，仅修改某一属性时，tomcat-redis-session-manager无法监测到，因此也无法将修改后的session序列化到redis中，因此出现了session同步的问题。

因此，修改的方式就是在修改session自定义对象属性的地方，增加session.setAttribute("__changed__","");调用，前提是RedisSession.setManualDirtyTrackingSupportEnabled(true);

即开启了手工设置dirty标志。

具体配置方法：

Add the following into your Tomcat context.xml (or the context block of the server.xml if applicable.)

<Valve className="com.radiadesign.catalina.session.RedisSessionHandlerValve" />
<Manager className="com.radiadesign.catalina.session.RedisSessionManager"
         host="localhost" <!-- optional: defaults to "localhost" -->
         port="6379" <!-- optional: defaults to "6379" -->
         database="0" <!-- optional: defaults to "0" -->
         maxInactiveInterval="60" <!-- optional: defaults to "60" (in seconds) --> />

The Valve must be declared before the Manager.

Copy the tomcat-redis-session-manager.jar and jedis-2.0.0.jar files into the lib directory of your Tomcat installation.

Reboot the server, and sessions should now be stored in Redis.

另外，tomcat-redis-session-manager的源码是2年前的，最高支持tomcat7，所以我修改了一下代码，增加了对tomcat8的支持。

github地址：

https://github.com/bsr1983/tomcat-redis-session-manager

在tomcat8下已测试通过。