Redis的java客戶端Jedis Client介紹（持續翻譯中）
1. 概覽
This article is an introduction to Jedis, a client library in Java for Redis – the popular in-memory data structure store that can persist on disk as well. It is driven by a keystore-based data structure to persist data and can be used as a database, cache, message broker, etc.
本文主要介紹Jedis，一個Redis的客戶端類庫，Redis是當下流行的記憶體資料庫，同時也可將資料持久化到磁碟。是基於鍵的資料結構像資料庫、快取、訊息一樣持久化資料。
First, we are going to explain in which kind of situations Jedis is useful and what it is about.
首先，我們將解釋Jedis是什麼且在什麼情況下有用的。
In the subsequent sections we are elaborating on the various data structures and explaining transactions, pipelining and the publish/subscribe feature. We conclude with connection pooling and Redis Cluster.
在隨後的章節，我們將從各種資料型別，並且介紹事務、pipeline、釋出/訂閱等功能。最後我們將介紹連線池和Redis叢集。
2. 為什麼是Jedis?
Redis lists the most well-known client libraries on their official site. There are multiple alternatives to Jedis, but only two more are currently worthy of their recommendation star, lettuce, and Redisson.
Redis在其官網上列出了比較有名的客戶端類庫。
These two clients do have some unique features like thread safety, transparent reconnection handling and an asynchronous API, all features of which Jedis lacks.
However, it is small and considerably faster than the other two. Besides, it is the client library of choice of the Spring Framework developers, and it has the biggest community of all three.
3. Maven Dependencies
Let’s start by declaring the only dependency we will need in the pom.xml:
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If you’re looking for the latest version of the library, check out this page.
4. Redis Installation
You will need to install and fire up one of the latest versions of Redis. We are running the latest stable version at this moment (3.2.1), but any post 3.x version should be okay.
Find here more information about Redis for Linux and Macintosh, they have very similar basic installation steps. Windows is not officially supported, but this port is well maintained.
After that we can directly dive in and connect to it from our Java code:
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The default constructor will work just fine unless you have started the service on a non-default port or a remote machine, in which case you can configure it correctly by passing the correct values as parameters into the constructor.
5. Redis Data Structures
Most of the native operation commands are supported and, conveniently enough, they normally share the same method name.
5.1. Strings
Strings are the most basic kind of Redis value, useful for when you need to persist simple key-value data types:
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The variable cachedResponse will hold the value 32,15,223,828. Coupled with expiration support, discussed later, it can work as a lightning fast and simple to use cache layer for HTTP requests received at your web application and other caching requirements.
5.2. Lists
Redis Lists are simply lists of strings, sorted by insertion order and make it an ideal tool to implement, for instance, message queues:
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The variable task will hold the value firstTask. Remember that you can serialize any object and persist it as a string, so messages in the queue can carry more complex data when required.
5.3. Sets
Redis Sets are an unordered collection of Strings that come in handy when you want to exclude repeated members:
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The Java Set nicknames will have a size of 2, the second addition of nickname#1 was ignored. Also, the exists variable will have a value of true, the method sismember enables you to check for the existence of a particular member quickly.
5.4. Hashes
Redis Hashes are mapping between String fields and String values:
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As you can see, hashes are a very convenient data type when you want to access object’s properties individually since you do not need to retrieve the whole object.
5.5. Sorted Sets
Sorted Sets are like a Set where each member has an associated ranking, that is used for sorting them:
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The variable player will hold the value PlayerThree because we are retrieving the top 1 player and he is the one with the highest score. The rank variable will have a value of 1 because PlayerOne is the second in the ranking and the ranking is zero-based.
6. Transactions
Transactions guarantee atomicity and thread safety operations, which means that requests from other clients will never be handled concurrently during Redis transactions:
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You can even make a transaction success dependent on a specific key by “watching” it right before you instantiate your Transaction:
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If the value of that key changes before the transaction is executed, the transaction will not be completed successfully.
7. Pipelining
When we have to send multiple commands, we can pack them together in one request and save connection overhead by using pipelines, it is essentially a network optimization. As long as the operations are mutually independent, we can take advantage of this technique:
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Notice we do not get direct access to the command responses, instead, we’re given a Response instance from which we can request the underlying response after the pipeline has been synced.
8. Publish/Subscribe
We can use the Redis messaging broker functionality to send messages between the different components of our system. Make sure the subscriber and publisher threads do not share the same Jedis connection.
8.1. Subscriber
Subscribe and listen to messages sent to a channel:
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Subscribe is a blocking method, you will need to unsubscribe from the JedisPubSub explicitly. We have overridden the onMessage method but there are many more useful methods available to override.
8.2. Publisher
Then simply send messages to that same channel from the publisher’s thread:
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9. Connection Pooling
It is important to know that the way we have been dealing with our Jedis instance is naive. In a real-world scenario, you do not want to use a single instance in a multi-threaded environment as a single instance is not thread-safe.
Luckily enough we can easily create a pool of connections to Redis for us to reuse on demand, a pool that is thread safe and reliable as long as you return the resource to the pool when you are done with it.
Let’s create the JedisPool:
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Since the pool instance is thread safe, you can store it somewhere statically but you should take care of destroying the pool to avoid leaks when the application is being shutdown.
Now we can make use of our pool from anywhere in the application when needed:
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We used the Java try-with-resources statement to avoid having to manually close the Jedis resource, but if you cannot use this statement you can also close the resource manually in the finally clause.
Make sure you use a pool like we have described in your application if you do not want to face nasty multi-threading issues. You can obviously play with the pool configuration parameters to adapt it to the best setup in your system.
10. Redis Cluster
This Redis implementation provides easy scalability and high availability, we encourage you to read their official specification if you are not familiar with it. We will not cover Redis cluster setup since that is a bit out of the scope for this article, but you should have no problems in doing so when you are done with its documentation.
Once we have that ready, we can start using it from our application:
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We only need to provide the host and port details from one of our master instances, it will auto-discover the rest of the instances in the cluster.
This is certainly a very powerful feature but it is not a silver bullet. When using Redis Cluster you cannot perform transactions nor use pipelines, two important features on which many applications rely for ensuring data integrity.
Transactions are disabled because, in a clustered environment, keys will be persisted across multiple instances. Operation atomicity and thread safety cannot be guaranteed for operations that involve command execution in different instances.
Some advanced key creation strategies will ensure that data that is interesting for you to be persisted in the same instance will get persisted that way. In theory, that should enable you to perform transactions successfully using one of the underlying Jedis instances of the Redis Cluster.
Unfortunately, currently you cannot find out in which Redis instance a particular key is saved using Jedis (which is actually supported natively by Redis), so you do not know which of the instances you must perform the transaction operation. If you are interested about this, you can find more information here.
11. Conclusion
The vast majority of the features from Redis are already available in Jedis and its development moves forward at a good pace.
It gives you the ability to integrate a powerful in-memory storage engine in your application with very little hassle, just do not forget to set up connection pooling to avoid thread safety issues.
You can find code samples in the GitHub project.