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This article focuses on "the principle of Hibernate caching mechanism", interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn the principle of Hibernate caching mechanism.

1. Why use Hibernate caching?

Hibernate is a persistence layer framework that frequently accesses physical databases.

In order to reduce the frequency of application access to physical data sources, so as to improve the performance of the application.

The data in the cache is the replication of the data in the physical data source. The application reads and writes data from the cache at run time, and synchronizes the data of the cache and the physical data source at a specific time or event.

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two。 Project actual combat

When the Session object calls the save () method to save an object, the object is placed in the Session cache.

When the Session object calls the get () or load () method to fetch an object from the database, the object is also put into the Session cache.

When you use the same Session to write HQL and QBC to query data from the database, the data will be read directly from the cache and the database will not be accessed.

Hibernate provides several methods (evit/clear/contains/flush....) To manage and judge the first-level cache.

Now in the JavaEE Dao layer, providing external database access, each time a new Session thread is obtained from the Session factory, resulting in the first-level cache being rarely utilized.

Source code of instance project: https://git.oschina.net/LanboEx/hiberdemo

/ / 1.Hibernate 's first-level cache. You can see that only one sql Session session = getSession (); UserPO userPO = session.get (UserPO.class, "031e7a36972e11e6acede16e8241c0fe"); System.out.println ("1. Visit DB for the first time: "+ userPO.getName () +", "+ userPO.getPasswd ()); UserPO userPO1 = session.get (UserPO.class," 031e7a36972e11e6acede16e8241c0fe "); System.out.println (" 2. Visit DB for the second time: "+ userPO1.getName () +", whether there is a specific object in the first-level cache "+ session.contains (userPO))

/ / 2. Using the evite/clear method to manually clear specific objects in the cache, you can see that hiber outputs two SQL Session session1 = getSession (); UserPO userPO3 = session1.get (UserPO.class, "031e7a36972e11e6acede16e8241c0fe"); and System.out.println ("3. Get object for the first time: "+ userPO3.getName () +", "+ userPO3.getPasswd ()); session1.evict (userPO3); UserPO userPO4 = session1.get (UserPO.class," 031e7a36972e11e6acede16e8241c0fe "); System.out.println (" 4. Get object for the second time: "+ userPO4.getName () +", "+ userPO4.getPasswd ())

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3. Hibernate caching principle

There are two main categories of Hibernate caching:

A.Hibernate first-level cache, also known as [Session cache].

Session built-in cannot be uninstalled, and Session's cache is a transaction-wide cache (the life cycle of Session objects usually corresponds to a database transaction or an application transaction).

In the first-level cache, each instance of the persistent class has a unique OID.

B.Hibernate secondary cache, also known as [SessionFactory cache].

Because the life cycle of the SessionFactory object corresponds to the entire process of the application.

Hibernate secondary cache is a process-wide or cluster-wide cache, which may have concurrency problems, so it is necessary to adopt an appropriate concurrent access strategy, which provides a transaction isolation level for the cached data.

The second-level cache is optional and is a configurable plug-in, which is not enabled by SessionFactory by default.

Hibernate provides the org.hibernate.cache.CacheProvider interface, which acts as an adapter between the cache plug-in and the Hibernate.

What kind of data is suitable for storage in the second-level cache?

1) data that is rarely modified

2) data that is not very important, allowing occasional concurrent data

3) data that will not be accessed concurrently

4) constant data

Data that is not suitable for storage in the second level cache?

1) data that is often modified

2) data accessed concurrently, such as financial data, is absolutely not allowed.

3) data shared with other applications.

The deferred loading implementation of c.Session solves two problems: closing the connection normally and ensuring that the same Session is accessed in the request.

Hibernate Session is a layer of advanced encapsulation of java.sql.Connection, and one Session corresponds to one Connection.

Properly shut down Session after the end of the Http request (the filter enables the normal shutdown of Session)

Deferred loading must be guaranteed to be the same Session (Session is bound to ThreadLocal).

How does the cache apply to d.Hibernate lookup objects?

When Hibernate accesses the data object according to ID, first look it up in the Session first-level cache

Cannot be found. If the second-level cache is configured, then check from the second-level cache.

If none can be found, query the database and put the results into the cache according to ID when deleting, updating, and adding data, and update the cache at the same time.

e. Comparison between first-level cache and second-level cache

First-level cache

Second-level cache

The form in which data is stored

Interrelated persistent objects

Bulk data of object

The scope of the cache

Transaction scope, each transaction has a separate first-level cache

Process-wide or cluster-wide, where the cache is shared by all transactions within the same process or cluster

Concurrent access strategy

Since each transaction has a separate first-level cache, there is no concurrency problem, so there is no need to provide a concurrent access strategy

Since multiple transactions access the same data in the secondary cache at the same time, an appropriate concurrent access strategy must be provided to ensure a specific transaction isolation level.

Data expiration policy

Objects in the first-level cache never expire unless the application displays emptying or emptying specific objects

Data expiration policies must be provided, such as the maximum number of objects in the memory-based cache, the maximum time that objects are allowed to be in the cache, and the maximum idle time that objects are allowed to stay in the cache

Physical medium

Memory

The bulk data of objects in memory and hard disk are first stored in the memory-based cache. When the number of objects in memory reaches the maxElementsInMemory value of the data expiration policy, the rest of the objects are written to the hard disk-based cache.

Cache software implementation

Included in the Session implementation of Hibernate

Provided by third parties, Hibernate provides only cache adapters to integrate specific cache plug-ins into Hibernate

How caching is enabled

As long as saving, updating, deleting, loading and querying are performed through the Session interface, Hibernate will enable first-level caching. For batch operations, if you do not want to enable first-level caching, execute directly through JDBCAPI.

Users can configure the second-level cache at the granularity of a single class or a single collection of classes. If an instance of a class is read frequently but rarely modified, you can consider using a secondary cache. Only if a class or collection is configured with a secondary cache, Hibernate will add its instance to the secondary cache at run time.

How the user manages the cache

The physical medium of the first-level cache is memory. Due to the limited capacity of memory, the number of loaded objects must be limited by appropriate retrieval strategies and retrieval methods. The evit () method of Session can display to clear specific objects in the cache, but it is not recommended.

The physical media of the second-level cache can make the memory and the hard disk, so the second-level cache can store a large amount of data, and the maxElementsInMemory attribute of the data expiration policy can control the number of objects in memory. Managing the second-level cache mainly includes two aspects: selecting the persistence class that needs to use the second-level cache and setting the appropriate concurrent access strategy; selecting the cache adapter and setting the appropriate data expiration policy. The evit () method of SessionFactory can also be displayed to empty specific objects in the cache, but it is not recommended

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