Hibernate Collection Cache如何工作

介绍

之前，我描述了Hibernate用于存储实体的二级缓存条目结构。 除了实体，Hibernate还可以存储实体关联，本文将阐明集合缓存的内部工作原理。

领域模型

对于即将进行的测试，我们将使用以下实体模型：

存储库具有一组Commit实体：

@org.hibernate.annotations.Cache(
    usage = CacheConcurrencyStrategy.READ_WRITE
)
@OneToMany(mappedBy = "repository", 
    cascade = CascadeType.ALL, orphanRemoval = true)
private List<Commit> commits = new ArrayList<>();

每个Commit实体都有一组Change可嵌入元素。

@ElementCollection
@CollectionTable(
    name="commit_change",
    joinColumns = @JoinColumn(name="commit_id")
)
@org.hibernate.annotations.Cache(
    usage = CacheConcurrencyStrategy.READ_WRITE
)
@OrderColumn(name = "index_id")
private List<Change> changes = new ArrayList<>();

现在，我们将插入一些测试数据：

doInTransaction(session -> {
    Repository repository = 
        new Repository("Hibernate-Master-Class");
    session.persist(repository);

    Commit commit1 = new Commit();
    commit1.getChanges().add(
        new Change("README.txt", "0a1,5...")
    );
    commit1.getChanges().add(
        new Change("web.xml", "17c17...")
    );

    Commit commit2 = new Commit();
    commit2.getChanges().add(
        new Change("README.txt", "0b2,5...")
    );

    repository.addCommit(commit1);
    repository.addCommit(commit2);
    session.persist(commit1);
});

直读缓存

集合缓存采用了一种通读同步策略 ：

doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    for (Commit commit : repository.getCommits()) {
        assertFalse(commit.getChanges().isEmpty());
    }
});

并且首次访问集合时将对其进行缓存：

select
    collection0_.id as id1_0_0_,
    collection0_.name as name2_0_0_ 
from
    Repository collection0_ 
where
    collection0_.id=1  
    
select
    commits0_.repository_id as reposito3_0_0_,
    commits0_.id as id1_1_0_,
    commits0_.id as id1_1_1_,
    commits0_.repository_id as reposito3_1_1_,
    commits0_.review as review2_1_1_ 
from
    commit commits0_ 
where
    commits0_.r  
        
select
    changes0_.commit_id as commit_i1_1_0_,
    changes0_.diff as diff2_2_0_,
    changes0_.path as path3_2_0_,
    changes0_.index_id as index_id4_0_ 
from
    commit_change changes0_ 
where
    changes0_.commit_id=1  
            
select
    changes0_.commit_id as commit_i1_1_0_,
    changes0_.diff as diff2_2_0_,
    changes0_.path as path3_2_0_,
    changes0_.index_id as index_id4_0_ 
from
    commit_change changes0_ 
where
    changes0_.commit_id=2

缓存存储库及其关联的提交之后，由于所有实体及其关联都由第二级缓存提供服务，因此加载存储库并遍历“ 提交和更改”集合将不会访问数据库：

LOGGER.info("Load collections from cache");
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    assertEquals(2, repository.getCommits().size());
});

运行先前的测试用例时，没有执行SQL SELECT语句：

CollectionCacheTest - Load collections from cache
JdbcTransaction - committed JDBC Connection

集合缓存条目结构

对于实体集合，Hibernate仅存储实体标识符，因此也需要缓存实体：

key = {[email protected]}
    key = {[email protected]} "1"
    type = {[email protected]} 
    entityOrRoleName = {[email protected]} "com.vladmihalcea.hibernate.masterclass.laboratory.cache.CollectionCacheTest$Repository.commits"
    tenantId = null
    hashCode = 31
value = {org.hibernate.cache.ehcach[email protected]3982} 
    value = {[email protected]} "CollectionCacheEntry[1,2]"
    version = null
    timestamp = 5858841154416640

CollectionCacheEntry存储与给定存储库实体关联的提交标识符。

由于元素类型没有标识符，因此Hibernate会存储其脱水状态。 更改可嵌入的内容缓存如下：

key = {[email protected]} "com.vladmihalcea.hibernate.masterclass.laboratory.cache.CollectionCacheTest$Commit.changes#1"
    key = {[email protected]} "1"
    type = {[email protected]} 
    entityOrRoleName = {[email protected]} "com.vladmihalcea.hibernate.masterclass.laboratory.cache.CollectionCacheTest$Commit.changes"
    tenantId = null
    hashCode = 31
value = {org.hibernate.cache.ehcach[email protected]3971} 
    value = {[email protected]}
        state = {java.io.Serializable[2]@3980} 
            0 = {java.lang.Object[2]@3981} 
                0 = {[email protected]} "0a1,5..."
                1 = {[email protected]} "README.txt"
            1 = {java.lang.Object[2]@3982} 
                0 = {[email protected]} "17c17..."
                1 = {[email protected]} "web.xml"
    version = null
    timestamp = 5858843026345984

集合缓存一致性模型

在使用缓存时 ，一致性是最大的问题 ，因此我们需要了解Hibernate Collection Cache如何处理实体状态更改。

CollectionUpdateAction负责所有Collection的修改，并且只要集合发生更改，就会将关联的缓存条目逐出：

protected final void evict() throws CacheException {
    if ( persister.hasCache() ) {
        final CacheKey ck = session.generateCacheKey(
            key, 
            persister.getKeyType(), 
            persister.getRole()
        );
        persister.getCacheAccessStrategy().remove( ck );
    }
}

CollectionRegionAccessStrategy规范也记录了此行为：

对于缓存的收集数据，所有修改操作实际上只会使条目无效。

根据当前的并发策略，收回集合缓存条目：

• 在提交当前事务之前 ，用于CacheConcurrencyStrategy.NONSTRICT_READ_WRITE
• 立即提交当前事务后 ，用于CacheConcurrencyStrategy.READ_WRITE
• 对于CacheConcurrencyStrategy.TRANSACTIONAL ， 确切地在何时提交当前事务

添加新的收藏夹条目

以下测试案例向我们的存储库添加了一个新的Commit实体：

LOGGER.info("Adding invalidates Collection Cache");
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    assertEquals(2, repository.getCommits().size());

    Commit commit = new Commit();
    commit.getChanges().add(
        new Change("Main.java", "0b3,17...")
    );
    repository.addCommit(commit);
});
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    assertEquals(3, repository.getCommits().size());
});

运行此测试将生成以下输出：

--Adding invalidates Collection Cache

insert 
into
   commit
   (id, repository_id, review) 
values
   (default, 1, false)
   
insert 
into
   commit_change
   (commit_id, index_id, diff, path) 
values
   (3, 0, '0b3,17...', 'Main.java')
 
--committed JDBC Connection

select
   commits0_.repository_id as reposito3_0_0_,
   commits0_.id as id1_1_0_,
   commits0_.id as id11_1_1_,
   commits0_.repository_id as reposito3_1_1_,
   commits0_.review as review2_1_1_ 
from
   commit commits0_ 
where
   commits0_.repository_id=1
   
--committed JDBC Connection

保留新的Commit实体后，将清除Repository.commits集合缓存，并从数据库中获取关联的Commits实体（下次访问该集合）。

删除现有的集合条目

删除Collection元素遵循相同的模式：

LOGGER.info("Removing invalidates Collection Cache");
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    assertEquals(2, repository.getCommits().size());
    Commit removable = repository.getCommits().get(0);
    repository.removeCommit(removable);
});
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    assertEquals(1, repository.getCommits().size());
});

生成以下输出：

--Removing invalidates Collection Cache

delete 
from
   commit_change 
where
   commit_id=1
   
delete 
from
   commit 
where
   id=1
   
--committed JDBC Connection

select
   commits0_.repository_id as reposito3_0_0_,
   commits0_.id as id1_1_0_,
   commits0_.id as id1_1_1_,
   commits0_.repository_id as reposito3_1_1_,
   commits0_.review as review2_1_1_ 
from
   commit commits0_ 
where
   commits0_.repository_id=1
   
--committed JDBC Connection

一旦更改其结构，便会收回集合缓存。

直接删除集合元素

只要Hibernate知道目标缓存集合要进行的所有更改，它就可以确保缓存的一致性。 Hibernate使用其自己的Collection类型（例如PersistentBag ， PersistentSet ）来允许延迟加载或检测脏状态 。

如果删除内部Collection元素而不更新Collection状态，则Hibernate将无法使当前缓存的Collection条目无效：

LOGGER.info("Removing Child causes inconsistencies");
doInTransaction(session -> {
    Commit commit = (Commit) 
        session.get(Commit.class, 1L);
    session.delete(commit);
});
try {
    doInTransaction(session -> {
        Repository repository = (Repository) 
            session.get(Repository.class, 1L);
        assertEquals(1, repository.getCommits().size());
    });
} catch (ObjectNotFoundException e) {
    LOGGER.warn("Object not found", e);
}

--Removing Child causes inconsistencies

delete 
from
   commit_change 
where
   commit_id=1
   
delete 
from
   commit 
where
   id=1

-committed JDBC Connection

select
   collection0_.id as id1_1_0_,
   collection0_.repository_id as reposito3_1_0_,
   collection0_.review as review2_1_0_ 
from
   commit collection0_ 
where
   collection0_.id=1

--No row with the given identifier exists: 
-- [CollectionCacheTest$Commit#1]

--rolled JDBC Connection

当Commit实体被删除时，Hibernate不知道它必须更新所有关联的Collection Cache。 下次加载Commit集合时，Hibernate将意识到某些实体不再存在，并且将引发异常。

使用HQL更新Collection元素

通过HQL执行批量更新时，Hibernate可以保持缓存一致性：

LOGGER.info("Updating Child entities using HQL");
doInTransaction(session -> {
    Repository repository = (Repository)
         session.get(Repository.class, 1L);
    for (Commit commit : repository.getCommits()) {
        assertFalse(commit.review);
    }
});
doInTransaction(session -> {
    session.createQuery(
        "update Commit c " +
        "set c.review = true ")
    .executeUpdate();
});
doInTransaction(session -> {
    Repository repository = (Repository)
        session.get(Repository.class, 1L);
    for(Commit commit : repository.getCommits()) {
        assertTrue(commit.review);
    }
});

运行此测试用例将生成以下SQL：

--Updating Child entities using HQL

--committed JDBC Connection

update
   commit 
set
   review=true
   
--committed JDBC Connection

select
   commits0_.repository_id as reposito3_0_0_,
   commits0_.id as id1_1_0_,
   commits0_.id as id1_1_1_,
   commits0_.repository_id as reposito3_1_1_,
   commits0_.review as review2_1_1_ 
from
   commit commits0_ 
where
   commits0_.repository_id=1
   
--committed JDBC Connection

第一个事务不需要命中数据库，仅依赖于第二级缓存。 HQL UPDATE清除了集合缓存，因此，在随后访问集合时，Hibernate将不得不从数据库中重新加载它。

使用SQL更新Collection元素

Hibernate还可以使批量SQL UPDATE语句的缓存条目无效：

LOGGER.info("Updating Child entities using SQL");
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    for (Commit commit : repository.getCommits()) {
        assertFalse(commit.review);
    }
});
doInTransaction(session -> {
    session.createSQLQuery(
        "update Commit c " +
        "set c.review = true ")
    .addSynchronizedEntityClass(Commit.class)
    .executeUpdate();
});
doInTransaction(session -> {
    Repository repository = (Repository) 
        session.get(Repository.class, 1L);
    for(Commit commit : repository.getCommits()) {
        assertTrue(commit.review);
    }
});

生成以下输出：

--Updating Child entities using SQL

--committed JDBC Connection

update
   commit 
set
   review=true
   
--committed JDBC Connection
   
select
   commits0_.repository_id as reposito3_0_0_,
   commits0_.id as id1_1_0_,
   commits0_.id as id1_1_1_,
   commits0_.repository_id as reposito3_1_1_,
   commits0_.review as review2_1_1_ 
from
   commit commits0_ 
where
   commits0_.repository_id=1  
   
--committed JDBC Connection

BulkOperationCleanupAction负责清理大容量DML语句上的二级缓存。 尽管Hibernate在执行HQL语句时可以检测到受影响的缓存区域，但是对于本机查询，您需要指示Hibernate该语句应使哪些区域无效。 如果您未指定任何此类区域，则Hibernate将清除所有第二级缓存区域。

结论

集合缓存是一项非常有用的功能，是对第二级实体缓存的补充。 这样，我们可以存储整个实体图，从而减少了只读应用程序中的数据库查询工作量。 像使用AUTO刷新一样 ，Hibernate在执行本机查询时无法自省受影响的表空间。 为了避免一致性问题（使用AUTO刷新时）或缓存未命中（二级缓存），每当我们需要运行本机查询时，我们都必须显式声明目标表，因此Hibernate可以采取适当的措施（例如刷新或使缓存无效）地区）。

• 代码可在GitHub上获得 。

翻译自: https://www.javacodegeeks.com/2015/05/how-does-hibernate-collection-cache-work.html