我遇到了一个问题，我感到非常迷茫，不知道该怎么办。我使用查询缓存+二级缓存，我想正确缓存结果10秒。这是我的
ehcache.xml:
<ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../config/ehcache.xsd">
<diskStore path="java.io.tmpdir"/>
<cache name = "TestEntity"
maxElementsInMemory="100"
eternal="false"
timeToLiveSeconds="11"
memoryStoreEvictionPolicy="LRU">
</cache>
<cache name="org.hibernate.cache.internal.StandardQueryCache"
maxElementsInMemory="100"
eternal="false"
timeToLiveSeconds="10"
memoryStoreEvictionPolicy="LRU">
</cache>
<defaultCache
maxElementsInMemory="10000"
eternal="false"
timeToLiveSeconds="120"
maxElementsOnDisk="100"
diskExpiryThreadIntervalSeconds="120"
memoryStoreEvictionPolicy="LRU">
<persistence strategy="localTempSwap"/>
</defaultCache>
</ehcache>
首先，我使用 insert() 方法填充数据库。 然后，我第一次调用我的 select() 方法来获取数据。一切工作正常 - 查询和实体被缓存，如果我在 2 秒后调用 select() 方法，我将在不向数据库发出任何请求的情况下获取数据。 然后我等待 12 秒(为了让缓存完全过期)，调用 select() 并在再次调用 select() 后等待 2 秒。这就是我得到 n+1 选择的地方:
2019-02-13 18:52:17,101 [DEBUG] org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:92) select testentity0_.id as id1_0_0_, testentity0_.value as value2_0_0_ from test testentity0_ where testentity0_.id=? 2019-02-13 18:52:17,107 [DEBUG] org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:92) select testentity0_.id as id1_0_0_, testentity0_.value as value2_0_0_ from test testentity0_ where testentity0_.id=? 2019-02-13 18:52:17,108 [DEBUG] org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:92) select testentity0_.id as id1_0_0_, testentity0_.value as value2_0_0_ from test testentity0_ where testentity0_.id=? 2019-02-13 18:52:17,108 [DEBUG] org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:92) select testentity0_.id as id1_0_0_, testentity0_.value as value2_0_0_ from test testentity0_ where testentity0_.id=? 2019-02-13 18:52:17,109 [DEBUG] org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:92) select testentity0_.id as id1_0_0_, testentity0_.value as value2_0_0_ from test testentity0_ where testentity0_.id=?
我知道发出这些请求是因为查询缓存仅缓存 id，并且二级缓存中似乎缺少这些 id 的实体。但他们为什么失踪了？当我启用完整日志记录时，我看到在第三次调用 select() 后，出现了类似
的日志条目Adding entity to second-level cache: [TestEntity#1]
因此，如果实体被添加到二级缓存并且它们应该仅在 11 秒内过期，为什么它们在 2 秒后就丢失了？
我的pom.xml的一部分:
<dependencies>
<dependency>
<groupId>com.h2database</groupId>
<artifactId>h2</artifactId>
<version>1.4.194</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>5.2.7.Final</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-ehcache</artifactId>
<version>5.2.7.Final</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-c3p0</artifactId>
<version>5.2.7.Final</version>
</dependency>
</dependencies>
persistence.xml:
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="main">
<class>TestEntity</class>
<exclude-unlisted-classes>true</exclude-unlisted-classes>
<properties>
<property name="hibernate.hbm2ddl.auto" value="create-drop"/>
<property name="hibernate.use_sql_comments" value="true"/>
<property name="hibernate.cache.use_second_level_cache" value="true"/>
<property name="hibernate.cache.region.factory_class" value="org.hibernate.cache.ehcache.EhCacheRegionFactory"/>
<property name="hibernate.cache.use_query_cache" value="true"/>
<property name="net.sf.ehcache.configurationResourceName" value="ehcache.xml"/>
</properties>
</persistence-unit>
</persistence>
TestEntity.java:
import org.hibernate.annotations.CacheConcurrencyStrategy;
import javax.persistence.*;
/**
* User: Kirill Smirnov (<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="b5de9bc6d8dcc7dbdac3f5c6dcc7d0dbd4878585859bc7c0" rel="noreferrer noopener nofollow">[email protected]</a>)
* Date: 18.12.18
* Time: 19:20
*/
@Entity
@Table(name = "test")
@Cacheable
@org.hibernate.annotations.Cache(usage = CacheConcurrencyStrategy.READ_ONLY)
public class TestEntity {
@Id
@GeneratedValue(generator = "test_seq")
@SequenceGenerator(name = "test_seq", sequenceName="TEST_SEQ")
@Column(name = "id")
private int id;
@Column(name = "value", nullable = false)
private String value;
public TestEntity() {
}
public TestEntity(String value) {
this.value = value;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getValue() {
return value;
}
public void setValue(String value) {
this.value = value;
}
}
Main.java:
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import java.util.Properties;
/**
* User: Kirill Smirnov (<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="2e45005d43475c4041586e5d475c4b404f1c1e1e1e005c5b" rel="noreferrer noopener nofollow">[email protected]</a>)
* Date: 14.11.14
* Time: 15:55
*/
public class Main {
public static void main(String[] args) throws Exception {
Properties entityManagerFactoryProperties = new Properties();
entityManagerFactoryProperties.setProperty("javax.persistence.jdbc.driver", "org.h2.Driver");
entityManagerFactoryProperties.setProperty("javax.persistence.jdbc.url", "jdbc:h2:mem:");
entityManagerFactoryProperties.setProperty("javax.persistence.jdbc.user", "sa");
entityManagerFactoryProperties.setProperty("javax.persistence.jdbc.password", "");
entityManagerFactoryProperties.setProperty("hibernate.c3p0.min_size", "" + 1);
entityManagerFactoryProperties.setProperty("hibernate.c3p0.max_size", "" + 1);
entityManagerFactoryProperties.setProperty("hibernate.c3p0.timeout", "" + 5000);
EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("main", entityManagerFactoryProperties);
insert(entityManagerFactory);
select(entityManagerFactory);
Thread.sleep(2000);
select(entityManagerFactory);
Thread.sleep(12000);
select(entityManagerFactory);
Thread.sleep(2000);
select(entityManagerFactory);
entityManagerFactory.close();
}
private static void insert(EntityManagerFactory entityManagerFactory) {
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();
try {
entityManager.persist(new TestEntity("1"));
entityManager.persist(new TestEntity("2"));
entityManager.persist(new TestEntity("3"));
entityManager.persist(new TestEntity("4"));
entityManager.persist(new TestEntity("5"));
entityManager.getTransaction().commit();
} catch (Exception e) {
entityManager.getTransaction().rollback();
throw e;
} finally {
entityManager.close();
}
}
private static void select(EntityManagerFactory entityManagerFactory) {
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();
try {
String queryText = "FROM TestEntity";
TypedQuery<TestEntity> query = entityManager.createQuery(queryText, TestEntity.class).setHint("org.hibernate.cacheable", true);
query.getResultList();
entityManager.getTransaction().commit();
} catch (Exception e) {
entityManager.getTransaction().rollback();
throw e;
} finally {
entityManager.close();
}
}
}
附注我猜问题的原因是 Hibernate 中的一个错误。如果我从 5.2 升级到 5.4，问题就会消失。不过，我接受弗拉德的回答，因为它总体上包含有用的信息。
最佳答案
这就是臭名昭著的N+1 Query Cache issue .
您必须确保实体缓存区域的 TTL(生存时间)高于查询缓存或集合缓存的 TTL。
否则，Hibernate 将在查询缓存或集合缓存中查找实体标识符，并假定实体已存储在实体缓存区域中。但如果实体缓存中没有找到实体，则只能从数据库中获取，从而触发 N+1 查询问题。
现在，返回您的设置。这是您为实体缓存区域设置的内容:
<cache name = "TestEntity"
maxElementsInMemory="100"
eternal="false"
timeToLiveSeconds="10"
memoryStoreEvictionPolicy="LRU">
</cache>
请注意，timeToLiveSeconds 仅设置为 10 秒。
QueryCache 设置如下:
<cache name="org.hibernate.cache.internal.StandardQueryCache"
maxElementsInMemory="100"
eternal="false"
timeToLiveSeconds="10"
memoryStoreEvictionPolicy="LRU">
</cache>
因此，timeToLiveSeconds 也设置为 10 秒，并确保实体查询缓存的过期时间不会早于查询缓存和关联的集合缓存。
接下来，将 TestEntity 的 timeToLiveSeconds 提高到 60 或 120 秒。或者将其设置为eternal = true 并禁用TTL，因为该实体正在使用 CacheConcurrencyStartegy.READ_ONLY .
<cache name = "TestEntity"
maxElementsInMemory="100"
eternal="false"
timeToLiveSeconds="60"
memoryStoreEvictionPolicy="LRU">
</cache>
关于java - 使用查询缓存时，Hibernate 执行 N+1 个选择而不是 1 个查询，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/54674418/