Hibernate sending superfluous queries to the database

问题

I have an odd problem where Hibernate is running more queries than I've asked for, and can't see the need for.

Here is my controller:

@Autowired UserService users;

@RequestMapping("/test")
@ResponseBody
public String test() {
    User user = users.findUser(1L);
    return "Found user: "+user.getEmail();
}

Here is the UserService:

@Component
public class UserService {

    @javax.persistence.PersistenceUnit private EntityManagerFactory emf;

    private JpaTemplate getJpaTemplate() {
        return new JpaTemplate(emf);
    }

    public User findUser(long id) {
        long start = System.currentTimeMillis();
        JpaTemplate jpaTemplate = getJpaTemplate();
        User user = jpaTemplate.find(User.class, id);
        System.out.println(System.currentTimeMillis() - start);
        return user;
    }
}

The call to findUser() takes about 140ms... Rather baffling. The database performs just fine for other queries, including this one in some handlers (I suspect when it is not the first query run).

JProfiler suggests that every time it is called, four queries are sent to the database (not necessarily in this order):

1) [5ms] select user... (the actual query)

2) [7ms] SHOW COLLATION

3) [14ms] /* mysql-connector-java-5.1.7 ( Revision: ${svn.Revision} ) */SELECT @@session.auto_increment_increment

4) [70ms] /* mysql-connector-java-5.1.7 ( Revision: ${svn.Revision} ) */SHOW VARIABLES WHERE Variable_name ='language' OR Variable_name = 'net_write_timeout' OR Variable_name = 'interactive_timeout' OR Variable_name = 'wait_timeout' OR Variable_name = 'character_set_client' OR Variable_name = 'character_set_connection' OR Variable_name = 'character_set' OR Variable_name = 'character_set_server' OR Variable_name = 'tx_isolation' OR Variable_name = 'transaction_isolation' OR Variable_name = 'character_set_results' OR Variable_name = 'timezone' OR Variable_name = 'time_zone' OR Variable_name = 'system_time_zone' OR Variable_name = 'lower_case_table_names' OR Variable_name = 'max_allowed_packet' OR Variable_name = 'net_buffer_length' OR Variable_name = 'sql_mode' OR Variable_name = 'query_cache_type' OR Variable_name = 'query_cache_size' OR Variable_name = 'init_connect'

Its clear that the actual query takes no time at all, and most of the time is spend in that fourth one. What can I do about this? It doesn't show up in the hibernate log output, only the first, actual query does. Incidentally, all the time is spent after the call to getJpaTemplate() - i.e actually in the jpa.find() method.

Any ideas?

UPDATE: I've worked out that it is hibernate doing some sort of initial connection to the database multiple times, since somebody else posted the same set of queries (http://ondra.zizka.cz/stranky/programovani/java/hibernate_netbeans_howto_tutorial.texy). Why would hibernate make its initial connection repeatedly, isn't it using a connection pool - how can I check?

回答1:

I've solved it. I had a non-pooled datasource:

<bean name="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">

From the Javadoc at: http://static.springsource.org/spring/docs/2.0.x/api/org/springframework/jdbc/datasource/DriverManagerDataSource.html

Simple implementation of the standard JDBC DataSource interface, configuring the plain old JDBC DriverManager via bean properties, and returning a new Connection from every getConnection call. NOTE: This class is not an actual connection pool; it does not actually pool Connections. It just serves as simple replacement for a full-blown connection pool, implementing the same standard interface, but creating new Connections on every call.

So I've now replaced this with:

<bean name="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
    <property name="driverClass" value="com.mysql.jdbc.Driver"/> 
    <property name="jdbcUrl" value="jdbc:mysql://server.domain/database"/> 
    <property name="user" value="theUsername"/> 
    <property name="password" value="thePassword"/> 
</bean>

I had to throw in c3p0-0.9.1.2.jar too, since it uses that connection pool.

来源：https://stackoverflow.com/questions/2588435/hibernate-sending-superfluous-queries-to-the-database

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java

mysql

hibernate

Spring

JProfiler