persistence.xml 中 jpa 的 C3p0 连接池不起作用？

Question

在尝试按照我能找到的所有示例配置 c3p0 后，我仍然遇到 JDBC 超时问题。我正在使用 JPA、Hibernate、Tomcat、MySQL [AWS RDS]。这是 persistence.xml 的片段：

<property name="javax.persistence.jdbc.url"
value="jdbc:mysql://url..." />
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
<property name="connection.provider_class"  value="org.hibernate.connection.C3P0ConnectionProvider"/>
<property name="hibernate.c3p0.acquire_increment" value="4" />
<property name="hibernate.c3p0.idle_test_period" value="3000" /> 
<property name="hibernate.c3p0.max_size" value="100" />
<property name="hibernate.c3p0.max_statements" value="15" />
<property name="hibernate.c3p0.min_size" value="5" />
<property name="hibernate.c3p0.timeout" value="100" /> 

以及错误日志：

09:18:51.776  WARN   org.hibernate.util.JDBCExceptionReporter  - SQL Error: 0, SQLState: 08S01
09:18:51.777  ERROR  org.hibernate.util.JDBCExceptionReporter  - The last packet successfully received from the server was 38,491,585 milliseconds ago.  ...

Answer 1

我从这个博客中找到了答案：
http://blog.hpxn.net/2009/02/05/using -c3p0-and-hibernate-3/

persistence.xml 中的属性是错误的，你应该以 hibernate 开头拼写它们：

<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_1_0.xsd"
  version="1.0">
    <persistence-unit name="mypersistenceunitname">
      <properties>
        <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver" />
        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
        <property name="hibernate.hbm2ddl.auto" value="create-drop" />

        <!-- Important -->
        <property name="hibernate.connection.provider_class"
          value="org.hibernate.connection.C3P0ConnectionProvider" />

        <property name="hibernate.c3p0.max_size" value="100" />
        <property name="hibernate.c3p0.min_size" value="0" />
        <property name="hibernate.c3p0.acquire_increment" value="1" />
        <property name="hibernate.c3p0.idle_test_period" value="300" />
        <property name="hibernate.c3p0.max_statements" value="0" />
        <property name="hibernate.c3p0.timeout" value="100" />
      </properties>
    </persistence-unit>
</persistence>

然后，你会看到这个神话般的日志出现：

2011-08-15 08:58:33 com.mchange.v2.c3p0.C3P0Registry banner
INFO: Initializing c3p0-0.9.1 [built 16-January-2007 14:46:42; debug? true; trace: 10]
2011-08-15 08:58:33 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
INFO: Initializing c3p0 pool... com.mchange.v2.c3p0.PoolBackedDataSource@f3a94e12 [     connectionPoolDataSource -> com.mchange.v2.c3p0.WrapperConnectionPoolDataSource@6297f706     ....

问候，
Stéphane（抱歉赏金）

Answer 2

配置看起来没问题。激活日志记录并确保 C3P0 以适当的设置启动。

2010-10-16 11:58:31,271 INFO [main] o.h.c.ConnectionProviderFactory [ConnectionProviderFactory.java:173] Initializing connection provider: org.hibernate.connection.C3P0ConnectionProvider
...