c3p0的配置解释

找到了一个关于c3p0的配置 
<c3p0-config> 
<default-config> 
<!--当连接池中的连接耗尽的时候c3p0一次同时获取的连接数。Default: 3 --> 
<property name="acquireIncrement">3</property> 

<!--定义在从数据库获取新连接失败后重复尝试的次数。Default: 30 --> 
<property name="acquireRetryAttempts">30</property> 

<!--两次连接中间隔时间，单位毫秒。Default: 1000 --> 
<property name="acquireRetryDelay">1000</property> 

<!--连接关闭时默认将所有未提交的操作回滚。Default: false --> 
<property name="autoCommitOnClose">false</property> 

<!--c3p0将建一张名为Test的空表，并使用其自带的查询语句进行测试。如果定义了这个参数那么 
属性preferredTestQuery将被忽略。你不能在这张Test表上进行任何操作，它将只供c3p0测试 
使用。Default: null--> 
<property name="automaticTestTable">Test</property> 

<!--获取连接失败将会引起所有等待连接池来获取连接的线程抛出异常。但是数据源仍有效 
保留，并在下次调用getConnection()的时候继续尝试获取连接。如果设为true，那么在尝试 
获取连接失败后该数据源将申明已断开并永久关闭。Default: false--> 
<property name="breakAfterAcquireFailure">false</property> 

<!--当连接池用完时客户端调用getConnection()后等待获取新连接的时间，超时后将抛出 
SQLException,如设为0则无限期等待。单位毫秒。Default: 0 --> 
<property name="checkoutTimeout">100</property> 

<!--通过实现ConnectionTester或QueryConnectionTester的类来测试连接。类名需制定全路径。 
Default: com.mchange.v2.c3p0.impl.DefaultConnectionTester--> 
<property name="connectionTesterClassName"></property> 

<!--指定c3p0 libraries的路径，如果（通常都是这样）在本地即可获得那么无需设置，默认null即可 
Default: null--> 
<property name="factoryClassLocation">null</property> 

<!--Strongly disrecommended. Setting this to true may lead to subtle and bizarre bugs. 
（文档原文）作者强烈建议不使用的一个属性--> 
<property name="forceIgnoreUnresolvedTransactions">false</property> 

<!--每60秒检查所有连接池中的空闲连接。Default: 0 --> 
<property name="idleConnectionTestPeriod">60</property> 

<!--初始化时获取三个连接，取值应在minPoolSize与maxPoolSize之间。Default: 3 --> 
<property name="initialPoolSize">3</property> 

<!--最大空闲时间,60秒内未使用则连接被丢弃。若为0则永不丢弃。Default: 0 --> 
<property name="maxIdleTime">60</property> 

<!--连接池中保留的最大连接数。Default: 15 --> 
<property name="maxPoolSize">15</property> 

<!--JDBC的标准参数，用以控制数据源内加载的PreparedStatements数量。但由于预缓存的statements 
属于单个connection而不是整个连接池。所以设置这个参数需要考虑到多方面的因素。 
如果maxStatements与maxStatementsPerConnection均为0，则缓存被关闭。Default: 0--> 
<property name="maxStatements">100</property> 

<!--maxStatementsPerConnection定义了连接池内单个连接所拥有的最大缓存statements数。Default: 0 --> 
<property name="maxStatementsPerConnection"></property> 

<!--c3p0是异步操作的，缓慢的JDBC操作通过帮助进程完成。扩展这些操作可以有效的提升性能 
通过多线程实现多个操作同时被执行。Default: 3--> 
<property name="numHelperThreads">3</property> 

<!--当用户调用getConnection()时使root用户成为去获取连接的用户。主要用于连接池连接非c3p0 
的数据源时。Default: null--> 
<property name="overrideDefaultUser">root</property> 

<!--与overrideDefaultUser参数对应使用的一个参数。Default: null--> 
<property name="overrideDefaultPassword">password</property> 

<!--密码。Default: null--> 
<property name="password"></property> 

<!--定义所有连接测试都执行的测试语句。在使用连接测试的情况下这个一显著提高测试速度。注意： 
测试的表必须在初始数据源的时候就存在。Default: null--> 
<property name="preferredTestQuery">select id from test where id=1</property> 

<!--用户修改系统配置参数执行前最多等待300秒。Default: 300 --> 
<property name="propertyCycle">300</property> 

<!--因性能消耗大请只在需要的时候使用它。如果设为true那么在每个connection提交的 
时候都将校验其有效性。建议使用idleConnectionTestPeriod或automaticTestTable 
等方法来提升连接测试的性能。Default: false --> 
<property name="testConnectionOnCheckout">false</property> 

<!--如果设为true那么在取得连接的同时将校验连接的有效性。Default: false --> 
<property name="testConnectionOnCheckin">true</property> 

<!--用户名。Default: null--> 
<property name="user">root</property> 

<!--早期的c3p0版本对JDBC接口采用动态反射代理。在早期版本用途广泛的情况下这个参数 
允许用户恢复到动态反射代理以解决不稳定的故障。最新的非反射代理更快并且已经开始 
广泛的被使用，所以这个参数未必有用。现在原先的动态反射与新的非反射代理同时受到 
支持，但今后可能的版本可能不支持动态反射代理。Default: false--> 
<property name="usesTraditionalReflectiveProxies">false</property> 

<property name="automaticTestTable">con_test</property> 
<property name="checkoutTimeout">30000</property> 
<property name="idleConnectionTestPeriod">30</property> 
<property name="initialPoolSize">10</property> 
<property name="maxIdleTime">30</property> 
<property name="maxPoolSize">25</property> 
<property name="minPoolSize">10</property> 
<property name="maxStatements">0</property> 
<user-overrides user="swaldman"> 
</user-overrides> 
</default-config> 
<named-config name="dumbTestConfig"> 
<property name="maxStatements">200</property> 
<user-overrides user="poop"> 
<property name="maxStatements">300</property> 
</user-overrides> 
</named-config> 
</c3p0-config> 

===================== 
报错误： 
APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks! 

一般设置maxStatements=0解决该问题 
但是： 
把max_statements设置为0。 
c3p0在同时关闭statement和connection的时候，或者关闭他们之间的时间很短的时候，有时候connection并没有被关闭，因为有些preparedstatement还在被cached住。这是c3p0的作者自己说的。 
http://forum.hibernate.org/viewtopic.php?t=947246&highlight=apparent+deadlock+c3p0 

C3P0增加以下配置信息: 

//set to 'SELECT 1'       
preferredTestQuery = 'SELECT 1'     
//set to something much less than wait_timeout, prevents connections from going stale    
idleConnectionTestPeriod = 18000      
//set to something slightly less than wait_timeout, preventing 'stale' connections from being handed out    
maxIdleTime = 25000     
//if you can take the performance 'hit', set to "true"    
testConnectionOnCheckout = true     

A c3p0 pool with the settings you have should recover from a database reset, but that doesn't mean you will never see an Exception. Stale Connections from the old database session will still be broken, and if those Connections have already been checked out,
or if they are in the pool and not tested on checkout, the application will see the broken Connection, in the form of an Exception. 

You can use c3p0 to minimize the likelihood that your application will see a stale Connection on database shutdown/restart. The most reliable means of preventing this is to set hibernate.c3p0.validate to true (in a hibernate application -- all other c3p0 apps
should use the c3p0-native property c3p0.testConnectionOnCheckout). If you set this property to true, c3p0 will test Connections prior to checkout, and your app will never see a stale Connection on database restart unless the Connection had already been
checked out when the database went down. 

Another less reliable, but potentially less expensive, strategy is to set c3p0.testConnectionsOnCheckin and hibernate.c3p0.idle_test_period (c3p0-native c3p0.idleConnectionTestPeriod) to a low value, in which case all connection tests
are asynchronous and you are guanteed that no Connection will be checked out that hasn't been tested in the last idle_test_period seconds. Thus, your app will only see broken Connections from the pool if Connections are checked out during a short window of
time. 

In either case, I recommend setting "c3p0.preferredTestQuery" or "c3p0.automaticTestTable" in your c3p0 properties file, as c3p0's default Connection test is often slow. 

See "Configuring Connection Testing" in c3p0's docs for more information. 

在使用c3p0作为连接池时，其中的一些配置参数需要修改。主要是maxIdleTime和idleConnectionTestPeriod。 MySQL默认是8小时（28800秒）后自动关闭已打开的连接，所以c3p0要在8小时内关闭不使用的连接，上面的2参数要小于28800秒。附上在 hibernate中配置c3p0的关键字。 

c3p0-native property name hibernate configuration key 
c3p0.acquireIncrement hibernate.c3p0.acquire_increment 
c3p0.idleConnectionTestPeriod hibernate.c3p0.idle_test_period 
c3p0.initialPoolSize not available -- uses minimum size 
c3p0.maxIdleTime hibernate.c3p0.timeout 
c3p0.maxPoolSize hibernate.c3p0.max_size 
c3p0.maxStatements hibernate.c3p0.max_statements 
c3p0.minPoolSize hibernate.c3p0.min_size