C3P0在最近的demo中也用了（我用的是0.9.2.1版本），因为单例很难应付大量并发。
用法详见文档：http://www.mchange.com/projects/c3p0/
基本的用法在http://www.mchange.com/projects/c3p0/#quickstart，
以及http://www.mchange.com/projects/c3p0/#using_c3p0中。
在项目中更为方便的做法是将配置写在配置文件中。
C0P0的配置文件名为c3p0-config.xml，详见http://www.mchange.com/projects/c3p0/#configuration_files
一个示例的配置文件如下：
<?xml version="1.0" encoding="utf-8"?> <c3p0-config>
<default-config>
<property name="automaticTestTable">t_c3p0_test</property>
<property name="checkoutTimeout">10000</property>
<property name="idleConnectionTestPeriod">10</property>
<property name="initialPoolSize">5</property>
<property name="maxIdleTime">600</property>
<property name="maxPoolSize">20</property>
<property name="minPoolSize">2</property>
<property name="maxStatements">60</property>
<property name="maxStatementsPerConnection">3</property>
<user-overrides user="test-user">
<property name="maxPoolSize">10</property>
<property name="minPoolSize">1</property>
<property name="maxStatements">0</property>
</user-overrides>
</default-config>
<named-config name="metkb">
<property name="acquireIncrement">20</property>
<property name="initialPoolSize">10</property>
<property name="minPoolSize">10</property>
<property name="maxPoolSize">100</property>
<property name="maxStatements">0</property>
<property name="maxStatementsPerConnection">10</property>
<!-- he's important, but there's only one of him -->
<user-overrides user="master-of-the-universe">
<property name="acquireIncrement">1</property>
<property name="initialPoolSize">1</property>
<property name="minPoolSize">1</property>
<property name="maxPoolSize">5</property>
<property name="maxStatementsPerConnection">50</property>
</user-overrides>
</named-config>
</c3p0-config>
放在eclipse Web工程的src目录下即可（eclipse会将该目录下的配置文件复制到tomcat的webapps/XXX/WEB-INF/classes下）
一个C3P0的单例的示例代码如下（连接池的单例并不妨碍并发，因为从连接池中取连接并不费时，而且取出的连接是线程安全的）：
package cn.edu.ruc.metkb.util; import java.beans.PropertyVetoException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement; import com.mchange.v2.c3p0.ComboPooledDataSource; public class PooledDBA implements DBA {
private ComboPooledDataSource cpds = null;
private static Log dblog = null;
private static String DBClassName = null;
private static String DBName = null;
private static String DBUrl = null;
private static String DBUser = null;
private static String DBPassword = null; protected PooledDBA() {
cpds = new ComboPooledDataSource("metkb");
try {
cpds.setDriverClass(DBClassName);
} catch (PropertyVetoException e) {
dblog.exception(e);
}
cpds.setJdbcUrl(DBUrl + DBName);
cpds.setUser(DBUser);
cpds.setPassword(DBPassword);
} static {
try {
DBClassName = ConfigFactory.getInstance().get("db.classname");
DBName = ConfigFactory.getInstance().get("db.name");
DBUrl = ConfigFactory.getInstance().get("db.url");
DBUser = ConfigFactory.getInstance().get("db.user");
DBPassword = ConfigFactory.getInstance().get("db.password");
dblog = LogFactory.getInstance().getLog("db");
} catch (Exception e) {
e.printStackTrace();
}
} @Override
public synchronized Connection getConnection() throws SQLException,
ClassNotFoundException, InterruptedException {
return cpds.getConnection();
} @Override
public synchronized void close(Connection conn) {
try {
if (conn != null) {
conn.close();
conn = null;
}
} catch (SQLException e) {
dblog.exception(e);
}
} @Override
public synchronized void close(Statement stat) {
try {
if (stat != null) {
stat.close();
stat = null;
}
} catch (SQLException e) {
dblog.exception(e);
}
} @Override
public synchronized void close(ResultSet rest) {
try {
if (rest != null) {
rest.close();
rest = null;
}
} catch (SQLException e) {
dblog.exception(e);
}
}
}