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java学习笔记41（数据库连接池 C3p0连接池）

在之前的学习中，我们发现，我们需要频繁的创建连接对象，用完之后还需要在关闭资源，因为这些连接对象都是占资源的，但是又不得不创建，比较繁琐，为了解决这种情况，Java 出现了数据库连接池；

数据库连接池的概念：

　　定义：本质上就是一个容器（集合，java 没有容器）用来存放数据库连接的内容，当系统初始化以后，容器被创建，容器中就会申请一些连接对象，当用户来访问数据库的时候，从容器中取出对象，用完之后归还。

通俗理解连接池：就像一个饭店，本来是来一个客人就招一个服务员，然后客人走后，服务员就被辞退，下次再来客人就再招一个服务员，然后用了连接池以后就像是招了好几个服务员在一个屋里等着，来一个客人就派一个服务员去接待，客人走后服务员在回到屋内。数据库连接池就是这样，连接池里有若干个连接对象， 需要时就拿去用， 用完后归还连接。

数据库的连接池有好多种，我们今天学习 C3p0 连接池，

需要导入的 jar 包有：

　　c3p0-0.9.5.2.jar

 　　mchange-commons-java-0.2.12.jar

　　 mysql-connector-java-5.1.45-bin.jar

jar 包放入 lib 文件夹，导 jar 包的方法在之前写过，这里就不介绍了

C3p0 连接池需要配置外部文件，注意：文件名必须是  c3p0.properties 或 c3p0-config.xml  必须放在 src 文件夹下

这里是写的 c3p0-config.xml 配置文件，代码如下：

<c3p0-config>
  <!-- 使用默认的配置读取连接池对象 -->
  <default-config>
      <!--  连接参数 -->
    <property name="driverClass">com.mysql.jdbc.Driver</property>
    <property name="jdbcUrl">jdbc:mysql://localhost:3306/qy97</property>
    <property name="user">root</property>
    <property name="password">123456</property>

<span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)"> 连接池参数 </span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="initialPoolSize"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>5<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="maxPoolSize"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>10<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="checkoutTimeout"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>3000<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>



</default-config>


<span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)">另一个连接池</span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>



<named-config name="otherc3p0">
<!--  连接参数 -->
<property name="driverClass">com.mysql.jdbc.Driver</property>
<property name="jdbcUrl">jdbc:mysql://localhost:3306/mybase</property>
<property name="user">root</property>
<property name="password">123456</property>


<span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)"> 连接池参数 </span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="initialPoolSize"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>5<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="maxPoolSize"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>8<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="checkoutTimeout"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>1000<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>



</named-config>
</c3p0-config>

连接池的用法：

package com.zs.cp30;

import com.mchange.v2.c3p0.ComboPooledDataSource;


import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;


/创建出 c3p0 连接池/
public class C3p0Demo {
public static void main(String[] args) throws SQLException {
//    1. 导入两个包 c3p0-0.9.5.2.jar  mchange-commons-java-0.2.12.jar
//    2. 创建连接池核心对象, 数据库连接池对象 ComboPooledDataSource</span>
DataSource ds=new ComboPooledDataSource();
//    3. 获取连接
Connection conn=ds.getConnection();
//        4. 输出连接地址，查看是否连接成功
        System.out.println(conn);
}
}

在之前的配置文件种，我们写了最大连接数是 10 个，现在我们来获得这十个连接：

package com.zs.cp30;

import com.mchange.v2.c3p0.ComboPooledDataSource;


import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;


public class C3p0Demo1 {
public static void main(String[] args) throws SQLException {
DataSource ds=new ComboPooledDataSource();


    </span><span style="color: rgba(0, 0, 255, 1)">for</span> (<span style="color: rgba(0, 0, 255, 1)">int</span> i = 1; i &lt;= 10; i++<span style="color: rgba(0, 0, 0, 1)">) {
        Connection conn </span>=<span style="color: rgba(0, 0, 0, 1)"> ds.getConnection();
        System.out.println(conn);
    }
}



}
/结果:
com.mchange.v2.c3p0.impl.NewProxyConnection@7a419da4 [wrapping: com.mysql.jdbc.JDBC4Connection@14555e0a]
com.mchange.v2.c3p0.impl.NewProxyConnection@759d26fb [wrapping: com.mysql.jdbc.JDBC4Connection@3c73951]
com.mchange.v2.c3p0.impl.NewProxyConnection@6f46426d [wrapping: com.mysql.jdbc.JDBC4Connection@73700b80]
com.mchange.v2.c3p0.impl.NewProxyConnection@10d307f1 [wrapping: com.mysql.jdbc.JDBC4Connection@4d5b6aac]
com.mchange.v2.c3p0.impl.NewProxyConnection@4a7f959b [wrapping: com.mysql.jdbc.JDBC4Connection@429bffaa]
com.mchange.v2.c3p0.impl.NewProxyConnection@483f6d77 [wrapping: com.mysql.jdbc.JDBC4Connection@7e5afaa6]
com.mchange.v2.c3p0.impl.NewProxyConnection@28f3b248 [wrapping: com.mysql.jdbc.JDBC4Connection@1b1426f4]
com.mchange.v2.c3p0.impl.NewProxyConnection@581ac8a8 [wrapping: com.mysql.jdbc.JDBC4Connection@6d4e5011]
com.mchange.v2.c3p0.impl.NewProxyConnection@76c3e77a [wrapping: com.mysql.jdbc.JDBC4Connection@78123e82]
com.mchange.v2.c3p0.impl.NewProxyConnection@fba92d3 [wrapping: com.mysql.jdbc.JDBC4Connection@662b4c69]/

在上面我们可以看出，10 个连接对象，都不相同，这时，连接池内所有的连接对象都被取出来了，当取第 11 个对象时，就会报错：

public class C3p0Demo1 {
    public static void main(String[] args) throws SQLException {
        DataSource ds=new ComboPooledDataSource();

    </span><span style="color: rgba(0, 0, 255, 1)">for</span> (<span style="color: rgba(0, 0, 255, 1)">int</span> i = 1; i &lt;= 11; i++<span style="color: rgba(0, 0, 0, 1)">) {
        Connection conn </span>=<span style="color: rgba(0, 0, 0, 1)"> ds.getConnection();
        System.out.println(conn);
    }
}



}

上面的 代码会报错，因为没有第十一个连接对象，那么我们在循环种还回去一个连接对象，那么就不会报错了：

package com.zs.cp30;

import com.mchange.v2.c3p0.ComboPooledDataSource;


import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;


public class C3p0Demo1 {
public static void main(String[] args) throws SQLException {
DataSource ds=new ComboPooledDataSource();


    </span><span style="color: rgba(0, 0, 255, 1)">for</span> (<span style="color: rgba(0, 0, 255, 1)">int</span> i = 1; i &lt;= 11; i++<span style="color: rgba(0, 0, 0, 1)">) {
        Connection conn </span>=<span style="color: rgba(0, 0, 0, 1)"> ds.getConnection();
        System.out.println(conn);
        </span><span style="color: rgba(0, 0, 255, 1)">if</span> (i==5<span style="color: rgba(0, 0, 0, 1)">) {
            conn.close();
        }
    }
}



}
/结果:
com.mchange.v2.c3p0.impl.NewProxyConnection@7a419da4 [wrapping: com.mysql.jdbc.JDBC4Connection@14555e0a]
com.mchange.v2.c3p0.impl.NewProxyConnection@759d26fb [wrapping: com.mysql.jdbc.JDBC4Connection@3c73951]
com.mchange.v2.c3p0.impl.NewProxyConnection@6f46426d [wrapping: com.mysql.jdbc.JDBC4Connection@73700b80]
com.mchange.v2.c3p0.impl.NewProxyConnection@10d307f1 [wrapping: com.mysql.jdbc.JDBC4Connection@4d5b6aac]
com.mchange.v2.c3p0.impl.NewProxyConnection@4a7f959b [wrapping: com.mysql.jdbc.JDBC4Connection@429bffaa]
com.mchange.v2.c3p0.impl.NewProxyConnection@28f3b248 [wrapping: com.mysql.jdbc.JDBC4Connection@1b1426f4]
com.mchange.v2.c3p0.impl.NewProxyConnection@581ac8a8 [wrapping: com.mysql.jdbc.JDBC4Connection@6d4e5011]
com.mchange.v2.c3p0.impl.NewProxyConnection@76c3e77a [wrapping: com.mysql.jdbc.JDBC4Connection@429bffaa]
com.mchange.v2.c3p0.impl.NewProxyConnection@67c33749 [wrapping: com.mysql.jdbc.JDBC4Connection@fba92d3]
com.mchange.v2.c3p0.impl.NewProxyConnection@fa49800 [wrapping: com.mysql.jdbc.JDBC4Connection@71238fc2]
com.mchange.v2.c3p0.impl.NewProxyConnection@16a0ee18 [wrapping: com.mysql.jdbc.JDBC4Connection@3d6f0054]/

注意上面结果中红色标识的两个连接对象，可以看出这两个是一个对象，也就是这个对象被调用了两次，连接对象用完要及时归还，在连接池中，为了方便也用 close 方法，不过此时表示的是归还对象，并不是释放资源。

 

用完要及时归还。