When I put jdbc connecting  in  DefaultContext, and When I put my WAR file in
webapps dir ,tomcat will auto deploy it, but tomcat can't release connecting
pool before it undeploy the old WAR. the connecting size will increase more and
more ,don't reduce. 
You can test it, use netstat |wc -l to see how many connecting to database. then
for a long time , system file description will be used over, then system has no
resource to use.

How can I release the connecting pool by manual in my program?

attachment is the server.xml that I use.

<Server port="8005" shutdown="SHUTDOWN">
  <GlobalNamingResources>
    <!-- Used by Manager webapp -->
    <Resource name="UserDatabase" auth="Container"
              type="org.apache.catalina.UserDatabase"
      description="User database that can be updated and saved">
    </Resource>
    <ResourceParams name="UserDatabase">
      <parameter>
        <name>factory</name>
        <value>org.apache.catalina.users.MemoryUserDatabaseFactory</value>
      </parameter>
      <parameter>
        <name>pathname</name>
        <value>conf/tomcat-users.xml</value>
      </parameter>
    </ResourceParams>
  </GlobalNamingResources>

  <Service name="Catalina">
    <Connector port="80" />

    <!-- This is here for compatibility only, not required -->
    <Connector port="8009" protocol="AJP/1.3" />

    <Engine name="Catalina" defaultHost="localhost">
      <Logger className="org.apache.catalina.logger.FileLogger" />

      <Realm className="org.apache.catalina.realm.UserDatabaseRealm"
             resourceName="UserDatabase" />

        <Realm  className="org.apache.catalina.realm.JDBCRealm" debug="0"
             driverName="oracle.jdbc.driver.OracleDriver"
          connectionURL="jdbc:oracle:thin:@localhost:1521:nms"
         connectionName="test" connectionPassword="test"
              userTable="view_users" userNameCol="username" userCredCol="password"
          userRoleTable="tb_user_roles" roleNameCol="rolename" />

        <DefaultContext>

        <Resource name="jdbc/AdminDB" auth="Container" type="javax.sql.DataSource"/>

        <ResourceParams name="jdbc/AdminDB">
<parameter>
                <name>factory</name>
                <value>org.apache.commons.dbcp.BasicDataSourceFactory</value>
            </parameter>
            <parameter>
            <name>driverClassName</name>
                <value>oracle.jdbc.driver.OracleDriver</value>
            </parameter>
            <parameter>
                <name>url</name>
                <value>jdbc:oracle:thin:@localhost:1521:nms</value>
            </parameter>
            <parameter>
                <name>username</name>
                <value>test</value>
            </parameter>
            <parameter>
                <name>password</name>
                <value>test</value>
            </parameter>
            <parameter>
                <name>maxActive</name>
                <value>20</value>
            </parameter>
            <parameter>
                <name>maxIdle</name>
                <value>10</value>
            </parameter>
            <parameter>
                <name>maxWait</name>
                <value>-1</value>
            </parameter>
            <parameter>
                    <name>removeAbandoned</name>
                    <value>true</value>
            </parameter>

            <parameter>
              <name>removeAbandonedTimeout</name>
              <value>200</value>
             </parameter>

            <parameter>
              <name>logAbandoned</name>
              <value>true</value>
            </parameter>

        </ResourceParams>

        </DefaultContext>

      <Host name="localhost" appBase="/home/smsc/proxy_nms/webapps"
unpackWARs="true" >

        <Context path="" docBase="/opt/tomcat/webapps/ROOT"
            reloadable="true" crossContext="true"
            debug="0" privileged="false" >

            <Logger className="org.apache.catalina.logger.FileLogger"
                    prefix="localhost_root_log." suffix=".txt"
                timestamp="true"/>
         </Context>

        <Context path="/proxy_nms" docBase="proxy_nms"
            reloadable="true" crossContext="true"
            debug="0" privileged="false" >


            <Logger className="org.apache.catalina.logger.FileLogger"
                    prefix="localhost_proxy_nms_log." suffix=".txt"
                timestamp="true"/>
         </Context>

      </Host>
    </Engine>
  </Service>
</Server>