hibernate插入问题

为什么插入22条记录 就再也插不进去了?配置如下<hibernate-mapping>
    <class name="com.icss.oa.entity.Reader" table="READER" schema="HT">
        <id name="readerid" type="java.lang.Long">
            <column name="READERID" precision="22" scale="0" />
            <generator class="sequence" >
            <param name="sequence">userid_seq</param>
            </generator>
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="10" />
        </property>
        <property name="age" type="java.lang.Long">
            <column name="AGE" precision="22" scale="0" />
        </property>
        <property name="sex" type="java.lang.String">
            <column name="SEX" length="2" />
        </property>
        <set name="books" order-by="bookid" inverse="true" cascade="all">
            <key>
                <column name="READERID" precision="22" scale="0" />
            </key>
            <one-to-many class="com.icss.oa.entity.Book" />
        </set>
    </class>
</hibernate-mapping>
=====================================================================================
<hibernate-mapping>
    <class name="com.icss.oa.entity.Book" table="BOOK" schema="HT">
        <id name="bookid" type="java.lang.Long">
            <column name="BOOKID" precision="22" scale="0" />
            <generator class="sequence" >
            <param name="sequence">userid_seq</param>
            </generator>
        </id>
        <many-to-one name="reader" class="com.icss.oa.entity.Reader" fetch="select">
            <column name="READERID" precision="22" scale="0" />
        </many-to-one>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="10" not-null="true" />
        </property>
        <property name="publishTime" type="java.util.Date">
            <column name="PUBLISH_TIME" length="7" />
        </property>
    </class>
</hibernate-mapping>
======================================================================================
为什么插入22条记录 就再也插不进去了?
public static void insertReader() { Session session = null;
Transaction tx = null;
try {
session = HibernateSessionFactory.getSession();
tx = session.beginTransaction(); Reader reader = new Reader();
reader.setAge(new Long(23));
reader.setName("hts3x");
reader.setSex("男");
session.save(reader);

Set books = new HashSet();
for (int i = 0; i < 5; i++) {
Book book = new Book();
book.setName("java" + i);
book.setPublishTime(Date.valueOf("2008-09-11"));
book.setReader(reader);
session.save(book);
books.add(book);
}


tx.commit();
} catch (Exception e) {
e.printStackTrace();
tx.rollback();
} finally {
session.close();
}