public class Project {
    private int prj_id;
    private String prj_name;
    private Set<Developer> devs = new HashSet<Developer>();
    set...
    get...
}

public class Developer {
    private int d_id;
    private String d_name;
    private Set<Project> projects = new HashSet<Project>();
    set...
    get...
}

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.project">
    <class name="Project" table="t_project">
        <!--主键-->
        <id name="prj_id" >
            <generator class="native"/>
        </id>
        <property name="prj_name"  type="string"/>
        <set cascade="save-update" inverse="true" name="devs" table="t_relation">
            <key column="prjid"></key>
            <many-to-many column="devid" class="Developer"/>
        </set>
    </class>

</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.project">
    <class name="Developer" table="t_developer">
        <!--主键-->
        <id name="d_id">
            <generator class="native"/>
        </id>
        <property name="d_name" type="string"/>
        <set name="projects" table="t_relation" >
            <!--外键字段-->
            <key column="devid"></key>
            <many-to-many column="prjid" class="Project"/>
        </set>
    </class>

</hibernate-mapping>

    @Test
    public void test1(){
        Session session = sf.openSession();
        session.beginTransaction();

        Project project_wuliu = new Project();
        project_wuliu.setPrj_name("物流系统");
        Project project_oa = new Project();
        project_oa.setPrj_name("OA系统");

        Developer developer_1 = new Developer();
        developer_1.setD_name("曹集");
        Developer developer_2 = new Developer();
        developer_2.setD_name("王睿");
        Developer developer_3 = new Developer();
        developer_3.setD_name("老张");

        project_wuliu.getDevs().add(developer_1);
        project_wuliu.getDevs().add(developer_2);

        project_oa.getDevs().add(developer_3);
        project_oa.getDevs().add(developer_2);

        session.save(project_wuliu);
        session.save(project_oa);
        session.save(developer_1);
        session.save(developer_2);
        session.save(developer_3);


        session.getTransaction().commit();
        session.clear();

    }

        Hibernate: insert into t_project (prj_name) values (?)
        Hibernate: insert into t_project (prj_name) values (?)
        Hibernate: insert into t_developer (d_name) values (?)
        Hibernate: insert into t_developer (d_name) values (?)
        Hibernate: insert into t_developer (d_name) values (?)
        Hibernate: insert into t_relation (prjid, devid) values (?, ?)
        Hibernate: insert into t_relation (prjid, devid) values (?, ?)
        Hibernate: insert into t_relation (prjid, devid) values (?, ?)
        Hibernate: insert into t_relation (prjid, devid) values (?, ?)