get NonUniqueObjectException when update entity using struts2 and hibernate

I use hibernate as the ORM framework,and this is my first time use it for writing operation.

Befor this appliction,I just use hibernate to read data from the db.

In my struts action,I try to update a entity named "Task",this is the form in the update page:

<s:form action="task_update" namespace="/common" cssStyle="width:95%">
    <s:textfield value="%{task.id}" cssStyle="display:none" name="task.id"></s:textfield>
    <s:textfield name="task.name" value="%{task.name}" label="TaskName"/>

    <s:select list="task.managers" 
        listKey="id" listValue="name" label="Manager" value="%{task.manage}" name="task.department.id">
    </s:se开发者_运维问答lect>

    <s:select list="#session.current_user.departments" 
        listKey="id" listValue="name" label="Departmentn of this task" value="%{task.department.{id}}" name="task.department.id">
    </s:select>
    <table>
        <caption align="left">Steps</caption>
        <tr>
            <th>Name</th>
            <th>End Time</th>
            <th>Operators</th>
            <th>Status</th>
            <th>Set the order</th>
            <th><span id="addStep" style="cursor:pointer" >Add Steps</span></th>
        </tr>
        <s:iterator value="task.steps">
            <tr class="step">
                <td>
                    <s:textfield name="task.steps[0].name" value="%{#this.name}" theme="simple"/>
                    <s:textfield name="task.steps[0].id" value="%{#this.id}" theme="simple" cssStyle="display:none"/>
                    <s:textfield name="task.steps[0].position" value="%{#this.position}" theme="simple" cssStyle="display:none" class="position"/>
                </td>
                <td><s:textfield name="task.steps[0].end" value="%{#this.end}" theme="simple"/></td>
                <td>        
                    <s:select list="allOpIndb"  listKey="id" listValue="name" value="%{#this.operator.{id}}"
                        name="task.steps[0].operator.id" multiple="true" theme="simple" id="a">
                    </s:select>
                </td>
                <td>
                    <s:select list="@com.infomanager.entity.TaskStepStatus@values()"  theme="simple" 
                        name="task.steps[0].status" listValue="cnValue" value="%{#this.status}" id="b"/>
                </td>
                <td>
                    <span class="up">up</span>
                    <span class="down">down</span>
                    <span class="del">del</span>
                </td>
                <td></td>
            </tr>
        </s:iterator>
        <tr>
            <td colspan="6">
                <s:submit value="Submit"></s:submit>
            </td>
        </tr>
    </table>
</s:form>

The whole codes of this page can be found here:

Then in the struts2 action,I get the task object created by struts2(The "task" object in the following example),and find the being updated object in the db(the "task_db" object in the following example):

public String task_update{
    DozerBeanMapper dbm = new DozerBeanMapper();
    // the 'task' object is created by struts2
    taskid = task.getId();
    String name_st = task.getName();
    int dep_id_st = task.getDepartment().getId();
    List<TaskStep> steps_st = task.getSteps();

    TaskDaoImpl tkDao = new TaskDaoImpl();
    TaskStepDaoImpl tsDao = new TaskStepDaoImpl();
    OperatorDaoImpl opDao = new OperatorDaoImpl();
    DepartmentDaoImpl depDao = new DepartmentDaoImpl();

    List<TaskStep> step_db = new ArrayList<TaskStep>();
    for (TaskStep step_st : steps_st) {
        int tsid = step_st.getId();
        TaskStep ts_db = tsDao.queryStepById(tsid);
        if (ts_db == null) {
            ts_db = step_st;
        } else
            dbm.map(step_st, ts_db);
        // sest the operators
        List<Operator> ops_to_db = new ArrayList<Operator>();
        for (Operator op_st : step_st.getOperator()) {
            ops_to_db.add(opDao.queryOperatorById(op_st.getId()));
        }
        ts_db.setOperator(ops_to_db);
        step_db.add(ts_db);
    }
    //set the id of the task have the same id with task_db,so set its id to a unimpossible value
    task.setId(-100);
    //set it to null!
    task=null;
    Task task_db = tkDao.queryTaskById(taskid);
    task_db.setName(name_st);
    task_db.setDepartment(depDao.queryDepartById(dep_id_st));
    task_db.setSteps(step_db);

    tkDao.updateTask(task_db);
}

When I submit the form,I got the "org.hibernate.NonUniqueObjectException:": a different object with the same identifier value was already associated with the session: [com.infomanager.entity.Task#4].

I wonder why? I have set the task's id to -100,and set it to null.

What is the problem?

THe following is the realation ship of the tabls in my db:

And the whole project can be found here:https://github.com/hguser/TaskManager

Basically, what hibernate is saying is that you have two objects which have the same identifier (same primary key) but they are not the same object.

Follow this post may be it will help you. NonUniqueObjectException

This exception is only there when hibernate find two objects in the session context with the same identifier.