因為
Servlet是預設在多執行緒環境中運行的，所以若在
Servlet使用下列方式來注入
EntityManager物件，是違反執行安全的(NOT thread-safe)：
@PersistenceContext(unitName="QueryPaging")
private EntityManager em;
所以，若要在
Servlet若要直接存取
EntityManager的話，應該用本文(context)查找(lookup)的方式來取得
EntityManager，再做相關的操作
package examples.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import examples.model.Employee;
@PersistenceContext(unitName="QueryPaging",name="myapp/QueryPaging")
@WebServlet("/EmAccessServlet")
public class EmAccessServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doPost(request, response);
}
@SuppressWarnings("unchecked")
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter pw = response.getWriter();
try
{
EntityManager em = (EntityManager)new InitialContext()
.lookup("java:comp/env/myapp/QueryPaging");
List empList = em.createQuery("select e from Employee e").getResultList();
for(Employee e : empList)
{
pw.println(e);
}
} catch (NamingException e)
{
e.printStackTrace();
}
}
}
使用上面的方式，對每個請求(request)都查找一個
EntityManager，應用程式容器(container)會確保不會有相同的要求共享這些
EntityManager。
