EntityManger & EntityManagerFactory


	EntityManger & EntityManagerFactory

baljinder

8-29-2008 11:23 AM

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<div style="margin: 12px 0px; font-family: arial; color: #333333; background: #ffffff; border: solid 4px #e5e5e5; width: 100%; clear: left;"><div class="CM_CTB_Content_Wrap" style="margin: 0px; padding: 0px;background-color: #ffffff;"><div style="border-bottom: solid 1px #dcdcdc; white-space: nowrap; margin-bottom: 8px; background-color: #eeeeee ;background-image: url(http://clipmarks.com/images/source-bg.gif); background-repeat: repeat-x; height: 24px; line-height: 24px; vertical-align: middle; padding-bottom: 4px; color: #666666; font-size: 10px;" ><a href="http://clipmarks.com/clip-to-blog/" title="see clips that are hot right now"><img src="http://content.clipmarks.com/blog_embed/a6f473bf-8f39-476d-9bfd-c40b342e459e/208D9785-B27F-4595-A34E-8AE858075320/" alt="" width="19" height="19" border="0" style="vertical-align: middle; margin: 0px 4px; display: inline; border: none; float:none;" /></a>clipped from <a title="http://javahowto.blogspot.com/2007/10/mind-thread-safety-when-injecting.html" href="http://javahowto.blogspot.com/2007/10/mind-thread-safety-when-injecting.html" style="font-size: 11px;">javahowto.blogspot.com</a></div><blockquote style="text-align: left; padding: 0px 8px; margin: 4px 0px 8px 0px; background: transparent; border: none;" cite="http://javahowto.blogspot.com/2007/10/mind-thread-safety-when-injecting.html"><P><DIV>As I wrote in post <A href="http://javahowto.blogspot.com/2007/10/why-we-need-type-level-injections-in.html">Why we need type-level injections in JavaEE</A>, injecting EJB 3 stateful beans into servlet instance fields is not thread-safe.  Along the same line, injecting <CODE>EntityManager</CODE> with <CODE>@PersistenceContext</CODE> into servlet instance variables is not thread-safe, either.  <CODE>EntityManager</CODE> is just not designed to be thread-safe.</DIV><BR />For example, the following code snippet of a servlet class is incorrect:</P></blockquote><div style="height: 2px; font-size: 2px; background: #dcdcdc; border-bottom: solid 1px #f5f5f5; margin: 2px 4px;"></div><blockquote style="text-align: left; padding: 0px 8px; margin: 4px 0px 8px 0px; background: transparent; border: none;" cite="http://javahowto.blogspot.com/2007/10/mind-thread-safety-when-injecting.html"><CODE>public class FooServlet extends HttpServlet {<BR />  //This field injection is not thread-safe.<BR />  //FIXME<BR />  @PersistenceContext<BR />  private EntityManager em;<BR />...<BR />}</CODE></blockquote><div style="height: 2px; font-size: 2px; background: #dcdcdc; border-bottom: solid 1px #f5f5f5; margin: 2px 4px;"></div><blockquote style="text-align: left; padding: 0px 8px; margin: 4px 0px 8px 0px; background: transparent; border: none;" cite="http://javahowto.blogspot.com/2007/10/mind-thread-safety-when-injecting.html">One way to fix this is to inject <CODE>EntityManagerFactory</CODE> instead.  <CODE>EntityManagerFactory</CODE> is guaranteed to be thread-safe.  For example:</blockquote><div style="height: 2px; font-size: 2px; background: #dcdcdc; border-bottom: solid 1px #f5f5f5; margin: 2px 4px;"></div><blockquote style="text-align: left; padding: 0px 8px; margin: 4px 0px 8px 0px; background: transparent; border: none;" cite="http://javahowto.blogspot.com/2007/10/mind-thread-safety-when-injecting.html"><CODE>public class FooServlet extends HttpServlet {<BR />  //This field injection is thread-safe<BR />  @PersistenceUnit<BR /><DIV>  private EntityManagerFactory emf;</DIV><BR />  protected void doGet(HttpServletRequest request,<BR />             HttpServletResponse response)<BR />  throws ServletException, IOException {<BR />  EntityManager em = emf.createEntityManager();<BR />  //work with em<BR />}<BR />}</CODE></blockquote></div><div style="margin: 0px 6px 6px 4px;"><table style="font-size: 11px;border-spacing: 0px;padding: 0px;" cellpadding="0" cellspacing="0" width="100%"><tr><td style="background:transparent;border-width:0px;padding:0px;">&nbsp;</td><td align="right" style="background:transparent;border-width:0px;padding:0px;width:107px" width="107"><a href="http://clipmarks.com/share/208D9785-B27F-4595-A34E-8AE858075320/blog/" title="blog or email this clip"><img src="http://content7.clipmarks.com/images/c2b-foot.png" border="0" alt="blog it" width="107" height="17" style="border-width:0px;padding:0px;margin:0px;" /></a></td></tr></table></div></div>

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