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| <title>7. Entity Locking</title><base href="display"><link rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator" content="DocBook XSL Stylesheets V1.79.1"><link rel="home" href="manual.html" title="Apache OpenJPA 2.4 User's Guide"><link rel="up" href="jpa_overview_em.html" title="Chapter 8. EntityManager"><link rel="prev" href="jpa_overview_em_query.html" title="6. Query Factory"><link rel="next" href="jpa_overview_em_properties.html" title="8. Retrieving Properties Information"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">7. |
| Entity Locking |
| </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="jpa_overview_em_query.html">Prev</a> </td><th width="60%" align="center">Chapter 8. |
| EntityManager |
| </th><td width="20%" align="right"> <a accesskey="n" href="jpa_overview_em_properties.html">Next</a></td></tr></table><hr></div><div class="section" id="jpa_overview_em_locking"><div class="titlepage"><div><div><h2 class="title" style="clear: both">7. |
| Entity Locking |
| </h2></div></div></div> |
| |
| <a class="indexterm" name="d5e2694"></a> |
| <p> |
| In the area of concurrency control, the JPA specification supports |
| optimistic and pessimistic locking. |
| </p> |
| <pre class="programlisting"> |
| public void lock(Object entity, LockModeType mode); |
| </pre> |
| <p> |
| <a class="indexterm" name="d5e2700"></a> |
| <a class="indexterm" name="d5e2703"></a> |
| This method locks the given entity using the named mode. The |
| <a class="ulink" href="http://download.oracle.com/javaee/6/api/javax/persistence/LockModeType.html" target="_top"> |
| <code class="classname">javax.persistence.LockModeType</code></a> enum defines eight |
| modes: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <p> |
| <code class="literal">OPTIMISTIC</code>: Optimistic locking. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">OPTIMISTIC_FORCE_INCREMENT</code>: Optimistic locking. |
| When a transaction is committed, the entity's version column |
| will be incremented even if the entity's state did not change in the transaction. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">PESSIMISTIC_READ</code>: Pessimistic locking. Other transactions |
| may concurrently read the entity, but cannot concurrently update it. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">PESSIMISTIC_WRITE</code>: Pessimistic locking. Other transactions |
| cannot concurrently read or write the entity. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">PESSIMISTIC_FORCE_INCREMENT</code>: Pessimistic locking. Other transactions |
| cannot concurrently read or write the entity. |
| When a transaction is committed, the entity's version column |
| will be incremented even if the entity's state did not change in the transaction. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">READ</code>: A synonym for <code class="literal">OPTIMISTIC</code>. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">WRITE</code>: A synonym for <code class="literal">OPTIMISTIC_FORCE_INCREMENT</code>. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <code class="literal">NONE</code>: No locking is performed. |
| </p> |
| </li></ul></div> |
| <p> |
| Entities can also be locked at the time when entity state gets loaded from the datastore. |
| This is achieved by supplying a lock mode to the respective versions of |
| <code class="methodname">find</code> and <code class="methodname">refresh</code> methods. |
| If an entity state is to be loaded by a query, a lock mode can be passed to the |
| <code class="methodname">Query.setLockMode</code> and <code class="methodname">TypedQuery.setLockMode</code> |
| methods. |
| </p> |
| <pre class="programlisting"> |
| public LockModeType getLockMode(Object entity); |
| </pre> |
| <p> |
| Returns the lock mode currently held by the given entity. |
| </p> |
| <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <p> |
| OpenJPA differentiates between <code class="literal">PESSIMISTIC_READ</code> and |
| <code class="literal">PESSIMISTIC_WRITE</code> lock modes only with DB2 databases. |
| While running with other databases, there is no distinction between these |
| two modes because |
| <code class="literal">PESSIMISTIC_READ</code> lock mode |
| is upgraded to <code class="literal">PESSIMISTIC_WRITE</code>. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| OpenJPA has additional APIs for controlling entity locking. See |
| <a class="xref" href="ref_guide_locking.html" title="3. Object Locking">Section 3, “ |
| Object Locking |
| ”</a> in the Reference Guide for details. |
| </p> |
| </li></ul></div> |
| </div> |
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