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Ehcache获取分布式事务支持可从net.sf.ehcache.transaction.manager. DefaultTransactionManagerLookup 类中知晓：
private
final
JndiSelector defaultJndiSelector =
new
JndiSelector("genericJNDI", "java:/TransactionManager");
private
final
Selector[] transactionManagerSelectors =
new
Selector[] {defaultJndiSelector,
new
JndiSelector("Weblogic", "javax.transaction.TransactionManager"),
new
FactorySelector("Bitronix", "bitronix.tm.TransactionManagerServices"),
new
ClassSelector("Atomikos", "com.atomikos.icatch.jta.UserTransactionManager")};
默认获取JNDI名“java:/TransactionManager”。JBoss JTA事务。
现跟踪Ehcache PUT操作时是如何加入事务的。
当Ehcache配置成xa或者xa-strict时，内部使用net.sf.ehcache.transaction.xa.XATransactionStore存储逻辑，put操作如下：
public
boolean
put(Element element)
throws
CacheException {
LOG.debug("cache {} put {}", cache.getName(), element);
//
this forces enlistment so the XA transaction timeout can be propagated to the XA resource
getOrCreateTransactionContext();
Element oldElement = getQuietFromUnderlyingStore(element.getObjectKey());
return
internalPut(
new
StorePutCommand(oldElement, copyElementForWrite(element)));
}
红色部分是事务关键操作：初始化事务上下文
private
XATransactionContext getOrCreateTransactionContext() {
try
{
EhcacheXAResourceImpl xaResource = getOrCreateXAResource();
XATransactionContext transactionContext = xaResource.getCurrentTransactionContext();
if
(transactionContext ==
null
) {
transactionManagerLookup.register(xaResource);
LOG.debug("creating new XA context");
transactionContext = xaResource.createTransactionContext();
xaResource.addTwoPcExecutionListener(
new
UnregisterXAResource());
}
else
{
transactionContext = xaResource.getCurrentTransactionContext();
}
LOG.debug("using XA context {}", transactionContext);
return
transactionContext;
}
catch
(SystemException e) {
throw
new
TransactionException("cannot get the current transaction", e);
}
catch
(RollbackException e) {
throw
new
TransactionException("transaction rolled back", e);
}
}
net.sf.ehcache.transaction.xa.EhcacheXAResourceImpl为Ehcache XAResource的实现。
研究一下XAResource都干了些啥。无非是该事务源相关属性及提交、回滚等操作吧。具体看一下实现代码：
关键属性：
private
final
Ehcache cache;
private
final
Store underlyingStore;
private
final
TransactionIDFactory transactionIDFactory;
private
final
TransactionManager txnManager;
private
final
SoftLockFactory softLockFactory;
private
final
ConcurrentMap<Xid, XATransactionContext> xidToContextMap =
new
ConcurrentHashMap<Xid, XATransactionContext>();
private
final
XARequestProcessor processor;
private
volatile
Xid currentXid;
private
volatile
int
transactionTimeout;
private
final
List<XAExecutionListener> listeners =
new
ArrayList<XAExecutionListener>();
private
final
ElementValueComparator comparator;
......
关键方法：
public
void
commit(Xid xid,
boolean
onePhase)
public
void
rollback(Xid xid)
throws
XAException
/**
* Add a listener which will be called back according to the 2PC lifecycle
*
@param
listener the XAExecutionListener
*/
void
addTwoPcExecutionListener(XAExecutionListener listener);
/**
* Obtain the already associated {
@link
XATransactionContext} with the current Transaction,
* or create a new one should none be there yet.
*
@return
The associated Transaction associated {
@link
XATransactionContext}
*/
XATransactionContext createTransactionContext()
throws
SystemException, RollbackException;
前两个为XAResource接口抽象方法，完成事务的基本操作，在JTA事务提交或是回滚时会被调用；后面是Ehcache抽象出来的接口方法。
接着关心这里的rollback()方法都做了啥。关键代码如下：
int rc = prepareInternal(xid);
if (rc == XA_RDONLY) {
return;
}
public
int
prepareInternal(Xid xid)
throws
XAException {
fireBeforePrepare();
XATransactionContext twopcTransactionContext = xidToContextMap.get(xid);
if
(twopcTransactionContext ==
null
) {
throw
new
EhcacheXAException("transaction never started: " + xid, XAException.XAER_NOTA);
}
XidTransactionID xidTransactionID = transactionIDFactory.createXidTransactionID(xid);
List<Command> commands = twopcTransactionContext.getCommands();
List<Command> preparedCommands =
new
LinkedList<Command>();
boolean
prepareUpdated =
false
;
LOG.debug("preparing {} command(s) for [{}]", commands.size(), xid);
for
(Command command : commands) {
try
{
prepareUpdated |= command.prepare(underlyingStore, softLockFactory, xidTransactionID, comparator);
preparedCommands.add(0, command);
}
catch
(OptimisticLockFailureException ie) {
for
(Command preparedCommand : preparedCommands) {
preparedCommand.rollback(underlyingStore);
}
preparedCommands.clear();
throw
new
EhcacheXAException(command + " failed because value changed between execution and 2PC",
XAException.XA_RBINTEGRITY, ie);
}
}
xidToContextMap.remove(xid);
if
(!prepareUpdated) {
rollbackInternal(xid);
}
LOG.debug("prepared xid [{}] read only? {}", xid, !prepareUpdated);
return
prepareUpdated ? XA_OK : XA_RDONLY;
}
可以看到进行了底部存储逻辑的回滚处理。
实例化好XAResource后回到初始化事务上下文方法getOrCreateTransactionContext中，可以知道通过该XAResource直接取得XATransactionContext。并且给XAResource注册了两个监听器UnregisterXAResource和CleanupXAResource（在事务提交或是混回滚时启动）。
至此事务的初始化工作完成。
理一下put过程。外部调用
XATransactionStore
的put方法，向当前XID的XATransactionContext中添加addCommand（封装了针对指定cache具体的提交与回滚操作），接着控制权到事务管理中，事务管理器管理
EhcacheXAResourceImpl
对象，提交与回滚操作通过取得XATransactionContext中的Commands对象并执行来完成。
欢迎大家批评指正！
