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Ehcache獲取分布式事務支持可從net.sf.ehcache.transaction.manager. DefaultTransactionManagerLookup 類中知曉：
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")};
默認獲取JNDI名“java:/TransactionManager”。JBoss JTA事務。
現跟蹤Ehcache PUT操作時是如何加入事務的。
當Ehcache配置成xa或者xa-strict時，內部使用net.sf.ehcache.transaction.xa.XATransactionStore存儲邏輯，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)));
}
紅色部分是事務關鍵操作：初始化事務上下文
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為Ehcache XAResource的實現。
研究一下XAResource都干了些啥。無非是該事務源相關屬性及提交、回滾等操作吧。具體看一下實現代碼：
關鍵屬性：
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;
......
關鍵方法：
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;
前兩個為XAResource接口抽象方法，完成事務的基本操作，在JTA事務提交或是回滾時會被調用；后面是Ehcache抽象出來的接口方法。
接著關心這里的rollback()方法都做了啥。關鍵代碼如下：
?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;
}
可以看到進行了底部存儲邏輯的回滾處理。
實例化好XAResource后回到初始化事務上下文方法getOrCreateTransactionContext中，可以知道通過該XAResource直接取得XATransactionContext。并且給XAResource注冊了兩個監聽器UnregisterXAResource和CleanupXAResource（在事務提交或是混回滾時啟動）。
至此事務的初始化工作完成。
?
理一下put過程。外部調用
XATransactionStore
的put方法，向當前XID的XATransactionContext中添加addCommand（封裝了針對指定cache具體的提交與回滾操作），接著控制權到事務管理中，事務管理器管理
EhcacheXAResourceImpl
對象，提交與回滾操作通過取得XATransactionContext中的Commands對象并執行來完成。
歡迎大家批評指正！
?
?
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