Ehcache 事务管理源码探析
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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。如果需要其他类型的事务管理获取方式，
通过配置TransactionManagerLookup来替代默认的。
配置如下：
<transactionManagerLookup
class= "com.mycompany.transaction.manager.MyTransactionManagerLookupClass"
properties="" propertySeparator=":"/>
另外当使用默认的事务管理查找类，但是不同的JNDI查找名称，可以配置：
<transactionManagerLookup
class="net.sf.ehcache.transaction.manager.DefaultTransactionManagerLookup"
properties="jndiName=java:appserver/TransactionManager" propertySeparator=";"/>
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。事务上下文维护着该事务的所有cache操作、remove操作的keys、put操作的keys和underlyingStore。
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);
}
}
构建事务上下文的操作：
public XATransactionContext createTransactionContext() throws SystemException, RollbackException {
XATransactionContext ctx = getCurrentTransactionContext();
if (ctx != null) {
return ctx;
}
Transaction transaction = txnManager.getTransaction();
LOG.debug("enlisting {} in {}", this, transaction);
transaction.enlistResource(this);//关键操作：注册当前XAResource到事务中
// currentXid is set by a call to start() which itself is called by transaction.enlistResource(this)
if (currentXid == null) {
throw new CacheException("enlistment of XAResource of cache named '" + getCacheName() +
"' did not end up calling XAResource.start()");
}
ctx = xidToContextMap.get(currentXid);
if (ctx == null) {
LOG.debug("creating new context for XID [{}]", currentXid);
ctx = new XATransactionContext(underlyingStore);
xidToContextMap.put(currentXid, ctx);
}
return ctx;
}
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！
