TopLink源代码分析 JTA的部分技术实现细节

TopLink源代码分析 JTA的部分技术实现细节

[什么是JTA]

Java事务API（JTA；Java Transaction API）和它的同胞Java事务服务(JTS；Java TransactionService)，为J2EE平台提供了分布式事务服务。一个分布式事务（distributedtransaction）包括一个事务管理器（transaction manager）和一个或多个资源管理器(resource manager)。一个资源管理器（resource manager）是任意类型的持久化数据存储。事务管理器（transaction manager）承担着所有事务参与单元者的相互通讯的责任。下图显示了事务管理器和资源管理的间的关系。

　　JTA事务比JDBC事务更强大。一个JTA事务可以有多个参与者，而一个JDBC事务则被限定在一个单一的数据库连接。下列任一个Java平台的组件都可以参与到一个JTA事务中：

　　.JDBC连接

　　.JDO PersistenceManager 对象

　　.JMS 队列

　　.JMS 主题

　　.企业JavaBeans（EJB）

　　.一个用J2EE Connector Architecture 规范编译的资源分配器。

简单的说，JTA实现了在分布式系统中对单一事务的提交和管理。

[源代码分析]

第一步：当连接数据库的时候，前期的准备函数，这里面都发生了什么？

/**

* INTERNAL:

* This method includes all of the code that is issued before the datasource

* is connected to.

*/

protected void preConnectDatasource(){

//Bug#3440544 Check if logged in already to stop the attempt to login more than once

if (isLoggedIn) {

throw ValidationException.alreadyLoggedIn(this.getName());

}

this.platform = null;

if (isInProfile()) {

getProfiler().initialize();

}

updateProfile(SessionProfiler.LoginTime, new Date(System.currentTimeMillis()));

// Login and initialize

getEventManager().preLogin(this);

//setup the external transaction controller

getServerPlatform().initializeExternalTransactionController();

log(SessionLog.INFO, null, "topLink_version", DatasourceLogin.getVersion());

if (getServerPlatform().getServerNameAndVersion() != null) {

log(SessionLog.FINE, null, "application_server_name_and_version", getServerPlatform().getServerNameAndVersion());

}

}

第二步：上面的程序中，有这样一行代码

//setup the external transaction controller getServerPlatform().initializeExternalTransactionController();

它是干什么的呢？？？

public void initializeExternalTransactionController() {

this.ensureNotLoggedIn();

//BUG 3975114: Even if JTA is disabled, override if we're in CMP

//JTA must never be disable during CMP (WLS/Oc4j)

if (!isJTAEnabled() && !isCMP()) {

return;

}

//BUG 3975114: display a warning if JTA is disabled and we're in CMP

if (!isJTAEnabled() && isCMP()) {

AbstractSessionLog.getLog().warning("jta_cannot_be_disabled_in_cmp");

}

//check if the transaction controller class is overridden by a preLogin or equivalent,

//or if the transaction controller was already defined, in which case they should have written

//a subclass. Show a warning

try {

if (getDatabaseSession().getExternalTransactionController() != null) {

this.externalTransactionControllerNotNullWarning();

return;

}

ExternalTransactionController controller = null;

if (PrivilegedAccessHelper.shouldUsePrivilegedAccess()){

try {

controller = (ExternalTransactionController)AccessController.doPrivileged(new PrivilegedNewInstanceFromClass(this.getExternalTransactionControllerClass()));

} catch (PrivilegedActionException exception) {

Exception throwableException = exception.getException();

if (throwableException instanceof InstantiationException) {

throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());

} else {

throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());

}

}

} else {

controller = (ExternalTransactionController)PrivilegedAccessHelper.newInstanceFromClass(this.getExternalTransactionControllerClass());

}

getDatabaseSession().setExternalTransactionController(controller);

} catch (InstantiationException instantiationException) {

throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());

} catch (IllegalAccessException illegalAccessException) {

throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());

}

}

乖乖~~~~~这么长的函数，鬼知道它做的是什么。。。。

不过，貌似它里面有这样一行话。

getDatabaseSession().setExternalTransactionController(controller);

乖乖~~~~原来是用来启动JTA事务管理控制器的。。。快点去看看它干了什么。。

第三步：啊哈，奥秘在这里

public class SunAS9TransactionController extends JTATransactionController {

// Use "java:appserver/TransactionManager" instead of java:pm/TransactionManager

// as the former one is available in ACC as well as server.

// See com.sun.enterprise.naming.java.javaURLContext in GlassFish appserv-core

public static final String JNDI_TRANSACTION_MANAGER_NAME = "java:appserver/TransactionManager";

public SunAS9TransactionController() {

super();

}

/**

* INTERNAL:

* Obtain and return the JTA TransactionManager on this platform

*/

protected TransactionManager acquireTransactionManager() throws Exception {

return (TransactionManager)jndiLookup(JNDI_TRANSACTION_MANAGER_NAME);

}

}

同志们看见了吗~~~~

public static final String JNDI_TRANSACTION_MANAGER_NAME = "java:appserver/TransactionManager";

搞了半天，原来它用JMX启动了一个交易服务来管理JTA~~~

？？？什么是<span styl

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• 2010-01-25 14:56
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