qr code generator java program Figure 11-7: Two scenarios for managing concurrent access to an entity bean in Java

Maker QR Code ISO/IEC18004 in Java Figure 11-7: Two scenarios for managing concurrent access to an entity bean

Figure 11-7: Two scenarios for managing concurrent access to an entity bean
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Better performance can be achieved Database synchronization is usually more robust and has more features than container synchronization In complex applications, the database is often used by many parts in the system such as the reporting module, data extraction procedures, and so on It is much safer, therefore, to rely on transaction and locking management at the database level than within the EJB world Containers do not need to incorporate complex locking and serialization code
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The EJB Transaction Model Support for distributed transactions is perhaps the primary feature making EJB attractive to enterprise software developers Building support for distributed transactions into an application is a difficult task that is alleviated by using EJB By moving the responsibility for managing transactions from the bean developer to the container and server provider, EJB simplifies application development and ensures that the difficult issues involving transactions are handled by experts 191
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EJB transaction support mirrors the CORBA Object Transaction Service (OTS) and supports the flat transaction model OTS defines required support for a flat transaction model and optional support for nested transactions, which allow for transactions to be started within the context of another transaction Each transaction manages its own change set When a nested transaction is rolled back, all changes made within that nested transactions are discarded If the nested transaction is committed, the changes made within it become part of the enclosing transaction's change set, to be committed or rolled back by the enclosing transaction In a nested model, the top-level transactions often correspond to the database transactions In this case, the nested transactions are limited in that often they are not durable (that is, persistent) because right up to the moment in which a top-level commit occurs, the information is not stored in the database EJB transaction support stays with the flat transaction model; this decision is based on the fact that today's database systems and transaction monitors support the flat transaction model EJB uses the Java Transaction Service (JTS), and specifically the Java Transaction API (JTA) JTS maps the OTS API to the Java programming language; that is, it maps the OTS IDL definitions to Java using the IDL-to-Java language mapping In addition, JTS is a Java mapping for the X/Open XA interface standard The JTA provides a convenient set of Java interfaces to be used for transaction processing Such conformance to standards ensures that JTS will remain the standard for transaction handling in Java The JTS javaxtransaction package defines the application-level demarcation APIs that allow transaction bracketing to be defined by the application developer EJB relies on this interface and, more specifically, on the javaxtransactionUserTransaction interface (The EJB container provider needs to implement only this interface) Bean developers and clients make demarcation calls using APIs in this package Transaction demarcation can be performed by the client, the bean, or the container Demarcation performed by the container is the simplest because both client and bean are oblivious to transactions Obviously, the container must use policy information defined by the bean because the container is a generic entity that must adapt its behavior to the bean In the case of transactions, the bean defines a demarcation policy using the transaction attribute The container uses the fact that access to bean functionality must go through the container to manage transaction demarcation The actual decision about what to do is based on the transaction attribute defined by the bean Note that the policy may be defined globally for the entire bean or for one bean method at a time; it is therefore possible for different transactional behaviors to occur for different methods For example, a bean can set its transaction attribute so that every method invocation must reside within its own transaction In this case, the container will start a new transaction for every call made on the bean and manage the propagation of that transaction context if the bean makes additional calls during the processing of the original invocation This scenario is shown in Figure 11-8
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Figure 11-8: An example of container demarcation Possible transaction attribute values are as follows: TX_NOT_SUPPORTED By using this value, the bean notifies the container that it does not support transactions Invocations coming through the container will not be within a transaction The container guarantees this by suspending any transaction that is active during calls made on the bean TX_BEAN_MANAGED This attribute is used by the bean when it intends to use explicit demarcation and wants to notify the container that it should not manage transactions on its behalf TX_REQUIRED This attribute is used by beans that want to ensure that any processing is performed within a transaction If the call is made within a transaction started by a client, the bean invocation is performed within the transaction Otherwise, the container is responsible for starting a transaction before handing control over to the bean The container is also responsible for committing the transaction before returning control to the caller TX_SUPPORTS Used by beans that want to be invoked within the same scope as the client If the client has a transaction open when it calls the bean, the bean will perform within the transaction of the caller; otherwise, it will perform without an active transaction TX_REQUIRES_NEW Beans using this value inform their container that they need to be invoked within the scope of a new transaction No matter what the transaction state of the client, the container always opens a new transaction that is active throughout the operation of the bean When the bean completes, the container commits the transaction and resumes the suspended client transaction TX_MANDATORY This attribute is similar to TX_SUPPORTS in that the bean is activated within the transaction of the client However, if a client without an open transaction tries to make the call, a TransactionRequired exception is raised This attribute is used by beans that need to be invoked within the context of the client transaction but want to guarantee that they cannot be invoked without a transaction Another attribute that must be defined by beans for the container to manage transactions on its behalf is the transaction isolation level that controls the locking 193
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options for reads These values mirror the options available in the javasqlConnection interface Possible values for transaction isolation levels are TRANSACTION_READ_UNCOMMITTED It is possible to read data that is marked as dirty (dirty reads); uncommitted data can be read TRANSACTION_READ_COMMITTED Dirty reads are prevented; only committed data can be read TRANSACTION_REPEATABLE_READ Dirty reads and nonrepeatable reads are prevented TRANSACTION_SERIALIZABLE Transactions are serialized; one transaction must complete for another to proceed TRANSACTION_NONE No transaction is used Although container demarcation is certainly the simplest model for the client and for bean developers, sometimes the set of possible values allowed as transaction attributes does not provide fine enough control over transaction demarcation Both bean-managed and client-managed demarcation are allowed under the EJB transaction model For bean-managed demarcation, the bean will be defined using the TX_BEAN_MANAGED transaction attribute, which informs the container that it should not manage transactions on behalf of the bean The bean can then use the javaxtransactionUserTransaction interface Beans that do not define TX_BEAN_MANAGED as their transaction attribute are not allowed to make direct calls to the underlying transaction manager Alternatively, the client itself can use the javaxtransactionUserTransaction interface to begin, commit, and roll back transactions Because the container and the server can manage transactions, transaction-related exceptions (specifically, TransactionRolledbackException, Transaction RequiredException, and InvalidTransactionException) can be raised Any exception-handling mechanism is generally a mechanism designed f r situations in o which the error detector (the object that raises the exception) can be a different object than the error handler (the object that catches the exception and tries to take corrective measures) Therefore, raising an exception within the context of a container-managed transaction cannot automatically mean that the transaction must be marked for rollback only It is quite possible that the client having caught the exception can fix the problem and continue with a follow-up call to the business method The only case in which the client can assume that the current transaction has been marked for rollback is when the TransactionRolledbackException exception is raised Alternatively, the exception handler can use the explicit test of getRollbackOnly() to verify the status of the active transaction Beans that manage their own transaction processing (that is, those that use TX_BEAN_MANAGED) should use the setRollbackOnly() call before raising the exception, depending on whether the error they have discovered is such that it should not allow the changes made thus far to commit The EJB Distribution Model EJB targets large-scale, distributed object applications Distribution is therefore fundamental to the definition of EJB Application programming in the EJB world involves building the enterprise beans and the clients using the business functionality implemented by the beans The containers, servers, and other EJB infrastructure objects are provided by vendors and are used to simplify the building of distributed applications Therefore, the EJB distribution model must be simple enough so that applications programmers can use it, yet powerful enough to build mission-critical and large-scale applications To accomplish this, the EJB distribution model uses a proven distribution technology based on proxies and object stubs The EJB distribution model is not a new model; it 194
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uses standard Java RMI based on the CORBA distribution model Distribution transparency is complete from both sides Enterprise bean functionality may be accessed locally or remotely, and clients can access enterprise beans with no regard to their actual physical location Neither the client nor the enterprise bean is aware of its peer's location The Java RMI stubs created involve the EJB object and the home interface object Since the EJB object is the interface object through which functionality is accessed, it is only natural for it to be the object for which a Java RMI stub is created In addition, the home interface gets its own RMI stub; this is necessary to initiate the connection with the enterprise bean, either through creation or through a finder process Using Java RMI as the stubs and interfaces for supporting distribution in EJB does not infer the underlying wire-level protocol In fact, the preferred solution for EJB in terms of the network protocol is CORBA IIOP By using IIOP, EJB ensures that its distribution model remains interoperable and open to interfacing with non-Java objects and legacy systems The basing of EJB on CORBA IIOP is quite natural; after all, the distribution model is identical, EJB transactions are based on the CORBA OTS model, and both use the same object model Even the naming issue is resolved by mapping the CORBA naming service, which is part of Common Object Services (COS), to the JNDI API Finally, EJB security, discussed in the following section, is mapped to CORBA security using CORBA::Principal for mapping the user ID, IIOP over the Secure Socket Layer (SSL) for client authentication and encryption, and Secure IIOP (SECIIOP) for mapping to a specific security mechanism such as GSSKerberos
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EJB Security As in transactions, the primary goal of EJB in terms of its security infrastructure is to provide a set of interfaces that allow the responsibility for implementing security to be moved from the enterprise bean developer to the container and server implementer This does not mean that an enterprise bean cannot control security; it means that mechanisms are provided so that the typical enterprise bean can declaratively define what its security attributes are, allowing the implementation details to be handled by the server and container As in transactions, this is achieved through a set of security attributes used at deployment time The security attributes in the EJB deployment descriptor define a list of authorized users, each authorized entry being of type javasecurityPrincipal the standard interface in Java used for access control identification and authorization Additional deployment attributes related to security include the RunAsMode and RunAsIdentity attributes The RunAsMode attribute defines the security identity that will be propagated when the bean makes additional calls while servicing a client request Possible values that may be assumed by this attribute are as follows: CLIENT_IDENTITY This value is used when the call made by the bean should assume the security information of the client making the original call; the bean servicing the second call will think that the original client made the call (in terms of security) This is shown in Figure 11-9
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Figure 11-9: How the CLIENT_IDENTITY attribute is used SYSTEM_IDENTITY The call will use the identity of a privileged system account SPECIFIED_IDENTITY The call will be made using a specific user account The RunAsIdentity attribute is relevant only when the RunAsMode attribute has a SPECIFIED_IDENTITY value; in this case, the RunAsIdentity attribute defines the actual user identity to be used when making the second call
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Summary The EJB component model is the main programming model used in server-side applications It brings structure and consistency to the most complex aspect of enterprise applications server-side processing, which includes transaction management and persistence management We have described the lifecycle of entity beans and session beans and have outlined when you should use each type We have also described the architecture in terms of the EJB container, the EJB server, the client, and the developer roles More detailed discussions on the development and usage of EJBs within WebSphere are covered in s 29 through 34
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12: Extensible Markup Language (XML) Overview Extensible Markup Language (XML) is one of the newer members of WebSphere's technology foundation Starting with version 3x, all WebSphere Application Servers include many of the XML libraries and maintain a lot of their own metadata in XML Version 40 has taken this to a new level; it uses XML throughout the product, both by supporting XML parsing and generating and by using XML as the basis for more advanced technologies, such as Web services This chapter provides information on the various libraries with which you should be familiar when working with XML Later chapters will delve into the uses of XML for building and deploying applications and will discuss the use of the Document Object Model (DOM), the Simple API for XML (SAX), and XSL Transformations (XSLT) While working with XML within WebSphere means working with Java libraries that create or process XML documents, the topics discussed in this chapter hold true for all XML processing, regardless of the programming language being used The New "Silver Bullet" XML is the new "silver bullet" of the information technology (IT) world that is, it is the solution for many difficult IT problems XML is being used today as the basis for integration and automation two keys to efficient deployment of complex information systems In addition, XML is perhaps the single most important infrastructure component for e-business applications, because it is the key to allowing systems created by different vendors to talk to one another and share data It is also the basis for Enterprise Application Integration (EAI) systems such as Vitria, TIBCO, MQSeries, Web Methods, Cross Worlds, and more Everywhere you look, you see vendors and developers scurrying to XML and for good reason XML is a standard that was developed by the World Wide Web Consortium (W3C) to improve data sharing and formatting over the Web; in general, it supports the notion of tagged data in a way that preserves the semantics of the domain XML is an application (that is, a subset) of the Standard Generalized Markup Language (SGML), but it is much simpler to use It is more powerful than Hypertext Markup Language (HTML); in fact, HTML is a subset of XML Therefore, XML is just the right combination of SGML's powerful expression options and HTML's simplicity, and it is already slated to become the lingua franca of the Web In addition, it promises advances in other areas, such as business-to-business applications, Electronic Data Interchange (EDI), and document management XML is the center of intensive research and development, both within the W3C and throughout the industry As a standard, it is the basis for many other standards that are being developed by the W3C such as the Extensible Linking Language (XLL), the DOM, SAX, and XSLT All of these specifications can be found on the W3C site at http://wwww3org, as well as on other sites such as those for OASIS and most major vendors, including IBM, Microsoft, Sun, and Oracle XML is already an important technology used for e-business applications, and we are sure this trend will accelerate quickly to the point where XML and e-business are almost synonymous A huge number of applications use XML today, and the sources for these applications include more than the W3C and commercial vendors For example, the Financial Products Markup Language (FpML) is an application of XML that describes financial products such as derivatives and swaps It is being developed by JPMorgan 197
and PricewaterhouseCoopers and is being distributed under the Mozilla license; an example of an FpML document is shown in Listing 12-1 Note An obvious question is "Why would JPMorgan publish source code in which it has invested, which could then be used by its competitors to a technological advantage " The answer is, obviously, not simply that the people at JPMorgan are "nice" It is because companies such as JPMorgan understand that they exist in a new Web and ebusiness paradigm in which open standards enable services and systems that were never possible before XML is leading this charge Listing 12-1: FpML example < xml version="10" standalone="no" > <!-- Copyright (c) 1999 by JPMorgan and PricewaterhouseCoopers PricewaterhouseCoopers refers to the individual member firms of the world wide PricewaterhouseCoopers organization All rights reserved --> <!-- version 10b2 : August 6, 1999 --> <fpml:FpML xmlns:fpml="urn:fpml-FpML" xmlns:t="urn:fpml-type" xmlns:d="urn:fpml-date" xmlns:m="urn:fpml-money" xmlns:r="urn:fpml-rate">
<fpml:Trade> <fpml:tradeIDs> <tid:TradeIDs xmlns:tid="urn:fpml-TradeID"> <tid:TradeID> <tid:partyReference>ABC Trust</tid:partyReference> <tid:transactionReferenceNumber>237732</tid:transactionReferenceNu mber> </tid:TradeID> <tid:TradeID> <tid:partyReference>XYZ Group</tid:partyReference> 198
<tid:transactionReferenceNumber>1230</tid:transactionReferenceNumb er> </tid:TradeID> <tid:TradeID> <tid:partyReference>CDF Inc</tid:partyReference> <tid:transactionReferenceNumber>237732</tid:transactionReferenceNu mber> </tid:TradeID> <tid:TradeID> <tid:partyReference>TUV Ltd</tid:partyReference>
<tid:transactionReferenceNumber>1230</tid:transactionReferenceNumb er> </tid:TradeID> <tid:TradeID> <tid:partyReference>MNO LLC</tid:partyReference> <tid:transactionReferenceNumber>237732</tid:transactionReferenceNu mber> </tid:TradeID> <tid:TradeID> <tid:partyReference>HIJcom</tid:partyReference> <tid:transactionReferenceNumber>1230</tid:transactionReferenceNumb er> </tid:TradeID> </tid:TradeIDs> </fpml:tradeIDs> </fpml:Trade>
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