How do ejb access the database

I'm sure its something to do with setting up ODBC data source in control panel I am using J2EE 1. Raj Chila. I like You will have to create a System DSN preferably as it will be visible to all users and other services applies for windows Environment. I hope this helps. Thanks - I have set up the data source, but it is the "Then you will have to configure your J2EE server to use the correct driver for the Database" part that I do not understand how to do Setting up databases in sun one I have not worked on this before so I dont really know if I can add a new database configuration.

This is due to a technical limitation that does not allow the EJB container to distinguish duplicates in the underlying JDBC result set. With relationship caching enabled, changes to the relationship are automatically reflected in cache. For instance, if an instance is added on the "many" side of a one-to-many relationship, that change is reflected in the cached relationship—a subsequent query to the bean on the "one" side of the relationship causes the relationship to be refreshed in cache, so that the new instance is available to the query.

Specify the caching-name element in the weblogic-query element of the weblogic-cmp-jar. If a caching-name element is specified in the weblogic-query element, when the finder query is executed, WebLogic Server loads data for related beans as specified by the relationship-caching element that the caching-name element specifies. Make sure that the finders-load-bean element in weblogic-ejb-jar. The default value of finder-load-bean is True. Specify the database-type element in the weblogic-cmp-jar.

Relationship caching uses outer joins for queries and outer join syntax can vary by database type. Because relationship caching uses join queries, the number of caching-element elements in the relationship-caching element can increase duplicates in the result set. Specify one one-to-many relationships per caching-element to avoid duplicates in the result set.

Specify the relationship-caching element in weblogic-cmp-jar. Using nested caching-element elements enables the bean to load more than one level of related beans. In the above sample, addressBean is the second level related bean because it is nested in the accountBean. Currently, there is no limitation on the number of caching-element s that you can specify. However, setting too many caching-element levels could have an impact on the performance of the current transaction.

An entity bean's concurrency strategy specifies how the EJB container should manage concurrent access to the bean; it determines how and when WebLogic Server synchronizes its cached copy of the entity with the database. With exclusive concurrency, the container places an exclusive lock on cached EJB instances when the bean is associated with a transaction.

Other requests for the EJB instance are blocked until the transaction completes. Exclusive locks are local to the server instance, so this strategy is most appropriate for a single server architecture.

When used in a cluster, exclusive concurrency serializes all requests to a bean instance within a server instance, but concurrency between servers in the cluster that access the same bean concurrently is governed by the database. Multiple clients can use the exclusive concurrency option to access entity EJBs in a serial fashion. Using this exclusive option means that if two clients simultaneously attempt to access the same entity EJB instance an instance having the same primary key , the second client is blocked until the EJB is available.

With database concurrency, concurrency control for an EJB for each transaction is deferred to the underlying datastore. WebLogic Server allocates a separate entity bean instance to each transaction and allows concurrency control to be handled by the database. This is the default option. With the Database mechanism, the EJB container continues to cache instances of entity bean instances. However, the container does not cache the intermediate state of a bean instance between transactions.

The request to commit data is subsequently passed along to the database. The database, therefore, handles all concurrency control and deadlock detection for the EJB's data. As with the Database concurrency strategy, Optimistic concurrency gives each transaction its own bean instance.

The Optimistic concurrency strategy does not hold any locks in the EJB container or the database while the transaction is in process. To prevent optimistic data from going stale, the container activates a new bean instance for each transaction so that requests proceed in parallel. WebLogic Server calls ejbLoad for entity beans based on the value specified in read-timeout-seconds , provided the value of cache-between-transactions is True.

You can invalidate optimistic beans explicitly when the underlying data is updated outside the EJB container this is also known as a "backdoor update". By default, when a bean that has a concurrency-strategy of Optimistic is deployed in a cluster and a member of the cluster updates the bean, the EJB container attempts to invalidate all copies of the bean in all servers in the cluster.

This invalidation allows you to avoid optimistic concurrency failures, but can be a drain on performance because it is a resource-intensive operation. If you prefer, you can set cluster-invalidation-disabled in weblogic-cmp-jar. You can configure the EJB container to validate an Optimistic bean's transaction data before committing the transaction, to verify that no data read or updated by the transaction has bean changed by another transaction. If it detects changed data, the EJB container rolls back the transaction.

Configure validity checking for a bean with Optimistic concurrency using the verify-columns and verify-rows elements in the table-map element for the bean in weblogic-cmp-jar. The verify-rows element specifies the rows in a table that the EJB container should check when optimistic concurrency is used; the verify-columns element specifies how columns in a table are checked for validity when you use the optimistic concurrency strategy.

Read —To check all rows in the table that have been read during the transaction. This includes both rows that are simply read and rows that are read and then updated or deleted by the transaction. Checking all rows entails additional overhead because it generally increases the amount of optimistic checking that must be performed by the EJB container.

With the Read option, committed transactions read a set of rows that are guaranteed not to be modified by another transaction until after the transaction commits. Modified —To check only the rows that have been updated or deleted by the current transaction. This setting ensures that two transactions will not update the same row concurrently, resulting in a lost update, but it allows reads and updates of different transactions to be interleaved. Read —To check all columns in the table that have been read during the transaction.

Modified —To check only the columns that have been updated or deleted by the current transaction. Version —To check that a version column exists in the table and that this column is used to implement optimistic concurrency. A version column must be created with an initial value of 0 or any positive integer, and must increment by 1 whenever the row is modified. For more information, see version-column-initial-value. Additionally, if you use database triggers to update version columns in the database and you set trigger-updates-optimistic-column to True , the database triggers must initialize the version column in the database when the bean is created.

Timestamp —To check that a timestamp column exists in the table and that this column is used to implement optimistic concurrency. Timestamp-based optimistic concurrency requires a 1 second granularity for the database column. By default, the EJB container manages the version and timestamp columns and ensures that these columns are kept up to date.

If you choose to instead let database triggers maintain version and timestamp columns, set the value of trigger-updates-optimistic-column to True. If verify-columns is set to Version or Timestamp :. Specify the version or timestamp column using the optimistic-column in the table-map element in the weblogic-cmp-jar. Mapping this column to a cmp-field is optional. The optimistic-column element identifies a database column that contains a version or timestamp value used to implement optimistic concurrency.

This element is case maintaining, though not all databases are case sensitive. The value of this element is ignored unless verify-columns is set to Version or Timestamp. Specify the initial value of the version column using the version-column-initial-value element in the weblogic-cmp-jar. If the EJB is mapped to multiple tables, optimistic checking is only performed on the tables that are updated during the transaction. This problem occurs because of a mismatch in date value precision between the Oracle DATE column and the java.

Date type. The java. There are two ways to avoid this error:. Set the Oracle database column type to Timestamp , a higher precision type introduced in Oracle9i.

Include logic in your application to zero out the milliseconds of a java. Date value. To accomplish this, prepare a date field for an entity bean java.

Date field in this way:. Used to signify that a bean is read-only. The container activates a new bean instance for each transaction so that requests proceed in parallel. WebLogic Server calls ejbLoad for read-only beans based on the read-timeout-seconds parameter.

To allow generation of more efficient code for read-only beans, create and remove operations are not allowed for EJBs that use ReadOnly concurrency in this release of WebLogic Server. To allow read-only beans to use create and remove operations—for instance, to support legacy behavior—set the allow-readonly-create-and-remove element in weblogic-cmp-jar. Concurrency strategies present a trade-off between performance and freshness of data.

You should choose a concurrency strategy based on which of these factors weighs more heavily in your application. The trade-offs are summarized in Table Table Concurrency Strategy Trade-offs. Deferring concurrency control to the database improves throughput, compared to exclusive concurrency, for concurrent access to data and provides deadlock detection.

Mitigate deadlock risk by setting use-select-for-update in weblogic-cmp-jar. This causes the database to take out an exclusive lock when the read is done, avoiding the deadlock that can occur when a read lock is upgraded to an exclusive lock. Makes the bean more dependent on the database's lock policies, which might reduce the bean's portability. If the database concurrency control is sufficient for your application and you do not require additional features provided by the container.

Note : Use the transaction-isolation element in combination with Database concurrency to achieve the desired concurrency behavior. Provides highest level of concurrent access, as it holds no locks in the EJB container or database during a transaction. If multiple transactions are unlikely to attempt to modify the same application data at the same time.

Serializes access to EJB data in a single server non-clustered environment for a high level of consistency. Avoids deadlocks due to lock upgrades, and prevents unnecessary calls to ejbLoad to refresh the bean instance's persistent fields. Performance degradation can result. Once a client has locked an EJB instance, other clients are blocked from the EJB's data, even those who require only read-access.

For applications in which a high level of concurrency is essential, and more important than performance. Specify the concurrency mechanism for a bean by setting the concurrency-strategy element in the entity-cache element in weblogic-ejb-jar. Because concurrency-strategy is defined at the bean level, different beans in the same application can use different concurrency strategies, as appropriate. If you do not specify a concurrency-strategy , WebLogic Server uses Database c oncurrency by default.

In situations of high throughput, transactions that use an exclusive concurrency strategy can encounter deadlocks if a transaction that performs a cascade delete needs access to the same entity bean as a transaction that does not perform a cascade delete. You can prevent such deadlocks with the lock-order element of weblogic-cmp-jar.

Its value is an integer value. The bean with the lowest lock-order value is processed first, the bean with the next lowest lock-order value is processed next, and so on.

The locking order specified should be the same as the locking order used by other transactions in the application. If the normal lock order is BeanA, then BeanB, specify this lock-order , and cascade delete will use it.

For persistent data that is only occasionally updated, you can implement a "read-mostly pattern" in WebLogic Server by mapping a read-only and a read-write entity bean to the same data. You use the read-only entity bean for reads and the read-write entity bean for writes.

The read-only entity EJB loads bean data at intervals specified by the read-timeout-seconds element in the entity-cache or entity-cache-ref element for the bean in weblogic-ejb-jar. To ensure that the read-only bean always returns current data, the read-only bean must be invalidated when the read-write bean changes the entity bean data. In a WebLogic Server cluster, the read-mostly pattern gives clients of the read-only EJB the performance advantage of reading from cache, while clients of the read-write EJB enjoy true transactional behavior—the read-write EJB's cached state always matches the persistent data in the database.

These practices will reduce the likelihood of data consistency problems with the read-mostly pattern. Configuring the read-only beans' read-timeout-seconds element in weblogic-ejb-jar. To update the minimum data set necessary; avoid implementing beans that write numerous, unchanged fields to the datastore at each ejbStore. To update their data on a timely basis; do not use a read-write bean in lengthy transactions that might run longer than the read-timeout-seconds setting for its read-only counterparts.

To invalidate the read-only counterpart of a read-write bean when the read-write bean updates bean data. If you are running EJB 2. An optimistic bean acts like a read-only bean when performing a read—it reads from the cache and can return stale data. However, when an optimistic bean performs a write, the container ensures that the data being updated has not changed—providing the same level of consistency for writes as a bean that uses Database concurrency. See Choosing a Concurrency Strategy.

The invalidation-target element in the entity-descriptor element in weblogic-ejb-jar. The target ejb-name must be a read-only entity EJB. In this release of WebLogic Server, you can invalidate any optimistic entity bean that has cache-between-transactions enabled, by calling the following invalidate method on either the CachingHome or CachingLocalHome interface.

The following example code shows how to cast the home to CachingHome and then call the method:. Example Casting the Home and Calling the Method. The invalidate method causes the entity beans to be invalidated in the local server instance. If the server instance is a member of a cluster, it multicasts a message to the other clustered servers to invalidate their cached copies of the bean.

Upon the next getXXX to an invalidated entity bean, the container loads the current version of the bean's persistent data to the entity cache from the database, as described in Understanding ejbLoad and ejbStore Behavior. WebLogic Server calls invalidate after the update transaction has completed.

If the invalidation occurs during a transaction update, the previous version might be read if the isolation-level for transactions does not permit reading uncommitted data. Each section contains the elements related to a particular type of feature or behavior. The table in each section defines relevant elements terms of: the behavior it controls, the parent element in weblogic-cmp-jar. The following lists the container-managed relationship elements in weblogic-cmp-jar. Table Container-managed Relationship Elements in weblogic-cmp-jar.

Note : If an ejb-relation-name for the relationship is specified in ejb-jar. The name of the relationship role. A relationship has two roles—one for each side of the relationship. For a or 1-m relationship, specify only the role on the foreign-key side. For a m:m relationship, specify the roles on both sides of the relationship.

For an example, see Defining a Many-to-Many Relationship. Specifies the target side of the key column mapping for the relationship—the foreign key column. Specifies the initiating side of the key column mapping for the relationship—the primary key column. Table Primary Key Elements in weblogic-cmp-jar. Identifies the facility used to generate primary keys. Understanding Entity Pooling If you specify a non-zero value for the initial-beans-in-free-pool element in weblogic-ejb-jar.

Understanding Entity Caching When a business method is called on a bean, the container obtains an instance from the pool, calls ejbActivate , and the instance services the method call. How many instances with same primary key can exist in cache simultaneously? Understanding Passivation of Entity Beans Entity beans that are involved in transactions can be passivated when necessary in order to attempt to prevent a CacheFullException when an attempt is made to insert an entity bean into a cache that is full.

To optionally program EJBs to notify their cache that they are done performing operations in the current transaction, you can use the operationsComplete Java API as follows: weblogic. EJBLocalObject public. EJBObject public. Note: When used with CMP 1. Controlling the Behavior of ejbLoad and ejbStore For applications in which multiple clients can currently access and modify a bean's underlying data, the default behavior of ejbLoad and ejbStore described in Understanding ejbLoad and ejbStore Behavior ensures database integrity by: Ensuring that each new transaction uses the latest version of the EJB's persistent data, and Updating the database upon transaction commitment.

Disabling Cache Flushing According to the EJB specification, updates made by a transaction must be reflected in the results of query-finders and ejbSelect s issued during the transaction. Configuring Application-Level Caching Application-level caching—also known as "combined caching"—allows multiple entity beans that are part of the same Java EE enterprise application to share a single runtime cache. Application-level caching offers the following advantages: Reduces the number of entity bean caches, and hence the effort to configure the cache.

Provides better scalability Application-level caching is not the best choice, however, for applications that experience high throughput. To configure an application-level cache: Verify that the weblogic-application. Note that: entity-cache-name should be the name of the application-level cache, as specified in weblogic-application.

Guidelines for Primary Keys Follow these suggestions when using primary keys with WebLogic Server: Do not construct a new primary key class with an ejbCreate.

If you need to use BigDecimal as the primary key, you should: Implement a primary key class. In the primary key class, implement the boolean equal Object x method. In the equal method, use boolean BigDecimal. WebLogic Server supports two methods of automatic primary key generation: Native database primary key generation—The database generates the primary key. EJBException: nested exception is: java. Instead, use the AutoKey option with Oracle databases. Generating Primary Keys with a Named Sequence Table A sequence table is a database-neutral way to generate primary keys.

Declaring Primary Key Field Type For both native DBMS primary key generation, or key generation using a named sequence table, in the abstract get and set methods of the associated entity bean, declare the primary field type to be either: java.

Integer java. Long In weblogic-cmp-jar. Benefits of String Trimming Untrimmed primary key fields can cause comparison operators to fail and can result in non-portable behavior. For instance, suppose: a primary key field is mapped to the char 10 datatype in database you insert the value " smith " into the database column since "smith" is five characters in length and the char datatype is a fixed-length—of ten characters, in this case—the database pads the value by appending five blank spaces, resulting in " smith " being inserted into the database you issue a SELECT statement to retrieve " smith " from the database, only, due to the database-appended characters, " smith " is retrieved A comparison of the retrieved " smith " value with the original " smith " string would fail unless the retrieved value was first trimmed of its trailing spaces.

Disabling String Trimming Automatic string trimming is enabled by default in this release. Automatic Table Creation Development Only To make iterative development easier, the WebLogic Server EJB container can be configured to automatically change the underlying table schema as entity beans change, ensuring that tables always reflect the most recent object relationship mapping.

Note: This feature is disabled when a server instance is running in production mode, as a production environment may require the use of more precise table schema definitions. Results in this behavior Disabled The EJB container takes no action when underlying table schema changes. CreateOnly For each CMP bean in the JAR , if there is no table in the database for the bean, the container attempts to create the table based on information found in the deployment descriptor, including join tables, sequence tables, and Oracle database sequences.

Delaying Database Inserts Because of timing issues that may occur when creating a CMP bean, WebLogic Server enables you to specify at what point during the bean creation process the bean is inserted into the database. Why Delay Database Inserts? This factor in combination with other conditions can lead to these problems: Problem 1: Non-null foreign key constraint If a database row is inserted after ejbCreate that is, before ejbPostCreate , then the foreign key is given a null value.

Problem 2: Creating beans during ejbPostCreate When a related bean is created, the database insert for that bean happens before the create call finishes.

Note: In a one-to-one relationship, if the parent bean's primary key is embedded in the child bean's CMR field, when the EJB container creates the beans, it will not check if the parent bean has children, for performance reasons. To avoid a duplicationKeyException database exception, you must set the foreign key constraint on the child table in the database.

Configuring Delayed Database Inserts You can delay database inserts until the end of the ejbCreate method or ejbPostCreate method, using the delay-database-insert-until element in weblogic-cmp-jar. Limiting Database Reads with cache-between-transactions As described in Understanding ejbLoad and ejbStore Behavior , by default, WebLogic Server calls ejbLoad each time a transaction is initiated for an entity bean.

When long-term caching is configured for a: ReadOnly bean, WebLogic Server ignores the value of the cache-between-transactions. Note: If a bean with Exclusive concurrency is deployed in a cluster long-term caching is automatically disabled because any server instance in the cluster may update the bean data. This would make caching between transactions impossible.

Note: In a cluster, when a bean with Optimistic concurrency is updated, notifications are broadcast to other cluster members to prevent optimistic conflicts.

Optimistic concurrency is not available for BMP beans. Human Resources Skills. Health Care Skills. Finance Skills. All Courses. All Practice Tests. Ejb enterprise Javabeans Tutorial. Job Recommendation Latest. Jobs in Meghalaya Jobs in Shillong. View All Locations. How to design your resume? Have you ever lie on your resume? Read This Tips for writing resume in slowdown What do employers look for in a resume? Interview Tips 5 ways to be authentic in an interview Tips to help you face your job interview Top 10 commonly asked BPO Interview questions 5 things you should never talk in any job interview Best job interview tips for job seekers 7 Tips to recruit the right candidates in 5 Important interview questions techies fumble most What are avoidable questions in an Interview?

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Ejb Exception Handling. Ejb Web Services. Create a project with a name EjbComponent under a package com. Create Book. Create LibraryPersistentBean. Clean and Build the application to make sure business logic is working as per the requirements. Finally, deploy the application in the form of jar file on JBoss Application Server. JBoss Application server will get started automatically if it is not started yet. Modify it as shown below.