java qr code reader Transitioning out of the Method-Ready Pool: The death of a stateless bean instance in Java

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Transitioning out of the Method-Ready Pool: The death of a stateless bean instance
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Bean instances leave the Method-Ready Pool for the Does Not Exist state when the server no longer needs them that is, when the server decides to reduce the total size of the Method-Ready Pool by evicting one or more instances from memory. The process begins when a PreDestroy event on the bean is triggered. The bean class can register for this event by annotating a method with @javax.annotation.PreDestroy. The container calls this annotated method when the PreDestroy event is fired. This callback method can be of any name, but it must return void, have no parameters, and throw no checked exceptions. The bean class may define only one @PreDestroy method (but it is not required to do so). An @PreDestroy callback method can perform any cleanup operations, such as closing open resources.
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The duration of a stateless bean instance s life is assumed to be very long. However, some EJB servers may actually destroy and create instances with every method invocation, making this strategy less attractive. Consult your vendor s documentation for details on how your EJB server handles stateless instances.
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@Stateless public class MyBean implements MyLocalBusiness { @PreDestroy public void cleanup( ) { ... }
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Alternatively, you can declare your @PreDestroy method in the EJB s XML deployment descriptor:
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<ejb-jar> <enterprise-beans> <session> <ejb-name>MyEJBName</ejb-name> <pre-destroy> <lifecycle-callback-method>cleanup</lifecycle-callback-method> </pre-destroy> </session> </enterprise-beans> </ejb-jar>
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As with @PostConstruct, @PreDestroy is invoked only once: when the bean is about to transition to the Does Not Exist state. During this callback method, the SessionContext and access to the JNDI ENC are still available to the bean instance. Following the execution of the @PreDestroy method, the bean is dereferenced and eventually garbage-collected.
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Example: The EncryptionEJB
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Often we write applications that require users to choose a password or enter a credit card number. Storing this sensitive data in its raw form (called cleartext) represents a security risk: if someone were to gain unauthorized access to our database, he or she could query for this information and we d be liable for any damage done. A nice alternative to persisting these fields in human-legible format is to encrypt them using our application. Because this is a simple request-response model requiring no conversational state, the stateless session bean is ideally suited to handle these requirements efficiently. Other EJBs or standalone clients may later leverage the generic encryption service we provide in this component. The EncryptionEJB example is discussed in full detail in Appendix B. There are two techniques we ll consider: Cryptographic hashing Cipher-based symmetrical encryption Cryptographic hashing is the process in which some input is transformed into a reproducible, fixed-size result. The algorithm is unidirectional, meaning that it s not mathematically possible to un-hash the output back to its original form. By storing the
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hash of passwords only, we may compare hashes of login attempts against the stored value (see Figure 5-3).
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Sometimes we need to be able to get the cleartext back out of an encrypted result. For example, perhaps our application offers storage of a previously used credit card number for future purchases. For this, we may employ symmetrical encryption, which dictates that some key (backed by a passphrase) may both encrypt and decrypt the data. Without the correct passphrase, attempts to decode become very tricky for potential attackers.
The Contract: Business Interfaces
Using the requirements described in the previous section, we can flesh out the contract for our EncryptionEJB:
public interface EncryptionCommonBusiness { /** * Encrypts the specified String, returning the result * * @param input * @return * @throws IllegalArgumentException If no input was provided (null) */ String encrypt(String input) throws IllegalArgumentException; /** * Decrypts the specified String, returning the result. The general * contract is that the result of decrypting a String encrypted with * {@link EncryptionCommonBusiness#encrypt(String)} will be equal * by value to the original input (round trip). * * @param input * @return * @throws IllegalArgumentException If no input was provided (null) */ String decrypt(String input) throws IllegalArgumentException;
Here we account for our symmetrical encryption functions:
/** * Returns a one-way hash of the specified argument. Useful * for safely storing passwords. * * @param input * @return * @throws IllegalArgumentException If no input was provided (null) */ String hash(String input) throws IllegalArgumentException; /** * Returns whether or not the specified input matches the specified * hash. Useful for validating passwords against a * securely stored hash. * * @param hash * @param input * @return * @throws IllegalArgumentException If either the hash or input is not provided (null) */ boolean compare(String hash, String input) throws IllegalArgumentException; }
And these methods will handle our one-way cryptographic hashing. The Encryption CommonBusiness interface again becomes the base for both our local business and remote business views:
public interface EncryptionLocalBusiness extends EncryptionCommonBusiness {} public interface EncryptionRemoteBusiness extends EncryptionCommonBusiness {}
A business interface can be a remote or local interface but not both. Remote business interfaces are able to receive method invocations from networked clients. When a client invokes a method on a session bean s remote interface, the parameter values and return value are copied. This is true regardless of whether the client is running in the same VM or on another machine in the network. This is known as call-by-value semantics. Local interfaces are available only within the same JVM as the session bean. Invoking on a local interface does not copy the parameters or return value. Because of this, local interfaces are said to follow what is termed call-by-reference semantics.
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