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Any business interface can throw application exceptions. These should describe a business logic problem in this case, a problem in encryption/decryption. Application exceptions should be meaningful to the client, providing a brief and relevant identification of the error. Our new EncryptionException will describe a specific business problem that is possibly recoverable; this makes it an application exception. The EJB container treats any
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exception that does not extend RuntimeException as an application exception. Here is the definition of EncryptionException:
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@ApplicationException // Explicit annotation, though this is inferred as default because we extend Exception public class EncryptionException extends Exception{...}
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An application exception is propagated to the calling client as is. Any instance variables you include in these exceptions should be serializable. Nonapplication exceptions are always wrapped, by the Container, in an EJBException. This means that any exception you throw that is or extends RuntimeException will be caught by the EJB container and wrapped in an EJBException. This is especially important for session beans that interact with entity beans. All exceptions thrown by Java Persistence interfaces are RuntimeExceptions. Your client code must be aware of this if it needs to take action on specific persistence exceptions. Exception behavior, preventing the wrapping in EJBEx ception, can be declared explicitly using the @javax.ejb.ApplicationException and the <application-exception> XML deployment descriptor metadata. These constructs are discussed in detail in 17, as they have a huge impact on transactional behavior.
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Bean Implementation Class
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The EncryptionEJB models a specific business process, so it is an excellent candidate for a stateless session bean. This bean really represents a set of independent operations another indication that it is a good candidate for a stateless session bean. First we ll reveal any nonobvious types to be imported (all will be discussed in turn):
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import import import import import import javax.annotation.PostConstruct; javax.annotation.Resource; javax.ejb.Local; javax.ejb.Remote; javax.ejb.SessionContext; javax.ejb.Stateless;
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Annotations upon the bean implementation class provide the metadata declaring our SLSB and business interfaces:
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@Stateless(name = "EncryptionEJB") @Local(EncryptionLocalBusiness.class) @Remote(EncryptionRemoteBusiness.class) public class EncryptionBean implements EncryptionLocalBusiness, EncryptionRemoteBusiness { ...
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The bean class is annotated with the @javax.ejb.Stateless annotation to identify that it is a stateless session bean. The name() attribute identifies the EJB name of the session bean. The EJB name defaults to the unqualified (simple) name of the bean class if you initialize this attribute; in this case, the EJB name would default to EncryptionBean. In most cases, you don t have to be aware of the concept of an EJB name, but it s useful
when you want to override or augment metadata with an XML deployment descriptor. This will allow us to externalize properties and provide configuration for the service without requiring recompilation of the Java sources. When used on the bean class, the @Local and @Remote annotations take an array of interface classes. It is not recommended that you use this approach unless you have to, as implementing your business interfaces directly enforces the contract between the bean class and these interfaces. These examples opt for the bean implementation class approach simply to centralize annotation metadata for readability, and the contract is honored by ensuring the bean implementation class explicitly implements all business interfaces. The EncryptionEJB identifies its remote and local interfaces via the class-level annotations upon the bean implementation class. As we ve mentioned before, @Remote and @Local could also go directly upon the EncryptionBusinessRemote and EncryptionBusinessLocal interfaces:
@Local public interface EncryptionLocalBusiness extends EncryptionCommonBusiness {} @Remote public interface EncryptionRemoteBusiness extends EncryptionCommonBusiness {}
When an EJB is deployed, the container looks at the interfaces of the bean class to see whether they are annotated as business interfaces. This introspection may determine the business remote and business local interfaces of the bean class. The art of creating a truly secure encryption service is beyond scope of this exercise, so we ll leave the implementation of the business methods for you to explore in runnable examples detailed in the appendix. Worthy of note, however, is that our bean implementation class must perform some initialization before it s able to do any real work. For this purpose, we introduce:
/** * Initializes this service before it may handle requests * * @throws Exception If some unexpected error occurred */ @PostConstruct public void initialize() throws Exception { // Log that we're here log.info("Initializing, part of " + PostConstruct.class.getName() + " life cycle"); // Get the passphrase for symmetric encryption final String passphrase = this.getCiphersPassphrase(); // Do initialization tasks here ...
}
The @PostConstruct annotation defines a callback method to be executed at the proper point within the SLSB lifecycle. In this case, initialize() will be invoked by the Container after it has created the bean instance and performed any injection operations. We ve also defined the internal method getCiphersPassphrase(), which we ll use to obtain an externalized parameter as the passphrase (secret key) for use in symmetric encryption.
Accessing Environment Properties (Injection and Lookup)
Our bean implementation class declares a few instance members as part of its internal state:
/** * Passphrase to use for the key in cipher operations; lazily initialized * and loaded via SessionContext.lookup */ private String ciphersPassphrase; /** * Algorithm to use in message digest (hash) operations, injected * via @Resource annotation */ @Resource private String messageDigestAlgorithm;
Remember that the internal state for each instance is permissible, so long as these don t leak out to the client. Stateless session beans may contain no conversational state! Two of these properties, ciphersPassphrase and messageDigestAlgorithm, represent configurable attributes that we ll provide via externalized variables called environment entries. An environment entry is a value placed into JNDI under a private namespace called the Enterprise Naming Context (ENC), which is visible only to our EJB. There are a variety of ways to get at the values within the ENC, and this example will illustrate both manual lookup and injection. The messageDigestAlgorithm is annotated with @Resource; this tells the EJB container that when an instance of the bean class is created, this field must be initialized with values in the container s ENC. When the EJB container is deployed, the ENC is populated both with metadata embedded in annotations such as @Resource and with information stored in the optional EJB XML deployment descriptor. For example, the annotation alone suggests a named value within the ENC should be used to initialize the field externally. This named value can be configured by defining environment entries within the EJB s XML deployment descriptor:
<ejb-jar xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/ejb-jar_3_1.xsd"
version="3.1"> <enterprise-beans> <!-In this section we'll bolster our EncryptionEJB with some additional metadata to complement the info defined via annotations. --> <session> <!-This will match the value of @Stateless.name upon our bean implementation class --> <ejb-name>EncryptionEJB</ejb-name> <!-- Override the ciphers default passphrase --> <env-entry> <env-entry-name>ciphersPassphrase</env-entry-name> <env-entry-type>java.lang.String</env-entry-type> <env-entry-value>OverriddenPassword</env-entry-value> </env-entry> <!-- Override the default unidirectional hash MessageDigest algorithm --> <env-entry> <env-entry-name>messageDigestAlgorithm</env-entry-name> <env-entry-type>java.lang.String</env-entry-type> <env-entry-value>SHA</env-entry-value> </env-entry> </session> </enterprise-beans> </ejb-jar>
When this file, META-INF/ejb-jar.xml, is included in the packaging of the EJB, the Container will place the designated values within the ENC of our EncryptionEJB upon deployment. The messageDigestAlgorithm field may then be injected by the Container. But we ve defined no annotation upon the field ciphersPassphrase, so we ll look it up manually. The @Resource annotation is very versatile, and when placed upon a known type such as SessionContext, the Container knows how to handle this injection request. Through the lookup() method upon a SessionContext, we can poke into the ENC to obtain the value we re looking for:
private String getEnvironmentEntryAsString(final String envEntryName) throws IllegalStateException { // Get the SessionContext from the injected instance member final SessionContext context = this.context;
// Lookup in the Private JNDI ENC via the injected SessionContext Object lookupValue = null; try { lookupValue = context.lookup(envEntryName); log.debug("Obtained environment entry \"" + envEntryName + "\": " + look upValue); } catch (final IllegalArgumentException iae) { // Not found defined within this EJB's Component Environment, // so return null and let the caller handle it log.warn("Could not find environment entry with name: " + envEntryName); return null; } // Cast String returnValue = null; try { returnValue = String.class.cast(lookupValue); } catch (final ClassCastException cce) { throw new IllegalStateException("The specified environment entry, " + lookupValue + ", was not able to be represented as a " + String.class.getName(), cce); } // Return return returnValue;
}
We may use something similar to the previous code to extract out the cipher passphrase from the ENC in a manual fashion. Because of this extra burden upon the developer, it s easy to see why injection is the preferred mechanism to obtain information from the Container. Externalization of values is incredibly important. In this case, the application assembler or deployer of the EJB may choose a secret passphrase for the encryption scheme in production environments, and developers of the application never have to be exposed to this privileged information. We ve also used environment entries to make our service configurable.
It s worth noting: never, ever design to store passwords in a file to be placed on the filesystem. By definition, this is insecure. This example is in place to illustrate the absence of conversational state while maintaining internal state and should not be used as a real-world encryption system.
The exact semantics of the <env-entry> and @Resource annotation are discussed in detail in 16.
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