POJO Cache

The section lists some basic terminology that will be used throughout this guide.

JBoss Cache consists of two components, Core Cache, and POJO Cache. Core Cache provides efficient memory storage, transactions, replication, eviction, persistent storage, and many other "core" features you would expect from a distributed cache. The Core Cache API is tree based. Data is arranged on the tree using nodes that each offer a map of attributes. This map-like API is intuitive and easy to use for caching data, but just like the Java Collection API, it operates only off of simple and serializable types. Therefore, it has the following constraints:

POJO Cache attempts to address these issues by building a layer on top of Core Cache which transparently maps normal Java object model operations to individual Node operations on the cache. This offers the following improvements:

In POJO Cache, these are the typical development and programming steps:

More details on these steps will be given in later chapters.

Since POJO Cache is a layer on-top of Core Cache, all features available in Core Cache are also available in POJO Cache. Furthermore, you can obtain an instance to the underlying Core Cache by calling PojoCache.getCache(). This is useful for resusing the same cache instance to store custom data, along with the POJO model.

Here are the current features and benefits of PojoCache:

To use POJO Cache, you obtain the instance from the PojoCacheFactory by supplying a config file that is used by the delegating Cache implementation. Once the PojoCache instance is obtained, you can call the cache life cycle method to start the cache. Below is a code snippet that creates and starts the cache:

String configFile = "replSync-service.xml";
boolean toStart = false;
PojoCache pcache = PojoCacheFactory.createCache(configFiel, toStart);
pcache.start(); // if toStart above is true, it will starts the cache automatically.
pcache.attach(id, pojo);
// ...
pcache.stop(); // stop the cache. This will take PojoCache out of the clustering group, if any, e.g.

POJO Cache is currently supported on JDK 5 (since release 2.0). It requires the following libraries (in addition to jboss-cache.jar and the required libraries for Core Cache) to start up:

As mentioned, the JBoss Aop framework is used to provide a configurable interceptor stack. In the current implementation, the main POJO Cache methods have their own independant stack. These are specified in META-INF/pojocache-aop.xml In most cases, this file should be left alone, although advanced users may wish to add their own interceptors. The Following is the default configuration:

   <!-- Check id range validity -->
   <interceptor name="CheckId" class="org.jboss.cache.pojo.interceptors.CheckIdInterceptor"
         scope="PER_INSTANCE"/>

   <!-- Track Tx undo operation -->
   <interceptor name="Undo" class="org.jboss.cache.pojo.interceptors.PojoTxUndoInterceptor"
         scope="PER_INSTANCE"/>

   <!-- Begining of interceptor chain -->
   <interceptor name="Start" class="org.jboss.cache.pojo.interceptors.PojoBeginInterceptor"
         scope="PER_INSTANCE"/>

   <!-- Check if we need a local tx for batch processing -->
   <interceptor name="Tx" class="org.jboss.cache.pojo.interceptors.PojoTxInterceptor"
         scope="PER_INSTANCE"/>

   <!--
      Mockup failed tx for testing. You will need to set PojoFailedTxMockupInterceptor.setRollback(true)
      to activate it.
   -->
   <interceptor name="MockupTx" class="org.jboss.cache.pojo.interceptors.PojoFailedTxMockupInterceptor"
         scope="PER_INSTANCE"/>

   <!-- Perform parent level node locking -->
   <interceptor name="TxLock" class="org.jboss.cache.pojo.interceptors.PojoTxLockInterceptor"
         scope="PER_INSTANCE"/>

   <!-- Interceptor to perform Pojo level rollback -->
   <interceptor name="TxUndo" class="org.jboss.cache.pojo.interceptors.PojoTxUndoSynchronizationInterceptor"
                scope="PER_INSTANCE"/>

   <!-- Interceptor to used to check recursive field interception. -->
   <interceptor name="Reentrant" class="org.jboss.cache.pojo.interceptors.MethodReentrancyStopperInterceptor"
                scope="PER_INSTANCE"/>

   <!-- Whether to allow non-serializable pojo. Default is false. -->
   <interceptor name="MarshallNonSerializable" class="org.jboss.cache.pojo.interceptors.CheckNonSerializableInterceptor"
                scope="PER_INSTANCE">
         <attribute name="marshallNonSerializable">false</attribute>
                </interceptor>

   <stack name="Attach">
      <interceptor-ref name="Start"/>
      <interceptor-ref name="CheckId"/>
      <interceptor-ref name="Tx"/>
      <interceptor-ref name="TxLock"/>
      <interceptor-ref name="TxUndo"/>
   </stack>

   <stack name="Detach">
      <interceptor-ref name="Start"/>
      <interceptor-ref name="CheckId"/>
      <interceptor-ref name="Tx"/>
      <interceptor-ref name="TxLock"/>
      <interceptor-ref name="TxUndo"/>
   </stack>

   <stack name="Find">
      <interceptor-ref name="Start"/>
      <interceptor-ref name="CheckId"/>
   </stack>

The stack should be self-explanatory. For example, for the Attach stack, we currently have Start, CheckId, Tx, TxLock, and TxUndo interceptors. The stack always starts with a Start interceptor such that initialization can be done properly. CheckId is to ensure the validity of the Id (e.g., it didn't use any internal Id string). Finally, Tx, TxLock, and TxUndo are handling the the proper transaction locking and rollback behavior (if needed).

POJO Cache currently uses JBoss AOP to intercept field operations. If a class has been properly instrumented (by either using the @Replicable annotation, or if the object has already been advised by JBoss AOP), then a cache interceptor is added during an attach() call. Afterward, any field modification will invoke the corresponding CacheFieldInterceptor instance. Below is a schematic illustration of this process.

Only fields, and not methods are intercepted, since this is the most efficient and accurate way to gaurantee the same data is visible on all nodes in the cluster. Further, this allows for objects that do not conform to the JavaBean specficiation to be replicable. There are two important aspects of field interception:

The figure below illustrates both field read and write operations. Once an POJO is managed by POJO Cache (i.e., after an attach() method has been called), JBoss Aop will invoke the CacheFieldInterceptor every time a class operates on a field. The cache is always consulted, since it is in control of the mapped data (i.e. it gaurantess the state changes made by other nodes in the cluster are visible). Afterwords, the in-memmory copy is updated. This is mainly to allow transaction rollbacks to restore the previous state of the object.

As previously mentioned, unlike a traditional cache system, POJO Cache preserves object identity. This allows for any type of object relationship available in the Java language to be transparently handled.

During the mapping process, all object references are checked to see if they are already stored in the cache. If already stored, instead of duplicating the data, a reference to the original object is written in the cache. All referenced objects are reference counted, so they will be removed once they are no longer referenced.

To look at one example, let's say that multiple Persons ("joe" and "mary") objects can own the same Address (e.g., a household). The following diagram is a graphical representation of the pysical cache data. As can be seen, the "San Jose" address is only stored once.

In the following code snippet, we show programmatically the object sharing example.

import org.jboss.cache.pojo.PojoCache;
import org.jboss.cache.pojo.PojoCacheFactory;
import org.jboss.test.cache.test.standAloneAop.Person;
import org.jboss.test.cache.test.standAloneAop.Address;

String configFile = "META-INF/replSync-service.xml";
PojoCache cache = PojoCacheFactory.createCache(configFile); // This will start PojoCache automatically

Person joe = new Person(); // instantiate a Person object named joe
joe.setName("Joe Black");
joe.setAge(41);

Person mary = new Person(); // instantiate a Person object named mary
mary.setName("Mary White");
mary.setAge(30);

Address addr = new Address(); // instantiate a Address object named addr
addr.setCity("Sunnyvale");
addr.setStreet("123 Albert Ave");
addr.setZip(94086);

joe.setAddress(addr); // set the address reference
mary.setAddress(addr); // set the address reference

cache.attach("pojo/joe", joe); // add aop sanctioned object (and sub-objects) into cache.
cache.attach("pojo/mary", mary); // add aop sanctioned object (and sub-objects) into cache.

Address joeAddr = joe.getAddress();
Address maryAddr = mary.getAddress(); // joeAddr and maryAddr should be the same instance

cache.detach("pojo/joe");
maryAddr = mary.getAddress(); // Should still have the address.

If joe is removed from the cache, mary should still have reference the same Address object in the cache store.

To further illustrate this relationship management, let's examine the Java code under a replicated environment. Imagine two separate cache instances in the cluster now (cache1 and cache2). On the first cache instance, both joe and mary are attached as above. Then, the application fails over to cache2. Here is the code snippet for cache2 (assume the objects were already attached):

/**
 * Code snippet on cache2 during fail-over
 */
import org.jboss.cache.PropertyConfigurator;
import org.jboss.cache.pojo.PojoCache;
import org.jboss.test.cache.test.standAloneAop.Person;
import org.jboss.test.cache.test.standAloneAop.Address;

String configFile = "META-INF/replSync-service.xml";
PojoCache cache2 = PojoCacheFactory.createCache(configFile); // This will start PojoCache automatically

Person joe = cache2.find("pojo/joe"); // retrieve the POJO reference.
Person mary = cache2.find("pojo/mary"); // retrieve the POJO reference.

Address joeAddr = joe.getAddress();
Address maryAddr = mary.getAddress(); // joeAddr and maryAddr should be the same instance!!!

maryAddr = mary.getAddress().setZip(95123);
int zip = joeAddr.getAddress().getZip(); // Should be 95123 as well instead of 94086!

POJO Cache preserves the inheritance hierarchy of all attached objects. For example, if a Student extends Person with an additional field year, then once Student is put into the cache, all the class attributes of Person are mapped to the cache as well.

Following is a code snippet that illustrates how the inheritance behavior of a POJO is maintained. Again, no special configuration is needed.

import org.jboss.test.cache.test.standAloneAop.Student;

Student joe = new Student(); // Student extends Person class
joe.setName("Joe Black"); // This is base class attributes
joe.setAge(22); // This is also base class attributes
joe.setYear("Senior"); // This is Student class attribute

cache.attach("pojo/student/joe", joe);

//...

joe = (Student)cache.attach("pojo/student/joe");
Person person = (Person)joe; // it will be correct here
joe.setYear("Junior"); // will be intercepted by the cache
joe.setName("Joe Black II"); // also intercepted by the cache

The previous sections describe the logical object mapping model. In this section, we will explain the physical mapping model, that is, how do we map the POJO into Core Cache for transactional state replication. However, it should be noted that the physical structure of the cache is purely an internal implementation detail, it should not be treated as an API as it may change in future releases. This information is provided solely to aid in better understanding the mapping process in POJO Cache.

When an object is first attached in POJO Cache, the Core Cache node representation is created in a special internal area. The Id fqn that is passed to attach() is used to create an empty node that references the internal node. Future references to the same object will point to the same internal node location, and that node will remain until all such references have been removed (detached).

The example below demonstrates the mapping of the Person object under id "pojo/joe" and "pojo/mary" as metioned in previous sections. It is created from a two node replication group where one node is a Beanshell window and the other node is a Swing Gui window (shown here). For clarity, multiple snapshots were taken to highlight the mapping process.

The first figure illustrates the first step of the mapping approach. From the bottom of the figure, it can be seen that the PojoReference field under pojo/joe is pointing to an internal location, /__JBossInternal__/5c4o12-lpaf5g-esl49n5e-1-esl49n5o-2. That is, under the user-specified Id string, we store only an indirect reference to the internal area. Please note that Mary has a similar reference.

Then by clicking on the referenced internal node (from the following figure), it can seen that the primitive fields for Joe are stored there. E.g., Age is 41 and Name is Joe Black. And similarly for Mary as well.

Under the /__JBossInternal__/5c4o12-lpaf5g-esl49n5e-1-esl49n5o-2, it can be seen that there is an Address node. Clicking on the Address node shows that it references another internal location: /__JBossInternal__/5c4o12-lpaf5g-esl49n5e-1-esl49ngs-3 as shown in the following figure. Then by the same token, the Address node under /__JBossInternal__/5c4o12-lpaf5g-esl49n5e-1-esl49na0-4 points to the same address reference. That is, both Joe and Mary share the same Address reference.

Finally, the /__JBossInternal__/5c4o12-lpaf5g-esl49n5e-1-esl49ngs-3 node contains the various various primitive fields of Address, e.g., Street, Zip, and City. This is illustrated in the following figure.

Due to current Java limitations, Collection classes that implement Set, List, and Map are substituted with a Java proxy. That is, whenever POJO Cache encounters any Collection instance, it will:

The drawback to this approach is that the calling application must re-get any collection references that were attached. Otherwise, the cache will not be aware of future changes. If the collection is referenced from another object, then the calling app can obtain the proxy by using the publishing mechanism provided by the object (e.g. Person.getHobbies()). If, however, the collection is directly attached to the cache, then a subsequent find() call will need to be made to retrieve the proxy.

The following code snippet illustrates obtaining a direct Collection proxy reference:

List list = new ArrayList();
list.add("ONE");
list.add("TWO");

cache.attach("pojo/list", list);
list.add("THREE"); // This won't be intercepted by the cache!

List proxyList = cache.find("pojo/list"; // Note that list is a proxy reference
proxyList.add("FOUR"); // This will be intercepted by the cache

This snippet illustrates obtaining the proxy reference from a refering object:

Person joe = new Person();
joe.setName("Joe Black"); // This is base class attributes
List lang = new ArrayList();
lang.add("English");
lang.add("Mandarin");
joe.setLanguages(lang);

// This will map the languages List automatically and swap it out with the proxy reference.
cache.attach("pojo/student/joe", joe);
lang = joe.getLanguages(); // Note that lang is now a proxy reference
lang.add("French"); // This will be intercepted by the cache

Finally, when a Collection is removed from the cache (e.g., via detach), you still can use the proxy reference. POJO Cache will just redirect the call back to the in-memory copy. See below:

List list = new ArrayList();
list.add("ONE");
list.add("TWO");

cache.attach("pojo/list", list);
List proxyList = cache.find("pojo/list"); // Note that list is a proxy reference
proxyList.add("THREE"); // This will be intercepted by the cache

cache.detach("pojo/list"); // detach from the cache
proxyList.add("FOUR"); // proxyList has 4 elements still.

As of 2.2, array fields of any attached object are updated transparently, provided that the array is written/read from a class marked with @Replicable. If this is the case, only the indexes of the array that are modified are replicated. However, if the array is passed externally to a class that is not marked as @Replicable, then the changes will not be noticed. For this reason, it is recommended to abstract access to the array where possible (i.e. setItem(item, index)). If an external, non-replicable class needs access to the array, then it is recommended to pass a copy, and add a method to the container object that reapplies the changes. Also, due to JVM limitations, an array can not be monitored if it is directly attached to the cache (i.e. a first class object). POJO Cache still allows this, but they are treated as a serializable type. As with other serializable type, they must be reattached after every change.

The following code snippet illustrates accessing a replicated array through abstraction:

@Replicable public class Team
{
   private String[] members = new String[10];

   public String getMember(int index)
   {
      return members[index];
   }

   public void setMember(int index, String member)
   {
      members[index] = member;
   }
}
   
public class SomeExternalClass
{
   ...
   public void someMethod() 
   {
     Team team = new Team(); 
     cache.attach("/team/1", team);
     
     team.setMember(0, "John");
     team.setMember(1, "Joe");
   }
}    

PojoCacheFactory provides a couple of static methods to instantiate and obtain a PojoCache instance.

/**
 * Create a PojoCache instance. Note that this will start the cache life cycle automatically.
 * @param config A configuration string that represents the file name that is used to
 * configure the underlying Cache instance.
 * @return PojoCache
 */
public static PojoCache createInstance(String config);

/**
 * Create a PojoCache instance.
 * @param config A configuration string that represents the file name that is used to
 * configure the underlying Cache instance.
 * @param start If true, it will start the cache life cycle.
 * @return PojoCache
 */
public static PojoCache createInstance(String config, boolean start);

/**
 * Create a PojoCache instance.
 * @param config A configuration object that is used to configure the underlying Cache instance.
 * @param start If true, it will start the cache life cycle.
 * @return PojoCache
 */
public static PojoCache createInstance(Configuration config, boolean start);

For example, to obtain a PojoCache instance and start the cache lifestyle automatically, we can do:

   String configFile = "META-INF/replSync-service.xml";
   PojoCache cache = PojoCacheFactory.createInstance(configFile);

PojoCache is the main interface for POJO Cache operations. Since most of the cache interaction is performed against the application domain model, there are only a few methods on this interface.

Load-time instrumentation uses a Java agent to intercept all classes loaded by the JVM. As they are loaded JBoss AOP instruments them, allowing POJO Cache to monitor field changes. To enable load time instrumentation the JVM must be started with the following specified:

These requirements lead to the following example ant task:

<java classname="Foo" fork="yes">
   <jvmarg value="-javaagent:lib/jboss-aop.jar"/>
   <jvmarg value="-Djboss.aop.path=etc/META-INF/pojocache-aop.xml"/>
   <classpath refid="test.classpath"/>
</java>

Once the JVM is executed in this manner, any class with the @Replicable annotation will be instrumented when it is loaded.

While load-time is the preffered approach, it is also possible to instrument classes at compile-time. To do this, the aopc tool is used, with the following requirements:

The following is an example ant task which performs compile-time instrumentation:

<taskdef name="aopc" classname="org.jboss.aop.ant.AopC" classpathref="aop.classpath"/>
<target name="aopc" depends="compile" description="Precompile aop class">
    <aopc compilerclasspathref="aop.classpath" verbose="true">
       <src path="${build}"/>
       <include name="org/jboss/cache/aop/test/**/*.class"/>
       <aoppath path="${output}/resources/pojocache-aop.xml"/>
       <classpath path="${build}"/>
       <classpath refid="lib.classpath"/>
    </aopc>
</target>

In this example, once the aopc target is executeed the clasess in the build directory are modified. They can then be packaged in a jar and loaded using the normal Java mechanisms.

The advanced user might decide to alter the provided AOP descritor. In order to do this, it is important to understand the reaons behind what is provided, and what is required by POJO Cache. Previous sections have mentioned that any class with the @Replicable annotation will be instrumented. This happens, because the provided AOP descriptor, pojocache-aop.xml, has a perpare statement which matches any class (or subclass) using the annotation. This is shown in the following snippet:

<prepare expr="field(* $instanceof{@org.jboss.cache.pojo.annotation.Replicable}->*)"/>

More specifically, any code which accesses a field on a class which has been annotated with @Replicable, will be instrumented: The "field" pointcut in the expression matches both read and write operations. The wildcard "*" indicates that all java protection modes are intercepted (private, package, protected, public). The $instanceof expression refers to any annotation that subclasses @Replicable. Finally, the ending wildcard allows the matched field to have any name.

Annotation is a new feature in Java 5.0 that when declared can contain metadata at compile and run time. It is well suited for aop declaration since there will be no need for external metadata xml descriptor.

As already mentioned, a user can use the aop precompiler ( aopc ) to precompile the POJO classes such that, during runtime, there is no additional system class loader needed. The precompiler will read in pojocache-aop.xml and weave the POJO byte code at compile time. This is a convenient feature to make the aop less intrusive.

Below is an Ant snippet that defines the library needed for the various Ant targets that we are listing here. User can refer to the build.xml in the distribution for full details.

<path id="aop.classpath">
   <fileset dir="${lib}">
      <include name="**/*.jar" />
      <exclude name="**/jboss-cache.jar" />
      <exclude name="**/j*unit.jar" />
      <exclude name="**/bsh*.jar" />
   </fileset>
</path>
         

The example POJO classes used for are: Person, Student, and Address. Below are their definition (note that neither class implements Serializable ) along with the annotation.

@org.jboss.cache.pojo.annotation.Replicable
public class Person {
   String name=null;
   int age=0;
   Map hobbies=null;
   Address address=null;
   Set skills;
   List languages;

   public String getName() { return name; }
   public void setName(String name) { this.name=name; }

   public int getAge() { return age; }
   public void setAge(int age) { this.age = age; }

   public Map getHobbies() { return hobbies; }
   public void setHobbies(Map hobbies) { this.hobbies = hobbies; }

   public Address getAddress() { return address; }
   public void setAddress(Address address) { this.address = address; }

   public Set getSkills() { return skills; }
   public void setSkills(Set skills) { this.skills = skills; }

   public List getLanguages() { return languages; }
   public void setLanguages(List languages) { this.languages = languages; }
}

public class Student extends Person {
   String year=null;

   public String getYear() { return year; }
   public void setYear(String year) { this.year=year; }
}

   @org.jboss.cache.pojo.annotation.Replicable
   public class Address {
   String street=null;
   String city=null;
   int zip=0;

   public String getStreet() { return street; }
   public void setStreet(String street) { this.street=street; }
   // ...
}

Below is a sample xml configuration for Cache that you can use for PojoCache creation.

<?xml version="1.0" encoding="UTF-8" ?>

<server>
   <mbean code="org.jboss.cache.pojo.jmx.PojoCacheJmxWrapper" 
          name="jboss.cache:service=PojoCache">
      
      <depends>jboss:service=TransactionManager</depends>

      <!-- Configure the TransactionManager -->
      <attribute name="TransactionManagerLookupClass">
         org.jboss.cache.transaction.DummyTransactionManagerLookup
      </attribute>

      <!-- Isolation level : SERIALIZABLE
                             REPEATABLE_READ (default)
                             READ_COMMITTED
                             READ_UNCOMMITTED
                             NONE
      -->
      <attribute name="IsolationLevel">REPEATABLE_READ</attribute>

      <!-- Valid modes are LOCAL, REPL_ASYNC and REPL_SYNC -->
      <attribute name="CacheMode">REPL_SYNC</attribute>

      <!-- Name of cluster. Needs to be the same for all caches, 
           in order for them to find each other
      -->
      <attribute name="ClusterName">PojoCacheCluster</attribute>

   	  <!-- JGroups protocol stack properties. -->
      <attribute name="ClusterConfig">
         <config>
            <!-- UDP: if you have a multihomed machine, set the bind_addr 
                 attribute to the appropriate NIC IP address -->
            <!-- UDP: On Windows machines, because of the media sense feature
                 being broken with multicast (even after disabling media sense)
                 set the loopback attribute to true -->
            <UDP mcast_addr="228.1.2.3" mcast_port="48866"
                 ip_ttl="64" ip_mcast="true"
                 mcast_send_buf_size="150000" mcast_recv_buf_size="80000"
                 ucast_send_buf_size="150000" ucast_recv_buf_size="80000"
                 loopback="false"/>
            <PING timeout="2000" num_initial_members="3"/>
            <MERGE2 min_interval="10000" max_interval="20000"/>
            <FD shun="true"/>
            <FD_SOCK/>
            <VERIFY_SUSPECT timeout="1500"/>
            <pbcast.NAKACK gc_lag="50" retransmit_timeout="600,1200,2400,4800"
                           max_xmit_size="8192"/>
            <UNICAST timeout="600,1200,2400",4800/>
            <pbcast.STABLE desired_avg_gossip="400000"/>
            <FC max_credits="2000000" min_threshold="0.10"/>
            <FRAG2 frag_size="8192"/>
            <pbcast.GMS join_timeout="5000" join_retry_timeout="2000"
                        shun="true" print_local_addr="true"/>
            <pbcast.STATE_TRANSFER/>
         </config>
      </attribute>

      <!-- Whether or not to fetch state on joining a cluster -->
      <attribute name="FetchInMemoryState">true</attribute>

      <!-- The max amount of time (in milliseconds) we wait until the
           initial state (ie. the contents of the cache) are retrieved from
           existing members in a clustered environment
      -->
      <attribute name="InitialStateRetrievalTimeout">15000</attribute>

      <!-- Number of milliseconds to wait until all responses for a
           synchronous call have been received.
      -->
      <attribute name="SyncReplTimeout">15000</attribute>

      <!--  Max number of milliseconds to wait for a lock acquisition -->
      <attribute name="LockAcquisitionTimeout">10000</attribute>
   
   </mbean>
</server>

Attached is a full listing for pojocache-aop.xml.

   <?xml version="1.0" encoding="UTF-8"?>
   <!--
       This is the PojoCache configuration file that specifies:
         1. Interceptor stack for API
         2. Annotation binding for POJO (via "prepare" element)

       Basically, this is a variant of jboss-aop.xml. Note that
       except for the customization of interceptor stack, you should
       not need to modify this file.

       To run PojoCache, you will need to define a system property:
       jboss.aop.path that contains the path to this file such that JBoss Aop
       can locate it.
   -->
   <aop>

      <!--
      This defines the PojoCache 2.0 interceptor stack. Unless necessary, don't modify the stack here!
      -->

      <!-- Check id range validity -->
      <interceptor name="CheckId" class="org.jboss.cache.pojo.interceptors.CheckIdInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Track Tx undo operation -->
      <interceptor name="Undo" class="org.jboss.cache.pojo.interceptors.PojoTxUndoInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Begining of interceptor chain -->
      <interceptor name="Start" class="org.jboss.cache.pojo.interceptors.PojoBeginInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Check if we need a local tx for batch processing -->
      <interceptor name="Tx" class="org.jboss.cache.pojo.interceptors.PojoTxInterceptor"
                   scope="PER_INSTANCE"/>

      <!--
         Mockup failed tx for testing. You will need to set PojoFailedTxMockupInterceptor.setRollback(true)
         to activate it.
       -->
      <interceptor name="MockupTx" class="org.jboss.cache.pojo.interceptors.PojoFailedTxMockupInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Perform parent level node locking -->
      <interceptor name="TxLock" class="org.jboss.cache.pojo.interceptors.PojoTxLockInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Interceptor to perform Pojo level rollback -->
      <interceptor name="TxUndo" class="org.jboss.cache.pojo.interceptors.PojoTxUndoSynchronizationInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Interceptor to used to check recursive field interception. -->
      <interceptor name="Reentrant" class="org.jboss.cache.pojo.interceptors.MethodReentrancyStopperInterceptor"
                   scope="PER_INSTANCE"/>

      <!-- Whether to allow non-serializable pojo. Default is false. -->
      <interceptor name="MarshallNonSerializable"
                   class="org.jboss.cache.pojo.interceptors.CheckNonSerializableInterceptor"
                   scope="PER_INSTANCE">
            <attribute name="marshallNonSerializable">false</attribute>
                   </interceptor>

      <!-- This defines the stack macro -->
      <stack name="Attach">
         <interceptor-ref name="Start"/>
         <interceptor-ref name="CheckId"/>
         <interceptor-ref name="MarshallNonSerializable"/>
         <interceptor-ref name="Tx"/>
         <!-- NOTE: You can comment this out during production although leaving it here is OK. -->
         <interceptor-ref name="MockupTx"/>
         <interceptor-ref name="TxLock"/>
         <interceptor-ref name="TxUndo"/>
      </stack>

      <stack name="Detach">
         <interceptor-ref name="Start"/>
         <interceptor-ref name="CheckId"/>
         <interceptor-ref name="Tx"/>
         <!-- NOTE: You can comment this out during production although leaving it here is OK. -->
         <interceptor-ref name="MockupTx"/>
         <interceptor-ref name="TxLock"/>
         <interceptor-ref name="TxUndo"/>
      </stack>

      <stack name="Find">
         <interceptor-ref name="Start"/>
         <interceptor-ref name="CheckId"/>
      </stack>

      <!--
         The following section should be READ-ONLY!! It defines the annotation binding to the stack.
      -->

      <!-- This binds the jointpoint to specific in-memory operations. Currently in PojoUtil. -->
      <bind pointcut="execution(*
         @org.jboss.cache.pojo.annotation.Reentrant->toString())">
         <interceptor-ref name="Reentrant"/>
      </bind>

      <bind pointcut="execution(*
         org.jboss.cache.pojo.PojoUtil->@org.jboss.cache.pojo.annotation.TxUndo(..))">
         <interceptor-ref name="Undo"/>
      </bind>

      <bind pointcut="execution(* org.jboss.cache.pojo.impl.PojoCacheImpl->@org.jboss.cache.pojo.annotation.Attach(..))">
         <stack-ref name="Attach"/>
      </bind>

      <bind pointcut="execution(* org.jboss.cache.pojo.impl.PojoCacheImpl->@org.jboss.cache.pojo.annotation.Detach(..))">
         <stack-ref name="Detach"/>
      </bind>

      <bind pointcut="execution(* org.jboss.cache.pojo.impl.PojoCacheImpl->@org.jboss.cache.pojo.annotation.Find(..))">
         <stack-ref name="Find"/>
      </bind>


      <!--
         Following is declaration for JDK50 annotation. You use the specific annotation on your
         POJO such that it can be instrumented. Idea is user will then need only to annotate like:
   @org.jboss.cache.pojo.annotation.Replicable
   in his POJO. There will be no need of jboss-aop.xml from user's side.
      -->

      <!-- If a POJO has PojoCachable annotation, it will be asepctized. -->
      <prepare expr="field(* $instanceof{@org.jboss.cache.pojo.annotation.Replicable}->*)" />

      <!-- Observer and Observable to monitor field modification -->
      <bind pointcut="
         set(* $instanceof{@org.jboss.cache.pojo.annotation.Replicable}->*)
         ">
            <interceptor class="org.jboss.cache.pojo.observable.SubjectInterceptor"/>
      </bind>

      <introduction class="$instanceof{@org.jboss.cache.pojo.annotation.Replicable}">
         <mixin>
            <interfaces>org.jboss.cache.pojo.observable.Subject</interfaces>
            <class>org.jboss.cache.pojo.observable.SubjectImpl</class>
            <construction>new org.jboss.cache.pojo.observable.SubjectImpl(this)</construction>
         </mixin>
      </introduction>
   </aop>