Cayenne Guide

CayenneModeler GUI tool is intended to work with object relational mapping projects. While you can edit your XML by hand, it is rarely needed, as the Modeler is a pretty advanced tool included in Cayenne distribution. To obtain CayenneModeler, download Cayenne distribution archive from https://cayenne.apache.org/download/ matching the OS you are using. Of course Java needs to be installed on the machine where you are going to run the Modeler.

The Modeler can also be started from Maven. While it may look like an exotic way to start a GUI application, it has its benefits - no need to download Cayenne distribution, the version of the Modeler always matches the version of the framework, the plugin can find mapping files in the project automatically. So it is an attractive option to some developers. Maven option requires a declaration in the POM:

And then can be run as

A Cayenne project is an XML representation of a model connecting database schema with Java classes. A project is normally created and manipulated via CayenneModeler GUI and then used to initialize Cayenne runtime. A project is made of one or more files. There’s always a root project descriptor file in any valid project. It is normally called cayenne-xyz.xml, where "xyz" is the name of the project.

Project descriptor can reference DataMap files, one per DataMap. DataMap files are normally called xyz.map.xml, where "xyz" is the name of the DataMap. For legacy reasons this naming convention is different from the convention for the root project descriptor above, and we may align it in the future versions. Here is how a typical project might look on the file system:

DataMap are referenced by name in the root descriptor:

Map files are resolved by Cayenne by appending ".map.xml" extension to the map name, and resolving the resulting string relative to the root descriptor URI. The following sections discuss varios ORM model objects, without regards to their XML representation. XML format details are really unimportant to the Cayenne users.

DataMap is a container of persistent entities and other object-relational metadata. DataMap provides developers with a scope to organize their entities, but it does not provide a namespace for entities. In fact all DataMaps present in runtime are combined in a single namespace. Each DataMap must be associated with a DataNode. This is how Cayenne knows which database to use when running a query.

DataNode is model of a database. It is actually pretty simple. It has an arbitrary user-provided name and information needed to create or locate a JDBC DataSource. Most projects only have one DataNode, though there may be any number of nodes if needed.

DbEntity is a model of a single DB table or view. DbEntity is made of DbAttributes that correspond to columns, and DbRelationships that map PK/FK pairs. DbRelationships are not strictly tied to FK constraints in DB, and should be mapped for all logical "relationships" between the tables.

ObjEntity is a model of a single persistent Java class. ObjEntity is made of ObjAttributes and ObjRelationships. Both correspond to entity class properties. However ObjAttributes represent "simple" properties (normally things like String, numbers, dates, etc.), while ObjRelationships correspond to properties that have a type of another entity.

ObjEntity maps to one or more DbEntities. There’s always one "root" DbEntity for each ObjEntity. ObjAttribiute maps to a DbAttribute or an Embeddable. Most often mapped DbAttribute is from the root DbEntity. Sometimes mapping is done to a DbAttribute from another DbEntity somehow related to the root DbEntity. Such ObjAttribute is called "flattened". Similarly ObjRelationship maps either to a single DbRelationship, or to a chain of DbRelationships ("flattened" ObjRelationship).

ObjEntities may also contain mapping of their lifecycle callback methods.

Embeddable is a model of a Java class that acts as a single attribute of an ObjEntity, but maps to multiple columns in the database.

A model of a stored procedure in the database.

A model of a query. Cayenne allows queries to be mapped in Cayenne project, or created in the code. Depending on the circumstances the users may take one or the other approach.

Normally each ObjEntity is mapped to a specific Java class (such as Artist or Painting) that explicitly declare all entity properties as pairs of getters and setters. However Cayenne allows to map a completly generic class to any number of entities. The only expectation is that a generic class implements org.apache.cayenne.DataObject. So an ideal candidate for a generic class is CayenneDataObject, or some custom subclass of CayenneDataObject.

If you don’t enter anything for Java Class of an ObjEntity, Cayenne assumes generic mapping and uses the following implicit rules to determine a class of a generic object. If DataMap "Custom Superclass" is set, runtime uses this class to instantiate new objects. If not, org.apache.cayenne.CayenneDataObject is used.

Class generation procedures (either done in the Modeler or with Ant or Maven) would skip entities that are mapped to CayenneDataObject explicitly or have no class mapping.

Cayenne distribution contains the following core runtime jars in the distribution lib directory:

When using cayenne-server-x.x.jar you’ll need a few third party jars (all included in lib/third-party directory of the distribution):

Cayenne integrates with various caching, clustering and other frameworks. These optional integrations will require other third-party jars that the users will need to obtain on their own.

If you are using Maven, you won’t have to deal with figuring out the dependencies. You can simply include cayenne-server artifact in your POM:

Additionally Cayenne provides a Maven plugin with a set of goals to perform various project tasks, such as synching generated Java classes with the mapping, described in the following subsection. The full plugin name is org.apache.cayenne.plugins:cayenne-maven-plugin.

In runtime Cayenne is accessed via org.apache.cayenne.configuration.server.ServerRuntime. ServerRuntime is created by calling a convenient builder:

The parameter you pass to the builder is a location of the main project file. Location is a '/'-separated path (same path separator is used on UNIX and Windows) that is resolved relative to the application classpath. The project file can be placed in the root package or in a subpackage (e.g. in the code above it is in "com/example" subpackage).

ServerRuntime encapsulates a single Cayenne stack. Most applications will just have one ServerRuntime using it to create as many ObjectContexts as needed, access the Dependency Injection (DI) container and work with other Cayenne features. Internally ServerRuntime is just a thin wrapper around the DI container. Detailed features of the container are discussed in "Customizing Cayenne Runtime" chapter. Here we’ll just show an example of how an application might turn on external transactions:

It is a good idea to shut down the runtime when it is no longer needed, usually before the application itself is shutdown:

When a runtime object has the same scope as the application, this may not be always necessary, however in some cases it is essential, and is generally considered a good practice. E.g. in a web container hot redeploy of a webapp will cause resource leaks and eventual OutOfMemoryError if the application fails to shutdown CayenneRuntime.

ServerRuntime requires at least one mapping project to run. But it can also take multiple projects and merge them together in a single configuration. This way different parts of a database can be mapped independenlty from each other (even by different software providers), and combined in runtime when assembling an application. Doing it is as easy as passing multiple project locations to ServerRuntime constructor:

When the projects are merged, the following rules are applied:

Web applications can use a variety of mechanisms to configure and start the "services" they need, Cayenne being one of such services. Configuration can be done within standard Servlet specification objects like Servlets, Filters, or ServletContextListeners, or can use Spring, JEE CDI, etc. This is a user’s architectural choice and Cayenne is agnostic to it and will happily work in any environment. As described above, all that is needed is to create an instance of ServerRuntime somewhere and provide the application code with means to access it. And shut it down when the application ends to avoid container leaks.

Still Cayenne includes a piece of web app configuration code that can assist in quickly setting up simple Cayenne-enabled web applications. We are talking about CayenneFilter. It is declared in web.xml:

When started by the web container, it creates a instance of ServerRuntime and stores it in the ServletContext. Note that the name of Cayenne XML project file is derived from the "filter-name". In the example above CayenneFilter will look for an XML file "cayenne-project.xml". This can be overridden with "configuration-location" init parameter.

When the application runs, all HTTP requests matching the filter url-pattern will have access to a session-scoped ObjectContext like this:

Of course the ObjectContext scope, and other behavior of the Cayenne runtime can be customized via dependency injection. For this another filter init parameter called "extra-modules" is used. "extra-modules" is a comma or space-separated list of class names, with each class implementing Module interface. These optional custom modules are loaded after the the standard ones, which allows users to override all standard definitions.

For those interested in the DI container contents of the runtime created by CayenneFilter, it is the same ServerRuntime as would’ve been created by other means, but with an extra org.apache.cayenne.configuration.web.WebModule module that provides org.apache.cayenne.configuration.web.RequestHandler service. This is the service to override in the custom modules if you need to provide a different ObjectContext scope, etc.

ObjectContext is an interface that users normally work with to access the database. It provides the API to execute database operations and to manage persistent objects. A context is obtained from the ServerRuntime:

The call above creates a new instance of ObjectContext that can access the database via this runtime. ObjectContext is a single "work area" in Cayenne, storing persistent objects. ObjectContext guarantees that for each database row with a unique ID it will contain at most one instance of an object, thus ensuring object graph consistency between multiple selects (a feature called "uniquing"). At the same time different ObjectContexts will have independent copies of objects for each unique database row. This allows users to isolate object changes from one another by using separate ObjectContexts.

These properties directly affect the strategies for scoping and sharing (or not sharing) ObjectContexts. Contexts that are only used to fetch objects from the database and whose objects are never modified by the application can be shared between mutliple users (and multiple threads). Contexts that store modified objects should be accessed only by a single user (e.g. a web application user might reuse a context instance between multiple web requests in the same HttpSession, thus carrying uncommitted changes to objects from request to request, until he decides to commit or rollback them). Even for a single user it might make sense to use mutliple ObjectContexts (e.g. request-scoped contexts to allow concurrent requests from the browser that change and commit objects independently).

ObjectContext is serializable and does not permanently hold to any of the application resources. So it does not have to be closed. If the context is not used anymore, it should simply be allowed to go out of scope and get garbage collected, just like any other Java object.

Cayenne can persist Java objects that implement org.apache.cayenne.Persistent interface. Generally persistent classes are generated from the model as described above, so users do not have to worry about superclass and property implementation details.

Persistent interface provides access to 3 persistence-related properties - objectId, persistenceState and objectContext. All 3 are initialized by Cayenne runtime framework. Application code should not attempt to change them. However it is allowed to read them, which provides valuable runtime information. E.g. ObjectId can be used for quick equality check of 2 objects, knowing persistence state would allow highlighting changed objects, etc.

Each persistent object belongs to a single ObjectContext, and can be in one of the following persistence states (as defined in org.apache.cayenne.PersistenceState) :

One of the first things users usually want to do with an ObjectContext is to select some objects from a database. This is done by calling "performQuery" method:

We’ll discuss queries in some detail in the following chapters. The example above is self-explanatory - we create a SelectQuery that matches all Artist objects present in the database, and then call "performQuery", getting a list of Artist objects.

Some queries can be quite complex, returning multiple result sets or even updating the database. For such queries ObjectContext provides "performGenericQuery"method. While not nearly as commonly-used as "performQuery", it is nevertheless important in some situations. E.g.:

An application might modify selected objects. E.g.:

The first time the object property is changed, the object’s state is automatically set to "MODIFIED" by Cayenne. Cayenne tracks all in-memory changes until a user calls "commitChanges":

At this point all in-memory changes are analyzed and a minimal set of SQL statements is issued in a single transaction to synchronize the database with the in-memory state. In our example "commitChanges" commits just one object, but generally it can be any number of objects.

If instead of commit, we wanted to reset all changed objects to the previously committed state, we’d call rollbackChanges instead:

"newObject" method call creates a persistent object and sets its state to "NEW":

It will only exist in memory until "commitChanges" is issued. On commit Cayenne might generate a new primary key (unless a user set it explicitly, or a PK was inferred from a relationship) and issue an INSERT SQL statement to permanently store the object.

deleteObjects method takes one or more Persistent objects and marks them as "DELETED":

Additionally "deleteObjects" processes all delete rules modeled for the affected objects. This may result in implicitly deleting or modifying extra related objects. Same as insert and update, delete operations are sent to the database only when "commitChanges" is called. Similarly "rollbackChanges" will undo the effect of "newObject" and "deleteObjects".

localObject returns a copy of a given persistent object that is "local" to a given ObjectContext:

Since an application often works with more than one context, "localObject" is a rather common operation. E.g. to improve performance a user might utilize a single shared context to select and cache data, and then occasionally transfer some selected objects to another context to modify and commit them:

Often an appliction needs to inspect mapping metadata. This information is stored in the EntityResolver object, accessible via the ObjectContext:

Here we discussed the most commonly used subset of the ObjectContext API. There are other useful methods, e.g. those allowing to inspect registered objects state in bulk, etc. Check the latest JavaDocs for details.

There is a useful helper class called "Cayenne" (fully-qualified name "org.apache.cayenne.Cayenne") that builds on ObjectContext API to provide a number of very common operations. E.g. get a primary key (most entities do not model PK as an object property) :

It also provides the reverse operation - finding an object given a known PK:

If a query is expected to return 0 or 1 object, Cayenne helper class can be used to find this object. It throws an exception if more than one object matched the query:

Feel free to explore Cayenne class API for other useful methods.

In all the examples shown so far an ObjectContext would directly connect to a database to select data or synchronize its state (either via commit or rollback). However another context can be used in all these scenarios instead of a database. This concept is called ObjectContext "nesting". Nesting is a parent/child relationship between two contexts, where child is a nested context and selects or commits its objects via a parent.

Nesting is useful to create isolated object editing areas (child contexts) that need to all be committed to an intermediate in-memory store (parent context), or rolled back without affecting changes already recorded in the parent. Think cascading GUI dialogs, or parallel AJAX requests coming to the same session.

In theory Cayenne supports any number of nesting levels, however applications should generally stay with one or two, as deep hierarchies will most certainly degrade the performance of the deeply nested child contexts. This is due to the fact that each context in a nesting chain has to update its own objects during most operations.

Cayenne ROP is an extreme case of nesting when a child context is located in a separate JVM and communicates with its parent via a web service. ROP is discussed in details in the following chapters. Here we concentrate on the same-VM nesting.

To create a nested context, use an instance of ServerRuntime, passing it the desired parent:

From here a nested context operates just like a regular context (you can perform queries, create and delete objects, etc.). The only difference is that commit and rollback operations can either be limited to synchronization with the parent, or cascade all the way to the database:

As described in the CayenneModeler chapter, Cayenne supports mapping of completely generic classes to specific entities. Although for conveniece most applications should stick with entity-specific class mappings, the generic feature offers some interesting possibilities, such as creating mappings completely on the fly in a running application, etc.

Generic objects are first class citizens in Cayenne, and all common persistent operations apply to them as well. There are some pecularities however, described below.

When creating a new generic object, either cast your ObjectContext to DataContext (that provides "newObject(String)" API), or provide your object with an explicit ObjectId:

SelectQuery for generic object should be created passing entity name String in constructor, instead of a Java class:

Use DataObject API to access and modify properties of a generic object:

This is how an application can obtain entity name of a generic object:

Considering how much attention is given to managing transactions in most other ORMs, transactions have been conspicuously absent from the ObjectContext discussion till now. The reason is that transactions are seamless in Cayenne in all but a few special cases. ObjectContext is an in-memory container of objects that is disconnected from the database, except when it needs to run an operation. So it does not care about any surrounding transaction scope. Sure enough all database operations are transactional, so when an application does a commit, all SQL execution is wrapped in a database transaction. But this is done behind the scenes and is rarely a concern to the application code.

Two cases where transactions need to be taken into consideration are container-managed and application-managed transactions.

If you are using an EJB container (or some other JTA environment), you’ll likely need to switch Cayenne runtime into "external transactions mode". This is either done in the Modeler (check DataDomain > 'Container-Managed Transactions' checkbox), or in the code:

In this case Cayenne assumes that JDBC Connections obtained by runtime whenever that might happen are all coming from a transactional DataSource managed by the container. In this case Cayenne does not attempt to commit or rollback the connections, leaving it up to the container to do that when appropriate.

In the second scenario, an application might need to define its own transaction scope that spans more than one Cayenne operation. E.g. two sequential commits that need to be rolled back together in case of failure. This can be done with an explicit thread-bound transaction that surrounds a set of operations. Application is responsible for committing or rolling it back:

Cayenne provides a simple yet powerful object-based expression language. The most common use of expressions are to build qualifiers and orderings of queries that are later converted to SQL by Cayenne and to evaluate in-memory against specific objects (to access certain values in the object graph or to perform in-memory object filtering and sorting). Cayenne provides API to build expressions in the code and a parser to create expressions from strings.

Before discussing how to build expressions, it is important to understand one group of expressions widely used in Cayenne - path expressions. There are two types of path expressions - object and database, used for navigating graphs of connected objects or joined DB tables respectively. Object paths are much more commonly used, as after all Cayenne is supposed to provide a degree of isolation of the object model from the database. However database paths are helpful in certain situations. General structure of path expressions is the following:

An object path expression represents a chain of property names rooted in a certain (unspecified during expression creation) object and "navigating" to its related value. E.g. a path expression artist.name might be a property path starting from a Painting object, pointing to the related Artist object, and then to its name attribute. A few more examples:

Similarly a database path expression is a dot-separated path through DB table joins and columns. In Cayenne joins are mapped as DbRelationships with some symbolic names (the closest concept to DbRelationship name in the DB world is a named foreign key constraint. But DbRelationship names are usually chosen arbitrarily, without regard to constraints naming or even constraints presence). A database path therefore might look like this - db:dbrelationshipX.dbrelationshipY.COLUMN_Z. More specific examples:

Cayenne supports "aliases" in path Expressions. E.g. the same expression can be written using explicit path or an alias:

SelectQuery using the second form of the path expression must be made aware of the alias via SelectQuery.aliasPathSplits(..), otherwise an Exception will be thrown. The main use of aliases is to allow users to control how SQL joins are generated if the same path is encountered more than once in any given Expression. Each alias for any given path would result in a separate join. Without aliases, a single join will be used for a group of matching paths.

While in most cases users are likely to rely on API from the following section for expression creation, we’ll start by showing String expressions, as this will help to understand the semantics. A Cayenne expression can be represented as a String, which can be converted to an expression object using ExpressionFactory.exp static method. Here is an example:

This particular expression may be used to match Paintings whose names that start with "A" and whose price is less than $1000. While this example is pretty self-explanatory, there are a few points worth mentioning. "name" and "price" here are object paths discussed earlier. As always, paths themselves are not attached to a specific root entity and can be applied to any entity that has similarly named attributes or relationships. So when we are saying that this expression "may be used to match Paintings", we are implying that there may be other entities, for which this expression is valid. Now the expression details…​

Character constants that are not paths or numeric values should be enclosed in single or double quotes. Two of the expressions below are equivalent:

Case sensitivity. Expression operators are case sensitive and are usually lowercase. Complex words follow the Java camel-case style:

Grouping with parenthesis:

Path prefixes. Object expressions are unquoted strings, optionally prefixed by obj: (usually they are not prefixed at all actually). Database expressions are always prefixed with db:. A special kind of prefix, not discussed yet is enum: that prefixes an enumeration constant:

Binary conditions are expressions that contain a path on the left, a value on the right, and some operation between them, such as equals, like, etc. They can be used as qualifiers in SelectQueries:

Named parameters. Expressions can have named parameters (names that start with "$"). Parameterized expressions allow to create reusable expression templates. Also if an Expression contains a complex object that doesn’t have a simple String representation (e.g. a Date, a DataObject, an ObjectId), parameterizing such expression is the only way to represent it as String. Here are some examples:

To create a named parameterized expression with a LIKE clause, SQL wildcards must be part of the values in the Map and not the expression string itself:

When matching on a relationship, parameters can be Persistent objects or ObjectIds:

Uninitialized parameters will be automatically pruned from expressions, so a user can omit some parameters when creating an expression from a parameterized template:

Null handling. Handling of Java nulls as operands is no different from normal values. Instead of using special conditional operators, like SQL does (IS NULL, IS NOT NULL), "=" and "!=" expressions are used directly with null values. It is up to Cayenne to translate expressions with nulls to the valid SQL.

Creating expressions from Strings is a powerful and dynamic approach, however a safer alternative is to use Java API. It provides some degree of compile-time checking of expressions validity. The API is cenetred around ExpressionFactory class, and the Expression class. ExpressionFactory contains a number of rather self-explanatory factory methods. We won’t be going over all of them in detail, but will rather show a few general examples and some gotchas.

The following code recreates the expression from the previous chapter, but now using expression API:

This is more verbose than creating it from String, but it is also more resilient to the entity properties renaming and precludes semantic errors in the expression String.

As discussed earlier, Cayenne supports aliases in path Expressions, allowing to control how SQL joins are generated if the same path is encountered more than once in the same Expression. Two ExpressionFactory methods allow to implicitly generate aliases to "split" match paths into individual joins if needed:

"Path" argument to both of these methods can use a split character (a pipe symbol '|') instead of dot to indicate that relationship following a path should be split into a separate set of joins, one per collection value. There can only be one split at most in any given path. Split must always precede a relationship. E.g. |exhibits.paintings, exhibits|paintings, etc. Internally Cayenne would generate distinct aliases for each of the split expressions, forcing separate joins.

When used in a query, an expression is converted to SQL WHERE clause (or ORDER BY clause) by Cayenne during query execution. Thus the actual evaluation against the data is done by the database engine. However the same expressions can also be used for accessing object properties, calculating values, in-memory filtering.

Checking whether an object satisfies an expression:

Reading property value:

Filtering a list of objects:

SelectQuery is the most commonly used query in user applications. This may be the only query you will need in most appplications. It returns a list of persistent objects of a certain type specified in the query:

This returned all rows in the "ARTIST" table. If the logs were turned on, you might see the following SQL printed:

This SQL was generated by Cayenne from the SelectQuery above. SelectQuery can have a qualifier to select only the data that you care about. Qualifier is simply an Expression (Expressions where discussed in the previous chapter). If you only want artists whose name begins with 'Pablo', you might use the following qualifier expression:

The SQL will look different this time:

SelectQuery allows to append parts of qualifier to self:

To order the results of SelectQuery, one or more Orderings can be applied. Ordering were already discussed earlier. E.g.:

There’s a number of other useful properties in SelectQuery that define what to select and how to optimize database interaction (prefetching, caching, fetch offset and limit, pagination, etc.). Some of them are discussed in separate chapters on caching and performance optimization. Others are fairly self-explanatory. Please check the API docs for the full extent of the SelectQuery features.

EJBQLQuery was created as a part of an experiment in adopting some of Java Persistence API (JPA) approaches in Cayenne. It is a parameterized object query that is created from query String. A String used to build EJBQLQuery must conform to JPQL (JPA query language):

JPQL details can be found in any JPA manual. Here we’ll mention only how this fits into Cayenne and what are the differences between EJBQL and other Cayenne queries.

Although most frequently EJBQLQuery is used as an alternative to SelectQuery, there are also DELETE and UPDATE varieties available.

In most cases SelectQuery is preferred to EJBQLQuery, as it is API-based, and provides you with better compile-time checks. However sometimes you may want a completely scriptable object query. This is when you might prefer EJBQL. A more practical reason for picking EJBQL over SelectQuery though is that the former offers some extra selecting capabilities, namely aggregate functions and subqueries:

This also demonstrates a previously unseen type of select result - a List of Object[] elements, where each entry in an Object[] is either a DataObject or a scalar, depending on the query SELECT clause. A result can also be a list of scalars:

While Cayenne Expressions discussed previously can be thought of as identical to JPQL WHERE clause, and indeed they are very close, there are a few noteable differences:

SQLTemplate is a query that allows to run native SQL from a Cayenne application. It comes handy when the standard ORM concepts are not sufficient for a given query or an update. SQL is too powerful and allows to manipulate data in ways that are not easily described as a graph of related entities. Cayenne acknowledges this fact and provides this facility to execute SQL, mapping the result to objects when possible. Here are examples of selecting and non-selecting SQLTemplates:

Cayenne doesn’t make any attempt to make sense of the SQL semantics, so it doesn’t know whether a given query is performing a select or update, etc. It is the the user’s decision to run a given query as a selecting or "generic".

Another point to note is that the first argument to the SQLTemplate constructor - the Java class - has the same meaning as in SelectQuery only when the result can be converted to objects (e.g. when this is a selecting query and it is selecting all columns from one table). In this case it denotes the "root" entity of this query result. If the query does not denote a single entity result, this argument is only used for query routing, i.e. determining which database it should be run against. You are free to use any persistent class or even a DataMap instance in such situation. It will work as long as the passed "root" maps to the same database as the current query.

To achieve interoperability between mutliple RDBMS a user can specify multiple SQL statements for the same SQLTemplate, each corresponding to a native SQL dialect. A key used to look up the right dialect during execution is a fully qualified class name of the corresponding DbAdapter. If no DB-specific statement is present for a given DB, a default generic statement is used. E.g. in all the examples above a default statement will be used regardless of the runtime database. So in most cases you won’t need to explicitly "translate" your SQL to all possible dialects. Here is how this works in practice:

Stored procedures are mapped as separate objects in CayenneModeler. ProcedureQuery provides a way to execute them with a certain set of parameters. Just like with SQLTemplate, the outcome of a procedure can be anything - a single result set, mutliple result sets, some data modification (returned as an update count), or a combination of these. So use "performQuery" to get a single result set, and use "performGenericQuery" for anything else:

A stored procedure can return data back to the application as result sets or via OUT parameters. To simplify the processing of the query output, QueryResponse treats OUT parameters as if it was a separate result set. If a stored procedure declares any OUT or INOUT parameters, QueryResponse will contain their returned values in the very first result list:

There maybe a situation when a stored procedure handles its own transactions, but an application is configured to use Cayenne-managed transactions. This is obviously conflicting and undesirable behavior. In this case ProcedureQueries should be executed explicitly wrapped in an "external" Transaction. This is one of the few cases when a user should worry about transactions at all. See Transactions section for more details.

NamedQuery is a query that is a reference to another query stored in the DataMap. The actual stored query can be SelectQuery, SQLTemplate, EJBQLQuery, etc. It doesn’t matter - the API for calling them is the same - via a NamedQuery:

If a user needs some extra functionality not addressed by the existing set of Cayenne queries, he can write his own. The only requirement is to implement org.apache.cayenne.query.Query interface. The easiest way to go about it is to subclass some of the base queries in Cayenne.

E.g. to do something directly in the JDBC layer, you might subclass AbstractQuery:

To delegate the actual query execution to a standard Cayenne query, you may subclass IndirectQuery:

In fact many internal Cayenne queries are IndirectQueries, delegating to SelectQuery or SQLTemplate after some preprocessing.

Cayenne defines the following 8 types of lifecycle events for which callbacks can be regsitered:

Callback methods on Persistent classes are mapped in CayenneModeler for each ObjEntity. Empty callback methods are automatically created as a part of class generation (either with Maven, Ant or the Modeler) and are later filled with appropriate logic by the programmer. E.g. assuming we mapped a 'post-add' callback called 'onNewOrder' in ObjEntity 'Order', the following code will be generated:

As onNewOrder() is already declared in the mapping, it does not need to be registered explicitly. Implementing the method in subclass to do something meaningful is all that is required at this point.

As a rule callback methods do not have any knowledge of the outside application, and can only access the state of the object itself and possibly the state of other persistent objects via object’s own ObjectContext.

A listener is simply some application class that has one or more annotated callback methods. A callback method signature should be void someMethod(SomePersistentType object). It can be public, private, protected or use default access:

Notice that the example above contains an annotation on the callback method that defines the type of the event this method should be called for. Before we go into annotation details, we’ll show how to create and register a listener with Cayenne. It is always a user responsibility to register desired application listeners, usually right after ServerRuntime is started. Here is an example:

First let’s define 2 simple listeners.

Ignore the annotations for a minute. The important point here is that the listeners are arbitrary classes unmapped and unknown to Cayenne, that contain some callback methods. Now let’s register them with runtime:

Listeners in this example are very simple. However they don’t have to be. Unlike Persistent objects, normally listeners initialization is managed by the application code, not Cayenne, so listeners may have knowledge of various application services, operation transactional context, etc. Besides a single listener can apply to multiple entities. As a consequence their callbacks can do more than just access a single ObjectContext.

Now let’s discuss the annotations. There are eight annotations exactly matching the names of eight lifecycle events. A callback method in a listener should be annotated with at least one, but possibly with more than one of them. Annotation itself defines what event the callback should react to. Annotation parameters are essentially an entity filter, defining a subset of ObjEntities whose events we are interested in:

As shown above, "value" (the implicit annotation parameter) can contain one or more entity classes. Only these entities' events will result in callback invocation. There’s also another way to match entities - via custom annotations. This allows to match any number of entities without even knowing what they are. Here is an example. We’ll first define a custom annotation:

Now we can define a listener that will react to events from ObjEntities annotated with this annotation:

As you see we don’t have any entities yet, still we can define a listener that does something useful. Now let’s annotate some entities:

A final touch in the listeners design is preserving the state of the listener within a single select or commit, so that events generated by multiple objects can be collected and processed all together. To do that you will need to implement a DataChannelFilter, and add some callback methods to it. They will store their state in a ThreadLocal variable of the filter. Here is an example filter that does something pretty meaningless - counts how many total objects were committed. However it demonstrates the important pattern of aggregating multiple events and presenting a combined result:

Now since this is both a filter and a listener, it needs to be registered as such:

Prefetching is a technique that allows to bring back in one query not only the queried objects, but also objects related to them. In other words it is a controlled eager relationship resolving mechanism. Prefetching is discussed in the "Performance Tuning" chapter, as it is a powerful performance optimization method. However another common application of prefetching is to refresh stale object relationships, so more generally it can be viewed as a technique for managing subsets of the object graph.

Prefetching example:

All types of relationships can be preftetched - to-one, to-many, flattened.

A prefetch can span multiple relationships:

A query can have multiple prefetches:

If a query is fetching DataRows, all "disjoint" prefetches are ignored, only "joint" prefetches are executed (see prefetching semantics discussion below for what disjoint and joint prefetches mean).

Converting result set data to Persistent objects and registering these objects in the ObjectContext can be an expensive operation compareable to the time spent running the query (and frequently exceeding it). Internally Cayenne builds the result as a list of DataRows, that are later converted to objects. Skipping the last step and using data in the form of DataRows can significantly increase performance.

DataRow is a simply a map of values keyed by their DB column name. It is a ubiqutous representation of DB data used internally by Cayenne. And it can be quite usable as is in the application in many cases. So performance sensitive selects should consider DataRows - it saves memory and CPU cycles. All selecting queries support DataRows option, e.g.:

Moreover DataRows may be converted to Persistent objects later as needed. So e.g. you may implement some in-memory filtering, only converting a subset of fetched objects:

While contemporary hardware may easily allow applications to fetch hundreds of thousands or even millions of objects into memory, it doesn’t mean this is always a good idea to do so. You can optimize processing of very large result sets with two techniques discussed in this and the following chapter - iterated and paginated queries.

Iterated query is not actually a special query. Any selecting query can be executed in iterated mode by the DataContext (like in the previous example, a cast to DataContext is needed). DataContext returns an object called ResultIterator that is backed by an open ResultSet. Data is read from ResultIterator one row at a time until it is exhausted. Data comes as a DataRows regardless of whether the orginating query was configured to fetch DataRows or not. A ResultIterator must be explicitly closed to avoid JDBC resource leak.

Iterated query provides constant memory performance for arbitrarily large ResultSets. This is true at least on the Cayenne end, as JDBC driver may still decide to bring the entire ResultSet into the JVM memory.

Here is a full example:

Also common sense tells us that ResultIterators should be processed and closed as soon as possible to release the DB connection. E.g. storing open iterators between HTTP requests and for unpredictable length of time would quickly exhaust the connection pool.

Enabling query pagination allows to load very large result sets in a Java app with very little memory overhead (much smaller than even the DataRows option discussed above). Moreover it is completely transparent to the application - a user gets what appears to be a list of Persistent objects - there’s no iterator to close or DataRows to convert to objects:

Having said that, DataRows option can be combined with pagination, providing the best of both worlds:

The way pagination works internally, it first fetches a list of IDs for the root entity of the query. This is very fast and initially takes very little memory. Then when an object is requested at an arbitrary index in the list, this object and adjacent objects (a "page" of objects that is determined by the query pageSize parameter) are fetched together by ID. Subsequent requests to the objects of this "page" are served from memory.

An obvious limitation of pagination is that if you eventually access all objects in the list, the memory use will end up being the same as with no pagination. However it is still a very useful approach. With some lists (e.g. multi-page search results) only a few top objects are normally accessed. At the same time pagination allows to estimate the full list size without fetching all the objects. And again - it is completely transparent and looks like a normal query.

By default when a single ObjectContext commits its changes, all other contexts in the same runtime receive an event that contains all the committed changes. This allows them to update their cached object state to match the latest committed data. There are however many problems with this ostensibly helpful feature. In short - it works well in environments with few contexts and in unclustered scenarios, such as single user desktop applications, or simple webapps with only a few users. More specifically:

So we’ve made a good case for disabling synchronization in most webapps. To do that, set to "false" the following DI property - Constants.SERVER_CONTEXTS_SYNC_PROPERTY, using one of the standard Cayenne DI approaches. E.g. from command line:

Or by changing the standard properties Map in a custom extensions module:

Cayenne runtime is built around a small powerful dependency injection (DI) container. Just like other popular DI technologies, such as Spring or Guice, Cayenne DI container manages sets of interdependent objects and allows users to configure them. These objects are regular Java objects. We are calling them "services" in this document to distinguish from all other objects that are not configured in the container and are not managed. DI container is responsible for service instantiation, injecting correct dependencies, maintaining service instances scope, and dispatching scope events to services.

The services are configured in special Java classes called "modules". Each module defines binding of service interfaces to implementation instances, implementation types or providers of implementation instances. There are no XML configuration files, and all the bindings are type-safe. The container supports injection into instance variables and constructor parameters based on the @Inject annotation. This mechanism is very close to Google Guice.

The discussion later in this chapter demonstrates a standalone DI container. But keep in mind that Cayenne already has a built-in Injector, and a set of default modules. A Cayenne user would normally only use the API below to write custom extension modules that will be loaded in that existing container when creating ServerRuntime. See "Starting and Stopping ServerRuntime" chapter for an example of passing an extension module to Cayenne.

Cayenne DI probably has ~80% of the features expected in a DI container and has no dependency on the rest of Cayenne, so in theory can be used as an application-wide DI engine. But it’s primary purpose is still to serve Cayenne. Hence there are no plans to expand it beyond Cayenne needs. It is an ideal "embedded" DI that does not interfere with Spring, Guice or any other such framework present elsewhere in the application.

The previous section discussed how Cayenne DI works in general terms. Since Cayenne users will mostly be dealing with an existing Injector provided by ServerRuntime, it is important to understand how to build custom extensions to a preconfigured container. As shown in "Starting and Stopping ServerRuntime" chapter, custom extensions are done by writing an aplication DI module (or multiple modules) that configures service overrides. This section shows all the configuration possibilities in detail, including changing properties of the existing services, contributing services to standard service lists and maps, and overriding service implementations. All the code examples later in this section are assumed to be placed in an application module "configure" method:

"Remote Object Persistence" is a low-overhead web services-based technology that provides lightweight object persistence and query functionality to 'remote' applications. In other words it provides familiar Cayenne API to applications that do not have direct access to the database. Instead such applications would access Cayenne Web Service (CWS). A single abstract data model (expressed as Cayenne XML DataMap) is used on the server and on the client, while execution logic can be partitioned between the tiers.The following picture compares a regular Cayenne web application and a rich client application that uses remote object persistence technology:

Persistence stack above consists of the following parts:

(The <Valve> tag can also be placed within the <Context> in any other locations used by Tomcat to load context configurations)