To have a working DI container, we need three things: service interfaces and classes, a module that describes service bindings, a container that loads the module, and resolves the depedencies. Let's start with service interfaces and classes:

public interface Service1 {
    public String getString();
}

public interface Service2 {
    public int getInt();
}

A service implementation using instance variable injection:

public class Service1Impl implements Service1 {
    @Inject
    private Service2 service2;

    public String getString() {
        return service2.getInt() + "_Service1Impl";
    }
}

Same thing, but using constructor injection:

public class Service1Impl implements Service1 {

    private Service2 service2;

    public Service1Impl(@Inject Service2 service2) {
        this.service2 = service2;
    }

    public String getString() {
        return service2.getInt() + "_Service1Impl";
    }
}

public class Service2Impl implements Service2 {
    private int i;

    public int getInt() {
        return i++;
    }
}

Now let's create a module implementing org.apache.cayenne.tutorial.di.Module interface that will contain DI configuration. A module binds service objects to keys that are reference. Binder provided by container implements fluent API to connect the key to implementation, and to configure various binding options (the options, such as scope, are demonstrated later in this chapter). The simplest form of a key is a Java Class object representing service interface. Here is a module that binds Service1 and Service2 to corresponding default implementations:

public class Module1 implements Module {

    public void configure(Binder binder) {
        binder.bind(Service1.class).to(Service1Impl.class);
        binder.bind(Service2.class).to(Service2Impl.class);
    }
}

Once we have at least one module, we can create a DI container. org.apache.cayenne.di.Injector is the container class in Cayenne:

Injector injector = DIBootstrap.createInjector(new Module1());

Now that we have created the container, we can obtain services from it and call their methods:

Service1 s1 = injector.getInstance(Service1.class);
for (int i = 0; i < 5; i++) {
    System.out.println("S1 String: " + s1.getString());
}

This outputs the following lines, demonstrating that s1 was Service1Impl and Service2 injected into it was Service2Impl:

0_Service1Impl
1_Service1Impl
2_Service1Impl
3_Service1Impl
4_Service1Impl

There are more flavors of bindings:

// binding to instance - allowing user to create and configure instance
// inside the module class
binder.bind(Service2.class).toInstance(new Service2Impl());

// binding to provider - delegating instance creation to a special
// provider class
binder.bind(Service1.class).toProvider(Service1Provider.class);

// binding to provider instance
binder.bind(Service1.class).toProviderInstance(new Service1Provider());

// multiple bindings of the same type using Key
// injection can reference the key name in annotation:
// @Inject("i1")
// private Service2 service2;
binder.bind(Key.get(Service2.class, "i1")).to(Service2Impl.class);
binder.bind(Key.get(Service2.class, "i2")).to(Service2Impl.class);

Another types of confiuguration that can be bound in the container are lists and maps. They will be discussed in the following chapters.

An important feature of the Cayenne DI container is instance scope. The default scope (implicitly used in all examples above) is "singleton", meaning that a binding would result in creation of only one service instance, that will be repeatedly returned from Injector.getInstance(..), as well as injected into classes that declare it as a dependency.

Singleton scope dispatches a "BeforeScopeEnd" event to interested services. This event occurs before the scope is shutdown, i.e. when Injector.shutdown() is called. Note that the built-in Cayenne injector is shutdown behind the scenes when ServerRuntime.shutdown() is invoked. Services may register as listeners for this event by annotating a no-argument method with @BeforeScopeEnd annotation. Such method should be implemented if a service needs to clean up some resources, stop threads, etc.

Another useful scope is "no scope", meaning that every time a container is asked to provide a service instance for a given key, a new instance will be created and returned:

binder.bind(Service2.class).to(Service2Impl.class).withoutScope();

Users can also create their own scopes, e.g. a web application request scope or a session scope. Most often than not custom scopes can be created as instances of org.apache.cayenne.di.spi.DefaultScope with startup and shutdown managed by the application (e.g. singleton scope is a DefaultScope managed by the Injector) .

Cayenne DI allows to override services already definied in the current module, or more commonly - some other module in the the same container. Actually there's no special API to override a service, you'd just bind the service key again with a new implementation or provider. The last binding for a key takes precedence. This means that the order of modules is important when configuring a container. The built-in Cayenne injector ensures that Cayenne standard modules are loaded first, followed by optional user extension modules. This way the application can override the standard services in Cayenne.

Many built-in Cayenne services change their behavior based on a value of some environment property. A user may change Cayenne behavior without even knowing which services are responsible for it, but setting a specific value of a known property. Supported property names are listed in "Appendix A".

There are two ways to set service properties. The most obvious one is to pass it to the JVM with -D flag on startup. E.g.

java -Dcayenne.server.contexts_sync_strategy=false ...

A second one is to contribute a property to org.apache.cayenne.configuration.DefaultRuntimeProperties.properties map (see the next section on how to do that). This map contains the default property values and can accept application-specific values, overrding the defaults.

Note that if a property value is a name of a Java class, when this Java class is instantiated by Cayenne, the container performs injection of instance variables. So even the dynamically specified Java classes can use @Inject annotation to get a hold of other Cayenne services.

If the same property is specified both in the command line and in the properties map, the command-line value takes precedence. The map value will be ignored. This way Cayenne runtime can be reconfigured during deployment.

Cayenne can be extended by adding custom objects to named maps or lists bound in DI. We are calling these lists/maps "service collections". A service collection allows things like appending a custom strategy to a list of built-in strategies. E.g. an application that needs to install a custom DbAdapter for some database type may contribute an instance of custom DbAdapterDetector to a org.apache.cayenne.configuration.server.DefaultDbAdapterFactory.detectors list:

public class MyDbAdapterDetector implements DbAdapterDetector {
    public DbAdapter createAdapter(DatabaseMetaData md) throws SQLException {
        // check if we support this database and retun custom adapter
        ...
    }
}

// since build-in list for this key is a singleton, repeated
// calls to 'bindList' will return the same instance 
binder.bindList(DefaultDbAdapterFactory.DETECTORS_LIST)
       .add(MyDbAdapterDetector.class);

Maps are customized using a similar "bindMap" method.

The names of built-in collections are listed in "Appendix B".

As mentioned above, custom modules are loaded by ServerRuntime after the built-in modules. So it is easy to redefine a built-in service in Cayenne by rebinding desired implementations or providers. To do that, first we need to know what those services to redefine are. While we describe some of them in the following sections, the best way to get a full list is to check the source code of the Cayenne version you are using and namely look in org.apache.cayenne.configuration.server.ServerModule - the main built-in module in Cayenne.

Now an example of overriding QueryCache service. The default implementation of this service is provided by MapQueryCacheProvider. But if we want to use EhCacheQueryCache (a Cayenne wrapper for the EhCache framework), we can define it like this:

binder.bind(QueryCache.class).to(EhCacheQueryCache.class);

org.apache.cayenne.log.JdbcEventLogger is the service that defines logging API for Cayenne internals. It provides facilities for logging queries, commits, transactions, etc. The default implementation is org.apache.cayenne.log.CommonsJdbcEventLogger that performs logging via commons-logging library. Cayenne library includes another potentially useful logger - org.apache.cayenne.log.FormattedCommonsJdbcEventLogger that produces formatted multiline SQL output that can be easier to read.