Service registry in java

A registry for service provider instances. A service is a well-known set of interfaces and (usually abstract) classes. A service provider is a specific implementation of a service. The classes in a provider typically implement the interface or subclass the class defined by the service itself. Service providers are stored in one or more categories, each of which is defined by a class of interface (described by a Class object) that all of its members must implement. The set of categories may be changed dynamically. Only a single instance of a given leaf class (that is, the actual class returned by getClass() , as opposed to any inherited classes or interfaces) may be registered. That is, suppose that the com.mycompany.mypkg.GreenServiceProvider class implements the com.mycompany.mypkg.MyService interface. If a GreenServiceProvider instance is registered, it will be stored in the category defined by the MyService class. If a new instance of GreenServiceProvider is registered, it will replace the previous instance. In practice, service provider objects are usually singletons so this behavior is appropriate. To declare a service provider, a services subdirectory is placed within the META-INF directory that is present in every JAR file. This directory contains a file for each service provider interface that has one or more implementation classes present in the JAR file. For example, if the JAR file contained a class named com.mycompany.mypkg.MyServiceImpl which implements the javax.someapi.SomeService interface, the JAR file would contain a file named:

META-INF/services/javax.someapi.SomeService

com.mycompany.mypkg.MyService

Nested Class Summary

A simple filter interface used by ServiceRegistry.getServiceProviders to select providers matching an arbitrary criterion.

Constructor Summary

Method Summary

Returns an Iterator containing service provider objects within a given category that satisfy a criterion imposed by the supplied ServiceRegistry.Filter object’s filter method.

Locates and incrementally instantiates the available providers of a given service using the context class loader.

Methods inherited from class java.lang.Object

Constructor Detail

ServiceRegistry

Method Detail

lookupProviders

public static Iterator lookupProviders(Class providerClass, ClassLoader loader)

Searches for implementations of a particular service class using the given class loader. This method transforms the name of the given service class into a provider-configuration filename as described in the class comment and then uses the getResources method of the given class loader to find all available files with that name. These files are then read and parsed to produce a list of provider-class names. The iterator that is returned uses the given class loader to look up and then instantiate each element of the list. Because it is possible for extensions to be installed into a running Java virtual machine, this method may return different results each time it is invoked.

lookupProviders

public static Iterator lookupProviders(Class providerClass)

Locates and incrementally instantiates the available providers of a given service using the context class loader. This convenience method is equivalent to:

ClassLoader cl = Thread.currentThread().getContextClassLoader(); return Service.providers(service, cl);

getCategories

Returns an Iterator of Class objects indicating the current set of categories. The iterator will be empty if no categories exist.

registerServiceProvider

Adds a service provider object to the registry. The provider is associated with the given category. If provider implements the RegisterableService interface, its onRegistration method will be called. Its onDeregistration method will be called each time it is deregistered from a category, for example if a category is removed or the registry is garbage collected.

registerServiceProvider

Adds a service provider object to the registry. The provider is associated within each category present in the registry whose Class it implements. If provider implements the RegisterableService interface, its onRegistration method will be called once for each category it is registered under. Its onDeregistration method will be called each time it is deregistered from a category or when the registry is finalized.

registerServiceProviders

Adds a set of service provider objects, taken from an Iterator to the registry. Each provider is associated within each category present in the registry whose Class it implements. For each entry of providers that implements the RegisterableService interface, its onRegistration method will be called once for each category it is registered under. Its onDeregistration method will be called each time it is deregistered from a category or when the registry is finalized.

deregisterServiceProvider

Removes a service provider object from the given category. If the provider was not previously registered, nothing happens and false is returned. Otherwise, true is returned. If an object of the same class as provider but not equal (using == ) to provider is registered, it will not be deregistered. If provider implements the RegisterableService interface, its onDeregistration method will be called.

deregisterServiceProvider

contains

getServiceProviders

public Iterator getServiceProviders(Class category, boolean useOrdering)

Returns an Iterator containing all registered service providers in the given category. If useOrdering is false , the iterator will return all of the server provider objects in an arbitrary order. Otherwise, the ordering will respect any pairwise orderings that have been set. If the graph of pairwise orderings contains cycles, any providers that belong to a cycle will not be returned.

getServiceProviders

public Iterator getServiceProviders(Class category, ServiceRegistry.Filter filter, boolean useOrdering)

Returns an Iterator containing service provider objects within a given category that satisfy a criterion imposed by the supplied ServiceRegistry.Filter object’s filter method. The useOrdering argument controls the ordering of the results using the same rules as getServiceProviders(Class, boolean) .

getServiceProviderByClass

Returns the currently registered service provider object that is of the given class type. At most one object of a given class is allowed to be registered at any given time. If no registered object has the desired class type, null is returned.

setOrdering

public boolean setOrdering(Class category, T firstProvider, T secondProvider)

Sets a pairwise ordering between two service provider objects within a given category. If one or both objects are not currently registered within the given category, or if the desired ordering is already set, nothing happens and false is returned. If the providers previously were ordered in the reverse direction, that ordering is removed. The ordering will be used by the getServiceProviders methods when their useOrdering argument is true .

unsetOrdering

public boolean unsetOrdering(Class category, T firstProvider, T secondProvider)

Sets a pairwise ordering between two service provider objects within a given category. If one or both objects are not currently registered within the given category, or if no ordering is currently set between them, nothing happens and false is returned. The ordering will be used by the getServiceProviders methods when their useOrdering argument is true .

deregisterAll

public void deregisterAll()

finalize

Finalizes this object prior to garbage collection. The deregisterAll method is called to deregister all currently registered service providers. This method should not be called from application code.

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Class ServiceRegistry

Service providers are stored in one or more categories, each of which is defined by a class or interface (described by a Class object) that all of its members must implement.

An attempt to load a provider that is not a subtype of one of the above types will result in IllegalArgumentException .

For the general mechanism to load service providers, see ServiceLoader , which is the underlying standard mechanism used by this class.

An application may customize the contents of a registry as it sees fit, so long as it has the appropriate runtime permission.

For information on how to create and deploy service providers, refer to the documentation on ServiceLoader

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