com.aquenos.cassandra.pvarchiver.server.util

Class FactoriesLoader

java.lang.Object

com.aquenos.cassandra.pvarchiver.server.util.FactoriesLoader

public abstract class FactoriesLoader
extends Object

Loader for automatically discovered factories. This loader provides a pluggable mechanism for discovering factories that are in the class-path. Despite its name, this mechanisms is not limited to factories, but can be used with any interfaces (or parent class). This class has been inspired by the SpringFactoriesLoader from Spring Boot.

This class loads the file specified by FACTORIES_RESOURCE_LOCATION from the class-path. Multiple instances of this file (originating from different modules) might exist in the class-path. This is intended and the entries found in all those files are merged to get a list of all available implementations in the class-path. The file itself follows the Java properties syntax, using interface names as keys and implementation names as values.

Author:

Sebastian Marsching

Field Summary

Fields

Modifier and Type	Field and Description
static String	FACTORIES_RESOURCE_LOCATION Location of the factories configuration file in the class-path.

Constructor Summary

Constructors

Constructor and Description
FactoriesLoader()

Method Summary

Modifier and Type	Method and Description
static <FactoryType> List<FactoryType>	loadFactories(Class<? extends FactoryType> factoryType, ClassLoader classLoader) Loads and instantiates all factories implementing a given base type.
static List<String>	loadFactoryNames(Class<?> factoryType, ClassLoader classLoader) Loads the list of class names of implementations for a specific interface or base class.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

FACTORIES_RESOURCE_LOCATION

public static final String FACTORIES_RESOURCE_LOCATION

Location of the factories configuration file in the class-path. Each module that provides one or several factories contributes an instance of this file, listing the factories provided by the module.

See Also:

Constant Field Values

Constructor Detail

FactoriesLoader

public FactoriesLoader()

Method Detail

loadFactoryNames

public static List<String> loadFactoryNames(Class<?> factoryType,
                                            ClassLoader classLoader)

Loads the list of class names of implementations for a specific interface or base class. This method scans all instances of the FACTORIES_RESOURCE_LOCATION file and looks up the property whose key is equal to the fully-qualified class name of factoryType. It builds a list containing the values listed under those keys. A single property file might contain multiple values for the same key if the values are separated by commas. The values are not verified in any way (for example, they might not represent the names of loadable classes) and simply returned.

Parameters:

factoryType - type of the interface or class whose fully qualified name should be used as the lookup key.

classLoader - class loader to be used when loading the FACTORIES_RESOURCE_LOCATION files. If null, the class loader that loaded this class is used.

Returns:

list of values (typically fully-qualified class-names) associated with the specified factory type. This list might be empty, but it is never null.

Throws:

RuntimeException - if there is an I/O error while trying to load one of the configuration files from the class-path.

loadFactories

public static <FactoryType> List<FactoryType> loadFactories(Class<? extends FactoryType> factoryType,
                                                            ClassLoader classLoader)

Loads and instantiates all factories implementing a given base type. This method uses the discovery mechanism provided by loadFactoryNames(Class, ClassLoader) to find all implementations of the specified base type. It then tries to create instances using the values returned by loadFactoryNames(Class, ClassLoader) as class names. This means that each implementation listed for the specified base type must have a default constructor.

Type Parameters:

FactoryType - type of the interface or base class for which factories are created. Typically, this is automatically inferred from factoryType.

Parameters:

factoryType - interface or base class that has to be implemented by all discovered implementations. The fully qualified name of this type is used when discovering the implementations.

classLoader - class loader used for discovering implementations and loading their classes. If null, the class loader that loaded this class is used.

Returns:

list of instances implementing factoryType. This list might be empty, but it is never null.

Throws:

IllegalArgumentException - if one of the discovered classes cannot be instantiated (e.g. because it cannot be loaded or it does not have a default constructor).

RuntimeException - if there is an I/O problem while trying to discover the implementations.