com.kdgregory.logging.aws.facade.FacadeFactory Maven / Gradle / Ivy
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// Copyright (c) Keith D Gregory
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package com.kdgregory.logging.aws.facade;
import java.lang.reflect.Constructor;
import com.kdgregory.logging.aws.internal.AbstractWriterConfig;
/**
* Creates new instances of AWS facade objects. Uses reflection to determine
* which implementation library is linked into the application.
*/
public class FacadeFactory
{
/**
* Instantiates the facade implementation corresponding to the provided
* interface class. This variant is for service facades, which must be
* configured.
*
* @throws IllegalArgumentException if unable to instantiate. Exception
* message will provide more information.
*/
public static T createFacade(Class facadeType, AbstractWriterConfig config)
{
Class implClass = findImplementationClass(facadeType);
return (T)instantiate(implClass, config);
}
/**
* Instantiates the facade implementation corresponding to the provided
* interface class. This variant is for InfoFacade, which
* uses default clients (so doesn't need configuration).
*
* @throws IllegalArgumentException if unable to instantiate. Exception
* message will provide more information.
*/
public static T createFacade(Class facadeType)
{
Class implClass = findImplementationClass(facadeType);
return (T)instantiate(implClass);
}
//----------------------------------------------------------------------------
// Internals
//----------------------------------------------------------------------------
private final static String[] FACADE_PACKAGES = new String[]
{
"com.kdgregory.logging.aws.facade.v1",
"com.kdgregory.logging.aws.facade.v2"
};
/**
* Determines whether a facade implementation class exists in the classpath,
* and returns it (or null).
*
* Implementation note: the facade implementation class names are based on
* the interface names, so it's easier to construct the names than to use
* a lookup table.
*/
private static Class findImplementationClass(Class facadeType)
{
for (String packageName : FACADE_PACKAGES)
{
String className = packageName + "." + facadeType.getSimpleName() + "Impl";
try
{
return Class.forName(className);
}
catch (ClassNotFoundException ignored)
{
// yes, it's using an exception for flow control; sue me
}
}
throw new IllegalArgumentException("no implementation class for " + facadeType.getName());
}
/**
* Instantiates the implementation class, with optional arguments.
*/
private static Object instantiate(Class implClass, Object... ctorArgs)
{
try
{
Constructor[] ctors = implClass.getConstructors();
if (ctors.length != 1)
{
throw new IllegalArgumentException("implementation class does not expose a single constructor: " + implClass.getName());
}
return ctors[0].newInstance(ctorArgs);
}
catch (Exception ex)
{
throw new IllegalArgumentException("unable to instantiate: " + implClass.getName(), ex);
}
}
}