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Units of Measurement Standard - This JSR specifies Java packages for modeling and working with measurement values, quantities and their corresponding units.
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/*
* Units of Measurement API
* Copyright (c) 2014-2023, Jean-Marie Dautelle, Werner Keil, Otavio Santana.
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without modification,
* are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions
* and the following disclaimer in the documentation and/or other materials provided with the distribution.
*
* 3. Neither the name of JSR-385 nor the names of its contributors may be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
* EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
package javax.measure.spi;
import java.lang.annotation.Annotation;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Objects;
import java.util.Optional;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Predicate;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
import javax.measure.Quantity;
import javax.measure.format.QuantityFormat;
import javax.measure.format.UnitFormat;
/**
* Service Provider for Units of Measurement services.
*
* All the methods in this class are safe to use by multiple concurrent threads.
*
*
* @version 2.3, May 19, 2023
* @author Werner Keil
* @author Martin Desruisseaux
* @since 1.0
*/
public abstract class ServiceProvider {
/**
* Class name of JSR-330 annotation for naming a service provider.
* We use reflection for keeping JSR-330 an optional dependency.
*/
private static final String LEGACY_NAMED_ANNOTATION = "javax.inject.Named";
/**
* Class name of JSR-250 annotation for assigning a priority level to a service provider.
* We use reflection for keeping JSR-250 an optional dependency.
*/
private static final String LEGACY_PRIORITY_ANNOTATION = "javax.annotation.Priority";
/**
* Class name of Jakarta Dependency Injection annotation for naming a service provider.
* We use reflection for keeping Jakata Injection an optional dependency.
*/
private static final String NAMED_ANNOTATION = "jakarta.inject.Named";
/**
* Class name of Jakarta Common Annotation for assigning a priority level to a service provider.
* We use reflection for keeping Jakarta Annotations an optional dependency.
*/
private static final String PRIORITY_ANNOTATION = "jakarta.annotation.Priority";
/**
* The current service provider, or {@code null} if not yet determined.
*
* Implementation Note: We do not cache a list of all service providers because that list depends
* indirectly on the thread invoking the {@link #available()} method. More specifically, it depends
* on the context class loader. Furthermore caching the {@code ServiceProvider}s can be a source of
* memory leaks. See {@link ServiceLoader#load(Class)} API note for reference.
*/
private static final AtomicReference current = new AtomicReference<>();
/**
* Creates a new service provider. Only to be used by subclasses.
*/
protected ServiceProvider() {
}
/**
* Allows to define a priority for a registered {@code ServiceProvider} instance.
* When multiple providers are registered in the system, the provider with the highest priority value is taken.
*
* If the {@value #PRIORITY_ANNOTATION} annotation (from Jakarta Annotations)
* or {@value #LEGACY_PRIORITY_ANNOTATION} annotation (from JSR-250) is present on the {@code ServiceProvider}
* implementation class, then that annotation (first if both were present) is taken and this {@code getPriority()} method is ignored.
* Otherwise – if a {@code Priority} annotation is absent – this method is used as a fallback.
*
* @return the provider's priority (default is 0).
*/
public int getPriority() {
return 0;
}
/**
* Returns the service to obtain a {@link SystemOfUnits}, or {@code null} if none.
*
* @return the service to obtain a {@link SystemOfUnits}, or {@code null}.
*/
public abstract SystemOfUnitsService getSystemOfUnitsService();
/**
* Returns the service to obtain {@link UnitFormat} and {@link QuantityFormat} or {@code null} if none.
*
* @return the service to obtain a {@link UnitFormat} and {@link QuantityFormat}, or {@code null}.
* @since 2.0
*/
public abstract FormatService getFormatService();
/**
* Returns a factory for the given {@link Quantity} type.
*
* @param
* the type of the {@link Quantity} instances created by the factory
* @param quantity
* the quantity type
* @return the {@link QuantityFactory} for the given type
*/
public abstract > QuantityFactory getQuantityFactory(Class quantity);
/**
* A filter and a comparator for processing the stream of service providers.
* The two tasks (filtering and sorting) are implemented by the same class,
* but the filter task shall be used only if the name to search is non-null.
* The comparator is used in all cases, for sorting providers with higher priority first.
*/
private static final class Selector implements Predicate, Comparator {
/**
* The name of the provider to search, or {@code null} if no filtering by name is applied.
*/
private final String toSearch;
/**
* The {@code value()} method in the {@value #NAMED_ANNOTATION} annotation,
* or {@code null} if that class is not on the classpath.
*/
private final Method nameGetter;
/**
* The {@code value()} method in the {@value #PRIORITY_ANNOTATION} annotation,
* or {@code null} if that class is not on the classpath.
*/
private final Method priorityGetter;
/**
* The {@code value()} method in the {@value #LEGACY_NAMED_ANNOTATION} annotation,
* or {@code null} if that class is not on the classpath.
*/
private final Method legacyNameGetter;
/**
* The {@code value()} method in the {@value #LEGACY_PRIORITY_ANNOTATION} annotation,
* or {@code null} if that class is not on the classpath.
*/
private final Method legacyPriorityGetter;
/**
* Creates a new filter and comparator for a stream of service providers.
*
* @param name name of the desired service provider, or {@code null} if no filtering by name is applied.
*/
Selector(String name) {
toSearch = name;
try {
if (name != null) {
nameGetter = getValueMethod(NAMED_ANNOTATION);
legacyNameGetter = getValueMethod(LEGACY_NAMED_ANNOTATION);
} else {
nameGetter = null;
legacyNameGetter = null;
}
priorityGetter = getValueMethod(PRIORITY_ANNOTATION);
legacyPriorityGetter = getValueMethod(LEGACY_PRIORITY_ANNOTATION);
} catch (NoSuchMethodException e) {
// Should never happen since value() is a standard public method of those annotations.
throw new ServiceConfigurationError("Cannot get annotation value", e);
}
}
/**
* Returns the {@code value()} method in the given annotation class.
*
* @param classname name of the class from which to get the {@code value()} method.
* @return the {@code value()} method, or {@code null} if the annotation class was not found.
*/
private static Method getValueMethod(final String classname) throws NoSuchMethodException {
try {
return Class.forName(classname).getMethod("value", (Class[]) null);
} catch (ClassNotFoundException e) {
// Ignore because JSR-330, JSR-250 and Jakarta are optional dependencies.
return null;
}
}
/**
* Invokes the {@code value()} method on the annotation of the given class.
* The annotation on which to invoke the method is given by {@link Method#getDeclaringClass()}.
*
* @param provider class of the provider on which to invoke annotation {@code value()}.
* @param getter the preferred {@code value()} method to invoke, or {@code null}.
* @param fallback an alternative {@code value()} method to invoke, or {@code null}.
* @return the value, or {@code null} if none.
*/
private static Object getValue(final Class provider, Method getter, Method fallback) {
if (getter == null) {
getter = fallback;
fallback = null;
}
while (getter != null) {
final Annotation a = provider.getAnnotation(getter.getDeclaringClass().asSubclass(Annotation.class));
if (a != null) try {
return getter.invoke(a, (Object[]) null);
} catch (IllegalAccessException | InvocationTargetException e) {
// Should never happen since value() is a public method and should not throw exception.
throw new ServiceConfigurationError("Cannot get annotation value", e);
}
getter = fallback;
fallback = null;
}
return null;
}
/**
* Returns {@code true} if the given service provider has the name we are looking for.
* This method shall be invoked only if a non-null name has been specified to the constructor.
* This method looks for the {@value #NAMED_ANNOTATION} and {@value #LEGACY_NAMED_ANNOTATION}
* annotations in that order, and if none are found fallbacks on {@link ServiceProvider#toString()}.
*/
@Override
public boolean test(final ServiceProvider provider) {
Object value = getValue(provider.getClass(), nameGetter, legacyNameGetter);
if (value == null) {
value = provider.toString();
}
return toSearch.equals(value);
}
/**
* Returns the priority of the given service provider.
* This method looks for the {@value #PRIORITY_ANNOTATION} and {@value #LEGACY_PRIORITY_ANNOTATION}
* annotations in that order, and if none are found falls back on {@link ServiceProvider#getPriority()}.
*/
private int priority(final ServiceProvider provider) {
Object value = getValue(provider.getClass(), priorityGetter, legacyPriorityGetter);
if (value != null) {
return (Integer) value;
}
return provider.getPriority();
}
/**
* Compares the given service providers for order based on their priority.
* The priority of each provider is determined as documented by {@link ServiceProvider#getPriority()}.
*/
@Override
public int compare(final ServiceProvider p1, final ServiceProvider p2) {
return Integer.compare(priority(p2), priority(p1)); // reverse order, higher number first.
}
/**
* Gets all {@link ServiceProvider}s sorted by priority and optionally filtered by the name in this selector.
* The list of service providers is not cached because it depends on the context class loader,
* which itself depends on which thread is invoking this method.
*/
private Stream stream() {
Stream stream = StreamSupport.stream(ServiceLoader.load(ServiceProvider.class).spliterator(), false);
if (toSearch != null) {
stream = stream.filter(this);
}
return stream.sorted(this);
}
}
/**
* Returns the list of all service providers available for the current thread's context class loader.
* The {@linkplain #current() current} service provider is always the first item in the returned list.
* Other service providers after the first item may depend on the caller thread
* (see {@linkplain ServiceLoader#load(Class) service loader API note}).
*
* @return all service providers available for the current thread's context class loader.
*/
public static final List available() {
ArrayList providers = new Selector(null).stream().collect(Collectors.toCollection(ArrayList::new));
final ServiceProvider first = current.get();
/*
* Make sure that 'first' is the first item in the 'providers' list. If that item appears
* somewhere else, we have to remove the second occurrence for avoiding duplicated elements.
* We compare the classes, not the instances, because new instances may be created each time
* this method is invoked and we have no guaranteed that implementors overrode 'equals'.
*/
setcur: if (first != null) {
final Class cf = first.getClass();
final int size = providers.size();
for (int i=0; iImplementors are encouraged to provide an {@code Named} annotation or to override {@link #toString()}
* and use a unique enough name, e.g. the class name or other distinct attributes.
* Should multiple service providers nevertheless use the same name, the one with the highest
* {@linkplain #getPriority() priority} wins.
*
* @param name
* the name of the service provider to return
* @return the {@link ServiceProvider} with the specified name
* @throws IllegalArgumentException
* if available service providers do not contain a provider with the specified name
* @throws NullPointerException
* if {@code name} is null
* @see #toString()
* @since 2.0
*/
public static ServiceProvider of(String name) {
Objects.requireNonNull(name);
Selector select = new Selector(name);
ServiceProvider p = current.get();
if (p != null && select.test(p)) {
return p;
}
Optional first = select.stream().findFirst();
if (first.isPresent()) {
return first.get();
} else {
throw new IllegalArgumentException("No Measurement ServiceProvider " + name + " found .");
}
}
/**
* Returns the current {@link ServiceProvider}. If necessary the {@link ServiceProvider} will be lazily loaded.
*
* If there are no providers available, an {@linkplain IllegalStateException} is thrown.
* Otherwise the provider with the highest priority is used
* or the one explicitly designated via {@link #setCurrent(ServiceProvider)}.
*
*
* @return the {@link ServiceProvider} used.
* @throws IllegalStateException
* if no {@link ServiceProvider} has been found.
* @see #getPriority()
* @see #setCurrent(ServiceProvider)
*/
public static final ServiceProvider current() {
ServiceProvider p = current.get();
if (p == null) {
Optional first = new Selector(null).stream().findFirst();
if (first.isPresent()) {
p = first.get();
} else {
throw new IllegalStateException("No Measurement ServiceProvider found.");
}
}
return p;
}
/**
* Replaces the current {@link ServiceProvider}.
*
* @param provider
* the new {@link ServiceProvider}
* @return the replaced provider, or null.
*/
public static final ServiceProvider setCurrent(ServiceProvider provider) {
Objects.requireNonNull(provider);
ServiceProvider old = current.getAndSet(provider);
if (old != provider) {
Logger.getLogger("javax.measure.spi").log(Level.CONFIG,
"Measurement ServiceProvider {1,choice,0#set to|1#replaced by} {0}.",
new Object[] {provider.getClass().getName(), (old == null) ? 0 : 1});
}
return old;
}
}