io.grpc.NameResolverProvider Maven / Gradle / Ivy
/*
* Copyright 2016, gRPC Authors All rights reserved.
*
* 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 io.grpc;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import java.net.URI;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
/**
* Provider of name resolvers for name agnostic consumption.
*
* Implementations should not throw. If they do, it may interrupt class loading. If
* exceptions may reasonably occur for implementation-specific reasons, implementations should
* generally handle the exception gracefully and return {@code false} from {@link #isAvailable()}.
*/
@ExperimentalApi
public abstract class NameResolverProvider extends NameResolver.Factory {
/**
* The port number used in case the target or the underlying naming system doesn't provide a
* port number.
*/
public static final Attributes.Key PARAMS_DEFAULT_PORT =
NameResolver.Factory.PARAMS_DEFAULT_PORT;
private static final List providers
= load(NameResolverProvider.class.getClassLoader());
private static final NameResolver.Factory factory = new NameResolverFactory(providers);
@VisibleForTesting
static List load(ClassLoader classLoader) {
Iterable candidates;
if (isAndroid()) {
candidates = getCandidatesViaHardCoded();
} else {
candidates = getCandidatesViaServiceLoader(classLoader);
}
List list = new ArrayList();
for (NameResolverProvider current : candidates) {
if (!current.isAvailable()) {
continue;
}
list.add(current);
}
// Sort descending based on priority.
Collections.sort(list, Collections.reverseOrder(new Comparator() {
@Override
public int compare(NameResolverProvider f1, NameResolverProvider f2) {
return f1.priority() - f2.priority();
}
}));
return Collections.unmodifiableList(list);
}
/**
* Loads service providers for the {@link NameResolverProvider} service using
* {@link ServiceLoader}.
*/
@VisibleForTesting
public static Iterable getCandidatesViaServiceLoader(
ClassLoader classLoader) {
Iterable i
= ServiceLoader.load(NameResolverProvider.class, classLoader);
// Attempt to load using the context class loader and ServiceLoader.
// This allows frameworks like http://aries.apache.org/modules/spi-fly.html to plug in.
if (!i.iterator().hasNext()) {
i = ServiceLoader.load(NameResolverProvider.class);
}
return i;
}
/**
* Load providers from a hard-coded list. This avoids using getResource(), which has performance
* problems on Android (see https://github.com/grpc/grpc-java/issues/2037). Any provider that may
* be used on Android is free to be added here.
*/
@VisibleForTesting
public static Iterable getCandidatesViaHardCoded() {
// Class.forName(String) is used to remove the need for ProGuard configuration. Note that
// ProGuard does not detect usages of Class.forName(String, boolean, ClassLoader):
// https://sourceforge.net/p/proguard/bugs/418/
List list = new ArrayList();
try {
list.add(create(Class.forName("io.grpc.internal.DnsNameResolverProvider")));
} catch (ClassNotFoundException ex) {
// ignore
}
return list;
}
@VisibleForTesting
static NameResolverProvider create(Class rawClass) {
try {
return rawClass.asSubclass(NameResolverProvider.class).getConstructor().newInstance();
} catch (Throwable t) {
throw new ServiceConfigurationError(
"Provider " + rawClass.getName() + " could not be instantiated: " + t, t);
}
}
/**
* Returns non-{@code null} ClassLoader-wide providers, in preference order.
*/
public static List providers() {
return providers;
}
public static NameResolver.Factory asFactory() {
return factory;
}
@VisibleForTesting
static NameResolver.Factory asFactory(List providers) {
return new NameResolverFactory(providers);
}
private static boolean isAndroid() {
try {
// Specify a class loader instead of null because we may be running under Robolectric
Class.forName("android.app.Application", /*initialize=*/ false,
NameResolverProvider.class.getClassLoader());
return true;
} catch (Exception e) {
// If Application isn't loaded, it might as well not be Android.
return false;
}
}
/**
* Whether this provider is available for use, taking the current environment into consideration.
* If {@code false}, no other methods are safe to be called.
*/
protected abstract boolean isAvailable();
/**
* A priority, from 0 to 10 that this provider should be used, taking the current environment into
* consideration. 5 should be considered the default, and then tweaked based on environment
* detection. A priority of 0 does not imply that the provider wouldn't work; just that it should
* be last in line.
*/
protected abstract int priority();
private static class NameResolverFactory extends NameResolver.Factory {
private final List providers;
public NameResolverFactory(List providers) {
this.providers = providers;
}
@Override
public NameResolver newNameResolver(URI targetUri, Attributes params) {
checkForProviders();
for (NameResolverProvider provider : providers) {
NameResolver resolver = provider.newNameResolver(targetUri, params);
if (resolver != null) {
return resolver;
}
}
return null;
}
@Override
public String getDefaultScheme() {
checkForProviders();
return providers.get(0).getDefaultScheme();
}
private void checkForProviders() {
Preconditions.checkState(!providers.isEmpty(),
"No NameResolverProviders found via ServiceLoader, including for DNS. "
+ "This is probably due to a broken build. If using ProGuard, check your configuration");
}
}
}