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/*
* Copyright The OpenTelemetry Authors
* SPDX-License-Identifier: Apache-2.0
*/
// Includes work from:
/*
* Copyright 2020 LINE Corporation
*
* LINE Corporation licenses this file to you 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:
*
* https://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.opentelemetry.context;
import java.util.function.Function;
import javax.annotation.Nullable;
/**
* The storage for storing and retrieving the current {@link Context}.
*
* If you want to implement your own storage or add some hooks when a {@link Context} is attached
* and restored, you should use {@link ContextStorageProvider}. Here's an example that sets MDC
* before {@link Context} is attached:
*
*
{@code
* > public class MyStorage implements ContextStorageProvider {
* >
* > @Override
* > public ContextStorage get() {
* > ContextStorage threadLocalStorage = ContextStorage.defaultStorage();
* > return new RequestContextStorage() {
* > @Override
* > public Scope T attach(Context toAttach) {
* > Context current = current();
* > setMdc(toAttach);
* > Scope scope = threadLocalStorage.attach(toAttach);
* > return () -> {
* > clearMdc();
* > setMdc(current);
* > scope.close();
* > }
* > }
* >
* > @Override
* > public Context current() {
* > return threadLocalStorage.current();
* > }
* > }
* > }
* > }
* }
*/
public interface ContextStorage {
/**
* Returns the {@link ContextStorage} being used by this application. This is only for use when
* integrating with other context propagation mechanisms and not meant for direct use. To attach
* or detach a {@link Context} in an application, use {@link Context#makeCurrent()} and {@link
* Scope#close()}.
*/
static ContextStorage get() {
return LazyStorage.get();
}
/**
* Returns the default {@link ContextStorage} which stores {@link Context} using a threadlocal.
*/
static ContextStorage defaultStorage() {
return ThreadLocalContextStorage.INSTANCE;
}
/**
* Adds the {@code wrapper}, which will be executed with the {@link ContextStorage} is first used,
* i.e., by calling {@link Context#makeCurrent()}. This must be called as early in your
* application as possible to have effect, often as part of a static initialization block in your
* main class.
*/
static void addWrapper(Function wrapper) {
ContextStorageWrappers.addWrapper(wrapper);
}
/**
* Sets the specified {@link Context} as the current {@link Context} and returns a {@link Scope}
* representing the scope of execution. {@link Scope#close()} must be called when the current
* {@link Context} should be restored to what it was before attaching {@code toAttach}.
*/
Scope attach(Context toAttach);
/**
* Returns the current {@link Context}. If no {@link Context} has been attached yet, this will
* return {@code null}.
*/
@Nullable
Context current();
/**
* Returns the root {@link Context} which all other {@link Context} are derived from.
*
* The default implementation returns the root {@code ArrayBasedContext}, but subclasses can
* override this method to return a root instance of a different {@link Context} implementation.
*/
default Context root() {
return ArrayBasedContext.root();
}
}