https://github.com/reactor/reactor-core: Non-Blocking Reactive Foundation for the JVM (reactor)
  • Stephane Maldini
  • David Karnok
  • Simon Baslé
  • Rajini Sivaram
  • Sébastien Deleuze 
/*
 * Copyright (c) 2011-2017 Pivotal Software Inc, 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 reactor.util.context;

import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Optional;
import java.util.stream.Stream;

import reactor.util.annotation.Nullable;


A key/value store that is propagated between components such as operators via the
 context protocol. Contexts are ideal to transport orthogonal information such as
 tracing or security tokens.

 Context implementations are thread-safe and immutable: mutative operations like put(Object, Object) will in fact return a new Context instance. 
 Note that contexts are optimized for low cardinality key/value storage, and a user might want to associate a dedicated mutable structure to a single key to represent his own context instead of using multiple put, which could be more costly. Past five user key/value pair, the Context will use a copy-on-write implementation backed by a new Map on each put. 
Author:Stephane Maldini
/**
 * A key/value store that is propagated between components such as operators via the
 *  context protocol. Contexts are ideal to transport orthogonal information such as
 *  tracing or security tokens.
 * <p>
 * {@link Context} implementations are thread-safe and immutable: mutative operations like
 * {@link #put(Object, Object)} will in fact return a new {@link Context} instance.
 * <p>
 * Note that contexts are optimized for low cardinality key/value storage, and a user
 * might want to associate a dedicated mutable structure to a single key to represent his
 * own context instead of using multiple {@link #put}, which could be more costly.
 * Past five user key/value pair, the {@link Context} will use a copy-on-write
 * implementation backed by a new {@link java.util.Map} on each {@link #put}.
 *
 * @author Stephane Maldini
 */
public interface Context {

	
Return an empty Context 
Returns:an empty Context
/**
	 * Return an empty {@link Context}
	 *
	 * @return an empty {@link Context}
	 */
	static Context empty() {
		return Context0.INSTANCE;
	}

	
Create a Context pre-initialized with one key-value pair. 
Params:
  • key – the key to initialize.
  • value – the value for the key.
Returns:a Context with a single entry.
/**
	 * Create a {@link Context} pre-initialized with one key-value pair.
	 *
	 * @param key the key to initialize.
	 * @param value the value for the key.
	 * @return a {@link Context} with a single entry.
	 */
	static Context of(Object key, Object value) {
		return new Context1(key, value);
	}

	
Create a Context pre-initialized with two key-value pairs. 
Params:
  • key1 – the first key to initialize.
  • value1 – the value for the first key.
  • key2 – the second key to initialize.
  • value2 – the value for the second key.
Returns:a Context with two entries.
/**
	 * Create a {@link Context} pre-initialized with two key-value pairs.
	 *
	 * @param key1 the first key to initialize.
	 * @param value1 the value for the first key.
	 * @param key2 the second key to initialize.
	 * @param value2 the value for the second key.
	 * @return a {@link Context} with two entries.
	 */
	static Context of(Object key1, Object value1,
			Object key2, Object value2) {
		return new Context2(key1, value1, key2, value2);
	}

	
Create a Context pre-initialized with three key-value pairs. 
Params:
  • key1 – the first key to initialize.
  • value1 – the value for the first key.
  • key2 – the second key to initialize.
  • value2 – the value for the second key.
  • key3 – the third key to initialize.
  • value3 – the value for the third key.
Returns:a Context with three entries.
/**
	 * Create a {@link Context} pre-initialized with three key-value pairs.
	 *
	 * @param key1 the first key to initialize.
	 * @param value1 the value for the first key.
	 * @param key2 the second key to initialize.
	 * @param value2 the value for the second key.
	 * @param key3 the third key to initialize.
	 * @param value3 the value for the third key.
	 * @return a {@link Context} with three entries.
	 */
	static Context of(Object key1, Object value1,
			Object key2, Object value2,
			Object key3, Object value3) {
		return new Context3(key1, value1, key2, value2, key3, value3);
	}

	
Create a Context pre-initialized with four key-value pairs. 
Params:
  • key1 – the first key to initialize.
  • value1 – the value for the first key.
  • key2 – the second key to initialize.
  • value2 – the value for the second key.
  • key3 – the third key to initialize.
  • value3 – the value for the third key.
  • key4 – the fourth key to initialize.
  • value4 – the value for the fourth key.
Returns:a Context with four entries.
/**
	 * Create a {@link Context} pre-initialized with four key-value pairs.
	 *
	 * @param key1 the first key to initialize.
	 * @param value1 the value for the first key.
	 * @param key2 the second key to initialize.
	 * @param value2 the value for the second key.
	 * @param key3 the third key to initialize.
	 * @param value3 the value for the third key.
	 * @param key4 the fourth key to initialize.
	 * @param value4 the value for the fourth key.
	 * @return a {@link Context} with four entries.
	 */
	static Context of(Object key1, Object value1,
			Object key2, Object value2,
			Object key3, Object value3,
			Object key4, Object value4) {
		return new Context4(key1, value1, key2, value2, key3, value3, key4, value4);
	}

	
Create a Context pre-initialized with five key-value pairs. 
Params:
  • key1 – the first key to initialize.
  • value1 – the value for the first key.
  • key2 – the second key to initialize.
  • value2 – the value for the second key.
  • key3 – the third key to initialize.
  • value3 – the value for the third key.
  • key4 – the fourth key to initialize.
  • value4 – the value for the fourth key.
  • key5 – the fifth key to initialize.
  • value5 – the value for the fifth key.
Returns:a Context with five entries.
/**
	 * Create a {@link Context} pre-initialized with five key-value pairs.
	 *
	 * @param key1 the first key to initialize.
	 * @param value1 the value for the first key.
	 * @param key2 the second key to initialize.
	 * @param value2 the value for the second key.
	 * @param key3 the third key to initialize.
	 * @param value3 the value for the third key.
	 * @param key4 the fourth key to initialize.
	 * @param value4 the value for the fourth key.
	 * @param key5 the fifth key to initialize.
	 * @param value5 the value for the fifth key.
	 * @return a {@link Context} with five entries.
	 */
	static Context of(Object key1, Object value1,
			Object key2, Object value2,
			Object key3, Object value3,
			Object key4, Object value4,
			Object key5, Object value5) {
		return new Context5(key1, value1, key2, value2, key3, value3, key4, value4, key5, value5);
	}

	
Resolve a value given a key that exists within the Context, or throw a NoSuchElementException if the key is not present. 
Params:
  • key – a lookup key to resolve the value within the context
Type parameters:
  • <T> – an unchecked casted generic for fluent typing convenience
Throws:
See Also:
Returns:the value resolved for this key (throws if key not found)
/**
	 * Resolve a value given a key that exists within the {@link Context}, or throw
	 * a {@link NoSuchElementException} if the key is not present.
	 *
	 * @param key a lookup key to resolve the value within the context
	 *
	 * @param <T> an unchecked casted generic for fluent typing convenience
	 *
	 * @return the value resolved for this key (throws if key not found)
	 * @throws NoSuchElementException when the given key is not present
	 * @see #getOrDefault(Object, Object)
	 * @see #getOrEmpty(Object)
	 * @see #hasKey(Object)
	 */
	<T> T get(Object key);

	
Resolve a value given a type key within the Context. 
Params:
  • key – a type key to resolve the value within the context
Type parameters:
  • <T> – an unchecked casted generic for fluent typing convenience
Throws:
See Also:
Returns:the value resolved for this type key (throws if key not found)
/**
	 * Resolve a value given a type key within the {@link Context}.
	 *
	 * @param key a type key to resolve the value within the context
	 *
	 * @param <T> an unchecked casted generic for fluent typing convenience
	 *
	 * @return the value resolved for this type key (throws if key not found)
	 * @throws NoSuchElementException when the given type key is not present
	 * @see #getOrDefault(Object, Object)
	 * @see #getOrEmpty(Object)
	 */
	default <T> T get(Class<T> key){
		T v = get((Object)key);
		if(key.isInstance(v)){
			return v;
		}
		throw new NoSuchElementException("Context does not contain a value of type "+key
				.getName());
	}

	
Resolve a value given a key within the Context. If unresolved return the passed default value. 
Params:
  • key – a lookup key to resolve the value within the context
  • defaultValue – a fallback value if key doesn't resolve
Returns:the value resolved for this key, or the given default if not present
/**
	 * Resolve a value given a key within the {@link Context}. If unresolved return the
	 * passed default value.
	 *
	 * @param key a lookup key to resolve the value within the context
	 * @param defaultValue a fallback value if key doesn't resolve
	 *
	 * @return the value resolved for this key, or the given default if not present
	 */
	@Nullable
	default <T> T getOrDefault(Object key, @Nullable T defaultValue){
		if(!hasKey(key)){
			return defaultValue;
		}
		return get(key);
	}

	
Resolve a value given a key within the Context. 
Params:
  • key – a lookup key to resolve the value within the context
Returns:an Optional of the value for that key.
/**
	 * Resolve a value given a key within the {@link Context}.
	 *
	 * @param key a lookup key to resolve the value within the context
	 *
	 * @return an {@link Optional} of the value for that key.
	 */
	default <T> Optional<T> getOrEmpty(Object key){
		if(hasKey(key)) {
			return Optional.of(get(key));
		}
		return Optional.empty();
	}

	
Return true if a particular key resolves to a value within the Context. 
Params:
  • key – a lookup key to test for
Returns:true if this context contains the given key
/**
	 * Return true if a particular key resolves to a value within the {@link Context}.
	 *
	 * @param key a lookup key to test for
	 *
	 * @return true if this context contains the given key
	 */
	boolean hasKey(Object key);

	
Return true if the Context is empty. 
Returns:true if the Context is empty.
/**
	 * Return true if the {@link Context} is empty.
	 *
	 * @return true if the {@link Context} is empty.
	 */
	default boolean isEmpty() {
		return this == Context0.INSTANCE || this instanceof Context0;
	}

	
Create a new Context that contains all current key/value pairs plus the given key/value pair. If that key existed in the current Context, its associated value is replaced in the resulting Context. 
Params:
  • key – the key to add/update in the new Context
  • value – the value to associate to the key in the new Context
Returns:a new Context including the provided key/value
/**
	 * Create a new {@link Context} that contains all current key/value pairs plus the
	 * given key/value pair. If that key existed in the current Context, its associated
	 * value is replaced in the resulting {@link Context}.
	 *
	 * @param key the key to add/update in the new {@link Context}
	 * @param value the value to associate to the key in the new {@link Context}
	 *
	 * @return a new {@link Context} including the provided key/value
	 */
	Context put(Object key, Object value);

	
Return a new Context that will resolve all existing keys except the removed one, key. 
 Note that if this Context doesn't contain the key, this method simply returns this same instance. 
Params:
  • key – the key to remove.
Returns:a new Context that doesn't include the provided key
/**
	 * Return a new {@link Context} that will resolve all existing keys except the
	 * removed one, {@code key}.
	 * <p>
	 * Note that if this {@link Context} doesn't contain the key, this method simply
	 * returns this same instance.
	 *
	 * @param key the key to remove.
	 * @return a new {@link Context} that doesn't include the provided key
	 */
	Context delete(Object key);

	
Stream key/value pairs from this Context 
Returns:a Stream of key/value pairs held by this context
/**
	 * Stream key/value pairs from this {@link Context}
	 *
	 * @return a {@link Stream} of key/value pairs held by this context
	 */
	Stream<Map.Entry<Object,Object>> stream();

	
Create a new Context by merging the content of this context and a given Context. If the other context is empty, the same Context instance is returned. 
Params:
  • other – the other Context to get values from
Returns:a new Context with a merge of the entries from this context and the given context.
/**
	 * Create a new {@link Context} by merging the content of this context and a given
	 * {@link Context}. If the other context is empty, the same {@link Context} instance
	 * is returned.
	 *
	 * @param other the other Context to get values from
	 * @return a new Context with a merge of the entries from this context and the given context.
	 */
	default Context putAll(Context other) {
		if (other.isEmpty()) return this;

		return other.stream()
		            .reduce(this,
				            (c, e) -> c.put(e.getKey(), e.getValue()),
				            (c1, c2) -> { throw new UnsupportedOperationException("Context.putAll should not use a parallelized stream");}
		            );
	}
}