org.osgi.service.condition.Condition Maven / Gradle / Ivy
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/*
* Copyright (c) OSGi Alliance (2020). 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 org.osgi.service.condition;
import org.osgi.annotation.versioning.ConsumerType;
/**
* Condition Service interface.
*
* In dynamic systems, such as OSGi, one of the more challenging problems can be
* to define when a system or part of it is ready to do work. The answer can
* change depending on the individual perspective. The developer of a web server
* might say, the system is ready when the server starts listening on port 80.
* An application developer however would define the system as ready when the
* database connection is up and all servlets are registered. Taking the
* application developers view, the web server should start listening on port 80
* when the application is ready and not beforehand.
*
* The {@code Condition} service interface is a marker interface designed to
* address this issue. Its role is to provide a dependency that can be tracked.
* It acts as a defined signal to other services.
*
* A {@code Condition} service must be registered with the
* {@link Condition#CONDITION_ID} service property.
*
* @ThreadSafe
* @author $Id: 9736e5e1c38c45254f733d73ed7ae2c0e253f544 $
*/
@ConsumerType
public interface Condition {
/**
* Service property identifying a condition's unique identifier.
*
* Since a {@code Condition} service can potentially describe more then one
* condition, the type of this service property is {@code String+}.
*/
String CONDITION_ID = "osgi.condition.id";
/**
* The unique identifier for the default True condition.
*
* The default True condition is registered by the framework during
* framework initialization and therefore can always be relied upon.
*
* @see Condition#CONDITION_ID
*/
String CONDITION_ID_TRUE = "true";
/**
* A condition instance that can be used to register {@code Condition}
* services.
*
* This can be helpful to avoid a bundle having to implement this interface
* to register a {@code Condition} service
*/
Condition INSTANCE = new ConditionImpl();
}
final class ConditionImpl implements Condition {
ConditionImpl() {
}
}