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There is a newer version: 3.3.2

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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
 *
 *     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.apache.dubbo.common;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Marker for extension interface
 * 
 * Changes on extension configuration file 

 * Use Protocol as an example, its configuration file 'META-INF/dubbo/com.xxx.Protocol' is changes from: 

 * 
 *     com.foo.XxxProtocol
 *     com.foo.YyyProtocol
 * 
 * 
 * to key-value pair 

 * 
 *     xxx=com.foo.XxxProtocol
 *     yyy=com.foo.YyyProtocol
 * 
 * 

 * The reason for this change is:
 * 
 * If there's third party library referenced by static field or by method in extension implementation, its class will
 * fail to initialize if the third party library doesn't exist. In this case, dubbo cannot figure out extension's id
 * therefore cannot be able to map the exception information with the extension, if the previous format is used.
 * 

 * For example:
 * 

 * Fails to load Extension("mina"). When user configure to use mina, dubbo will complain the extension cannot be loaded,
 * instead of reporting which extract extension implementation fails and the extract reason.
 * 
 *
 * @deprecated because it's too general, switch to use {@link org.apache.dubbo.common.extension.SPI}
 */
@Deprecated
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface Extension {

    /**
     * @deprecated
     */
    @Deprecated
    String value() default "";
}