1.Activate的作用

Activate：可以被框架中自动激活加载扩展，此Annotation用于配置扩展被自动激活加载条件。

以上定义有2个关键词，自动激活和加载条件

可以参考此篇文章:
解释:用户通过group和value配置激活条件,被activate注解的扩展点在满足某种条件时会被激活

2.Activate注解的功能定义

我们先看看其注解的定义,主要有3个功能定义,可以简化理解为筛选和排序

1. 分组(筛选条件)
2. key值(筛选条件)
3. 排序

代码示例1

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface Activate {
    /**
     * Activate the current extension when one of the groups matches. The group passed into
     * {@link ExtensionLoader#getActivateExtension(URL, String, String)} will be used for matching.
     *
     * @return group names to match
     * @see ExtensionLoader#getActivateExtension(URL, String, String)
     */
    String[] group() default {};

    /**
     * Activate the current extension when the specified keys appear in the URL's parameters.
     * <p>
     * For example, given <code>@Activate("cache, validation")</code>, the current extension will be return only when
     * there's either <code>cache</code> or <code>validation</code> key appeared in the URL's parameters.
     * </p>
     *
     * @return URL parameter keys
     * @see ExtensionLoader#getActivateExtension(URL, String)
     * @see ExtensionLoader#getActivateExtension(URL, String, String)
     */
    String[] value() default {};

    /**
     * Relative ordering info, optional
     *
     * @return extension list which should be put before the current one
     */
    String[] before() default {};

    /**
     * Relative ordering info, optional
     *
     * @return extension list which should be put after the current one
     */
    String[] after() default {};

    /**
     * Absolute ordering info, optional
     *
     * @return absolute ordering info
     */
    int order() default 0;
}

使用场景：
jdk spi中如果接口有多版本实现的话，会以迭代列表形式返回.
dubbo在此基础上添加了上面定义的3个概念.

3.代码示例

以Filter为例

代码示例2

public class Test1Filter implements Filter {
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        return null;
    }
}
@Activate(group = Constants.PROVIDER,order = 2)
public class Test2Filter implements Filter {
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        return null;
    }
}
@Activate(group = Constants.CONSUMER,order = 3)
public class Test3Filter implements Filter {
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        return null;
    }
}
@Activate(group = {Constants.PROVIDER,Constants.CONSUMER},order = 4)
public class Test4Filter implements Filter {
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        return null;
    }
}

@Activate(group = {Constants.PROVIDER,Constants.CONSUMER},order = 5,value = "test5")
public class Test5Filter implements Filter {
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        return null;
    }
}

services的配置文件

image

以上定义了5个自定义Filter

1. Test1Filter没有自动激活,所以不会自动加载激活
2. Test2Filter的group为Constants.PROVIDER,添加order以保证有序执行
3. Test3Filter的group为Constants.CONSUMER
4. Test4Filter的group为Constants.PROVIDER和Constants.CONSUMER
5. Test5Filter的value为test5

下面我们再来看示例代码

代码示例3

public static void main(String[] args){
    ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(Filter.class);

    URL url=new URL("","",3);
    url=url.addParameter("filterValue", "test5");
    List list=extensionLoader.getActivateExtension(url,"",Constants.PROVIDER);
    System.out.println(list);
    list=extensionLoader.getActivateExtension(url,"",Constants.CONSUMER);
    System.out.println(list);
    list=extensionLoader.getActivateExtension(url,"filterValue",Constants.CONSUMER);
    System.out.println(list);
}

除了返回内置的Filter之外,下面图片还返回了我们自己定义的Filter(在一定条件下自动激活)

图1 返回了Test2Filter和Test4Filter

image

图2 Test3Filter和Test4Filter

image

图3 Consumer的Filter和额外相关key的Filter

image

以Activate注解自动激活的方式可以方便地全局扩展dubbo Filter功能

示例代码