maven-osgi项目中的EventAdmin为null [英] EventAdmin is null in maven-osgi project

问题描述

我做了一个maven-osgi项目，其中一个激活器应该发送一个osgi事件但由于某种原因，EventAdmin总是为空。

I made an maven-osgi project where an activator should send an osgi event but for some reason EventAdmin is always null.

这是我的java类

package com.example.eventhandler;

import java.util.Dictionary;
import java.util.Hashtable;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventAdmin;

public class App implements BundleActivator {
    public void start(BundleContext context) throws Exception {
        ServiceReference ref = context.getServiceReference(EventAdmin.class.getName());
        if (ref != null) {
            EventAdmin eventAdmin = (EventAdmin) context.getService(ref);
            Dictionary properties = new Hashtable();

            eventAdmin.sendEvent(new Event("com/acme/reportgenerator/GENERATED", properties));
        } else {
            System.out.println("Ref is null!!");
        }
        System.out.println("Hello World!!");
    }

    public void stop(BundleContext context) throws Exception {
        System.out.println("Goodbye World!!");
    }
}

这是我的pom.xml：

And here is my pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>eventhandler</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>eventhandler</name>
    <url>http://maven.apache.org</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <bundle.symbolicName>com.example</bundle.symbolicName>
        <bundle.namespace>com.example</bundle.namespace>
    </properties>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.osgi</groupId>
            <artifactId>org.osgi.core</artifactId>
            <version>4.3.0</version>
        </dependency>
        <dependency>
            <groupId>org.eclipse.osgi</groupId>
            <artifactId>org.eclipse.osgi</artifactId>
            <version>3.7.1</version>
        </dependency>
        <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.osgi.compendium</artifactId>
            <version>1.4.0</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <extensions>true</extensions>
                <executions>
                    <execution>
                        <id>bundle-manifest</id>
                        <phase>process-classes</phase>
                        <goals>
                            <goal>manifest</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <manifestLocation>META-INF</manifestLocation>
                    <instructions>
                        <Bundle-SymbolicName>${bundle.symbolicName}</Bundle-SymbolicName>
                        <Bundle-Version>${pom.version}</Bundle-Version>
                        <Bundle-Activator>${bundle.namespace}.eventhandler.App</Bundle-Activator>
                        <Import-Package>
                            org.osgi.framework,
                            org.osgi.service.event
                        </Import-Package>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

EventAdmin为空的原因是什么？

What could be the reason EventAdmin is null?

推荐答案

您已在OSGi中实现了经典的反模式：您认为在捆绑包启动时，EventAdmin服务已经可用。这是一个本质上不安全的假设，因为你的bundle实际上可能在提供EventAdmin服务的bundle之前启动。

You have implemented a classic anti-pattern in OSGi: you assume that the EventAdmin service will already be available when your bundle starts. This is an inherently unsafe assumption because your bundle might in fact be started before the bundle that provides the EventAdmin service.

有一种错误的方法和正确的方法来解决这个问题。错误的方法是坚持必须在EventAdmin之后启动您的捆绑包。这种方式导致了启动顺序依赖性和极端脆弱性。请参阅： http://wiki.osgi.org/wiki/Avoid_Start_Order_Dependencies

There is a wrong way and a right way to fix this. The wrong way is to insist that your bundle must be started after EventAdmin. That way leads to start-order dependencies and extreme fragility. See: http://wiki.osgi.org/wiki/Avoid_Start_Order_Dependencies

正确的方法是使用声明服务（DS）等框架来声明捆绑包中引用EventAdmin服务的组件。然后DS将激活您的组件，并在它可用时立即将其注入EventAdmin实例。请参阅： http://wiki.osgi.org/wiki/Declarative_Services

The right way is to use a framework such as Declarative Services (DS) to declare a component inside your bundle that references the EventAdmin service. Then DS will activate your component and inject it with the EventAdmin instance as soon as it becomes available. See: http://wiki.osgi.org/wiki/Declarative_Services

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