服务跟踪（ServiceTracker）

当多个Bundle使用同一接口名注册服务时，服务的获取策略如下：OSGi容器会返回排行最高的服务，即，返回注册时SERVICE_RANKING属性值最大的服务。如果有多个服务的排行值相等，那么OSGi容器将返回PID值最小的那个服务。

 

如果服务消费者需要对服务进行跟踪，比如服务何时被注册，何时取消注册等，可以使用ServiceTracker类。以下是该类的使用范例源码：

 

1、接口及其实现类：

public
 
interface
 HelloService {
    public
 String sayHello(String name);
}

 

public
 
class
 HelloServiceImpl 
implements
 HelloService {
    public
 String sayHello(String name) {
        return
 
"Hello "
 + name;
    }
}

 

2、服务跟踪器子类

/**
 * 服务跟踪器：跟踪注册到HelloService接口下的所有服务
 */
public
 
class
 HelloServiceTracker 
extends
 ServiceTracker {
    public
 HelloServiceTracker(BundleContext context){
        super
(context, HelloService.
class
.getName(), 
null
);
    }

    @Override
    public
 Object addingService(ServiceReference reference) {
        System.out.println("adding service: "
 + reference.getBundle().getSymbolicName());
        return
 
super
.addingService(reference);
    }

    @Override
    public
 
void
 removedService(ServiceReference reference, Object service) {
        System.out.println("removed service: "
 + reference.getBundle().getSymbolicName());
        super
.removedService(reference, service);
    }
}

 

3、Bundle激活器类

public
 
class
 Activator 
implements
 BundleActivator {
    HelloServiceTracker helloServiceTracker;
    ServiceRegistration serviceRegistration;

    public
 
void
 start(BundleContext context) 
throws
 Exception {
        System.out.println("start..."
);

        //启动服务跟踪器
        helloServiceTracker = new
 HelloServiceTracker(context);
        helloServiceTracker.open();

        //注册服务
        serviceRegistration = context.registerService(HelloService.class
.getName(), 
new
 HelloServiceImpl(), 
null
);

        //获取被跟踪的服务
        HelloService helloService =  (HelloService)helloServiceTracker.getService();
        if
(helloService!=
null
){
            System.out.println(helloService.sayHello("cjm"
));
        }
    }

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

        //关闭服务跟踪器
        helloServiceTracker.close();

        serviceRegistration.unregister();
    }
}

 

4、运行结果

osgi>
start...
adding service: p1
Hello cjm

osgi> stop 86
stop
removed service: p1