我正在 Equinox OSGi 框架上构建一个 Java 应用程序，并且我一直在使用 DS（声明性服务）来声明引用和提供的服务。到目前为止，我实现的所有服务消费者恰好也是服务提供者，所以我很自然地将它们设为无状态（以便它们可以被多个消费者重用，而不是依附于一个消费者）并让它们成为由框架实例化（默认构造函数，在我的代码中没有调用）。
现在我有一个不同的情况：我有一个MyClass引用服务MyService但本身不是服务提供者的类。我需要能够实例化MyClass自己，而不是让 OSGi 框架实例化它。然后我希望框架将现有MyService实例传递给MyClass实例。像这样的东西：
public class MyClass {
private String myString;
private int myInt;
private MyService myService;
public MyClass(String myString, int myInt) {
this.myString = myString;
this.myInt= myInt;
}
// bind
private void setMyService(MyService myService) {
this.myService = myService;
}
// unbind
private void unsetMyService(MyService myService) {
this.myService = null;
}
public void doStuff() {
if (myService != null) {
myService.doTheStuff();
} else {
// Some fallback mechanism
}
}
}
public class AnotherClass {
public void doSomething(String myString, int myInt) {
MyClass myClass = new MyClass(myString, myInt);
// At this point I would want the OSGi framework to invoke
// the setMyService method of myClass with an instance of
// MyService, if available.
myClass.doStuff();
}
}
我的第一次尝试是使用 DS 创建组件定义并从那里MyClass引用：MyService
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="My Class">
<implementation class="my.package.MyClass"/>
<reference bind="setMyService" cardinality="0..1" interface="my.other.package.MyService" name="MyService" policy="static" unbind="unsetMyService"/>
</scr:component>
但是，MyClass它并不是真正的组件，因为我不希望管理它的生命周期——我想自己处理实例化。正如尼尔巴特利特在这里指出的那样：
例如，您可以说您的组件“依赖于”特定服务，在这种情况下，该组件将仅在该服务可用时创建和激活——并且当该服务不可用时它也将被销毁。
这不是我想要的。我想要没有生命周期管理的绑定。[注意：即使我将基数设置为0..1（可选和一元），框架仍然会尝试实例化MyClass（并且由于缺少无参数构造函数而失败）]
所以，我的问题是：有没有办法使用 DS 来获得我正在寻找的这种“仅绑定，无生命周期管理”功能？如果 DS 无法做到这一点，有哪些替代方案，您会推荐什么？
更新：使用ServiceTracker（由 Neil Bartlett 建议）
重要提示：我在下面发布了一个改进版本作为答案。我只是出于“历史”目的而将其保留在这里。
我不确定ServiceTracker在这种情况下如何申请。您会使用如下所示的静态注册表吗？
public class Activator implements BundleActivator {
private ServiceTracker<MyService, MyService> tracker;
@Override
public void start(BundleContext bundleContext) throws Exception {
MyServiceTrackerCustomizer customizer = new MyServiceTrackerCustomizer(bundleContext);
tracker = new ServiceTracker<MyService, MyService>(bundleContext, MyService.class, customizer);
tracker.open();
}
@Override
public void stop(BundleContext bundleContext) throws Exception {
tracker.close();
}
}
public class MyServiceTrackerCustomizer implements ServiceTrackerCustomizer<MyService, MyService> {
private BundleContext bundleContext;
public MyServiceTrackerCustomizer(BundleContext bundleContext) {
this.bundleContext = bundleContext;
}
@Override
public MyService addingService(ServiceReference<MyService> reference) {
MyService myService = bundleContext.getService(reference);
MyServiceRegistry.register(myService); // any better suggestion?
return myService;
}
@Override
public void modifiedService(ServiceReference<MyService> reference, MyService service) {
}
@Override
public void removedService(ServiceReference<MyService> reference, MyService service) {
bundleContext.ungetService(reference);
MyServiceRegistry.unregister(service); // any better suggestion?
}
}
public class MyServiceRegistry {
// I'm not sure about using a Set here... What if the MyService instances
// don't have proper equals and hashCode methods? But I need some way to
// compare services in isActive(MyService). Should I just express this
// need to implement equals and hashCode in the javadoc of the MyService
// interface? And if MyService is not defined by me, but is 3rd-party?
private static Set<MyService> myServices = new HashSet<MyService>();
public static void register(MyService service) {
myServices.add(service);
}
public static void unregister(MyService service) {
myServices.remove(service);
}
public static MyService getService() {
// Return whatever service the iterator returns first.
for (MyService service : myServices) {
return service;
}
return null;
}
public static boolean isActive(MyService service) {
return myServices.contains(service);
}
}
public class MyClass {
private String myString;
private int myInt;
private MyService myService;
public MyClass(String myString, int myInt) {
this.myString = myString;
this.myInt= myInt;
}
public void doStuff() {
// There's a race condition here: what if the service becomes
// inactive after I get it?
MyService myService = getMyService();
if (myService != null) {
myService.doTheStuff();
} else {
// Some fallback mechanism
}
}
protected MyService getMyService() {
if (myService != null && !MyServiceRegistry.isActive(myService)) {
myService = null;
}
if (myService == null) {
myService = MyServiceRegistry.getService();
}
return myService;
}
}
你会这样做吗？你能评论我在上面评论中写的问题吗？那是：
- 如果
Set服务实现没有正确实现equals和hashCode. - 竞争条件：经过我的
isActive检查，该服务可能会变为非活动状态。