Java 回退模式

Question

我正在尝试找到一种很好的方法来实现依赖于第三方库类的服务。我还有一个“默认”实现，可在库不可用或无法提供答案时用作回退。

public interface Service {

    public Object compute1();

    public Object compute2();
}

public class DefaultService implements Service {

    @Override
    public Object compute1() {
       // ...
    }

    @Override
    public Object compute2() {
        // ...
    }
}

服务的实际实现类似于:

public class ServiceImpl implements Service {
    Service defaultService = new DefaultService();
    ThirdPartyService thirdPartyService = new ThirdPartyService();

    @Override
    public Object compute1() {
        try {
            Object obj = thirdPartyService.customCompute1();
            return obj != null ? obj : defaultService.compute1();
        } 
        catch (Exception e) {
            return defaultService.compute1();
        }
    }

    @Override
    public Object compute2() {
        try {
            Object obj = thirdPartyService.customCompute2();
            return obj != null ? obj : defaultService.compute2();
        } 
        catch (Exception e) {
            return defaultService.compute2();
        }
    }
}

当前的实现似乎有点重复，只是对服务的实际调用不同，但 try/catch 和默认机制几乎相同。此外，如果在服务中添加另一种方法，实现看起来几乎是一样的。

是否有可能适用于此的设计模式(proxy，strategy)使代码看起来更好并减少进一步添加的复制粘贴？

Answer 1

代理可能会在这方面为您提供帮助。下面的示例未经测试，但应该让您了解可以放置什么:

public class FallbackService implements InvocationHandler {

    private final Service primaryService;
    private final Service fallbackService;

    private FallbackService(Service primaryService, Service fallbackService) {
        this.primaryService = primaryService;
        this.fallbackService = fallbackService;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            Object result = method.invoke(primaryService, args);
            if (result != null) return result;
        } catch (Exception ignore) {}
        return method.invoke(fallbackService, args);
    }

    public static Service createFallbackService(Service primaryService, Service fallbackService) {
        return (Service) Proxy.newProxyInstance(
                Service.class.getClassLoader(),
                new Class[] { Service.class },
                new FallbackService(primaryService, fallbackService)
        );
    }
}