使用@SPI的wrapper方式报空指针异常bug

[zhihuayu]

Environment

• Dubbo version: 2.7.8
• Operating System version: windows10
• Java version: 1.8

比如，有如下的wrapper方式的Filter，在服务启动的时候会报空指针异常。

public class DubboWrapperFilter implements Filter {

	private final Filter filter;

	public DubboWrapperFilter(Filter filter) {
		this.filter = filter;
	}

	@Override
	public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
		String name = filter.getClass().getName();
		System.out.println("wrapper:" + name);
		return filter.invoke(invoker, invocation);
	}

}

经查是因为active的排序导致，如果改成如下方式，则不会报错，但是其排序可能会无效，所以希望官方能做一个完整的方案，谢谢！

@Activate
public class WrapperHelloSPI implements HelloSPI {

	private final HelloSPI spi;

	public WrapperHelloSPI(HelloSPI spi) {
		this.spi = spi;
	}

	@Override
	public void say(String say) {
		System.out.println("wapper:" + say);
		spi.say(say);
	}

}

[AlbumenJ]

@zhihuayu hi, would you pls provide more detailed information for NPE.

The situation you use wrapper and the stack trace for NPE are both needed.

[zhihuayu]

@zhihuayu hi, would you pls provide more detailed information for NPE.

The situation you use wrapper and the stack trace for NPE are both needed.

非常感谢您的回复，也非常抱歉没有提供详细的堆栈信息。经调试，主要是因为duboo在初始化构建调用链时，即调用ProtocolFilterWrapper.buildInvokerChain方法时：

 private static <T> Invoker<T> buildInvokerChain(final Invoker<T> invoker, String key, String group) {
        Invoker<T> last = invoker;
        // 抛出异常
        List<Filter> filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);
   ......
}

其原因在ExtensionLoader的getActivateExtension方法中：

   public List<T> getActivateExtension(URL url, String[] values, String group) {
        List<T> activateExtensions = new ArrayList<>();
        List<String> names = values == null ? new ArrayList<>(0) : asList(values);
        if (!names.contains(REMOVE_VALUE_PREFIX + DEFAULT_KEY)) {
            getExtensionClasses();
            for (Map.Entry<String, Object> entry : cachedActivates.entrySet()) {
                String name = entry.getKey();
                Object activate = entry.getValue();

                String[] activateGroup, activateValue;

                if (activate instanceof Activate) {
                    activateGroup = ((Activate) activate).group();
                    activateValue = ((Activate) activate).value();
                } else if (activate instanceof com.alibaba.dubbo.common.extension.Activate) {
                    activateGroup = ((com.alibaba.dubbo.common.extension.Activate) activate).group();
                    activateValue = ((com.alibaba.dubbo.common.extension.Activate) activate).value();
                } else {
                    continue;
                }
                if (isMatchGroup(group, activateGroup)
                        && !names.contains(name)
                        && !names.contains(REMOVE_VALUE_PREFIX + name)
                        && isActive(activateValue, url)) {
                    activateExtensions.add(getExtension(name));
                }
            }
            // 排序抛出异常
            activateExtensions.sort(ActivateComparator.COMPARATOR);
        }
       ......
    }

实际上，抛出的空指针异常是在ActivateComparator.COMPARATOR比较排序的时候抛出的，如下所示ActivateComparator.parseActivate方法中，由于没有考虑Activate是否存在的情况，所以会抛出控制在异常：

  private ActivateInfo parseActivate(Class<?> clazz) {
        ActivateInfo info = new ActivateInfo();
        if (clazz.isAnnotationPresent(Activate.class)) {
            Activate activate = clazz.getAnnotation(Activate.class);
            info.before = activate.before();
            info.after = activate.after();
            info.order = activate.order();
        } else {
            com.alibaba.dubbo.common.extension.Activate activate = clazz.getAnnotation(
                    com.alibaba.dubbo.common.extension.Activate.class);
            //  没有考虑Activate不存在的情况，排除空指针异常
            info.before = activate.before();
            info.after = activate.after();
            info.order = activate.order();
        }
        return info;
    }

[xiaoheng1]

应该不是这个问题吧？getActivateExtension 方法遍历的是 cachedActivates.entrySet()，而 cachedActivates 缓存的是被 Activate 注解修饰的类，所以在排序的时候，应该全部是包含 Activate 注解的类.

public List<T> getActivateExtension(URL url, String[] values, String group) {
       ...
            for (Map.Entry<String, Object> entry : cachedActivates.entrySet()) {
                String name = entry.getKey();
                Object activate = entry.getValue();

                String[] activateGroup, activateValue;

                if (activate instanceof Activate) {
                    activateGroup = ((Activate) activate).group();
                    activateValue = ((Activate) activate).value();
                } else if (activate instanceof com.alibaba.dubbo.common.extension.Activate) {
                    activateGroup = ((com.alibaba.dubbo.common.extension.Activate) activate).group();
                    activateValue = ((com.alibaba.dubbo.common.extension.Activate) activate).value();
                } else {
                    continue;
                }
                if (isMatchGroup(group, activateGroup)
                        && !names.contains(name)
                        && !names.contains(REMOVE_VALUE_PREFIX + name)
                        && isActive(activateValue, url)) {
                    activateExtensions.add(getExtension(name));
                }
            }
            activateExtensions.sort(ActivateComparator.COMPARATOR);
        }

}

[zhihuayu]

ctivates.entrySet()) {

对的，但是Activate 注解修饰的类可以被没有Activate注解的wrapper类所修饰，原理是在调用getExtension(name)方法时会进行包装，包装之后再排序，所以会有空指针异常。

[AlbumenJ]

@zhihuayu thanks for your explanation

I have tried to reroduce this situation, and I found that the main reason which causes NPE in activateExtensions.sort(ActivateComparator.COMPARATOR); is this real Activate extension has been wrapped by getExtension(name) which will make extension being wrapped and the original Activate annotation unable to being fetch by ActivateComparator.COMPARATOR.

In order to slove this problem, maybe we can make getExtension(name) being called after sorted? Or would have any ideas for this?

We can fix it in the next release.

[zhihuayu]

@AlbumenJ 非常感谢您的回复
是的，排序之后再调用getExtension(name) 方法可以解决这个问题。我觉得，可以利用TreeMap数据结构的排序特性来完成这个功能，这样就只需要简单的修改ExtensionLoader.getActivateExtension方法以及排序策略类ActivateComparator即可，ExtensionLoader.getActivateExtension方法修改如下所示：



而ActivateComparator实现的Comparator的泛型为Object改成Class类型即可，如下所示：



上述修改方式仅是个人建议，仅供参考，再次感谢您的回复！

[chickenlj]

@zhihuayu @xiaoheng1 非常欢迎提个 pr 到主干仓库

[xiaoheng1]

already in process.

[zhihuayu]

@zhihuayu @xiaoheng1 非常欢迎提个 pr 到主干仓库

@xiaoheng1 已经提交了，谢谢！

[chickenlj]

不好意思，我没在这个 issue 中看到相关的 pull request 链接那

[zhihuayu]

@chickenlj 刚才已经提交pull request。

[AlbumenJ]

Related pr #7234