com.alibaba.dubbo.cache.filter.CacheFilter
when value is null,it will throw NPE(if use ehcache for jcache),why not check null here?
if the value is null ,maybe we have no need to cache it.



`package com.alibaba.dubbo.cache.filter;
public class CacheFilter implements Filter {

    private CacheFactory cacheFactory;

    public void setCacheFactory(CacheFactory cacheFactory) {
        this.cacheFactory = cacheFactory;
    }

    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        if (cacheFactory != null && ConfigUtils.isNotEmpty(invoker.getUrl().getMethodParameter(invocation.getMethodName(), Constants.CACHE_KEY))) {
            Cache cache = cacheFactory.getCache(invoker.getUrl().addParameter(Constants.METHOD_KEY, invocation.getMethodName()));
            if (cache != null) {
                String key = StringUtils.toArgumentString(invocation.getArguments());
                if (cache != null && key != null) {
                    Object value = cache.get(key);
                    if (value != null) {
                        return new RpcResult(value);
                    }
                    Result result = invoker.invoke(invocation);
                    **if (!result.hasException()) {**// should check value null or not ?
                        cache.put(key, result.getValue());
                    }
                    return result;
                }
            }
        }
        return invoker.invoke(invocation);
    }

}`
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.dubbo.cache.filter;
import com.alibaba.dubbo.cache.CacheFactory;
import com.alibaba.dubbo.cache.support.jcache.JCacheFactory;
import com.alibaba.dubbo.cache.support.lru.LruCacheFactory;
import com.alibaba.dubbo.cache.support.threadlocal.ThreadLocalCacheFactory;
import com.alibaba.dubbo.common.URL;
import com.alibaba.dubbo.rpc.Invoker;
import com.alibaba.dubbo.rpc.RpcInvocation;
import com.alibaba.dubbo.rpc.RpcResult;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;