Refer to constants instead of literals

[BurningCN]

What is the purpose of the change

Refer to constants instead of literals

[codecov-commenter]

Codecov Report

Merging #7688 (0d3c4fe) into master (f29663b) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master    #7688      +/-   ##
============================================
+ Coverage     59.18%   59.20%   +0.01%     
  Complexity      529      529              
============================================
  Files          1076     1076              
  Lines         43540    43540              
  Branches       6363     6363              
============================================
+ Hits          25771    25779       +8     
+ Misses        14903    14900       -3     
+ Partials       2866     2861       -5     

Impacted Files	Coverage Δ	Complexity Δ
...vent/listener/ServiceInstancesChangedListener.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...java/org/apache/dubbo/rpc/cluster/RouterChain.java	96.87% <100.00%> (ø)	0.00 <0.00> (ø)
...e/dubbo/remoting/transport/netty4/NettyClient.java	71.01% <100.00%> (ø)	0.00 <0.00> (ø)
...che/dubbo/remoting/transport/mina/MinaChannel.java	35.52% <0.00%> (-10.53%)	14.00% <0.00%> (-1.00%)
...ng/transport/dispatcher/all/AllChannelHandler.java	82.75% <0.00%> (-6.90%)	0.00% <0.00%> (ø%)
...apache/dubbo/common/extension/ExtensionLoader.java	79.95% <0.00%> (+0.22%)	0.00% <0.00%> (ø%)
...pache/dubbo/registry/support/AbstractRegistry.java	79.62% <0.00%> (+1.48%)	0.00% <0.00%> (ø%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	64.35% <0.00%> (+2.97%)	0.00% <0.00%> (ø%)
...he/dubbo/remoting/transport/netty/NettyServer.java	73.68% <0.00%> (+3.50%)	9.00% <0.00%> (+1.00%)
...dubbo/remoting/exchange/support/DefaultFuture.java	87.17% <0.00%> (+6.83%)	0.00% <0.00%> (ø%)

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[xiaoheng1]

LGTM