URLStreamHandler.getHostAddress() performance

[Tom Hawtin]

On 25/11/2014 12:02, Wang Weijun wrote:
> I am benchmarking security manager and notice this
>
> protected synchronized InetAddress getHostAddress(URL u) {
> [...]
>
> Is there any reason why the method is synchronized? Why not simply "synchronized (u)"?

Synchronizing on a public object is usually a bad idea. For instance 
Thread.join has had some interesting interactions with non-Java library 
code, and AFAIK that wasn't sprung in an update.

If the address resolution is not pinned, then unsynchonised access could 
result in the hostAddress changing, which is bad. However, it seems the 
major useful effect is to stop a large number of parallel DNS lookups 
for the same address. Neither InetAddress nor DNSNameService does not 
appear to do that itself, though I've not looked deeply into the mechanism.

So, I would suggest a lock/compare-and-set for just the write to the 
previously null URL.hostAddress, and locking based on the value of 
URL.getHost (don't synchronise on String (that'd be a little like 
synchronising on URL!)!). There's probably several places in the Java 
library doing something similar with Maps and Futures.

Tom