Avoid storing references of java.net.URL

Normally, I avoid writing something so obvious but since I'm bitten multiple times now, it might help future me.

Never ever store references of java.net.URL in Java collections. The reasoning is pretty simple, 'equals' and 'hashCode' methods of this class does extremely expensive synchronous DNS lookup on every call.

It is not uncommon to see most of your thread's time being spent on monitors:

"pool-2-thread-2" prio=10 tid=0x92061400 nid=0x1744 waiting for monitor entry [0x91fad000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at java.net.URLStreamHandler.getHostAddress(URLStreamHandler.java:429)
    - waiting to lock <0x9731b200> (a sun.net.www.protocol.http.Handler)
    at java.net.URLStreamHandler.hashCode(URLStreamHandler.java:354)
    at java.net.URL.hashCode(URL.java:875)
    - locked <0xaac87290> (a java.net.URL)
    at java.util.HashMap.getEntry(HashMap.java:361)
    at java.util.HashMap.containsKey(HashMap.java:352)
    at java.util.HashSet.contains(HashSet.java:201)


"pool-2-thread-1" prio=10 tid=0x9205e800 nid=0x1743 runnable [0x91ffe000]
   java.lang.Thread.State: RUNNABLE
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
    at java.net.InetAddress$1.lookupAllHostAddr(InetAddress.java:866)
    at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1258)
    at java.net.InetAddress.getAllByName0(InetAddress.java:1211)
    at java.net.InetAddress.getAllByName(InetAddress.java:1127)
    at java.net.InetAddress.getAllByName(InetAddress.java:1063)
    at java.net.InetAddress.getByName(InetAddress.java:1013)
    at java.net.URLStreamHandler.getHostAddress(URLStreamHandler.java:437)
    - locked <0x9731b200> (a sun.net.www.protocol.http.Handler)
    at java.net.URLStreamHandler.hashCode(URLStreamHandler.java:354)
    at java.net.URL.hashCode(URL.java:875)
    - locked <0xaac97228> (a java.net.URL)
    at java.util.HashMap.getEntry(HashMap.java:361)
    at java.util.HashMap.containsKey(HashMap.java:352)
    at java.util.HashSet.contains(HashSet.java:201)

This stack-trace just depicts hashCode but expect similar blocking code for 'equals' too. If you care a bit about performance, just stay away from this goddamned class.