Java Magic java.net.URL【 translate 】

Recently found a very interesting code segment ：

HashSet set = new HashSet();
set.add(new URL("http://google.com"));
System.out.println(set.contains(new URL("http://google.com")));
Thread.sleep(60000);
System.out.println(set.contains(new URL("http://google.com")));

Guess what , The first 3、5 What does the guild output ？

The result must not be true,true.

Okay , In most cases, the result may be true, false .

If you shut down the network and run it again, the result may be ：true,true 了 .

The reason for this is java.net.URL Class hashCode() and equals() The specific implementation of the method results in .

Let’s take a look at it hashCode Method ：

public synchronized int hashCode() {

if (hashCode != -1)
return hashCode;
hashCode = handler.hashCode(this);
return hashCode;
}
private int hashCode = -1;

We can see hashCode It’s a member variable , Calculate only once .

Be careful ,java.net.URL Class is immutable .

that handler What is it ？ yes URLStreamHandler A subclass of , Depends on the agreement （file、http、ftp…）.

If you are interested, please check ： java.net.URLStreamHandler#hashCode The logic of .

Let’s take a look at URL.hashCode() Of javadoc explain ：

Based on Web site comparison , It’s a blocking operation ！

OMG!! It’s a blocking operation ！！

Another exciting thing is ,handler Will parse the host ip Address to calculate the hash value . If it doesn’t work , Based on the domain name http://google.com Calculate the hash value .

If ip Is dynamic , Or there is request load balancing , Then the host’s ip It’s also dynamic —— So we might get out of sync hash values , In this way HashSet Namely 2 Different examples of .

It’s not good at all , By the way ,hashCode and equals The performance is terrible —— because URLStreamHandler Will open URLConnection.

So how to avoid this happening ？

• Use java.net.URI Replace java.net.URL; This is not the best choice , But there is also a definite hash implementation .
• Don’t use… In collections java.net.URL, If you really want to do this , It is recommended to use a representative URL Of String Objects in the collection .
• When you calculate the hash value , Broken net ！！！—— I’m kidding …
• Write your own URLStreamHandler Subclasses implement the appropriate hashCode Method .

appendix

Reference resources ：http://mishadoff.com/blog/java-magic-part-1-java-dot-net-dot-url/