什么是Java 8中String键的替代散列？

Question

Java 8 is providing alternative hashing for String keys to improve performance when a large number of key hash code collisions are encountered. Can anybody explain what is that and how it will work?

Answer 1

To bring more relevance to this question, the alternative hashing has been removed from JDK 8. Check out :

http://docs.oracle.com/javase/8/docs/technotes/guides/collections/changes8.html

http://openjdk.java.net/jeps/180

It is interesting to note that once the number of items in a hash bucket grows beyond a certain threshold, that bucket will switch from using a linked list of entries to a balanced tree.

The hash(Object key) function in the HashMap has been revised to follows with no special treatment to String objects:

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

Answer 2

From this email of core-lib-devs@openjkd :

• A new interface Hashable32 is introduced.
• Hashable32 provides a method hash32()
• String implements Hashable32 and hash32() method
• HashMap et al recognize String and invoke hash32() rather than hashCode()

The revisions of the code:

• Murmur3 : https://code.google.com/p/smhasher/wiki/MurmurHash3
• althashing "7" webrev : http://cr.openjdk.java.net/~mduigou/althashing7/8/webrev/
• althashing "8" webrev : http://cr.openjdk.java.net/~mduigou/althashing8/8/webrev/

Answer 3

值得注意的是，转向MurmurHash3不会阻止DoS攻击： http ： //emboss.github.com/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/