This is obviously the case for strings (and is special-cased in HashMap), but it should also be the case for every class used as a map key, especially if the data is potentially supplied by a hostile user. Now, if too many hash codes map to the same bucket in the map, the list of entries can be changed into a balanced binary tree, sorted first by hash code and then by each key’s compareTo method, as long as the keys are Comparable. This is fine for strings and HashMaps, but not so good for custom keys.Ī better approach was implemented in JEP 180 for Java 8. The String class gained a new hash32 method and field which used MurmurHash3 with a random seed. Java itself implemented a defense in the String class early in the Java 7 releases, described in the Collections Framework Enhancements in Java 7. It’s not that hard to defend against I know Jetty still limits form content to 200KB and 1,000 keys, and other servers have similar limits. Return Type: This method returns the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys. In Java, for example, the strings "fg", "gH" and "h)" all have a hash code of 3265, so an attacker can build query strings like fgfg=0&fggH=0&fgh)=0&gHfg=0 and so on to waste the server’s time and mount an effective denial of service. You can read about the original discovery in CERT VU#903934 and the original post in the Full Disclosure list archives, or on YouTube from 28c3. Since Java 5.0, the Comparator interface is generic that means when you implement it, you can specify what type of objects your comparator can compare.Ĭode listing 5.7: CustomerComparator.This article is inspired by the discovery years ago that an attacker could manipulate query parameters to turn the map data structure in many web servers into a linked list, by making all the parameter names end up in the same hash bucket, which CVE-2018-0875 reminded me of. These conditions have the goal of allowing objects to be fully sorted, much like the sorting of a database result set on all fields. The operator is used to compare the map keys and values. The compareTo method needs to satisfy the following conditions. It has to implement the Comparator interface. Two maps are equal if their key/value pairs are identical, regardless of the order of those pairs. We need to create a class for each way of ordering. We may want to sort by name or by address. We may want to order descending or ascending order. ![]() Sometimes we may want to change the ordering of a collection of objects from the same class. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method.Ĭhange Sorting/Ordering This is because sorted sets (and sorted maps) without explicit comparators behave "strangely" when they are used with elements (or keys) whose natural ordering is inconsistent with equals. It is strongly recommended (though not required) that natural orderings be consistent with equals. The comparator () method of interface is used to return the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys. Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false. However, its quite similar in nature to equals and. ![]() ![]() The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo((Object) e2) = 0 has the same boolean value as e1.equals((Object) e2) for every e1 and e2 of class C. The compareTo method is the sole member of the Comparable interface, and is not a member of Object. Objects that implement this interface can be used as keys in a sorted map or elements in a sorted set, without the need to specify a comparator. Customer cust1 = new Customer () Customer cust2 = new Customer () //.
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