| java.lang.Object | |
| ↳ | com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer |
Class Overview
This class is a kind of specialized type-safe Map, from char array to String value. Specialization means that in addition to type-safety and specific access patterns (key char array, Value optionally interned String; values added on access if necessary), and that instances are meant to be used concurrently, but by using well-defined mechanisms to obtain such concurrently usable instances. Main use for the class is to store symbol table information for things like compilers and parsers; especially when number of symbols (keywords) is limited.
For optimal performance, usage pattern should be one where matches
should be very common (especially after "warm-up"), and as with most hash-based
maps/sets, that hash codes are uniformly distributed. Also, collisions
are slightly more expensive than with HashMap or HashSet, since hash codes
are not used in resolving collisions; that is, equals() comparison is
done with all symbols in same bucket index.
Finally, rehashing is also more expensive, as hash codes are not
stored; rehashing requires all entries' hash codes to be recalculated.
Reason for not storing hash codes is reduced memory usage, hoping
for better memory locality.
Usual usage pattern is to create a single "master" instance, and either use that instance in sequential fashion, or to create derived "child" instances, which after use, are asked to return possible symbol additions to master instance. In either case benefit is that symbol table gets initialized so that further uses are more efficient, as eventually all symbols needed will already be in symbol table. At that point no more Symbol String allocations are needed, nor changes to symbol table itself.
Note that while individual SymbolTable instances are NOT thread-safe (much like generic collection classes), concurrently used "child" instances can be freely used without synchronization. However, using master table concurrently with child instances can only be done if access to master instance is read-only (i.e. no modifications done).
Summary
| Constants | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| int | DEFAULT_TABLE_SIZE | Default initial table size. | |||||||||
| int | HASH_MULT | ||||||||||
| int | MAX_TABLE_SIZE | Let's not expand symbol tables past some maximum size; this should protected against OOMEs caused by large documents with uniquer (~= random) names. | |||||||||
| Fields | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| _buckets | Overflow buckets; if primary doesn't match, lookup is done from here. | ||||||||||
| _canonicalize | Whether any canonicalization should be attempted (whether using intern or not) | ||||||||||
| _dirty | Flag that indicates if any changes have been made to the data; used to both determine if bucket array needs to be copied when (first) change is made, and potentially if updated bucket list is to be resync'ed back to master instance. | ||||||||||
| _indexMask | Mask used to get index from hash values; equal to
_buckets.length - 1, when _buckets.length is
a power of two. |
||||||||||
| _intern | Whether canonical symbol Strings are to be intern()ed before added to the table or not | ||||||||||
| _longestCollisionList | We need to keep track of the longest collision list; this is needed both to indicate problems with attacks and to allow flushing for other cases. | ||||||||||
| _parent | Sharing of learnt symbols is done by optional linking of symbol table instances with their parents. | ||||||||||
| _size | Current size (number of entries); needed to know if and when rehash. | ||||||||||
| _sizeThreshold | Limit that indicates maximum size this instance can hold before it needs to be expanded and rehashed. | ||||||||||
| _symbols | Primary matching symbols; it's expected most match occur from here. | ||||||||||
| Public Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Helper method that takes in a "raw" hash value, shuffles it as necessary,
and truncates to be used as the index.
| |||||||||||
Method for checking number of primary hash buckets this symbol
table uses.
| |||||||||||
Implementation of a hashing method for variable length
Strings.
| |||||||||||
Method mostly needed by unit tests; calculates number of
entries that are in collision list.
| |||||||||||
Method called to create root canonicalizer for a
JsonFactory
instance. | |||||||||||
"Factory" method; will create a new child instance of this symbol
table.
| |||||||||||
Method mostly needed by unit tests; calculates length of the
longest collision chain.
| |||||||||||
| Protected Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
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[Expand]
Inherited Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
From class
java.lang.Object
| |||||||||||
Constants
protected static final int DEFAULT_TABLE_SIZE
Default initial table size. Shouldn't be miniscule (as there's cost to both array realloc and rehashing), but let's keep it reasonably small nonetheless. For systems that properly reuse factories it doesn't matter either way; but when recreating factories often, initial overhead may dominate.
public static final int HASH_MULT
protected static final int MAX_TABLE_SIZE
Let's not expand symbol tables past some maximum size; this should protected against OOMEs caused by large documents with uniquer (~= random) names.
Fields
protected Bucket[] _buckets
Overflow buckets; if primary doesn't match, lookup is done from here.
Note: Number of buckets is half of number of symbol entries, on assumption there's less need for buckets.
protected final boolean _canonicalize
Whether any canonicalization should be attempted (whether using intern or not)
protected boolean _dirty
Flag that indicates if any changes have been made to the data; used to both determine if bucket array needs to be copied when (first) change is made, and potentially if updated bucket list is to be resync'ed back to master instance.
protected int _indexMask
Mask used to get index from hash values; equal to
_buckets.length - 1, when _buckets.length is
a power of two.
protected final boolean _intern
Whether canonical symbol Strings are to be intern()ed before added to the table or not
protected int _longestCollisionList
We need to keep track of the longest collision list; this is needed both to indicate problems with attacks and to allow flushing for other cases.
protected CharsToNameCanonicalizer _parent
Sharing of learnt symbols is done by optional linking of symbol
table instances with their parents. When parent linkage is
defined, and child instance is released (call to release),
parent's shared tables may be updated from the child instance.
protected int _size
Current size (number of entries); needed to know if and when rehash.
protected int _sizeThreshold
Limit that indicates maximum size this instance can hold before it needs to be expanded and rehashed. Calculated using fill factor passed in to constructor.
Public Methods
public final int _hashToIndex (int rawHash)
Helper method that takes in a "raw" hash value, shuffles it as necessary, and truncates to be used as the index.
public int bucketCount ()
Method for checking number of primary hash buckets this symbol table uses.
public int calcHash (char[] buffer, int start, int len)
Implementation of a hashing method for variable length Strings. Most of the time intention is that this calculation is done by caller during parsing, not here; however, sometimes it needs to be done for parsed "String" too.
Parameters
| len | Length of String; has to be at least 1 (caller guarantees this pre-condition) |
|---|
public int collisionCount ()
Method mostly needed by unit tests; calculates number of
entries that are in collision list. Value can be at most
(size() - 1), but should usually be much lower, ideally 0.
public static CharsToNameCanonicalizer createRoot ()
Method called to create root canonicalizer for a JsonFactory
instance. Root instance is never used directly; its main use is for
storing and sharing underlying symbol arrays as needed.
public int hashSeed ()
public CharsToNameCanonicalizer makeChild (boolean canonicalize, boolean intern)
"Factory" method; will create a new child instance of this symbol table. It will be a copy-on-write instance, ie. it will only use read-only copy of parent's data, but when changes are needed, a copy will be created.
Note: while this method is synchronized, it is generally not safe to both use makeChild/mergeChild, AND to use instance actively. Instead, a separate 'root' instance should be used on which only makeChild/mergeChild are called, but instance itself is not used as a symbol table.
public int maxCollisionLength ()
Method mostly needed by unit tests; calculates length of the
longest collision chain. This should typically be a low number,
but may be up to size() - 1 in the pathological case