Class CharsToNameCanonicalizer

java.lang.Object
org.codehaus.jackson.sym.CharsToNameCanonicalizer

public final class CharsToNameCanonicalizer extends Object
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).

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    protected org.codehaus.jackson.sym.CharsToNameCanonicalizer.Bucket[]
    Overflow buckets; if primary doesn't match, lookup is done from here.
    protected final boolean
    Whether any canonicalization should be attempted (whether using intern or not)
    protected boolean
    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
    Mask used to get index from hash values; equal to _buckets.length - 1, when _buckets.length is a power of two.
    protected final boolean
    Whether canonical symbol Strings are to be intern()ed before added to the table or not
    protected int
    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.
    Sharing of learnt symbols is done by optional linking of symbol table instances with their parents.
    protected int
    Current size (number of entries); needed to know if and when rehash.
    protected int
    Limit that indicates maximum size this instance can hold before it needs to be expanded and rehashed.
    protected String[]
    Primary matching symbols; it's expected most match occur from here.
    protected static final int
    Default initial table size.
    static final int
     
    protected static final int
    Let's not expand symbol tables past some maximum size; this should protected against OOMEs caused by large documents with uniquer (~= random) names.
  • Method Summary

    Modifier and Type
    Method
    Description
    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.
    int
    Method for checking number of primary hash buckets this symbol table uses.
    int
    calcHash(char[] buffer, int start, int len)
    Implementation of a hashing method for variable length Strings.
    int
     
    int
    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.
    protected static CharsToNameCanonicalizer
    createRoot(int hashSeed)
     
    findSymbol(char[] buffer, int start, int len, int h)
     
    int
     
    makeChild(boolean canonicalize, boolean intern)
    "Factory" method; will create a new child instance of this symbol table.
    int
    Method mostly needed by unit tests; calculates length of the longest collision chain.
    boolean
     
    void
     
    protected void
     
    int
     

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
  • Field Details

    • HASH_MULT

      public static final int HASH_MULT
      See Also:
    • DEFAULT_TABLE_SIZE

      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.
      See Also:
    • MAX_TABLE_SIZE

      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.
      See Also:
    • _parent

      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.
    • _intern

      protected final boolean _intern
      Whether canonical symbol Strings are to be intern()ed before added to the table or not
    • _canonicalize

      protected final boolean _canonicalize
      Whether any canonicalization should be attempted (whether using intern or not)
    • _symbols

      protected String[] _symbols
      Primary matching symbols; it's expected most match occur from here.
    • _buckets

      protected org.codehaus.jackson.sym.CharsToNameCanonicalizer.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.

    • _size

      protected int _size
      Current size (number of entries); needed to know if and when rehash.
    • _sizeThreshold

      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.
    • _indexMask

      protected int _indexMask
      Mask used to get index from hash values; equal to _buckets.length - 1, when _buckets.length is a power of two.
    • _longestCollisionList

      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.
      Since:
      1.9.9
    • _dirty

      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.
  • Method Details

    • createRoot

      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.
    • createRoot

      protected static CharsToNameCanonicalizer createRoot(int hashSeed)
    • makeChild

      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.

    • release

      public void release()
    • size

      public int size()
    • bucketCount

      public int bucketCount()
      Method for checking number of primary hash buckets this symbol table uses.
      Since:
      1.9.9
    • maybeDirty

      public boolean maybeDirty()
    • hashSeed

      public int hashSeed()
    • collisionCount

      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.
      Since:
      1.9.9
    • maxCollisionLength

      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
      Since:
      1.9.9
    • findSymbol

      public String findSymbol(char[] buffer, int start, int len, int h)
    • _hashToIndex

      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.
    • calcHash

      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)
    • calcHash

      public int calcHash(String key)
    • reportTooManyCollisions

      protected void reportTooManyCollisions(int maxLen)
      Since:
      1.9.9