Class CharsToNameCanonicalizer

java.lang.Object
tools.jackson.core.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 Details

    • HASH_MULT

      public static final int HASH_MULT
      See Also:

    • DEFAULT_T_SIZE

      private static final int DEFAULT_T_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. For systems that properly reuse factories it doesn't matter either way; but when recreating factories often, initial overhead may dominate.
      See Also:

    • MAX_T_SIZE

      private static final int MAX_T_SIZE
      Let's not expand symbol tables past some maximum size; this should protected against OOMEs caused by large documents with unique (~= random) names.
      See Also:

    • MAX_ENTRIES_FOR_REUSE

      public static final int MAX_ENTRIES_FOR_REUSE
      Let's only share reasonably sized symbol tables. Max size set to 3/4 of 16k; this corresponds to 64k main hash index. This should allow for enough distinct names for almost any case.
      See Also:

    • MAX_COLL_CHAIN_LENGTH

      static final int MAX_COLL_CHAIN_LENGTH
      Also: to thwart attacks based on hash collisions (which may or may not be cheap to calculate), we will need to detect "too long" collision chains.

      Note: longest chain we have been able to produce without malicious intent has been 38 (with "tools.jackson.core.main.TestWithTonsaSymbols"); our setting should be reasonable here.

      See Also:

    • _parent

      protected final 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.

    • _tableInfo

      protected final AtomicReference<CharsToNameCanonicalizer.TableInfo> _tableInfo
      Member that is only used by the root table instance: root passes immutable state info child instances, and children may return new state if they add entries to the table. Child tables do NOT use the reference.

    • _streamReadConstraints

      protected final StreamReadConstraints _streamReadConstraints
      Constraints used by TokenStreamFactory that uses this canonicalizer.
      Since:
      2.16

    • _seed

      protected final int _seed
      Seed value we use as the base to make hash codes non-static between different runs, but still stable for lifetime of a single symbol table instance. This is done for security reasons, to avoid potential DoS attack via hash collisions.

    • _factoryFeatures

      protected final int _factoryFeatures
      Feature flags of TokenStreamFactory that uses this canonicalizer.

    • _canonicalize

      protected boolean _canonicalize
      Whether any canonicalization should be attempted (whether using intern or not.

      NOTE: non-final since we may need to disable this with overflow.

    • _symbols

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

    • _buckets

      protected 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.

    • _hashShared

      protected boolean _hashShared
      Flag that indicates whether underlying data structures for the main hash area are shared or not. If they are, then they need to be handled in copy-on-write way, i.e. if they need to be modified, a copy needs to be made first; at this point it will not be shared any more, and can be modified.

      This flag needs to be checked both when adding new main entries, and when adding new collision list queues (i.e. creating a new collision list head entry)

    • _overflows

      protected BitSet _overflows
      Lazily constructed structure that is used to keep track of collision buckets that have overflowed once: this is used to detect likely attempts at denial-of-service attacks that uses hash collisions.

  • Constructor Details

  • Method Details

    • _thresholdSize

      private static int _thresholdSize(int hashAreaSize)

    • createRoot

      public static CharsToNameCanonicalizer createRoot(TokenStreamFactory owner)
      instance. Root instance is never used directly; its main use is for storing and sharing underlying symbol arrays as needed.
      Parameters:
      owner - Factory that will use the root instance; used for accessing configuration
      Returns:
      Root instance to use for constructing new child instances

    • createRoot

      public static CharsToNameCanonicalizer createRoot(TokenStreamFactory owner, int seed)

    • makeChild

      public CharsToNameCanonicalizer makeChild()
      "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: 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.

      Returns:
      Actual canonicalizer instance that can be used by a parser

    • release

      public void release()
      Method called by the using code to indicate it is done with this instance. This lets instance merge accumulated changes into parent (if need be), safely and efficiently, and without calling code having to know about parent information.

    • mergeChild

      private void mergeChild(CharsToNameCanonicalizer.TableInfo childState)
      Method that allows contents of child table to potentially be "merged in" with contents of this symbol table.

      Note that caller has to make sure symbol table passed in is really a child or sibling of this symbol table.

    • size

      public int size()
      Returns:
      Number of symbol entries contained by this canonicalizer instance

    • bucketCount

      public int bucketCount()
      Method for checking number of primary hash buckets this symbol table uses.
      Returns:
      number of primary slots table has currently

    • 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.
      Returns:
      Number of collisions in the primary hash area
      Since:
      2.1

    • 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
      Returns:
      Length of the collision chain
      Since:
      2.1

    • findSymbol

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

    • _findSymbol2

      private String _findSymbol2(char[] buffer, int start, int len, CharsToNameCanonicalizer.Bucket b)

    • _addSymbol

      private String _addSymbol(char[] buffer, int start, int len, int h, int index)

    • _handleSpillOverflow

      private void _handleSpillOverflow(int bucketIndex, CharsToNameCanonicalizer.Bucket newBucket, int mainIndex)
      Method called when an overflow bucket has hit the maximum expected length: this may be a case of DoS attack. Deal with it based on settings by either clearing up bucket (to avoid indefinite expansion) or throwing exception. Currently, the first overflow for any single bucket DOES NOT throw an exception, only second time (per symbol table instance)

    • _hashToIndex

      public 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.
      Parameters:
      rawHash - Raw hash value to use for calculating index
      Returns:
      Index value calculated

    • 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:
      buffer - Input buffer that contains name to decode
      start - Pointer to the first character of the name
      len - Length of String; has to be at least 1 (caller guarantees)
      Returns:
      Hash code calculated

    • calcHash

      public int calcHash(String key)

    • copyArrays

      private void copyArrays()
      Method called when copy-on-write is needed; generally when first change is made to a derived symbol table.

    • rehash

      private void rehash()
      Method called when size (number of entries) of symbol table grows so big that load factor is exceeded. Since size has to remain power of two, arrays will then always be doubled. Main work is really redistributing old entries into new String/Bucket entries.

    • _reportTooManyCollisions

      protected void _reportTooManyCollisions(int maxLen)

    • verifyInternalConsistency

      public void verifyInternalConsistency()