Class CharsToNameCanonicalizer
- java.lang.Object
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- com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer
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public final class CharsToNameCanonicalizer extends java.lang.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).
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Field Summary
Fields Modifier and Type Field Description protected com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer.Bucket[]
_buckets
Overflow buckets; if primary doesn't match, lookup is done from here.protected boolean
_canonicalize
Whether any canonicalization should be attempted (whether using intern or not.protected int
_flags
protected boolean
_hashShared
Flag that indicates whether underlying data structures for the main hash area are shared or not.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 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 java.util.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.protected CharsToNameCanonicalizer
_parent
Sharing of learnt symbols is done by optional linking of symbol table instances with their parents.protected 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.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.protected java.lang.String[]
_symbols
Primary matching symbols; it's expected most match occur from here.protected java.util.concurrent.atomic.AtomicReference<com.fasterxml.jackson.core.sym.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.static int
HASH_MULT
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description 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
bucketCount()
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
calcHash(java.lang.String key)
int
collisionCount()
Method mostly needed by unit tests; calculates number of entries that are in collision list.static CharsToNameCanonicalizer
createRoot()
Method called to create root canonicalizer for aJsonFactory
instance.protected static CharsToNameCanonicalizer
createRoot(int seed)
java.lang.String
findSymbol(char[] buffer, int start, int len, int h)
int
hashSeed()
CharsToNameCanonicalizer
makeChild(int flags)
"Factory" method; will create a new child instance of this symbol table.int
maxCollisionLength()
Method mostly needed by unit tests; calculates length of the longest collision chain.boolean
maybeDirty()
void
release()
Method called by the using code to indicate it is done with this instance.protected void
reportTooManyCollisions(int maxLen)
int
size()
protected void
verifyInternalConsistency()
Diagnostics method that will verify that internal data structures are consistent; not meant as user-facing method but only for test suites and possible troubleshooting.
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Field Detail
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HASH_MULT
public static final int HASH_MULT
- See Also:
- Constant Field Values
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_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 torelease
), parent's shared tables may be updated from the child instance.
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_tableInfo
protected final java.util.concurrent.atomic.AtomicReference<com.fasterxml.jackson.core.sym.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.
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_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.- Since:
- 2.1
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_flags
protected final int _flags
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_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.
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_symbols
protected java.lang.String[] _symbols
Primary matching symbols; it's expected most match occur from here.
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_buckets
protected com.fasterxml.jackson.core.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.
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_size
protected int _size
Current size (number of entries); needed to know if and when rehash.
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_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.
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_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.
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_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:
- 2.1
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_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)
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_overflows
protected java.util.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.- Since:
- 2.4
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Method Detail
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createRoot
public static CharsToNameCanonicalizer createRoot()
Method called to create root canonicalizer for aJsonFactory
instance. Root instance is never used directly; its main use is for storing and sharing underlying symbol arrays as needed.- Returns:
- Root instance to use for constructing new child instances
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createRoot
protected static CharsToNameCanonicalizer createRoot(int seed)
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makeChild
public CharsToNameCanonicalizer makeChild(int flags)
"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.
- Parameters:
flags
- Bit flags of activeJsonFactory.Feature
s enabled.- Returns:
- Actual canonicalizer instance that can be used by a parser
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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.
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size
public int size()
- Returns:
- Number of symbol entries contained by this canonicalizer instance
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bucketCount
public int bucketCount()
Method for checking number of primary hash buckets this symbol table uses.- Returns:
- number of primary slots table has currently
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maybeDirty
public boolean maybeDirty()
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hashSeed
public int hashSeed()
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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
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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 tosize()
- 1 in the pathological case- Returns:
- Length of the collision chain
- Since:
- 2.1
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findSymbol
public java.lang.String findSymbol(char[] buffer, int start, int len, int h)
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_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
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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 decodestart
- Pointer to the first character of the namelen
- Length of String; has to be at least 1 (caller guarantees)- Returns:
- Hash code calculated
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calcHash
public int calcHash(java.lang.String key)
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reportTooManyCollisions
protected void reportTooManyCollisions(int maxLen)
- Parameters:
maxLen
- Maximum allowed length of collision chain- Since:
- 2.1
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verifyInternalConsistency
protected void verifyInternalConsistency()
Diagnostics method that will verify that internal data structures are consistent; not meant as user-facing method but only for test suites and possible troubleshooting.- Since:
- 2.10
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