com.mongodb

Class DBCursor

java.lang.Object

com.mongodb.DBCursor

All Implemented Interfaces:

Closeable, AutoCloseable, Iterable<DBObject>, Iterator<DBObject>

public class DBCursor
extends Object
implements Iterator<DBObject>, Iterable<DBObject>, Closeable

An iterator over database results. Doing a find() query on a collection returns a DBCursor thus

 DBCursor cursor = collection.find( query );
 if( cursor.hasNext() )
     DBObject obj = cursor.next();
 

Warning: Calling toArray or length on a DBCursor will irrevocably turn it into an array. This means that, if the cursor was iterating over ten million results (which it was lazily fetching from the database), suddenly there will be a ten-million element array in memory. Before converting to an array, make sure that there are a reasonable number of results using skip() and limit().

For example, to get an array of the 1000-1100th elements of a cursor, use

 List obj = collection.find( query ).skip( 1000 ).limit( 100 ).toArray();
 

MongoDB Doc Links

• cursors

Constructor Summary

Constructors

Constructor and Description
DBCursor(DBCollection collection, DBObject q, DBObject k, ReadPreference preference) Initializes a new database cursor

Method Summary

Methods

Modifier and Type	Method and Description
DBCursor	addOption(int option) adds a query option - see Bytes.QUERYOPTION_* for list
DBCursor	addSpecial(String name, Object o) adds a special operator like $maxScan or $returnKey e.g.
DBCursor	batchSize(int n) Limits the number of elements returned in one batch.
void	close() kills the current cursor on the server.
DBCursor	copy() Creates a copy of an existing database cursor.
int	count() Counts the number of objects matching the query This does not take limit/skip into consideration
DBObject	curr() Returns the element the cursor is at.
DBObject	explain() Returns an object containing basic information about the execution of the query that created this cursor This creates a DBObject with the key/value pairs: "cursor" : cursor type "nScanned" : number of records examined by the database for this query "n" : the number of records that the database returned "millis" : how long it took the database to execute the query
DBCollection	getCollection() gets the collection
long	getCursorId() gets the cursor id.
DBDecoderFactory	getDecoderFactory()
DBObject	getKeysWanted() gets the fields to be returned
int	getOptions() gets the query options
DBObject	getQuery() gets the query
ReadPreference	getReadPreference() Gets the default read preference
ServerAddress	getServerAddress() Gets the Server Address of the server that data is pulled from.
List<Integer>	getSizes() gets a list containing the number of items received in each batch
boolean	hasNext() Checks if there is another object available
DBCursor	hint(DBObject indexKeys) Informs the database of indexed fields of the collection in order to improve performance.
DBCursor	hint(String indexName) Informs the database of an indexed field of the collection in order to improve performance.
int	itcount() for testing only! Iterates cursor and counts objects
Iterator<DBObject>	iterator() creates a copy of this cursor object that can be iterated.
int	length() pulls back all items into an array and returns the number of objects.
DBCursor	limit(int n) Limits the number of elements returned.
DBObject	next() Returns the object the cursor is at and moves the cursor ahead by one.
int	numGetMores() gets the number of times, so far, that the cursor retrieved a batch from the database
int	numSeen() Returns the number of objects through which the cursor has iterated.
void	remove() Not implemented.
DBCursor	resetOptions() resets the query options
DBCursor	setDecoderFactory(DBDecoderFactory fact)
DBCursor	setOptions(int options) sets the query option - see Bytes.QUERYOPTION_* for list
DBCursor	setReadPreference(ReadPreference preference) Sets the read preference for this cursor.
int	size() Counts the number of objects matching the query this does take limit/skip into consideration
DBCursor	skip(int n) Discards a given number of elements at the beginning of the cursor.
DBCursor	slaveOk() Deprecated. Replaced with ReadPreference.secondaryPreferred()
DBCursor	snapshot() Use snapshot mode for the query.
DBCursor	sort(DBObject orderBy) Sorts this cursor's elements.
List<DBObject>	toArray() Converts this cursor to an array.
List<DBObject>	toArray(int max) Converts this cursor to an array.
String	toString()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

DBCursor

public DBCursor(DBCollection collection,
        DBObject q,
        DBObject k,
        ReadPreference preference)

Initializes a new database cursor

Parameters:

collection - collection to use

q - query to perform

k - keys to return from the query

preference - the Read Preference for this query

Method Detail

copy

public DBCursor copy()

Creates a copy of an existing database cursor. The new cursor is an iterator, even if the original was an array.

Returns:

the new cursor

iterator

public Iterator<DBObject> iterator()

creates a copy of this cursor object that can be iterated. Note: - you can iterate the DBCursor itself without calling this method - no actual data is getting copied.

Specified by:

iterator in interface Iterable<DBObject>

Returns:

sort

public DBCursor sort(DBObject orderBy)

Sorts this cursor's elements. This method must be called before getting any object from the cursor.

Parameters:

orderBy - the fields by which to sort

Returns:

a cursor pointing to the first element of the sorted results

addSpecial

public DBCursor addSpecial(String name,
                  Object o)

adds a special operator like $maxScan or $returnKey e.g. addSpecial( "$returnKey" , 1 ) e.g. addSpecial( "$maxScan" , 100 )

Parameters:

name -

o -

Returns:

MongoDB Doc Links

• specialOperators

hint

public DBCursor hint(DBObject indexKeys)

Informs the database of indexed fields of the collection in order to improve performance.

Parameters:

indexKeys - a DBObject with fields and direction

Returns:

same DBCursor for chaining operations

hint

public DBCursor hint(String indexName)

Informs the database of an indexed field of the collection in order to improve performance.

Parameters:

indexName - the name of an index

Returns:

same DBCursor for chaining operations

snapshot

public DBCursor snapshot()

Use snapshot mode for the query. Snapshot mode assures no duplicates are returned, or objects missed, which were present at both the start and end of the query's execution (if an object is new during the query, or deleted during the query, it may or may not be returned, even with snapshot mode). Note that short query responses (less than 1MB) are always effectively snapshotted. Currently, snapshot mode may not be used with sorting or explicit hints.

Returns:

same DBCursor for chaining operations

explain

public DBObject explain()

Returns an object containing basic information about the execution of the query that created this cursor This creates a DBObject with the key/value pairs: "cursor" : cursor type "nScanned" : number of records examined by the database for this query "n" : the number of records that the database returned "millis" : how long it took the database to execute the query

Returns:

a DBObject

Throws:

MongoException

MongoDB Doc Links

• explain

limit

public DBCursor limit(int n)

Limits the number of elements returned. Note: parameter n should be positive, although a negative value is supported for legacy reason. Passing a negative value will call batchSize(int) which is the preferred method.

Parameters:

n - the number of elements to return

Returns:

a cursor to iterate the results

MongoDB Doc Links

• limit

batchSize

public DBCursor batchSize(int n)

Limits the number of elements returned in one batch. A cursor typically fetches a batch of result objects and store them locally. If batchSize is positive, it represents the size of each batch of objects retrieved. It can be adjusted to optimize performance and limit data transfer. If batchSize is negative, it will limit of number objects returned, that fit within the max batch size limit (usually 4MB), and cursor will be closed. For example if batchSize is -10, then the server will return a maximum of 10 documents and as many as can fit in 4MB, then close the cursor. Note that this feature is different from limit() in that documents must fit within a maximum size, and it removes the need to send a request to close the cursor server-side. The batch size can be changed even after a cursor is iterated, in which case the setting will apply on the next batch retrieval.

Parameters:

n - the number of elements to return in a batch

Returns:

skip

public DBCursor skip(int n)

Discards a given number of elements at the beginning of the cursor.

Parameters:

n - the number of elements to skip

Returns:

a cursor pointing to the new first element of the results

Throws:

IllegalStateException - if the cursor has started to be iterated through

getCursorId

public long getCursorId()

gets the cursor id.

Returns:

the cursor id, or 0 if there is no active cursor.

close

public void close()

kills the current cursor on the server.

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable

slaveOk

@Deprecated
public DBCursor slaveOk()

Deprecated. Replaced with ReadPreference.secondaryPreferred()

makes this query ok to run on a slave node

Returns:

a copy of the same cursor (for chaining)

See Also:

ReadPreference.secondaryPreferred()

addOption

public DBCursor addOption(int option)

adds a query option - see Bytes.QUERYOPTION_* for list

Parameters:

option -

Returns:

setOptions

public DBCursor setOptions(int options)

sets the query option - see Bytes.QUERYOPTION_* for list

Parameters:

options -

resetOptions

public DBCursor resetOptions()

resets the query options

getOptions

public int getOptions()

gets the query options

Returns:

numGetMores

public int numGetMores()

gets the number of times, so far, that the cursor retrieved a batch from the database

Returns:

getSizes

public List<Integer> getSizes()

gets a list containing the number of items received in each batch

Returns:

numSeen

public int numSeen()

Returns the number of objects through which the cursor has iterated.

Returns:

the number of objects seen

hasNext

public boolean hasNext()

Checks if there is another object available

Specified by:

hasNext in interface Iterator<DBObject>

Returns:

Throws:

MongoException

next

public DBObject next()

Returns the object the cursor is at and moves the cursor ahead by one.

Specified by:

next in interface Iterator<DBObject>

Returns:

the next element

Throws:

MongoException

curr

public DBObject curr()

Returns the element the cursor is at.

Returns:

the current element

remove

public void remove()

Not implemented.

Specified by:

remove in interface Iterator<DBObject>

length

public int length()

pulls back all items into an array and returns the number of objects. Note: this can be resource intensive

Returns:

the number of elements in the array

Throws:

MongoException

See Also:

count(), size()

toArray

public List<DBObject> toArray()

Converts this cursor to an array.

Returns:

an array of elements

Throws:

MongoException

toArray

public List<DBObject> toArray(int max)

Converts this cursor to an array.

Parameters:

max - the maximum number of objects to return

Returns:

an array of objects

Throws:

MongoException

itcount

public int itcount()

for testing only! Iterates cursor and counts objects

Returns:

num objects

Throws:

MongoException

See Also:

count()

count

public int count()

Counts the number of objects matching the query This does not take limit/skip into consideration

Returns:

the number of objects

Throws:

MongoException

See Also:

size()

size

public int size()

Counts the number of objects matching the query this does take limit/skip into consideration

Returns:

the number of objects

Throws:

MongoException

See Also:

count()

getKeysWanted

public DBObject getKeysWanted()

gets the fields to be returned

Returns:

getQuery

public DBObject getQuery()

gets the query

Returns:

getCollection

public DBCollection getCollection()

gets the collection

Returns:

getServerAddress

public ServerAddress getServerAddress()

Gets the Server Address of the server that data is pulled from. Note that this information may not be available until hasNext() or next() is called.

Returns:

setReadPreference

public DBCursor setReadPreference(ReadPreference preference)

Sets the read preference for this cursor. See the * documentation for ReadPreference for more information.

Parameters:

preference - Read Preference to use

getReadPreference

public ReadPreference getReadPreference()

Gets the default read preference

Returns:

setDecoderFactory

public DBCursor setDecoderFactory(DBDecoderFactory fact)

getDecoderFactory

public DBDecoderFactory getDecoderFactory()

toString

public String toString()

Overrides:

toString in class Object